Science Highlight




NGC 3256

Science Target Overview

The luminous infrared galaxy NGC 3256 is the brightest galaxy within ~40 Mpc. This galaxy, which is in the later stages of a merger of two gas-rich progenitors, hosts an extreme central starburst that can be seen across a wide range of wavelengths, but emits most strongly in the far infrared (Smith & Harvey 1996). Hubble Space Telescope optical imaging revealed hundreds of bright young clusters in the galactic center (Trancho et al. 2007). There is strong evidence for a superwind in NGC 3256, indicating strong star-burst driven blow-out of the interstellar medium (Heckman et al. 2000). Imaging in the infrared, radio and X-rays has shown that NGC 3256 has two distinct nuclei aligned in the north-south direction and separated by 5 arcsec, or 850 pc on the sky. The southern nucleus is highly obscured, rendering it invisible in the optical.

Because of its proximity and the fact that it is observed nearly face-on, NGC 3256 is an ideal target to study merger-induced starbursts in the local Universe. In fact, NGC 3256 could be regarded as the southern sky equivalent of Arp 220, the archetype of infrared-luminous merging galaxies.

Neutral gas in this galaxy was first studied by English et al (2003), who detected two HI tidal tails that extend up to 50 kpc. High-resolution observations of carbon-monoxide (CO 2-1) in NGC 3256 were made by Sakamoto et al. (2006) using the Submillimeter Array (SMA). This study revealed a large disk of molecular gas (r > 3 kpc) in the center of the merger with a strong gas concentration toward the double nucleus. This gas disk rotates around a point between the two nuclei. Also high-velocity molecular gas was discovered at the galaxy center, with velocities up to 420 km/s offset from the systemic velocity of the galaxy.

ALMA Data Overview

ALMA Science verification data on NGC3256 in Band 3 were taken in six different datasets over two consecutive nights: April 16-17, 2011. There are three datasets for April 16th and three for April 17th. These Band 3 observations utilized all four available basebands, which are associated with four different spectral windows: two in the Upper Sideband (USB) and two in the Lower Sideband (LSB). The first spectral window is centered on the CO(1-0) emission line in the galaxy NGC 3256 and is our highest frequency spectral window. There is one additional spectral window in the Upper Side Band (USB), and there are two spectral windows in the Lower Side Band (LSB). These additional spectral windows are used to measure the continuum emission in the galaxy, and may contain other emission lines as well. Each spectral window has a total bandwidth of 2 GHz divided over 128 channels, for a channel width of 15.625 MHz, corresponding to about 40 km/s. Online Hanning smoothing was applied to the data, resulting in a spectral resolution that is twice the channel separation. For the antenna configuration that was used during these observations, the angular resolution is expected to be about 6.5".

Due to unfortunate weather conditions during the days planned for these observations, the data were taken at Band 3 although no previous interferometric CO(1-0) observations of this galaxy exist. Casoli et al. (1990) observed the CO(1-0) line with the SEST 15 single dish telescope and were just able to measure a velocity field with 22" resolution. Interferometric observations were first made with the SMA in the CO(2-1) line by Sakamoto, Ho & Peck (2006). Because those observations were made at a  higher frequency, the angular resolution is higher (~2") than that of the ALMA observations shown here, taken in a very compact configuration. Nonetheless, it is possible to make a direct comparison of the distribution and velocity of the CO(2-1) and (1-0) gas looking, for example, at the south-western clump and the north-eastern 'arm', which are consistent in both data sets.

We thank the following people for suggesting NGC3256 for ALMA Science Verification: Kazushi Sakamoto, Satoki Matsushita, Alison Peck, & Martin Zwaan.


Using the data for publication

The following statement should be included in the acknowledgment of papers using the datasets listed above:

“This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00002.SV. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ."

Obtaining the Data

The data products are contained in three downloadable  files:

  • Uncalibrated data with tables for reduction
  • Calibrated data
  • Reference images

and can be downloaded here:  NGC3256 ALMA Science Verification Data

PLEASE make full use of the CASA Guides provided for this data set: NGC3256 Band 3 CASA Guide (This link will take you to an external web site, hosting the CASA Guides.)

NOTE: This script was developed in CASA version 3.3. It will not run in later version of CASA. For additional information see the following knowledge base article: "If my data were calibrated and imaged in CASA 3.3 and I want to redo it, are there resources to help?".