Science Highlight




Antennae Galaxies

Science Target Overview

The Antennae are a nearby (22 Mpc, Schweizer et al. 2008) pair of merging galaxies, NGC 4038 (RA=12h01m53.0s, Dec=−18°52′10″) in the north and NGC 4039 (RA=12h01m53.6s, Dec=−18°53′11″) in the south. These two spiral galaxies started to interact only a few hundred million years ago (Mihos et al. 1993; see also Karl et al. 2010, Renaud et al. 2009 and references therein), making the Antennae one of the nearest and youngest examples of a major galaxy merger. The nuclei of the original galaxies are thought to be composed mostly of old stars, dust filaments pervade the region between the two nuclei, and star-forming regions surrounded by HII regions can be seen throughout the system. Wilson et al. (2000) used OVRO to image CO(1-0) emission, a tracer of the bulk molecular gas distribution, with an angular resolution of 3.15″ x 4.91″, and more recently, Ueda et al (in prep) have used the SMA to image the CO(3-2) emission at ~1”.  Molecular emission is detected throughout the system and is particularly bright in the "interaction region" between the two nuclei, where it appears concentrated in five supergiant molecular complexes.

ALMA Data Overview

ALMA Science Verification data targeting the CO 3-2 line in the Antennae galaxy were obtained using the ALMA Band 7 receiver and observed in 10 separate blocks, each typically ~1 hour long, during May and June 2011. Each block observed one of two mosaic patterns, which we will refer to as the "Northern" and "Southern" mosaics. Within an individual observing block, the observations progress through individual pointings of the mosaic in rapid succession. One field was offset from the main body of the galaxy in each mosaic for calibration purposes. The observations used two basebands, each associated with one spectral window (see the ALMA Technical Handbook for a discussion of the distinction between basebands and spectral windows). The baseband in the lower sideband (LSB) is centered on the CO (3-2) transition, and the baseband in the upper sideband (USB) is used to measure continuum emission. The CASA guide presented here will focus on the reduction of the LSB CO (3-2) data. When these observations were taken, the ALMA antennas were in a configuration that is intermediate between the Cycle 0 "Extended" and "Compact" configurations. We expect this configuration to yield an angular resolution of about 1” near 345 GHz.
We thank the following people for suggesting NGC4038/9 for ALMA Science Verification: Francois Boulanger, Nicole Nesvadba, Cinthya Herrera.  We particularly thank Christine Wilson and Junko Ueda for providing both the suggestions and the OVRO (CO(1-0): Wilson et al. 2000)  and SMA (CO(3-2): Ueda et al., in prep) data for verification purposes.

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#2011.0.00003.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: Antennae ALMA Science Verification Data

PLEASE make full use of the CASA Guides provided for this data set: Antennae Band 7 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?".