AntennaeBand7 for CASA 3.3
Draft casaguide for the Antennae at Band 7
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 between 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 has revealed hundreds of bright young clusters in the galactic center (Trancho et al. 2007). There is also strong evidence for a superwind in NGC 3256, indicating strong starburst-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. In addition, high-velocity molecular gas was discovered at the galaxy's center, with velocities up to 420 km/s offset from the systemic velocity of the galaxy.
ALMA Data Overview
ALMA Science verification data on Antennae galaxies in Band 7 were taken in nine different datasets over several nights in May and June 2011, in order to get good uv-coverage. The datasets are listed next, classified as observations for the northern -NGC4038- and southern components -NGC4039- of Antennae.
These Band 7 observations utilized two basebands, which are associated with two different spectral windows: one in the Upper Sideband (USB) and one in the Lower Sideband (LSB). The first spectral window is centered on the CO(3-2) emission line in the galaxy Antenna. The additional spectral window in the USB is 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 3840 channels, for a channel width of 488.28 MHz, corresponding to 1634 km/s and 0.426 km/s respectively. For the antenna configuration that was used during these observations, the angular resolution is expected to be about 1".
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 by looking, for example, at the south-western clump and the north-eastern 'arm', which are consistent in both data sets.
We acknowledge the following people for suggesting Antennae for ALMA Science Verification:
Obtaining the Data
To download the data, click on the region closest to your location:
Here you will find three gzipped tar files which, after unpacking, will create three directories:
- Antennae_Band7_UnCalibratedMSandTablesForReduction - Here we provide you with "starter" datasets, where we have taken the raw data in ALMA Science Data Model (ASDM) format and converted them to CASA Measurement Sets (MS). We did this using the importasdm task in CASA. Along with the raw data, we also provide some tables that you will need for the calibration which cannot currently be generated inside of CASA. (This functionality will be available in CASA for Early Science.)
- Antennae_Band7_CalibratedData - The fully-calibrated uvdata, ready for imaging and self-calibration
- Antennae_Band7_ReferenceImages - The final continuum and spectral line images
To see which files you will need, read on below. The downloads to your local computer will take some time, so you may wish to begin them now.
NOTE: CASA 3.2.1 or later is required to follow this guide. For more information on obtaining the latest version of CASA, see http://casa.nrao.edu.
Antennae Band 7 Data Reduction Tutorial
In this tutorial, or "casaguide", we will guide you step-by-step through the reduction of the ALMA science verification data on the Antennae galaxies and its subsequent imaging. This casaguide consists of two parts:
To complete the Calibration section of the tutorial, you will need the data in the first directory: Antennae_Band7_UnCalibratedMSandTablesForReduction. For those wishing to skip the calibration section and proceed to Imaging, we also provide the fully-calibrated data in the Antennae_Band7_CalibratedData directory. Finally, we provide the final continuum and spectral line images in the Antennae_Band7_ReferenceImages directory.
How to use this casaguide
For both portions of the guide, we will provide you with the full CASA commands needed to carry out each step.
# In CASA The commands you need to execute will be displayed in regions like this.
Simply copy and paste the commands in order into your CASA terminal. You may also type the commands in by hand if desired, but be mindful of typos. Note that you may need to hit Enter twice in order for the process to start running. Also note that copying and pasting multiple commands at a time may not work, so only copy and paste the contents of one region at a time.
To learn how to extract the CASA commands into an executable python script, click here.
Occasionally we will also show output to the CASA logger:
This output will be displayed in regions like this.
For a brief introduction to the different ways CASA can be run, see the EVLA_Spectral_Line_Calibration_IRC+10216#How_to_Use_This_casaguide page. For further help getting started with CASA, see Getting_Started_in_CASA.
Daniel Espada 12:00 UT, 13 July 2011