Deep Optical Photometry of Six Fields in the Andromeda Galaxy

Brown et al. (2009)  ApJS, 184, 152



Thomas Brown (STScI) et al. obtained deep optical images reaching well below the oldest main sequence turnoff in six fields of the Andromeda Galaxy using

ACS. The fields fall at four positions on the southeast minor axis, one position in the giant stellar stream, and one position on the northeast major axis. 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. 




















Text Box: Left Fig.(1,  Brown et al. 2009): Shown is stellar density in the Andromeda vicinity, from counts of RGB stars (Ferguson et al. 
2002) and field studied by Brown et al. Right Fig (2, Brown et al. 
2009) shows the CMD in the six studied fields. 
Fig(3) shows the star location in CMD HST  press release.



 Step 1- Download the data from HLSP via the MAST-webpage via,



 Step 2- Plot the Density Map using Topcat cut Percentile Level 0.1 & 90%.

                   Fig(4) below, shows the Density Map of the CMD in STmag system; note that

               Brown et al. (2009) applied CTE correction, and used DaoPhot & PSF fitting

               for their data. They also suggested a simple conversion to the AB system:-








F606W expTime 138.6ks

F814W expTime 161.3ks


F606W expTime 52.8ks

F814W expTime 78.1ks


F606W expTime 52.8ks

F814W expTime 78.1ks


F606W expTime 28.7ks

F814W expTime 47.8ks


F606W expTime 28.1ks

F814W expTime 46.9ks


F606W expTime 28.1ks

F814W expTime 51.6ks


 Step 3- Download the SExCat tables for ProID 9453, 10265,  and 10816 from the HLA via  For this Test Case we show only the

               Halo11 data set  “hlsp_andromeda_hst_acs-wfc_halo11_f606w-f814w_v2_cat.txt”, and compare that with SExCat data from ProID 9453,

               in particular HST_9453_01_ACS_WFC_F606W_sexphot.txt and HST_9453_02_ACS_WFC_F814W_sexphot.txt.  Then match both tables to

              create F600W & F814W table. Note that the SExCat. is based  on visit-based individual drizzled images with exposure time of 5,060 sec only.


 Step 4- Select stars with errors in magAp2 for both filters with values less than 0.2 and CI values in the range of [0.8-1.3]. The selected stars are

               shown below.


  Step 5- Plot the sky positions for both catalogs as shown below. The Halo11-region from Brown et al.  is shown in blue, and  in red stars selected as above.

               Trim the outer area to insure a good agreement.   



   Step 6- Create the CMD, as shown below. Shown  in gray are HLPS STMags and shown in red are the selected stars (using the original ABmag system to offset the data).

                Use the above transformation to convert the HLA selected stars to STMag system, then plot them.



   Step 7- Plot the differential and cumulative  magnitude distribution for both data sets. The measured Halo Stars for the F606W filter are shown in green and for

                the F814W filter in red; the solid lines represent the stars selected as above, and the dash lines represent stars from  Brown et al. It is not surprising that

                the individual visits (with exposure time of only 5060sec for each filter) are shallower by ~4mag than the deep Brown Halo11 data set.   










   Step 8- Download the HSC data for the M31 stream via using “Search with Summary Form”.  Use a search radius of 2.5 arcmin

                 and select all objects with magAper2 around RA & Dec of (11.0749 & 39.7939). Save the file to disk.  Follow the above Steps 4-7. Note that a small delta ra and

                delta dec of 0.024&0.6  arcsec are needed to adjust the coordinates before matching with HLSP M31 stream data.  Select stars and match them with HLSP M31

                stream data. Estimate the conversion to AB system. Plot the CMD as Step 6 above.