Planetary Camera observations of NGC 1275 - Discovery of a central population of compact massive blue star clusters

Abstract

We have discovered a population of bright blue pointlike sources within 5 kpc of the nucleus of NGC 1275 using HST Planetary Camera observations. The typical object has Mv-12 to-14 (H 0 = 75 km s-1 Mpc-1); the brightest has M v-16. They are all blue, with V-R ;S 0.3. The color distribution and lack of excess Ha emission are consistent with nearly all being continuum sources. Many of the sources are unresolved even with the HST and consequently have sizes of ;S 15 pc. We suggest that these are young star clusters that will evolve to look like globular clusters. They are bluer than any clusters seen in the Milky Way or M87, and brighter than the blue clusters seen in the LMC. We derive ages of several hundred million years or less and corresponding masses of 10 5-10 8 1 0. The existence of these young clusters may be connected with a current or previous interaction with another galaxy, with the cooling flow in NGC 1275, or with some combination. Structure is detected in the underlying galaxy light that is suggestive of a merge between NGC 1275 and a second galaxy some 10 8 yr ago. If this merger triggered star formation, it would naturally account for the observed uniformity of cluster colors. Steady-state star formation in the x-ray cooling flow would imply a wider range in cluster age and color than is seen, unless the clusters disrupt. An interaction with the projected high-velocity, infalling system cannot explain the observations because this system has not yet reached the center of NGC 1275 where the clusters are concentrated, and because it has a total interaction time that is far too short for either the observed cluster lifetimes or the dynamical lifetime of structure in the galaxy. If the presence of recently formed protoglobulars around NGC 1275 is related to a previous merger, this would remove an important objection to the merger hypothesis for elliptical galaxy origins, provided that adequate gas is available in the merger for their formation.

Obscured clusters

“…SSCs have been of great interest and considerable study by extragalactic astronomers since their discovery some 15 or more years ago (Holtzman et al 1992). Critical questions about the cluster mass function (CMF), cluster disruption times, and whether there exists a global or locally dependent upper mass limit for stellar clusters, are central to the current debates of astronomers studying extragalactic stellar clusters (Gieles 2009).…”

Imaging of star clusters in unperturbed spiral galaxies with the Advanced Camera for Surveys

“…Full Table 11 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/949 Holtzman et al 1992;Whitmore & Schweizer 1995;Harris et al 2004;Bastian et al 2005c). But large-scale mergers are not necessarily needed for the formation of massive clusters.…”

Two Populations of Young Massive Star Clusters in Arp 220

“…While their older and usually more massive counterparts, the classic globular clusters, are found around almost every type of galaxy (Harris 2001), rich populations of luminous blue star clusters are found predominantly in starburst and interacting systems (Holtzman et al 1992;Whitmore et al 1993;Whitmore & Schweizer 1995;Bastian et al 2005, among many others). However, the facts that individual young clusters have been found in several nearby dwarf galaxies (e.g., Conti & Vacca 1994;O'Connell et al 1994;Billett et al 2002) and that small populations are found in several nearby spiral galaxies (Maoz et al 1996;Larsen 2000Larsen , 2002Larsen et al 2001) suggest that massive star cluster formation is a relatively widespread phenomenon, although it seems to occur with high efficiency only in the most active star-forming systems.…”