Ages of Star Clusters in the Tidal Tails of Merging Galaxies

Abstract

We study the stellar content in the tidal tails of three nearby merging galaxies, NGC 520, NGC 2623, and NGC 3256, using BVI imaging taken with the Advanced Camera for Surveys on board the Hubble Space Telescope. The tidal tails in all three systems contain compact and fairly massive young star clusters, embedded in a sea of diffuse, unresolved stellar light. We compare the measured colors and luminosities with predictions from population synthesis models to estimate cluster ages and find that clusters began forming in tidal tails during or shortly after the formation of the tails themselves. We find a lack of very young clusters (≤ 10 Myr old), implying that eventually star formation shuts off in the tails as the gas is used up or dispersed. There are a few clusters in each tail with estimated ages that are older than the modeled tails themselves, suggesting that these may have been stripped out from the original galaxy disks. The luminosity function of the tail clusters can be described by a single power-law, dN/dL ∝ L α , with −2.6 < α < −2.0. We find a stellar age gradient across some of the tidal tails, which we interpret as a superposition of 1) newly formed stars and clusters along the dense center of the tail and 2) a sea of broadly distributed, older stellar material ejected from the progenitor galaxies.

Massive Star Cluster Formation and Destruction in Luminous Infrared Galaxies in GOALS

“…ISHAPE iterates through different values for the effective radius until a minimum χ 2 is found. Similiar to the approach in Mulia et al (2015), we find that a conservative cut of 2 pixels FWHM effectively removes extended sources in both the nearest and furthest galaxies in the sample. Additionally, we made a cut of M B ≤ −9.5 mag, corresponding to the Humphreys & Davidson (1979) limit, where we might expect contamination of the cluster sample from single bright yellow supergiants in the Milky Way.…”

“…However, optical studies of other late stage mergers such as NGC 6240 (Pasquali, de Grijs & Gallagher 2003) and NGC 7673 (Homeier, Gallagher & Pasquali 2002) reveal only young star clusters, indicating that older star clusters, which would have formed earlier on in the merger, are either undetected or rare. In contrast, the lack of young star clusters in the tidal tails of NGC 520 and NGC 2623 (Mulia et al 2015) relative to what is observed for NGC 3256 (Trancho et al 2007) suggests that the remaining reservoirs of predominately neutral hydrogen (HI) gas in the tails cannot always form new clusters.…”

GASP XIII. Star formation in gas outside galaxies

“…At the present epoch, most new stars are formed in galaxy disks, but SF is known to take place also in more exotic environments, such as the tidal tails of merging systems (Elmegreen et al 1993;Boquien et al 2009;Bournaud et al 2004;Schweizer 2006;Duc 2012;Mullan et al 2011;Mulia et al 2015;Vulcani et al 2017).…”

Double Nuclei in NGC 908 and NGC 1187

“…It is clear from Figure 2 that both galaxies show a double stellar nucleus. In the case of NGC 908, we can even see elongated structures very similar to tidal tails (Knierman et al 2012;Mullan et al 2011;Struck & Smith 2012;Knierman et al 2013;Mulia et al 2015;Wen & Zheng 2016). One natural explanation, then, to the double stellar nucleus in NGC 908 is that it is the result of a merger.…”