Additional Newly Identified Star Clusters in M31

This is the final paper based on our searches for previously unrecognized star clusters in the Andromeda galaxy (M31, NGC 224). As in the case for our previous papers (Krienke & Hodge 2007; Hodge & Krienke 2008; Hodge et al. 2009, 2010), the searches have been made on archival Hubble Space Telescope images obtained for other purposes. Because much of the M31 disk is being studied by the Panchromatic Hubble Andromeda Treasury Program (Dalcanton et al. 2011) using new, deep, five-color HST exposures, we concentrated our searches on the available positions in other portions of the galaxy, primarily in the southern half and northwest quarter. Our searches covered those data that include at least two colors, preferably V and I (or equivalent) and sufficient exposure time to allow photometry to V ∼ 23. Table 1 lists the pointings chosen and gives their characteristics.

Cluster photometry was carried out using a program written in IDL (Krienke & Hodge 2007). Magnitudes were derived in the HST photometric system and transformed to the UBV system using the methods of Holtzman et al. (1995) and Sirianni et al. (2005). Because many of the clusters have both a low surface brightness and a significant size, the M31 background is often a significant fraction of the measured signal. Our program measures both a probable background level and its uncertainty by sampling several (10–24) similarly dimensioned fields on the image.

A single independent check on our photometry was provided by the inadvertent inclusion of a cluster coinciding with one of the clusters in the Johnson et al. (2012) catalog. Their cluster 640 has a quoted F475W magnitude (using the HST F475W filter) of 21.01 and a F814W magnitude of 19.66. We found a F555W magnitude of 20.65 and a F814W magnitude of 19.38. The F814W magnitude agreement is satisfactorily within the uncertainties. Our F555W magnitude lies between Johnson et al.'s F475W magnitude and their F814W magnitude, as expected, considering the shape of the spectral energy distribution (SED) indicated by their photometry.

Table 2 lists the clusters, their magnitudes (mostly V and I), their coordinates and the equivalent radii of our measurements. Many clusters are not perfectly circular and for them we derived equivalent radii from the measured area, given in the table. To eliminate clusters that were already known, we compared our data with the Revised Bologna Catalog (Galleti et al. 2004), the cluster list published by Caldwell et al. (2009), the catalog of Kim et al. (2007), and the Johnson et al. (2012) catalog. A referee generously supplied us with an additional list of duplicates that we had missed. Names assigned to the clusters continue with the acronym used in our earlier papers (KHM31).

This paper was based on archival observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. No public funding was expended in this research. We wish to express special appreciation for the referee's valuable help and very rapid response.