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Register a new userHST Observations of Young Star Clusters in Interacting Galaxies
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Authors
K. D. Borne
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We present early results from the analysis of HST imaging observations for several pairs of interacting galaxies. We include two cases that were specifically chosen to represent a strong early (young) encounter (Arp 81 = NGC 6621/6622) and a weak late (old) encounter (Arp 297 = NGC 5752/5754). The goals of the project include a determination of the timing, frequency, strength, and characteristics of the young star clusters formed in these two limiting cases of tidal encounters.
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We present Gemini optical spectroscopy of 23 young star clusters in NGC3256. We find that the cluster ages range are from few Myr to ~150 Myr. All these clusters are relatively massive (2--40)x 10^{5} msun$ and appear to be of roughly 1.5 zo metallicity. The majority of the clusters in our sample follow the same rotation curve as the gas and hence were presumably formed in the molecular-gas disk. However, a western subsample of five clusters has velocities that deviate significantly from the gas rotation curve. These clusters may either belong to the second spiral galaxy of the merger or may have formed in tidal-tail gas falling back into the system. We discuss our observations in light of other known cluster populations in merging galaxies, and suggest that NGC 3256 is similar to Arp 220, and hence may become an Ultra-luminous Infrared Galaxy as the merger progresses and the star-formation rate increases. Some of the clusters which appeared as isolated in our ground-based images are clearly resolved into multiple sub-components in the HST-ACS images. The same effect has been observed in the Antennae galaxies, showing that clusters are often not formed in isolation, but instead tend to form in larger groups or cluster complexes.
We present a spectroscopic survey of 21 young massive clusters and complexes and one tidal dwarf galaxy candidate (TDG) in Stephans Quintet, an interacting compact group of galaxies. All of the selected targets lie outside the main galaxies of the system and are associated with tidal debris. We find clusters with ages between a few and 125 Myr and confirm the ages estimated through HST photometry by Fedotov et al. (2011), as well as their modelled interaction history of the Quintet. Many of the clusters are found to be relatively long-lived, given their spectrosopically derived ages, while their high masses suggest that they will likely evolve to eventually become intergalactic clusters. One cluster, T118, is particularly interesting, given its age (sim 125 Myr), high mass (sim 2times10^6 Modot) and position in the extreme outer end of the young tidal tail. This cluster appears to be quite extended (Reff sim 12 - 15 pc) compared to clusters observed in galaxy disks (Reff sim 3 - 4 pc), which confirms an effect we previously found in the tidal tails of NGC 3256, where clusters are similarly extended. We find that star and cluster formation can proceed at a continuous pace for at least sim 150 Myr within the tidal debris of interacting galaxies. The spectrum of the TDG candidate is dominated by a young population (sim 7 Myr), and assuming a single age for the entire region, has a mass of at least 10^6 Modot.
We present Gemini optical spectroscopy of three young star clusters in the western tidal tail of NGC3256. Compact star clusters (as opposed to dwarf-galaxy candidates) in tidal tails are rare, with these three clusters the first for which detailed quantitative spectroscopy has ever been obtained. We find that two of these clusters appear to be coeval, while the third is approximately two times older~200 Myr vs.~80 Myr). All three clusters are massive (1-3 x 10^5 msun) and appear to be of roughly solar metallicity. Additionally, the three clusters appear to be relatively large (R_eff = 10-20 pc), possibly reflecting weak compression at the time of formation and/or the weak influence of the tidal field of the galaxy. All three clusters have velocities consistent with the general trend of the HI velocities in the tidal tail. We conclude that if the loosely bound tail material of NGC 3256 gets stripped during future interactions of this galaxy within its group, these three clusters may become part of the intra-group medium.
We present the first Advanced Camera for Surveys (ACS) observations of young star clusters in the colliding/merging galaxy UGC 10214. The observations were made as part of the Early Release Observation (ERO) program for the newly installed ACS during service mission SM3B for the Hubble Space Telescope (HST). Many young star clusters can be identified in the tails of UGC 10214, with ages ranging from ~3 Myr to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the star clusters found in the tail of UGC 10214 can only be explained if strong emission lines are included with a young stellar population. This has been confirmed by our Keck spectroscopy of some of these bright blue stellar knots. The most luminous and largest of these blue knots has an absolute magnitude of M_V = -14.45, with a half-light radius of 161 pc, and if it is a single star cluster, would qualify as a super star cluster (SSC). Alternatively, it could be a superposition of multiple scaled OB associations or clusters. With an estimated age of ~ 4-5 Myr, its derived mass is < 1.3 x 10^6 solar masses. Thus the young stellar knot is unbound and will not evolve into a normal globular cluster. The bright blue clusters and associations are much younger than the dynamical age of the tail, providing strong evidence that star formation occurs in the tail long after it was ejected. UGC 10214 provides a nearby example of processes that contributed to the formation of halos and intra-cluster media in the distant and younger Universe.
Star cluster formation is a major mode of star formation in the extreme conditions of interacting galaxies and violent starbursts. These newly-formed clusters are built from recycled gas, pre-enriched to various levels within the interacting galaxies. Hence, star clusters of different ages represent a fossil record of the chemical enrichment history of their host galaxy, as well as of the host galaxys violent star formation history. We present a new set of evolutionary synthesis models of our GALEV code, specifically developed to include the gaseous emission of presently forming star clusters, and a new tool to analyse multi-color observations with our models. First results for newly-born clusters in the dwarf starburst galaxy NGC 1569 are presented.
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