Hubble Source Catalog (HSC) Use Case # 4 - Photometry of Slightly Resolved Objects
Reference:- "The Color-Magnitude Relation for Metal-Poor Globular Clusters in M87"
Peng et al. (2009) investigated the color-magnitude relation for metal poor Globular Clusters (GCs) in M87 using HST/ACS observations with filters F606W and F814W. A color-magnitude diagram was constructed which displayed bimodality.
USE CASE GOAL: Construct a color magnitude diagram (CMD) of the globular clusters in M87 using the HSC and comparing the plot with Peng et al. (2009), in particular the bimodality.
HSC topics include:
Watch the Use Case 4 video tutorial
- How to select reasonable parameters for NumImages
- Combining ACS and WFPC2 magnitudes
- The effect of using MagAper2 or MagAuto
- Techniques for making the HSC sample more uniform
Part 1: View the Data
Part 2: Download and Correct the Data
- Download the HSC catalog for M87.
Go back to the HSC summary form, retain some of the previous parameters:
Target Name = M87
Radius = 5 arcmin
Mag Type = MagAper2
NumImages > 5
Maximum records = 5001
Output Coords = Decimal
Output Columns = Match ID, MatchRA, MatchDec, A_F814W, A_F814W_sigma, W2_F606W, W2_F606W_sigma, W2_814W, W2_814W_sigma, CI
Output Format = IRAF: space-separated w/INDEFs
Setting the output coordinates, columns and format is optional. You may prefer a different format or just use the defaults.
Note that both the A_F814W and W_814W observations have included since we will combine them in a later step. We check that the agreement between A_F814W and W_F814W is good enough to justify this approach in step 8.
STEP 5- Download the Peng et al. (2009) catalog from the
VizieR database. Enter M87 and follow the steps for Peng et al. (2009).
STEP 6- Make corrections for differences in photometric system and extinction.
Peng et al (2009) converted their data to the VEGAMAG magnitude system using zero points from Sirianni et al. (2005), whereas the HSC and HLA use ABMAG. From Sirianni et al. (2005), the correction to ABMAG for WFC is
ABmag_F606W = VEGAmag_F606W - 0.088
ABmag_F814W = VEGAmag_F814W - 0.436
Peng et al. used values of A(V) = 0.064 and A(I) = 0.042, to correct for extinction. We will adopt their values here.
STEP 7- Trim the data.
Due to problems identifying sources near the nucleus (see step 1), all those within a radius of 19.8" were removed, taking the total to 2763.
Based on inspection of stars all of which had CI <1.05 (see step 2) GCs were taken to be sources with CI between 1.05 and 1.9. After this correction, the total number of sources is 2229.
After trimming the remaining GCs with errors (ie the RMS scatter from the summary form) in magnitude of 0.2 or smaller, an additional 33 are removed, leaving 2196 sources remaining. 907 of which do not have both readings for F814W and F606W so the final total is 1289. This sample size is ~50% of Peng et al.'s 2250, using A_F606W, which is not currently available for the HSC.
Part 3: Plot and Compare the Data
STEP 8- Compare the HSC I filter data.
In this use case we combine the A_F814W and W_F814W magnitudes. We now check whether this step is justified by determining the agreement between the two measurements. This first plot shows the agreement in the raw HSC data; interestingly there appears to be a scattering of points below the main relation. These data are objects on the PC portion of the WFPC2. The problem arises because of the difference in resolution and pixel scale (0.05"/pix for PC and 0.10"/pix for WFC) between the PC and WFC in WFPC2. Hence the two filters require a different aperture correction. These outliers are removed by our CI < 1.05 cut in step 7.
The figure below (left) shows the result after the sample has been trimmed using steps 6-9. The residual plot (right) shows the scatter in more detail
The magnitudes were binned into sections and the average and scatter for each bin were calculated. Based on this comparison we add 0.03 magnitudes to the WFPC2 observations and the value of 23 mag was taken to be the cut off point. The inconsistency is probably due to some Malmquist bias.
STEP 9- Match sources and compare photometry.
After applying a small offset of +0.022° RA and +0.009° Dec to the Peng et al. positions we can match the two catalogs. A comparison of the color in ABMAG from both catalogs is shown below. Note the bimodality in the scatter of the points.
Now we separate the data into high and low quality subsamples. The left figure displays all sources in the matched catalog and the right figure displays only the points with uncertainties <0.1 mag. The dashed line has a slope of unity, whereas the solid lines are linear regression fits to the data points.
STEP 10- Create and compare the Color Magnitude Diagrams.
Below is the original Peng et al. CMD for M87 converted into ABMAG magnitudes, plotted alongside the CMDs for M87 constructed using HSC MagAper2 data, and on the far right constructed using HSC MagAuto. Below these diagrams are the corresponding color histograms.
Upon inspection the correspondence between Peng et al. and HSC MagAper data is good: both diagrams have a similar range of color and both display a clear bimodality. Peng et al. I magnitudes extend to fainter values because their photometry was gathered from co-added images. The bimodality is strongest for the Peng data as expected since both A_F606W and A_F814W magnitudes were used.
Our previous assumption that, using MagAuto for measuring magnitudes was detrimental, is proven correct in this diagram. Most of the bimodality in this diagram has been washed out.
STEP 11- Comparison of the size of the clusters in blue and red enhancements
The blue and red enhancements were separated and the mean CI values were calculated. Below the separation of the two colors is taken as the median.
The median value of the distribution, 0.513 mag, was taken to be the separation color. The average CI for the blue and red sub-populations were (1.316 ± 0.005) mag and (1.281 ± 0.006) mag respectively, giving a difference between the two of (0.035 ± 0.008) mag. This result indicates a statistically significant size difference between the two filters.
This result, that the radii of blue GCs are larger, is in agreement with Kundu et al. (1999) and also agrees with Webb et al. (2012)
STEP 12- Calculate the completeness in magnitude assuming that Peng et al. catalog is "the truth".
Note that ACS and WFPC2 source lists will not have the same coverage. The image (left) is from the footprints tab of the HLA, centered on M87, with A_F606W and W_F606W highlighted. The top left of the ACS square (yellow) has a section that is not included by the WFPC2 coverage and vice versa for other locations of the image. Seen in the image on the right is the image from step 1 with NumImages > 2 and the colors inverted, note the corresponding sky coverage between the two figures.
Before we can compute the completeness levels, the samples need to be trimmed so we have uniform coverage in both catalogs. In the figure below on the left, we use the same image from hla.stsci.edu from steps 1 & 2, and over-plotted the regions of Peng et al. (green) and HSC (blue). Note the lack of HSC sources in the nuclear region, due to the high background. We impose a maximum outer radius in order to make a fair comparison when computing the completeness, the distribution is shown in the right image.
The data was sorted into bins of 0.5 mag, and the ratio was taken between each catalog. The green line shows the mean value of all the points before the drop off.
The 50% threshold magnitude was calculated using interpolation to be 25.13 mag and 23.98 mag for F606W and F814W filters respectively.
The higher depth of the Peng et al data set is apparent when the total area of the histogram is compared with the HSC histogram. However the two catalogs have similar peaks in I, since the HSC is just deep enough to reach the peak of the luminosity function at I ~ 22.7 (e.g. Kundu et al. (1999))
- The HSC Beta version 0.2 catalog was used to recover the bimodality of the CMD of M87 seen in Peng et al. (2009).
- The magnitude cutoff was ~2 mag less that Peng et al. (2009) because the photometry of HSC is inferred from single visits rather than a co-added image.
- Foreground stars can be distinguished from GCs using the concentration index.
- Aperture magnitudes are the superior method (rather than MagAuto) for calculating the color of clusters.
- The sizes of the globular clusters in the blue and red sub-populations were calculated for M87 and the difference was (0.035 ± 0.008) mag with the blue being the larger of the two.
- Relative to the Peng et al. catalog, the HSC is complete to 99% down to ~24 in F606W and ~23 in F814W.
HSC WISH LIST:
- Use the HSC to measure the blue tilt of the CMD of M87. We refer the reader to Peng et al. (2009) and Waters et al. (2009) for examples of this topic.
- Measure the variability of sources (see HSC Use Case #2) in M87 similar to Baltz et al. (2004).
- Include photometry of more sources by not eliminating those filter magnitudes for sources with only one image, when they have other filter magnitudes with more than one image.