M100 Introduction Page.
Science Target Overview
M100 (NGC 4321; RA 12h 22m 54.8s, Dec +15 49' 19") is a nearby (~16 Mpc) 'grand-design' barred spiral galaxy in the Virgo cluster. It has long spiral arms dominating its optical disk and an abundance of molecular gas in its center. It has a relatively face-on inclination of ~30 degrees.
In the 1990s, M100 was mapped in CO (1-0) at an angular resolution of 6" using the Berkeley-Illinois-Maryland Association (BIMA) millimeter interferometer as part of the BIMA SONG survey (Regan et al. 2001; Helfer et al. 2003; Sheth et al. 2002). Using the Nobeyama mm-wave Array (NMA), the central 1' was mapped by Sakamoto et al. (1995, 1999) at 2.5" resolution. Its nucleus has also been mapped in CO using the IRAM interferometer (Garcia-Burillo et al. 1998). In this CASAguide, CO (1-0) emission is mapped with ALMA using the main (12-m) array, the 7-m array, and the single dishes (called Total Power), each of which corresponds to molecular gas structures with different physical size scales. Here, a method is described to combine these into a single image.
ALMA Data Overview
WARNING: Note that the calibration presented in this Guide is a new calibration (using CASA 4.3) of the same 12-m array data that were released previously (using CASA 3.3), and so the data reduction path has been updated to current best practices and starts from the raw data files called ASDMs (ALMA Science Data Model). The calibration described in this guide uses CASA 4.3. The previously released 12-m array data (reduced using CASA 3.3) is still available and from 28 July 2015 onwards can be identified by the suffix "_CASA3.3". Do not use the previously released CASA 3.3 version of the uncalibrated or calibrated 12-m data (either downloaded before 28 July 2015 or with the "_CASA3.3" suffix) for this tutorial.
ALMA Science Verification data at Band 3 was taken for M100 with the ALMA 12-m array on 10 August and 10 September, 2011.
The observations consisted of a 47 pointing mosaic centered at RA=12:22:54.6, Dec=+15:48:56.5 and used a spectral setup with four FDM (frequency domain mode) windows with 3840 channels each. The native channel width is 488.281 kHz, or 1.27 km/s, and one of the windows is centered at the CO (1-0) line with a rest frequency of 115.271 GHz in ALMA Band 3. There are three 12m datasets, comprising a total of 124.3 minutes on source. The integration interval for each visibility is 6.05 seconds.
The names of the three datasets are as follows:
In these observations, Titan is used at the amplitude calibrator, 3C273 is used as the bandpass calibrator, and J1224+213 is used as the gain calibrator. In contrast to the first calibration presented for this data, all calibration is done in CASA 4.3 and the gain calibration tables are applied to the data and the visibility weights in the final applycal step (with calwt=T). With these changes, each visibility is weighted appropriately for the final data combination.
Morita array (ACA) 7-m Data
ALMA Science Verification data at Band 3 was taken for M100 with the ACA on 17-18 March, 14 April, and 11 May, 2013. The observations consisted of a 23 pointing mosaic centered at RA=12:22:54.3, Dec=+15:48:51.4 and used a spectral setup with either two or four spectral windows with 4080 channels each. The native channel width is 488.281 kHz, or 1.27 km/s, and one of the windows is centered at the CO (1-0) line with a rest frequency of 115.271 GHz in ALMA Band 3; this setup mirrors the 12m observations. There are six datasets, comprising a total of 188.4 minutes on source. The integration interval for each visibility is 10.1 seconds.
The names of the six datasets are as follows:
In these observations, 3C273 (J1229+0203) is used as the amplitude and bandpass calibrator, and J1215+1654 is used as the gain calibrator. Titan is observed in each execution as the amplitude calibrator, but the quality of the data is not good (and one of the executions, _X4eb, is missing an observation of Titan). Instead, for the first five executions, the flux calibration is set using au.getALMAFluxForMS, which checks the ALMA measurements of calibrators to define the appropriate flux for a calibrator as a function of frequency and time of observation. For the final dataset (_X4eb), the flux of 3C273 was observed to change rapidly by both ALMA and CARMA, which is not reflected in the observations in the calibrator database, and so a best estimate has been made. In the 7m datasets, careful flagging of the Tsys tables was necessary in several cases, and the antenna CM01 had to be removed from almost all observations as it was not locked. Standard calibration scripts are used in the calibration of this data, and all extra flagging commands can be found within those files. Finally, all calibration is done using CASA 4.3, and the gain calibration tables are applied to the data and the visibility weights in the final applycal step (with calwt=T). With these changes, each visibility is weighted appropriately for the final data combination.
Note that in the 7m data, there is a weak continuum detection, but it is too weak to contaminate the line emission (the peak of the continuum detection is well less than the rms in a single line channel); the continuum is not even detected in the 12m data. Therefore in the combined data, no continuum subtraction is performed.
ALMA Science Verification Single-Dish data at Band 3 was taken for M100 on 1, 5, 7, and 17 July, 2014 with the Total-Power array which consisted of 2 or 3 12-m antennas and the ACA correlator. The rectangular area of 202" x 226" (RA x Dec) centered at RA=12:22:54.36, Dec=+15:48:50.6 was mapped using the on-the-fly technique. The frequency setup for the single dish observations mirrors that of the interferometer in that four 2-GHz-wide spectral windows are used, each having 4080 channels. The native channel width is set to 488.281 kHz, and one of the windows is centered at the CO (1-0) line with a rest frequency of 115.271 GHz.
There are four amplitude calibrator datasets (i.e., one per day) and nine science datasets (i.e., two or three per day). The integration interval for each visibility is 0.144 second for the amplitude calibrator and 1.008 seconds for the science datasets. The total time on source (M100) was 172.4 minutes.
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- uid___A002_X8602fa_Xc3 PM02, PM03, PM04
- uid___A002_X864236_Xe1 PM03, PM04
- uid___A002_X86fcfa_X3ae DV10, PM03, PM04
- uid___A002_X85c183_X36f DA61, PM03, PM04
- uid___A002_X85c183_X60b DA61, PM03, PM04
- uid___A002_X8602fa_X2ab PM02, PM03, PM04
- uid___A002_X8602fa_X577 PM02, PM03, PM04
- uid___A002_X864236_X2d4 PM03, PM04
- uid___A002_X864236_X693 PM03, PM04
- uid___A002_X86fcfa_Xd9 DV10, PM03, PM04
- uid___A002_X86fcfa_X664 DV10, PM03, PM04
- uid___A002_X86fcfa_X96c DV10, PM03, PM04
Acknowledgements and Data Usage
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#2011.0.00004.SV. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and 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."
We thank the following people for suggesting M100 for ALMA Science Verification: Preben Grosbol and Catherine Vlahakis.
Obtaining the Data
To download the data, follow one of the next links:
ACA (7m and Total Power data):
Total Power (Single Dish) Data Reduction Tutorial
A tutorial describing how to calibrate and image the M100 Total Power data can be found here: M100_Band3_SingleDish
M100 Data Combination Tutorial
A tutorial describing how to combine the M100 data from the 12m main array and the ACA (7m array + Total Power) can be found here: M100 Combine CASA 5.4.
How to Use A CASA Guide
For tips on using CASA and ways CASA can be run, see EVLA_Spectral_Line_Calibration_IRC+10216#How_to_Use_This_casaguide page.
To learn how to extract executable Python scripts from the tutorial, see Extracting_scripts_from_these_tutorials.
Within the guides:
# In CASA Regions of this color are CASA commands (or definitions) that need to be cut and pasted in sequence. Wait until one command is finished before pasting another. Tabs matter in python, make sure that commands that span more than one line and "for" loops keep their spacing. Sometimes (especially "for" loops) you may need to explicitly hit enter twice to get the command going.
Information in this color shows excerpts from the CASA Logger output
This color shows you background information about the data or other types of reference material