Star Clusters in Early-Type Galaxies

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  F225W F300W F330W F435W F450W F475W F555W F814W Cluster Detection Method/ Source Detection Threshold(s) Distance Modulus Minimum Log Mass for observability
NGC 524 WFC3   ACS       WFPC2 WFPC2 Cross-Compare/ F555W: 45\(\sigma\) F330W: 6\(\sigma\) 32.10 2.9926
NGC 2685 WFC3       WFPC2   WFPC2 WFPC2 Cross-Compare/ F555W: 45\(\sigma\) F450W: 40\(\sigma\) 30.57 1.81605
NGC 2768 WFC3     ACS     WFPC2 WFPC2 Cross-Compare/ F555W: 45\(\sigma\) F336W: 40\(\sigma\) 31.38 2.27633
NGC 3032 WFC3         WFC3 WFC3 WFC3 White light/ 5\(\sigma\) 30.48 1.83302
NGC 3156 WFC3         WFC3 WFC3 WFC3 White light/ 50\(\sigma\) 31.50 2.04135
NGC 3489 WFC3         WFC3 WFC3 WFC3 White light/ 200\(\sigma\) 29.60 1.68015
NGC 4459 WFC3 WFPC2       WFC3 WFC3 WFC3 White light/ 200\(\sigma\) 31.02 2.10882
NGC 4477 WFC3         WFC3 WFC3 WFC3 White light/ 100\(\sigma\) 31.34 2.32347
NGC 4550 WFC3 WFPC2       ACS WFPC2 WFPC2 Cross-Compare/ F555W: 45\(\sigma\) F300W: 6\(\sigma\) 31.06 2.13877

Source detection

All images examined were from the Hubble Space Telescope’s MAST archive, using different filters and cameras for each galaxy depending upon availability of data; Filter and camera choice for each galaxy is shown in Table 1. On these images, two point source detection methods were used: The primary method used was the “white light” detection method as in (Kaviraj 2012). The other method was to compile separate source catalogs for the F555W filter and the filter closest to F336W using the PYRAF command DAOFIND and take the overlapping sources as the output catalog. This Cross-comparison method was used whenever the galaxy had a combination of too many different cameras and low source counts leading to an unusable white light image.

Table Caption: This table details the information relevant to each individual galaxy surveyed. The central wavelengths of all filters used as well as the camera with which the image was taken motivated the choice of cluster detection method, where the presence of more than two different cameras in the sample led to difficulties in white light image creation. The source detection threshold in sigma above the background is included, as well as the distance modulus of the galaxy and the calculated minimum detectable mass of a cluster in that galaxy.

Cluster determination

In order to determine whether a detected point source was a star or a cluster, photometry for detected sources was obtained using the PYRAF task phot in the V-band image (F555W) at photometry apertures of 3 pixels and 0.5 pixels. The difference between these magnitudes was calculated as the V-band concentration index (C), where as with (CITE CHANDAR 2010 PAPER), a threshold of 2.3 was used; sources with C\(<\)2.3 were determined to not be clusters and were removed from the cluster catalog.

As an added precaution, source points were then observed manually on an RGB image of the galaxy in question to inspect for highly redshifted background galaxies. In the case where the field of the galaxy was quite cluttered, it was necessary to reduce centroiding areas to 2 pixel boxes, rather than 4 pixel boxes to prevent false identification of clusters due to the centroiding algorithm causing the star’s listed position to “