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BACKGROUND: Brown et al. (2009) obtained deep optical images reaching well below the oldest main sequence turnoff in six fields of the Andromeda Galaxy using ACS. The fields are located at four positions on the southeast minor axis, one position in the giant stellar stream, and one position on the northeast major axis (disk). These data were obtained as part of three large observing programs (9453, 10265, 10816) designed to probe the star formation history of the stellar population in various structures of the galaxy.


Left Fig. 1 (Brown et al. 2009; Reproduced by permission of the AAS): Shown is stellar density in the Andromeda vicinity, from counts of RGB stars (Ferguson et al. 2002) and fields studied by Brown et al.. Right Fig. 2 (Brown et al. 2009; Reproduced by permission of the AAS) shows the CMD in the six studied fields.

USE CASE GOAL: Construct the F606W - F814W vs F814W color magnitude diagram (CMD) using the HSC and compare it with the disk field from Brown et al. (2009).

NOTE: Several steps below require the use of software external to the HSC and HLA (e.g., to make calculations and plot data). This particular document uses TOPCAT to make most of these figures. Other popular analysis packages are PYRAF, IRAF, IDL, ... . Instructions for these steps will necessarily be less specific than the earlier steps using the HSC and HLA.

Part 1: View the Data

• STEP 1- Enter the Hubble Legacy Archive website (hla.stsci.edu) and view the HSC overlaid on the 10265_01 image (i.e., steps 1 - 7 of the "HSC Walkthrough").

Note: If you use NumImages > 0 in the interactive display you will find several blank sources. These are cosmic rays that have slipped through the processing for one of the 27 images. You will find that the decision of the criteria to use for NumImages is often one of the most important decisions when using the HSC.

Note: You can view the abstract and papers that have resulted from thethe 10265 data set by clicking on the blue 10265 in the PropID column of the inventory.

• STEP 2- Examine the HSC for the M31 disk field (i.e., similar to steps 8 - 9 of the "HSC Walkthrough").

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Part 2: Download and Trim the Data

• STEP 3- Download the HSC catalog for the M31 disk field (i.e., similar to step 10 of the "HSC Walkthrough using the Summary Search form").

Make a catalog to download by changing:
Maximum Records = 10001
Output Format = File: < your choice from the 8 options >
You may also want to change the output columns to customize your output (e.g., remove empty columns).

• STEP 4- Download the Brown et al. (2009) catalog via the MAST High Level Science Product (HLSP) webpage http://archive.stsci.edu/prepds/andromeda/
• STEP 5- Convert the photometric system Brown et al. (2009) used for the HLSP catalog to the AB system.

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+0.840 (ABMAG to STMAG)
-0.292 aperture correction (Sirianni et al. 2005, Table 3, i.e. 2.5log[Encircled Energy]=2.5log[0.764])
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25.632 compared with Brown's 25.60 (so Delta = 0.032)

• STEP 6- Plot the position on the sky for both data set. (Please use your favorite analysis and plotting package such as python, IRAF, IDL, sm, pgplot,...)

Shown in red is the Brown et al. Disk data set, and in blue the HSC. Note that the HSC provides all Sextractor objects including bright saturated stars which were removed by Brown et al.; in addition, Brown et al. excluded all data along the image edges and the detector gap since the number of available exposures in these area is smaller and hence the noise is larger.

• STEP 7 - Trim to the higher quality HSC data.

Select stars with errors in MagAper2 for both filters with values less than 0.2 and CI values in the range 1.0 - 1.3, typical of stars. After applying this selection, the total number of stars is 76497,649.

Part 3: Compare the Data

• STEP 8 - Visual inspection using ds9.

The figure above shows a comparison between the Brown et al. catalog (green circles) and the HSC catalog (blue circles – before the correction has been made to match positions). While the correspondence is in general very good, a few bright objects are missing from the HSC in the immediate vicinity of very bright stars.

• STEP 9 - Match sources and compare photometry.

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• Plot the residual from the fit and compare the magnitudes. Note that the enhancement of points near F606W=25.2 and F814W=24.8 is not an artifact, it is the "red clump", a feature in the stellar life of many stars (seen more clearly in the following figures).

• STEP 10 - Create and compare the CMD (i.e plot of F606W-F814W vs. F814W).

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Now measure the standard deviation (std) of V-I, and
calculate the ratio HSC/Brown of std
We find value of the ratio equal to 0.76 for the upper region (i.e. the HSC is actually slightly narrower) and 1.78 for the lower region (i.e. the HSC has more scatter at fainter magnitude as expected). Hence the HSC can be used to study the AGB up to F606W ~26 [AB] mag.

• STEP 11 - Calculate the completeness in magnitude assuming that Brown et al. (2009) catalog is "the truth".
  • Select all HSC objects within the same sky coverage as Brown et al. (i.e remove objects along the image edges and the detector gap).
  • Plot the ratio between the number counts in the HSC relative to the Brown et al. data set in 0.5 mag.
    
    
  • Shown below in blue is the distribution of the number counts in the HSC relative to the Brown et al. data set binned in 0.5mag. The right figure shows the data for F606W and the left figure shows the data for F814W. For both figures, the horizontal green line indicates our adopted median value of completeness estimated for the overlapping magnitude range above the completness threshold at ~26 mag. Relative to the Brown et al. catalog of photometrically classified stars the HSC completeness has a median values of 94% for F606W and 97% for F814W.

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