Below we present a science background for each of the five themes of the ALMA proposal categories, including some recent results when applicable. The intention with this expose is not to be exhaustive regarding the science that can be done with ALMA, but rather serve as a general guideline and inspiration to what is possible to achieve with the ALMA observatory. The recent results will be updated on a regular basis, so please stay tuned.
The details of the formation and evolution of galaxies is not completely understood, and remains one of the most important science goals of observational and theoretical cosmology today. ALMA will be a premier tool for studying various types of galaxies across the history of the universe. Understanding the true cosmic star formation history is important for constraining galaxy formation models. Multi-wavelength studies revealed that the cosmic star formation rate gradually rises by one order of magnitude from the present-day to redshift z=2 (~10 Gyr ago). However, at z>2, the contribution from the dusty star-forming galaxy population is still uncertain due to the lack of sensitivity of millimeter (mm) /submillimeter (submm) instruments before the ALMA-era. Searching for the first galaxies that emerged from the cosmic "dark ages" (at z>7) is also a big observational challenge for galaxy formation studies.
ALMA will trace the redshifted emission from near and beyond the peak of the dust emission. For high-z sources, negative k-correction overcomes the effect of the inverse square law and surface brightness dimming, making ALMA observations nearly independent of redshift for z ~ 0.5 – 10 (see Fig 1). The submm bright population is not the only target for ALMA, emission from fainter more normal (MW type) galaxies, selected through optical surveys (e.g. LBGs, LAEs etc.), will also be studied. The observed continuum emission can be used to trace the dust mass and temperature, the dusty star formation activity, and also the ISM mass. Because of its capability to make extremely deep, high-resolution images of high-redshift galaxies, ALMA will be an excellent and unique instrument for pinpointing SMGs selected from single-dish surveys with coarse beam-sizes, for studying gravitational lensing, and for resolving the mm / submm extragalactic background light which is likely originates from fainter normal galaxies.
Figure 1 - The predicted flux density of a dusty galaxy as a function of redshift in various submm / mm atmospheric windows (Blain et al. 2002).
In addition to studies of the dust continuum emission, ALMA’s strength is in its capability to detect and image line emission from molecular/atomic gas, which is an important diagnostic to trace the cold/warm component in distant galaxies. The cold gas component is thought to be the constituents for new stars, but its distribution and properties are likely regulated by the complex interplay between accretion (e.g. from major/minor mergers and cold mode accretion) and mass loss (e.g. from stellar/AGN outflows). Another advantage for ALMA, because of its superb sensitivity, is to determine the redshifts of dusty starburst galaxies by detecting one or more emission lines in the ALMA bands.
The FIR atomic lines are tracers of the neutral/ionized medium, and they are redshifted to the ALMA bands. Singly ionized carbon ([CII] 158 micron) is thought to be the dominant coolant in the ISM and the strongest fine structure line from galaxies. The [CI] line arises from similar regions as the CO emission. Other notable lines are [NII], [OI] and [OIII]; the ratios of these fine structure lines can be used to diagnose the strength of the radiation field or metallicity. This allows us to study very high-z sources from the reionization epoch, possibly enabling us to study the first galaxies.
Using the ALMA bands, we will be able to observe the CO emission from galaxies located at various redshifts across the Universe, with nearly continuous coverage in redshift. While the lowest-J CO line is redshifted out of ALMA Band 3 for z>0.37, higher-J rotational CO lines, which trace the the warm/dense gas directly associated with star-formation/AGN activity, are observable across a large redshift range. The very high-J rotational CO lines, observed with the Herschel satellite in nearby sources, can be observed with ALMA for high redshift galaxies. Example of the science that can be carried out using these lines are (to name a few): (1) the distribution and kinematics of the molecular emission, (2) molecular excitation, (3) gas/dynamical mass, (4) inflow/outflow, (5) properties of star formation, (6) metallicity. Other notable molecular lines include dense gas tracers HCN, HCO+, the shock tracer SiO, optically thin 13CO, and H2O (high critical densities (> 108 cm-3)), just to name a few.
Figure 2 – Redshifted frequencies of molecular and atomic lines observable in the ALMA bands. The filled circles correspond to lines detected at the time of publication (from Carilli & Walter 2013).
References: Blain et al. 2002, PhR, 369, 111; Carilli & Walter 2013, ARAA, 51, 105
Hodge et al., Karim et al., and Swinbank et al. observed 122 submm sources selected from the LABOCA Extended Chandra Deep Field South Submillimeter Survey (LESS) using Band 7 of ALMA. With 1.5” resolution, they were able to pinpoint the SMGs contributing the submillimeter emission in the LABOCA map, showing that the brightest sources in the original LESS sample comprise emission from multiple fainter SMGs. They also serendipitously detected bright emission lines in two of the SMG spectra which are likely [CII] 158 micron emission at z=4.42 and z=4.44, demonstrating that ALMA is able to detect the dominant fine-structure cooling lines from SMGs even with short (2 min) integrations. See Wang et al., Swinbank et al., Simpson et al., Thomson et al., and Chen et al. for related work on LESS.
Figure 3 – ALMA Band 7 observations reveal that the brightest submm sources (S870μm>12mJy) in the LESS survey comprise multiple, fainter galaxies (Karim et al. 2013).
Using ALMA Bands 3 and 7, Vieira et al., Weiss et al., and Hezaveh et al. observed strongly gravitationally lensed sources sampled originally from the South Pole Telescope (SPT) survey. They find that the sources are indeed composed of multiple components, indicative of gravitational lensing. Their gravitational lensing model suggests that the sources are amplified by factors of 4 – 22, which suggests that the lensed sources are ultra luminous starburst galaxies at high-z. Their blind redshift search in band 3 resulted in line detections in 23 sources, with 44 line features in the spectra, providing secure redshifts for ~70% of the sample. Their new analysis gave a mean redshift of z=3.5, and found that a significant portion of SMGs are indeed at high-z (z>4). These new findings will impact our current understanding of the formation of massive galaxies at high-z. Spilker et al. analyzed the stacked spectrum of 22 SPT sources from 250 to 770 GHz, and detected spectral features of 12CO, 13CO, HCN, HNC, HCO+, [CI], H2O.
Figure 4 – ALMA Band 3 redshift survey of 26 lensed galaxies selected from the South Pole Telescope (SPT) SZ survey (Vieira et al. 2013) [Reprinted by permission from Macmillan Publishers Ltd: Nature, Vieira J.D. et al. 2013, vol 495, Issue 7441, p. 344].
Using Band 6, Nagao et al. observed a z=4.8 SMG selected from the LESS survey. They detected the [NII] 205 micron emission line and assessed the metallicity of the SMG from the [NII] 205 micron and [CII] 158 micron flux ratio. They find that the metallicity in the SMG is consistent with solar, implying that the chemical evolution has progressed very rapidly in high-z SMGs.
A z=5.3 SMG called AzTEC-3 was observed in the [CII] and OH lines by Riechers et al. Their spatially resolved image allowed them to derive the surface density of the star formation rate (SFRD), and they find that it is comparable to the Eddington limit for radiation pressure supported disks. In addition to AzTEC-3, they detected the [CII] line in three Lyman Break Galaxies (LBGs) that are clustered around AzTEC-3 (Figure 5). These new measurements show that ALMA can detect the ISM in “typical” galaxies in the early universe. [CII] emission has been detected in other distant galaxies as well (De Breuck et al.).
Figure 5 - Artist's impression of the protocluster observed by ALMA. It shows the central starburst galaxy AzTEC-3 along with its labeled cohorts of smaller, less active galaxies. New ALMA observations suggest that AzTEC-3 recently merged with another young galaxy and that the whole system represents the first steps toward forming a galaxy cluster. | Credit: B. Saxton
References: Hodge et al. 2013, ApJ, 768, 91; Karim et al. 2013, MNRAS, 432, 2; Swinbank et al. 2012, 427, 1066; Nagao et al. 2012, A&A, 542, L34; Vieira et al. 2013, Nature, 495, 344; Weiss et al. 2013, ApJ, 767, 88; Hezaveh et al. 2013, ApJ, 767, 132; Riechers et al. ApJ, 2014, 796, 84; De Breuck et al. A&A, 2014, 565, 59; Wang et al. 2013, ApJ, 778, 179; Swinbank et al., 2014, MNRAS, 438, 1267; Simpson et al., 2014, ApJ, 788, 125; Thomson et al., 2014, MNRAS, 442, 577; Chen et al., 2015, ApJ, 299, 194; Spilker et al. 2014, ApJ, 785, 1492
Extragalactic Background Light
Hatsukade et al. serendipitously detected 15 faint "sub-mJy sources" in Band 6 data targeting 20 star-forming galaxies at z~1.4. They obtained source number counts at the faintest flux range among surveys at mm wavelengths, suggesting that ~80% of the extragalactic background light at mm /submm wavelengths come from such fainter galaxies. A similar result was obtained by Ono et al. 2014.
Figure 6 – ALMA Band 6 observations constrain the faint mm source number counts (Hatsukade et al. 2013).
References: Hatsukade et al. 2013, ApJ, 769, 27; Ono et al. 2014, ApJ, 795, 5
Using ALMA Band 7, Wang et al. observed the host galaxies of two gamma-ray bursts (GRBs) at z>2. The 345 GHz continuum observations show that the two host galaxies are at the faint end of the dusty galaxy population that gives rise to the submm extragalactic background light. Hatsukade et al. observed molecular gas and dust continuum in two GRB host galaxies, GRB 020819B and GRB 051022. Observations of the GRB 020819B revealed a remarkably dust-rich environment in the outskirts of the host galaxy, whereas molecular gas was found only around its centre.
References: Wang et al. 2012, ApJ, 761, L32; Hatsukade et al. 2014, Nature, 510, 247; Michalowski et al. 2014, A&A, 562, 70; Berger et al. 2014, ApJ, 796, 96
Wang et al. and Willott et al. observed z=5.8-6.4 QSOs using Band 6 and 7 of ALMA. They estimated the dynamical masses of the QSO host galaxies using the [CII] 158 micron emission line. They find that the dynamical masses are significantly lower than expected from the local black hole - velocity dispersion (or bulge mass) correlation, indicating that stellar mass growth lags black hole growth for high-z QSOs.
References: Willott et al. 2013, ApJ, 770, 13; Wang et al. 2013, ApJ, 773, 44
Ouchi et al. and Ota et al. observed z=6.6-7.0 LAEs using Band 6 of ALMA. They could not detect the 1.2mm continuum / [CII] 158 micron emission from these LAEs. The upper limit for the dust continuum and the [CII] emission flux suggest that these high-z galaxies in the reionization epoch have significantly lower gas and dust enrichment than SMGs and QSOs at similar/lower redshifts, as well as local star-forming galaxies.
References: Ouchi et al. 2013, ApJ, 778, 102; Ota et al. 2014, 792, 34
ALMA will make high-resolution, large-scale, fully-sampled images of the molecular gas in galaxies, and it will map in detail the main dynamical components of all galaxy types in various mass ranges, such as spiral galaxies, elliptical galaxies, dwarf galaxies, satellite galaxies, etc. These maps will give the information on both parsec and kpc scales needed to explore the relationship between star formation, gas density and gas kinematics. The small-scale structure of the molecular component will clarify the mechanisms of starbursts/AGNs in galaxies, and the associated feedback processes, such as outflows of molecular gas, bubbles and winds.
The high-resolution images from ALMA will allow us to study individual molecular clouds in nearby galaxies, including the Magellanic clouds which are known to have lower metallicity, lower dust content, and star formation rate per unit area which is about 10 times larger than the solar region. This will not only improve our understanding of the star formation processes in galaxies, but it will also allow us to constrain the H2/CO conversion factor, which still contains large uncertainties as it appears to vary depending on the environment and metal content, etc. ALMA will also improve our knowledge on the formation of globular clusters.
Figure 1. (Left) Distribution of molecular gas (Giant Molecular Clouds - GMCs) in the Whirlpool galaxy M51 (Credit: Koda et al. 2009, ApJL, 700, 132). (Right) The Small Magellanic Cloud observed from the Herschel Space Observatory. (Image credit: ESA/NASA/JPL-Caltech/STScI)
Multi-line analysis using diffuse to dense gas tracers will become possible with the superb sensitivity and wideband capabilities of ALMA. The CO lines (including its isotope) will be observed from J = 1-0 to J = 9 –8 (except for J = 5-4), and different J transitions from dense gas tracers such as HCN, HCO+ lines will also be available. The atomic carbon ([CI]) line will be observed in the 400 and 800 GHz bands. Molecular lines such as SiO and CH3CN may quantify the properties and location of the shocks in starbursting regions, merging galaxies, or in barred potential. Line surveys of fainter and exotic lines are also extremely important, and the chemical analysis of the ISM will be an integral part of ALMA in nearby galaxy studies.
Figure 2. 2 mm spectral line survey toward the nuclear region of NGC253 (Martin et al., 2006, ApJS, 164, 450)
The power of galactic nuclei spans a continuous range covering quasars, AGN, radio galaxy nuclei, IR luminous and starburst galaxies, and the more modest activity in nearby galaxies and the center of the Milky Way. While discs around massive black-holes are suspected to be the source of power for AGNs and quasars, it is clear that huge luminosities can be generated by starbursts triggered by galaxy-galaxy interactions and mergers that drive large quantities of gas towards the central regions. ALMA will provide much more light on these problems. It will allow us to determine the masses and kinematics of optically obscured galactic nuclei with a resolution of a few parsecs and image the distributions of a variety of molecules.
Figure 3. SMA CO(3-2) and 0.86mm emission in the nearby ULIRG Arp220 (Sakamoto et al., 2008, ApJ, 684, 957)
A key aspect of ALMA’s capabilities is its high sensitivity on the longest baselines. This will allow observations of a statistically significant sample of galactic nuclei and to resolve structures of a few parsecs in nearby nuclei and of ≈ 100 pc in luminous galaxies at redshifts of 0.1-0.2. Molecular imaging with ALMA will enable us to image directly a parsec-scale molecular torus in a large number of galaxies, and to determine accurately the column densities and optical depths of the torus. These observations are important to view directly how the gas loses sufficient angular momentum to bring it down to the scales where it can feed and/or obscure the AGN, testing and refining the unified models for AGN.
Figure 4. Artist's impression of an Active Galactic Nucleus (Copyright: ESA/NASA, the AVO project and Paolo Padovani)
Finally, our own Galactic center provides a unique opportunity to study at very high spatial resolution the physical processes occurring in a galactic nucleus, in particular the nature of the molecular cloud population and the physical phenomena occurring in the vicinity of a massive black hole. The central region of our galaxy hosts a crowded environment with strong shear, magnetic fields and frequent cloud-cloud collisions. At the dynamical center lies Sgr A*, a strong radio continuum source and the best black hole candidate in the known Universe. ALMA’s southern hemisphere location makes the Galactic center a key science target. Such observations are essential if we are to understand the nature of the ISM in the Galactic center and its star forming properties. Polarization measurements will also be extremely important in establishing the magnetic field geometry within the molecular gas – strong fields are thought to permeate much of the nuclear region.
Figure 5. Spatial distribution of molecular gas at the center of the Milky Way Galaxy in CO(3-2). The black cross mark indicates the position of “Sagittarius A*”. (Credit: Keio University) (Oka et al. 2012, ApJS, 201, 140)
Reference: ESO/ALMA document “Science with ALMA”
Starbursts and AGN
Using ALMA Band 3 to observe the CO(1-0) emission, Bolatto et al. (2013) discovered a molecular structure resembling a starburst driven wind in the nearby starburst galaxy NGC 253. The sensitivity of the ALMA data is an order of magnitude better than previous 12CO image of NGC253, allowing them to study the detailed structure of the molecular gas surrounding the Ha filaments. The molecular outflow rate determined from these observations is 3 – 9 Msun/yr, implying a ratio of mass-outflow rate to star-formation rate of at 1 – 3. This suggests that the star formation activity in the galaxy is regulated by the starburst-driven wind and will therefore determine the final stellar content.
Fig. 7 Starburst-driven outflow in NGC 253 (CREDT: Alberto Bolatto, University of Maryland)
Fathi et al. (2013) analyzed the ALMA Band 7 data of NGC 1097, a Seyfert 1 galaxy that is known to contain a central AGN and a starburst ring. The ALMA data suggests, after modeling the kinematic structure of the HCN(4-3) line, that dense gas is streaming down to 40pc distance from the supermassive BH in this galaxy. The dense gas is confined to a very thin disk, and they derive a dense gas inflow rate of 0.09 Msun/yr at 40pc radius. Izumi et al. (2013) used the HCN, HCO+ and CS lines to characterize the physical condition of gas, and found that the high-J lines (J = 4–3 and 3–2) are emitted from dense (104.5 cm−3≤nH2 ≤106 cm-3) and warm (70K≤Tkin≤550K) regions. They also suggest that the observed enhanced HCN emission arises from “high temperature chemistry” rather than pure gas phase PDR/XDR chemistry. A detailed investigation of the molecular lines detected in the 3mm band was conducted by Martin et al.
A well known Seyfert 2 galaxy NGC1068 was observed in ALMA Band 7 and 9 by Garcia-Burillo et al., with an aim to investigate the molecular fuelling and the feedback processes in this galaxy. They find that the CO(3-2) emission is distributed throughout the starburst ring and the bar/inter-bar region, but the dense gas tracers, HCO+, HCN, CS, are mainly found near the r~200pc circum-nuclear disk. From their kinematical analysis, they suggest a molecular outflow of dM/dt ~ 63 Msun/yr from the circum-nucler disk region. The same galaxy was observed in ALMA Band 3 by Takano et al., finding that the SO, HC3N, and CH3CN molecules are concentrated in the cicum-nuclear disk, CS and CH3OH molecules are distributed in both the cicumnucler disk and the starburst ring, and 13CO and C18O are distributed along the starburst ring.
References: Bolatto, A., et al., 2013, Nature, 499, 450; Fathi, K., et al., 2013, ApJ, 770, 27; Izumi, T., 2013, PASJ, 65, 100; Martin et al., 2015, A&A, 573, 116; Garcia-Burillo et al., 2014, A&A, 567, 125; Takano et al., 2014, PASJ, 66, 75
Merging galaxies and ULIRGs
The nearest face-on galaxy merger, the Antennae galaxy NGC4038/4039, is an ideal target for studying how galaxy interactions affect the interstellar medium and star formation. Herrera et al. (2012) used the Science Verification data of ALMA Band 7 to study the antennae galaxies in CO(3-2) emission. Their comparison with the VLT/SINFONI H2 image revealed that the distribution of CO and H2 are closely related, and suggests that the observed variations in the H2/CO line ratio may indicate that the SGMCs are dissipating their turbulent kinetic energy at different rates. Espada et al. (2013) also studied the same data-set in detail, and found 10 molecular clumps that are associated with the tidal arm south of NGC 4039, resembling a morphology of beads on a string with an almost equidistant separation between the beads of about 350 pc, which may represent a characteristic separation scale for giant molecular associations. A high spatial resolution (0.5”) CO(3-2) image toward the overlap region of the Antennae was obtained by Whitmore et al.
Fig. 8 ALMA (ESO/NAOJ/NRAO). Visible light image: the NASA/ESA Hubble Space Telescope
U/LIRGs are sources that show extreme IR luminosities, and are important sources to study merger triggered star formation and AGN activity. Iono et al. (2013) observed LIRGs VV114 using ALMA in dense gas tracers HCN and HCO+. They find a compact nuclear (< 200 pc) and extended (3–4 kpc) dense gas distribution across the eastern part of the galaxy pair. They found a significant enhancement of HCN (4–3) emission in an unresolved compact and broad component found in the eastern nucleus of VV 114, and suggested the presence of an AGN there. Imanishi et al. (2013) observed a LIRG NGC1614 in the same molecules, and found a velocity structure such that the northern (southern) side of the nucleus is redshifted (blueshifted) with respect to the nuclear velocity of this galaxy. Contrary to the nuclear region of VVV114, the HCN emission is weaker than HCO+, suggesting a pure starburst system. Using ALMA Band 9, the CO(6-5) line in NGC1614 (Xu et al) and in Arp220 (Wilson et al.) were observed.
References: Herrera, C., et al., 2012, A&A, 538, 9; Espada, D., et al., 2012, ApJ, 760, 25; Whitmore, B. et al., 2014, ApJ, 795, 156; Iono, D., et al., 2013, PASJ, 65, 7; Imanishi, M., et al., 2013, AJ, 146, 47; Xu et al. 2015, ApJ, 799, 11; Wilson et al., 2014, ApJL, 789, 36
Using the Science Verification data, Yusef-Zadeh et al. (2013) studied the SiO emission near the central region of the Milky way. They find form the ALMA observations that the interior of the circumnuclear molecular ring is not completely filled with ionized gas but it is a site of molecular clumps and on-going star formation. They find that these clumps are not gravitationally bound, and suggesting outflows from YSOs. This would be the first time that star formation was observed so close to the galactic center.
Fig. 9 A combined ALMA and Very Large Array (VLA) image of the galactic center. The supermassive black hole is marked by its traditional symbol Sgr A*. The red and blue areas, taken with ALMA, map the presence of silicon monoxide, an indicator of star formation. The blue areas have the highest velocities, blasting out at 150-200 kilometers per second. The green region, imaged with the VLA, traces hot gas around the black hole and corresponds to an area 3.5 by 4.5 light-years. Credit: Yusef-Zadeh et al., ALMA (ESO, NAOJ, NRAO), NRAO/AUI/NSF.
Referenes: Yusef-Zadeh, et al., 2013, ApJ, 767, 32
H2O maser traces at extreme conditions (Tk > 1000K). Hagiwara et al. (2013) presents the first detection of an extragalactic submillimeter H2O maser (321 GHz) toward the center of the Circinus galaxy, a nearby Type 2 Seyfert. They find that the 321 GHz maser occurs in a region similar to that of the 22 GHz maser.
References: Hagiwara, Y., et al., 2013, ApJ, 768, 38
Astrochemistry and complex organic molecules
ALMA sensitivity has allowed a number of important studies related to the chemistry of the interstellar medium, especially in the areas of deuterated molecules and complex organic molecules. The ALMA detection of abundant branched form of Propyl Cyanide in the Sgr B2 region reveals that the production route of branched isomers of linear molecules may be more efficient than previously thought in the interstellar medium. This supports the idea that complex branched pre-biotic molecules, such as amminoacids, may form on interstellar ices (http://www.almaobservatory.org/en/press-room/press-releases/754-alma-finds-that-organic-molecules-are-branching-out; http://adsabs.harvard.edu/abs/2014Sci...345.1584B)
Figure 1. Dust and molecules in the central region of our Galaxy: The background image shows the dust emission in a combination of data obtained with the APEX telescope and the Planck space observatory at a wavelength around 860 micrometers. The organic molecule iso-propyl cyanide with a branched carbon backbone (i-C3H7CN, left) as well as its straight-chain isomer normal-propyl cyanide (n-C3H7CN, right) were both detected with the Atacama Large Millimeter/submillimeter Array in the star-forming region Sgr B2, about 300 light years away from the Galactic center Sgr A*. © MPIfR/A. Weiß (background image), University of Cologne/M. Koerber (molecular models), MPIfR/A. Belloche (montage).
High mass star formation and infrared dark clouds
ALMA has provided convincing evidence for the presence of disks surrounding a number of high mass protostars. These findings, thanks to ALMA angular resolution and sensitivity, support the hypothesis that stars at least up to 10-20Msun may undergo a disk accretion phase, similar to solar-type stars. The improved angular resolution now offered by ALMA will allow to extend these studies to even more massive stars, which are typically located at larger distances from the Sun. (http://adsabs.harvard.edu/abs/2013A%26A...552L..10S ).
Figure 2. Velocity pattern derived from ALMA CH3CN observations of the young massive protostar G35.20-0.74N (left), compared with a Keplerian disk model (right). Adapted from http://www.aanda.org/articles/aa/pdf/2013/04/aa21134-13.pdf
The structure of massive proto-cluster cores at the distance of the Galactic Centre can also be studied in detail with ALMA. The famous “Brick” core (G0.253+0.016, also known as the “Lima Bean”) has been mapped in detail revealing the complex internal structure, which is consistent with the object being on the verge of forming stars. The complex clumpy structure has been interpreted as induced by cloud-cloud collisions or as the result of a relatively recent passage close to the supermassive black hole in the Galactic Centre. It is expected that the detailed study of massive clouds in extreme environment in our Galaxy, as is the case for the Brick, will allow us to shed light of how star formation may progress in other similarly extreme environments in the Universe (http://adsabs.harvard.edu/abs/2014ApJ...795L..25R).
Figure 3. ALMA 3mm observations of the dust structure within the Brick (orange), overlaid on an infrared map of the region from Spitzer observations (Green/Blue). Adapted from http://iopscience.iop.org/2041-8205/795/2/L25/pdf/apjl_795_2_25.pdf
Low mass star formation
The detailed physical and chemical structure of low mass protostars in nearby star forming regions can be studied in great detail with ALMA. The interaction between disks, jets, outflow cavities, and envelopes have been investigated in several studies. The ALMA sensitivity allows to trace the coherent kinematical pattern and the transition between the different structures. As an example, in the figure we show the observations of the famous HH212 jet (http://adsabs.harvard.edu/abs/2014A&A...568L...5C).
Figure 4. ALMA maps of the various physical and kinematical components in the low mass protostellar system HH212. The disk and jet are shown in the upper panel as traced by the dust continuum and the SiO emission, repsectively; the envelope and outflow cavity are shown in the bottom panel as traced by the C17O and C34S. Adapted from http://www.aanda.org/articles/aa/pdf/2014/08/aa24103-14.pdf
ALMA sensitivity and resolution also allows to reveal and compare with models the chemical effects of accretion variability onto the protostellar envelope. As accretion bursts release energy in the envelope, the gas phase chemistry is affected by the change of temperature and the release in the gas phase of molecules that are normally locked into the icy mantles of dust grains. These effects, as well as the enhancement of carbon chain molecules and methanol in the inner regions of the envelope of the protostar IRAS 15398-3359 are consistent as being the result of a recent accretion burst (http://adsabs.harvard.edu/abs/2013ApJ...779L..22J).
Figure 5. ALMA observations of IRAS 15398–3359. Left: CH3OH (red) and H13CO+ (blue) maps; Right comparison between the observed and modeled radial intensity profiles. Adapted from http://iopscience.iop.org/2041-8205/779/2/L22/pdf/apjl_779_2_22.pdf.
ALMA has detected molecular gas in several debris disks, this is an important finding as molecular gas is expected to be short lived in these systems. The currently favored explanation is that fresh gaseous material is constantly replenished in these systems through shattering of icy bodies. In particular, the observations of the nearby young star Beta Pictoris have revealed an asymmetric distribution of gas with most of the emission associated with a clump of gas. The mass and location of the molecular gas is consistent with being produced by the recent destruction of an icy body with a mass similar to Mars. (http://www.eso.org/public/news/eso1408/, http://adsabs.harvard.edu/abs/2014Sci...343.1490D)
Figure 1. The ALMA image of carbon monoxide around Beta Pictoris (above) can be deprojected (below) to simulate a view looking down on the system, revealing the large concentration of gas in its outer reaches. For comparison, orbits within the Solar System are shown for scale.
Protoplanetary disks and exoplanets
Detailed observations of young protostellar systems with ALMA have revealed the chemical signatures of the shock at the interface between the infalling envelope and the forming protoplanetary disk. The drastic change of the chemistry at the interface between the envelope and disk are believed to be caused by localised heating at the centrifugal barrier (http://www.almaobservatory.org/press-room/press-releases/670-astronomers-discovered-a-drastic-chemical-change-in-the-birth-of-a-planetary-system; http://adsabs.harvard.edu/abs/2014Natur.507...78S).
Figure 2. (left) Schematic illustration of the infalling-rotating envelope around the protostar. The gas can not go inside the centrifugal barrier due to the centrifugal force. The observer is on the left-hand side, looking at the envelope in an edge-on configuration (middle) : The highest and lowest velocities of the infalling-rotating gas calculated from the model. The emissions of the colored closed circles come from the corresponding colored closed circles in the left panel. (right): The highest and lowest velocities shown in the middle panel are superposed on the position-velocity diagram of cyclic-C3H2. The observation of cyclic-C3H2. showes a beautifull agreement with the model. The cyclic-C3H2. molecules completely disappear at a radius of ~100 AU (the centrifugal barrier) from the protostar, whereas SO appears at the radius. SO preferentially exists in the ring whose radius is that of the centrifugal barrier.
ALMA sensitivity also allows, for the first time, detailed studies of the chemical complexity of protoplanetary disks. The chemical effects of the transition of CO from the gas phase, in the inner warmer regions of the disk, to the solid state, in the outer cooler regions, has been observed through the chemical effect as destruction of certain molecules is suppressed where CO is not available in the gas. The prime molecular tracer of this effect has been shown to be N2H+ (http://www.eso.org/public/news/eso1333/; http://adsabs.harvard.edu/abs/2013Sci...341..630Q).
Figure 3. ALMA image of the N2H+(4-3) emission (green) shows the region where CO snow has formed around the star TW Hydrae (indicated at center). The blue circle represents where the orbit of Neptune would be when comparing it to the size of our solar system. Credit: Karin Oberg, Harvard University/University of Virginia.
At later stages of disk evolution ALMA allowed to reveal the detailed structure of the dust distribution as well as the molecular gas kinematics in the so-called transition disks, which may host young planetary systems. ALMA images revealed the probable effect of the disk planet interaction, which results in the efficient confinement of the large dust grains in the outer disk. The formation of large, cometary-size bodies in the outer disk are believed to be favored by the presence of these dust traps (http://www.eso.org/public/news/eso1325/; http://adsabs.harvard.edu/abs/2013Sci...340.1199V).
Figure 4. Annotated image from the Atacama Large Millimeter/submillimeter Array (ALMA) showing the dust trap in the disc that surrounds the system Oph-IRS 48. The dust trap provides a safe haven for the tiny dust particles in the disc, allowing them to clump together and grow to sizes that allow them to survive on their own. The green area is the dust trap, where the bigger particles accumulate. The size of the orbit of Neptune is shown in the upper left corner to show the scale. Credit: ALMA (ESO/NAOJ/NRAO)/Nienke van der Marel.
Signs of the possible presence of forming planetary systems were also found around the complex multiple system GG Tau, comprising at least five young stellar objects. The ALMA observations clearly revealed the inflow of material from the outer massive ring to the inner disk surrounding one of the young objects. This flow of material allows the inner disk to be long-lived and possibly host the formation of planets (http://www.eso.org/public/news/eso1434/; http://adsabs.harvard.edu/abs/2014Natur.514..600D).
Figure 5. a–c, ALMA; d–f, IRAM. a, 0.45-mm emission (black contours) and CO 6–5 flux (colour: see colour scale at top). b, 0.45-mm emission (black contours) and CO velocity field (colour). c, 0.45-mm emission (colour) with CO 6–5 flux (blue contours) and in inset H2 intensity (red). d, e, as a, b, but for 1.3-mm emission (contours) and CO 2–1 flux (colour). f, 1.3-mm emission (colour) and H2 intensity (contours). Positions are relative to right ascension (RA) 04 h 32 min 30.359 s and declination (dec.) 17° 31′ 40.38″ (J2000). Crosses are the locations of Aa (south) and Ab (north) components, and triangles and squares show the locations of the CO J = 6–5 and J = 2–1 peaks, respectively. Units on the colour scales are Jy per beam km s−1 (a, d), km s−1 (b, e) and mJy per beam (c, f). In a and d, the beam size is given in the inset. Adapted from http://www.nature.com/nature/journal/v514/n7524/full/nature13822.html.
The ALMA future capabilities for studying planet formation are impressively demonstrated by the recently released Science Verification datasets using the ALMA long baselines. The ALMA image of the young protoplanetary disk HL Tau reveals the complex structure of the dust distribution on the disk midplane. Potentially showing the effect on the disk of planetary embryos (http://www.eso.org/public/news/eso1436/).
Figure 6. This is the sharpest image ever taken by ALMA — sharper than is routinely achieved in visible light with the NASA/ESA Hubble Space Telescope. It shows the protoplanetary disc surrounding the young star HL Tauri at 1.3mm, with an angular resolution of about 35 milliarcsec. The observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system. In this picture the features seen in the HL Tauri system are labelled. Credit: ALMA (ESO/NAOJ/NRAO).
ALMA has started producing detailed images of Solar System bodies. Two bright comets were observed in 2013 (C/2012 F6, Lemmon, and C/2012 S1, ISON) revealing the detailed distribution of HCN, HNC and H2CO in the coma. The data are found to be in agreement with models that predict the formation of these compounds, especially H2CO, in the coma few hundred to few thousand kilometers from the nucleus from unidentified precursor molecules. The different formation lengths observed in the two comets at different heliocentric distances are consistent with the effects of photolysis or thermal degradation. (http://adsabs.harvard.edu//abs/2014ApJ...792L...2C).
Figure 7. Molecular line maps of comets Lemmon and ISON overplotted on the continuum images (adapted from http://iopscience.iop.org/2041-8205/792/1/L2/pdf/apjl_792_1_2.pdf)
The atmosphere of the Saturn moon Titan was spatially resolved with ALMA in the HNC and HC3N molecules. The data confirm that the two molecules are not co-located in the atmosphere, with HC3N being enhanced at the poles and spread over a wide range of altitudes (http://adsabs.harvard.edu//abs/2014ApJ...795L..30C).
The ALMA future capabilities for the study of Solar System objects and in particular minor bodies are very well demonstrated by the long baselines Science Verification data obtained for the Juno asteroid. The surface of the body is spatially resolved at 50milliarcsecond resolution at various rotational phases, allowing to constrain a detailed thermal model of the full surface of the asteroid.
Figure 8. (Left) ALMA long baselines Science Verification images of the asteroid Juno at 50milliarcsec angular resolution. (Right) Thermal model of the asteroid surface.
Millimeter continuum emission from stars
Over the last two decades, the high sensitivity and spatial resolution of the VLA has revolutionized stellar radio astronomy. Using the VLA, major advances were made in studies of non-thermal radiation from a wide variety of stellar types. Recent upgrades in the VLA bandwidth have increased sensitivity, and this revolution has continued. ALMA has extended imaging of stars to higher frequencies.
Photospheric emission becomes much easier to observe at millimeter wavelengths owing to the Rayleigh-Jeans Law. ALMA can resolve the photospheres and chromospheres of giant and supergiant stars within a few hundred parsecs. Moreover, in addition to free-free emission, ALMA will allow (sub)millimeter imaging of thermal emission from dust in stellar envelopes. ALMA will detect the photospheres of stars across the HR diagram, including those in the Bright Star Catalog, as it did recently for α Centauri (Liseau et al A&A 573, L4 2015).
ALMA’s ability to detect the photospheres of so many stars allows it to measure positions relatively often to astrometric accuracy. The orbit of any planet around its central star causes that star to undergo a reflexive circular motion around the star-planet barycenter. By taking advantage of the incredibly high resolution of ALMA in its widest configuration, we may be able to detect this motion. This will enable ALMA to indirectly detect planets which may orbit these stars.
Asymptotic Giant Branch Stars
As nuclear fuel deep within a star is burned, instabilities occur. These may result in rapid increase in heating, which may be seen by observers as a thermal pulse driven episode of mass loss. Nuclear-processing enriches the outer envelopes of the star; as dense warm material enriched in C, N, O, P, S and Si deep in the star cools, condensation produces stardust. The dust is accelerated outward during pulses and drags gas along with it into the circumstellar envelope, which may be imaged in dust continuum or in the lines of molecules such as CO, SiO and their isotopic variants. Thus, ALMA provides a sensitive probe of not only the physics of the stellar matter but of its nucleosynthetic origins. In several Early Science studies, ALMA’s high resolution has identified unknown components of stellar systems while its spectral line sensitivity has provided insight into isotopic variations within lost-mass shells and into dust formation processes near the stellar photosphere.
In one remarkable recent study, ALMA has imaged the detached molecular shell and circumstellar medium of the AGB star R Sculptoris with unprecedented detail during Cycle 0 (Maercker et al 2012). The ALMA data shows that the shell originates with change in mass loss rate and expansion velocity during a stellar thermal pulse. An ALMA image in a line of CO at 0.9mm wavelength reveals spiral structure in shell, suggesting an unseen companion star or massive planet modulates the loss of mass from the star. The ALMA observation demonstrates change in mass loss rate by a factor of thirty accompanied by a decrease in the expansion velocity.
Further observations (Vlemmings et al 2013 A&AP 556, L1) found variations in the 12CO/13CO intensity ratios in R Scl, signifying abundance variation in the isotopes. The ratio in the shell (~19) differs from that (>60) near the star; the ratio varies by more than a factor of ten within the shell. Isotopic fractionation in the cool (~35K) shell along with selective dissociation, perhaps owing to an embedded ultraviolet radiation source, may account for these variations although some part of them may arise from variation owing to nuclear processing in the star.
Decin et al. (2015A&A 574,5) showed that correlated structures seen in molecular images of the evolved carbon star CW Leo (IRC+10216) may result from spiral shell structure induced by an unseen binary companion, in a similar fashion to that seen in R Scl.
Figure 1. CW Leo spectra from ALMA in selected parts of a 20 GHz bandwidth measured by ALMA. Intensity is in Jy/bm and spectral resolution 1 MHz. Upper panels (red) show IRAM 30m spectra; lower panes the ALMA data.
Cernicharo et al (2013 ApJ 778 L25) imaged IRC+10216 with ALMA in nine excited vibrational states of HNC J=3-2 covering energies up to 5300 K. The radius of the unresolved emitting region is 0.6" or three stellar radii, similar to the event of dusty clumps observed in the infrared. The physical and chemical conditions in the dust formation zone should be characterized by modeling of inner envelope species. Owing to the high sensitivity, a briar patch of narrow and unidentified lines was seen.
Figure 2. Image of CO in the J=3-2 line from ALMA Cycle 0 data. The blue spiral connects emission components and shows the pattern of mass loss from the central star guided into the spiral by an unseen companion.
Figure 3. The Boomerang Nebula reveals its true shape with ALMA. The background blue structure, as seen in visible light (HST), shows a classic double-lobe shape with a very narrow central region. ALMA’s ability to see the cold molecular CO gas reveals the nebula’s more elongated shape, in contours. Contours levels shown are at 5s (white), 10s (blue), 20s (magenta), 30s (gray), 40s (red), and 50s (black), with s = 7.5 mJy/beam. The beam (FWHM) is shown as the white ellipse in the lower left corner. The ALMA data validate suggest a model incorporating two nested spherically symmetric shells: a warm inner shell extending 2.5–6'' with an expansion velocity of about 35 km/s and a cool, extended outer shell extending 6''–33'', with a velocity of about 164 km/s. The latter shell is cooled below the temperature of the cosmic microwave background through adiabatic expansion. The ALMA observations show that the inner component is bipolar, with a dense waist, and the outer component is patchy but roughly circular and similar in dimensions to the model, bearing in mind that a significant fraction of the flux in absorption has been resolved out. Credit: Bill Saxton; NRAO/AUI/NSF; NASA/Hubble; Raghvendra Sahai.
After the outer shell of the star is lost, the hot central stellar remnant is exposed, and its energetic radiation begins to ionize the remnant circumstellar shell, creating a planetary nebula. Many planetaries are markedly asymmetric, with material flowing outward at high velocity in the polar direction, and at lower velocities equatorially. Often a region of neutral gas and dust is found in the equatorial plane; ALMA provides excellent high spatial and kinematic sensitivity for imaging these regions. The Boomerang Nebula is one example of such an object. An ALMA image of the Boomerang obtained by Sahai, Vlemmings, Huggins, et al. (ApJ 777, 92) shows a central hourglass-shaped pre-planetary nebula surrounded by a patchy, but roughly round, cold high-velocity outflow centered on a dense waist containing large grains (yellow in the image above). As the flow from the central star expands, it cools through adiabatic expansion. This process has cooled the envelope substantially below the temperature of the Cosmic Microwave Background, the 2.7K relic radiation from the Big Bang which pervades the Universe. Thus the region appears to be among the coldest in the Universe, ironic for material so close to a hot central star. The ALMA observations also show that the outer regions of the CO flow are rewarmed, probably by photoelectric grain heating.
Supergiant and Hypergiant Stars
The Supergiant star VY Canis Majoris
An oxygen-rich red supergiant well-known for the complexity and large mass of its envelope is VY CMa. Using publicly available ALMA Science Verification data, Richards et al. (2014) and O’Gorman et al. (2015) imaged VY CMa with in the submm dust continuum and in the emission lines of water masers at 658, 321, and 325 GHz. The ALMA images at the highest frequency trace dust on spatial scales down to 11 R⋆ (71AU), locating two prominent components. One of these is associated with the star but the brighter, component ‘C’, lying about 400 AU to the southeast is massive, cool (Td ≲ 100 K) and lacks molecular emission near its peak. Overall the dust emission displays an anisotropic morphology, with 17% of the total dust mass located in clumps within a roughly spherical stellar wind. This pattern suggests continuous directional mass loss over decades. The masers extend over ~90 km s-1 and lie in irregular thick shells about the center of expansion, confirmed as the stellar position. Different transitions, however, form non-overlapping clumps with velocities consistent with expansion but exhibiting drastic and irregular departures from spherical symmetry. The maser inner rims lie at successively larger distances from the star, while the outer rims lie within the central 1.2” at ~2500, 6300, and 7500 AU in the order of the transitions given above. Bright elongated maser features suggestive of shocks are seen at up to 2000AU from the star, perhaps associated with dust formation.
The Hypergiant star &eta Carinae
The object &eta Carinae is considered to be an important short-lived unstable phase in the life of the most massive stars in galaxies, shortly before they explode as supernovae. It is one of the most luminous galactic sources and is regarded as an extreme case of a Luminous Blue Variable. This southern source may consist of two very massive (50-80 M) stars in a wide 5.5 year orbit. The binary is surrounded by a highly bipolar nebula containing 1-2 M of material which was ejected by one of the stars in 1843. No other stellar object (apart from supernovae) is known to have such extreme mass loss (i.e., ≈ 2x10−3Myr−1). At submillimeter and millimeter wavelengths, &eta Carinae is also avery strong source showing regular variations with a cycle of 5.5 years (it reaches about 40 Jy at maximum at 230 GHz). The variation in flux density is due to eclipsing events from the binary system. In addition, &eta Carinae is a strong emitter in the hydrogen
Millimeter observations of supernovae (SN) offer exciting possibilities, complementing observations at other wavelengths in important ways. As the radio spectrum of a supernova evolves, the higher peak flux density is expected to occur earlier in the millimeter wavelength regime. Thus at millimeter wavelengths one might observe supernovae more promptly and further away than at longer wavelengths. Very bright SNII could easily be detected and monitored with ALMA up to a distance of 350 Mpc, i.e. z ~ 0.1.
Figure 4. ALMA identified CO and SiO in the SN1987A inner ejecta. The CO clumps comtain at least 0.1Msun of 12CO, an order of magnitude more than measured in the first few years after the explosion.
As the ejecta expand and cool, dust is formed. Recent ALMA observations of Supernova 1987A in the Large Magellanic Cloud showed that dust continues to form. Furthermore, in the central regions CO and SiO molecules are also found. In fact, although the data were too fragmentary for an analysis, both abundant Si isotopes, 28Si and 29Si were imaged over partial velocity extents. This suggests that ALMA might probe the nucleosynthetic character of the debris around the remnant, illuminating the evolution of its central star. ALMA views the full velocity range of emission, unobscured by dust. Doppler tomography will be possible with the completed ALMA in CO and other molecules that will probe the spatial, chemical and kinetic environment within the inner ejecta. [Kamenetzky et al., 2013 ApJ, 773, 34.]
Gamma ray bursts
Gamma ray bursts (GRBs) are the brightest transient phenomena in the Universe, arising from poorly understood physical catastrophes which may involve the collapse of the densest core of a massive star to a Black Hole, or possibly the coalescence of two collapsed objects. Millimeter emission lags the high energy burst in time and can provide key insight into the development of the burst. The development of the burst can then be used to derive the energy, density and other key parameters of the explosion and help characterize the nature of the originating event. Even pre-Early Science ALMA was capable of measuring a GRB—test observations were made of GRB110715A at z= 0.8224 and made public as part of Science Verification (Sanchez-Ramirez, de Ugarte Postigo, Gorosabel et al. 2013, Highlights of Spanish Astrophysics VII, 399).
ALMA constitutes the largest ground-based instrument for solar research built in our decade. At a recent Workshop, attendees noted that ALMA will address several key solar physics problems posing particular observational challenges:
– Imaging the dynamic chromosphere: one of the greatest challenges to ALMA since it will entail imaging a source that fills the beam and varies on a time scale of tens of seconds.
– Imaging solar flares: A recent mystery has revolved around the so-called "sub-THz" spectral component to certain large flares. Unlike the expected synchrotron component, it shows an inverted spectrum.
– Radio Recombination Lines: High-n hydrogen RRLs have been detected in the 350 and 450 micron bands using an FTS. It is expected that certain ions (e.g., O VI) may also be detected as the result of overpopulation of states by dielectronic recombination.
– Prominences and filaments: imaging with high resolution these cool, filamentary structures suspended in the low corona and the precursors to spectacular eruptions into the interplanetary medium.
Figure 5. ALMA whole-disk test data from February 2011. Observations were taken through a high level of attenuating atmospheric water. Normally special solar filters would have been inserted to attenuate strong solar radiation.
Structure and dynamics of the chromosphere
The emission of the quiet Sun as seen in the millimeter and submillimeter (Fig 5) arises mainly from thermal emission from the chromosphere, which is a thin layer above the temperature minimum region, where non-radiative heating first becomes manifest. The chromosphere constitutes a complex and rapidly evolving plasma structure. It is dynamically driven by the convection of the underlying photosphere. Its ionization structure changes dramatically from the temperature minimum to the base of the corona. ALMA’s potential for measuring lower solar atmosphere temperatures directly without complex modeling enables studies of energy flow in the region. Interpretation is difficult however as shocks propagate through the region, and complex magnetic field structure mediates flows. ALMA can provide subarcsecond snapshot images of the dynamic chromosphere to probe these phenomena.
Millimeter and submillimeter Solar Flares
Electrons are accelerated in solar flares to energies beyond 70 MeV and emit synchrotron emission and gamma-ray bremsstrahlung. BIMA observations ([Kundu et al. 2000, 2001] suggest that the synchrotron emission continues beyond the millimeter waves into the submillimeter regime. A subset of large solar flares present a “sub-THz” component which shows an inverted spectrum up to 405 GHz and could be even more prominent in the higher frequency bands accessed by ALMA. ALMA offers only a limited field of view but options to improve flare observations with ALMA are available. For instance, ALMA could be split into sub-arrays to cover a larger area, at the cost of lower sensitivity and spatial resolution. ALMA could also be operated not as an interferometer, but in a single-dish mode. Each antenna may be pointed at slightly different locations within the active region so that an entire active region is covered by a rapidly sampled mosaic.
Solar Recombination Lines
Clark et al. 2001 have reported the detection of the Hydrogen recombination line emission in the 350µm and 450&mum band—ALMA Bands 9 and 10. They resport a dramatic increase of the line-to-continuum ratio moving from the center of the solar disk to its limb. These lines may provide a new probe of the altitude structure of the quiet solar atmosphere above the limb and in prominences.
Prominences and Filaments
Filaments in the sun appear as cool snakelike structures suspended in the corona above magnetic neutral lines. Seen in absorption against the disk at long wavelengths, they appear in emission above the limb. They may undergo eruption in association with coronal mass ejection. Across the ALMA Bands, the emission changes from optically thick to optically thin, suggesting that ALMA observations with high spatial and temporal resolution across the wide ALMA frequency window may allow new insights into their physics.
ALMA Design Reference Science Plan
The ALMA Design Reference Science Plan (DRSP) is an exercise to see how ALMA can provide science results for a suite of high-priority prototype science projects. The projects assume that ALMA is completed and is thus not directly applicable for the Early Science phase. Note also that the DRSP projects are not approved for execution with ALMA. Nevertheless, the DRSP provides an overview of potential projects and how these can be accommodated within the ALMA design.
The latest version of the ALMA Design Reference science Plan (Version 2.2) is hosted at ESO in Garching and can be accessed here: DRSPv2.2.