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, and the dusty star formation activity if the dust spectral energy distribution is well characterized. 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. 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.
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.
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].
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
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.
Figure 5 – ALMA Band 6 observations constrain the faint mm source number counts (Hatsukade et al. 2013).
References: Hatsukade et al. 2013, ApJ, 769, 27
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.
References: Wang et al. 2012, ApJ, 761, L32
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. observed a bright z=6.6 LAE, ‘Himiko’, using Band 6 of ALMA. They could not detect 1.2mm continuum / [CII] 158 micron emission from the LAE. The upper limits for the dust continuum and [CII] emission fluxes suggest that this high-z galaxy is a unique object with a very low dust content and perhaps nearly primordial interstellar gas.
References: Ouchi et al. 2013, arXiv1306.3572
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)
ESO/ALMA document “Science with ALMA”
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 Msyn/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)
References:Bolatto, A., et al., 2013, Nature, 499, 450
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.
Fig. 8 ALMA (ESO/NAOJ/NRAO). Visible light image: the NASA/ESA Hubble Space Telescope
Herrera, C., et al., 2012, A&A, 538, 9; Espada, D., et al., 2012, ApJ, 760, 25
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.
Iono, D., et al., 2013, PASJ, 65, 7; Imanishi, M., et al., 2013, AJ, 146, 47
AGN host galaxies
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.
Fathi, K., et al., 2013, ApJ, 770, 27; Izumi, T., 2013, PASJ in press
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.
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.
Hagiwara, Y., et al., 2013, ApJ, 768, 38
Outflows, jets and ionized winds
The HH 46/47 molecular outflow. The spectacular ALMA observations of this outflow were carried out by Hector Arce (Yale University) and collaborators during Early Science Cycle 0. The new high sensitivity and wide field ALMA observations show for the first time the full details of the flow and allowed the authors to reveal a much higher velocity component of the outflow than previously known. The results of this study have appeared on the Astrophysical Journal and are described in an ALMA press release (http://www.eso.org/public/news/eso1336/ http://adsabs.harvard.edu/abs/2013ApJ...774...39A )
Figure 1. ALMA mosaic of the molecular emission from the red and blue lobes of the HH 46/47 outflow (adapted from Arce et al. 2013).
High mass star formation and infrared dark clouds
ALMA results on IRDC SDC335.579-0.272. Some infrared dark clouds are thought to be the site for massive star formation. The ALMA sensitivity, even in Cycle 0, makes it the perfect instrument for investigating this early stage of massive star formation. Several different projects were carried out to study the structure of IRDCs, the image shows the N2H+ observations of SDC335.579-0.272 by the group led by Nicholas Peretto (Cardiff University). ALMA observations reveal for the first time the filaments of infalling material into the massive IRDC and feeding material for the formation of one of the most massive stars in our own Galaxy. The results of this study have been the subject of an ALMA press release and are published in Astronomy & Astrophysics (http://www.eso.org/public/news/eso1331/ http://adsabs.harvard.edu/abs/2013A%26A...555A.112P ).
Figure 2. ALMA N2H+ total intensity image of SDC335.579-0.272 (red) overlaid on the Spitzer image of the same region (adapted from Peretto et al. 2013).
Low mass star formation, ISM and astrochemistry
ALMA results on the low mass protobinary system IRAS 16293-2422. As part of the Science Verification activities ALMA observed the young solar-mass protostellar binary IRAS 16293-2422 in Ophiuchus. Six different research papers were published on these data, highlighting the broad potential for discovery in each ALMA observation. Among the many results on the structure and chemistry associated with this forming system, we highlight here one particular result relevant for the field of Astrobiology. The superb ALMA sensitivity at both Band 6 and Band 9 allowed the detection of the emission from a simple sugar-like molecule: Glycolaldehyde. The gas is observed in a region of about 30 AU around the protostar and slowly moving towards the inner regions of the system, close to the planet formation zone. These results have been published by the group led by Jes Joergensen (Niels Bohr Institute, University of Copenhagen) on the Astrophysical Journal Letters (http://www.eso.org/public/news/eso1234/ http://adsabs.harvard.edu/abs/2012ApJ...757L...4J )
Figure 3. a): band 9 continuum image of the binary protostar IRAS16293, the inset shows a spectum with one of the Glycolaldeheyde lines in red. b): Band 6 spectrum of IRAS16293, the Glycolaldehyde lines are marked in red (adapted from Joergensen et al. 2012).
The ALMA view of the Fomalhaut debris disk. The first paper published from ALMA Early Science Cycle 0 observations is the study of the Fomalhaut Debis Disk published on the Astrophysical Journal Letters by Aaron Boley (University of Florida). The sensitive ALMA data revealed that the structure of the large grains distribution in the debris disk is confined to a narrow ring, most likely shaped by the action of two unseen Super-Earth size planets orbiting the star (http://www.eso.org/public/news/eso1216/ http://adsabs.harvard.edu/abs/2012ApJ...750L..21B )
Figure 4. ALMA Band 7 continuum emission from the large grains in the Fomalhaut disks (orange) overlaid on the HST image (blue) of the scattered line emission from small grains (adapted from Boley et al. 2012)
Protoplanetary disks and exoplanets
The ALMA resolves the structure of the IRS 48 and HD 142527. ALMA Early Science Cycle 0 provided a very well suited set of angular resolution and sensitivity to study the structure and gas content of protoplanetary and transitional disks. In the first few million years of pre-main sequence evolution, the protoplanetary disks around young stars are thought to evolve and form planets. In this phase disks evolve under the effects of photoevaporation and the development of young planetary systems. The promise of ALMA as a transformational observatory has been proven correct by four milestone studies published by four independent groups. Luca Ricci (Caltech) and collaborators published a study on the Astrophysical Journal Letters to demonstrate that also disks around young Brown Dwarfs have the capabilities of forming planetary systems (http://www.eso.org/public/news/eso1248/ http://adsabs.harvard.edu/abs/2012ApJ...761L..20R ). The teams led by Simon Casassus (Universidad de Chile) and Nienke van der Marel (Leiden University) published two papers in Nature and Science to discuss the impact of young planets to the evolution of the gaseous and dust components of protoplanetary disks around intermediate mass stars (http://www.eso.org/public/images/eso1301b/ and http://www.eso.org/public/news/eso1325/ , http://adsabs.harvard.edu/abs/2013Natur.493..191C and http://adsabs.harvard.edu/abs/2013Sci...340.1199V ). The team led by Chunhua Qi (Harvard Smithsonian Center for Astrophysics) imaged the CO snowine, the transition from the CO being in the gas phase to the icy mantles of grains, on the midplane of the disk surrounding the young solar analogue TW Hya (http://www.eso.org/public/news/eso1333/ http://adsabs.harvard.edu/abs/2013Sci...341..630Q ).
Figure 5. From left to right: the velocity field of the dense gas in HD142527 (adapted from Casassus et al. 2013); the spatial distribution of different grain species in IRS 48: in orange the micron-size grains from VLT/VISIR observations ad in green the millimetre size grains as observed by ALMA in Band 9 (adapted from van der Marel et al. 2013); N2H+ image of the protoplanetary disk around TW Hya, showing the abundance enhancement of diazenylium corresponding to the transition of CO from the gaseous to the solid phase (adapted from Qi et al. 2013).
Millimeter continuum emission from stars
Over the last two decades, the high sensitivity and spatial resolution of the VLA has revo- lutionized stellar radio astronomy. Using the VLA, major advances were made in studies of non-thermal radiation from a wide variety of stellar types. With the recent upgrades in the VLA bandwidth and hence increased sensitivity, this revolution has continued.
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.
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 ths 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 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 at 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 bya factor of thirty accompanied by a decrease in the expansion velocity.
Figure 1. 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.
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 theBoomerang obtained by Sahai, Vlemmings, Huggins, et al. (ApJ, in press) 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.
The Hypergiant star η Carinae
The object η 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., ≈ 2 10−3Myr−1). At submillimeter and millimeter wavelengths, η 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, η Carinae is a strong emitter in the hydrogen and helium recombination lines displaying line widths of about 1000 kms−1 and maser activity.
It is clear that the high spatial resolution of ALMA will permit to measure in detail the (sub)millimeter emission of this outstanding stellar system and map the variations in the expanding ionized stellar envelope to an unprecedented level of detail.
Millimeter observations of supernovae (SN) offer exciting possibilities, complementing ob- servations 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. Thusat 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 3. 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 4. 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 1) 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 450µm 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.