The Star Cluster Population in the Tidal Tails of NGC 6872

We present a photometric analysis of the rich star cluster population in the tidal tails of NGC 6872. We find star clusters with ages between 1 - 100 Myr distributed in the tidal tails, while the tails themselves have an age of less than 150 Myr. Most of the young massive ($10^{4} \le M/M_{\odot} \le 10^{7}$) clusters are found in the outer regions of the galactic disk or the tidal tails. The mass distribution of the cluster population can be well described by power-law of the form $N(m) \propto m^{-\alpha}$, where $\alpha = 1.85 \pm 0.11$, in very good agreement with other young cluster populations found in a variety of different environments. We estimate the star formation rate for three separate regions of the galaxy, and find that the eastern tail is forming stars at $\sim 2$ times the rate of the western tail and $\sim 5$ times the rate of the main body of the galaxy. By comparing our observations with published N-body models of the fate of material in tidal tails in a galaxy cluster potential, we see that many of these young clusters will be lost into the intergalactic medium. We speculate that this mechanism may also be at work in larger galaxy clusters such as Fornax, and suggest that the so-called ultra-compact dwarf galaxies could be the most massive star clusters that have formed in the tidal tails of an ancient galactic merger.Comment: 12 pages, 10 figures, accepted A&

A detailed study of the enigmatic cluster M82F

We present a detailed study of the stellar cluster M82F, using multi-band high resolution HST imaging and deep ground based optical slit and integral field spectroscopy. Using the imaging we create colour maps of the cluster and surrounding region in order to search for substructure. We find a large amount of substructure, which we interpret as the result of differential extinction across the projected face of the cluster. With this interpretation, we are able to construct a spatially resolved extinction map across the cluster which is used to derive the intrinsic flux distribution. Fitting cluster profiles (King and EFF) to the intrinsic images we find that the cluster is 15-30% larger than previous estimates, and that no strong evidence of mass segregation in this cluster exists. Using the optical spectra, we find that the age of M82F is 60-80 Myr and from its velocity conclude that the cluster is not physically associated with a large HII region that it is projected upon, both in agreement with previous studies. The reconstructed integral field maps show that that majority of the line emission comes from a nearby HII region. The spatial dependence of the line widths (implying the presence of multiple components)measured corresponds to the extinction map derived from photometry, indicating that the gas/dust clouds responsible for the extinction are also partially ionised. Even with the wealth of observations presented here, we do not find a conclusive solution to the problem of the high light-to-mass ratio previously found for this cluster and its possible top-heavy stellar IMF.Comment: 12 pages, 7 figures, accepted MNRA

A spectroscopic census of the M82 stellar cluster population

We present a spectroscopic study of the stellar cluster population of M82, the archetype starburst galaxy, based primarily on new Gemini-North multi-object spectroscopy of 49 star clusters. These observations constitute the largest to date spectroscopic dataset of extragalactic young clusters, giving virtually continuous coverage across the galaxy; we use these data to deduce information about the clusters as well as the M82 post-starburst disk and nuclear starburst environments. Spectroscopic age-dating places clusters in the nucleus and disk between (7, 15) and (30, 270) Myr, with distribution peaks at ~10 and ~140 Myr respectively. We find cluster radial velocities in the range (-160, 220) km/s (wrt the galaxy centre) and line of sight Na I D interstellar absorption line velocities in (-75, 200) km/s, in many cases entirely decoupled from the clusters. As the disk cluster radial velocities lie on the flat part of the galaxy rotation curve, we conclude that they comprise a regularly orbiting system. Our observations suggest that the largest part of the population was created as a result of the close encounter with M81 ~220 Myr ago. Clusters in the nucleus are found in solid body rotation on the bar. The possible detection of WR features in their spectra indicates that cluster formation continues in the central starburst zone. We also report the potential discovery of two old populous clusters in the halo of M82, aged >8 Gyr. Using these measurements and simple dynamical considerations, we derive a toy model for the invisible physical structure of the galaxy, and confirm the existence of two dominant spiral arms.Comment: Accepted for publication in the Astrophysical Journa

Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies. II. NGC 3256 Clusters

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.Comment: 20 pages, 10 figures, 3 tables; Accepted Ap

Gemini Spectroscopy and HST Imaging of the Stellar Cluster Population in Region B of M82

We present new spectroscopic observations of the stellar cluster population of region B in the prototype starburst galaxy M82 obtained with the Gillett Gemini-North 8.1-metre telescope. By coupling the spectroscopy with UBVI photometry acquired with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST), we derive ages, extinctions and radial velocities for seven young massive clusters (YMCs) in region B. We find the clusters to have ages between 70 and 200 Myr and velocities in the range 230 to 350 km/s, while extinctions Av vary between ~1-2.5 mag. We also find evidence of differential extinction across the faces of some clusters which hinders the photometric determination of ages and extinctions in these cases. The cluster radial velocities indicate that the clusters are located at different depths within the disk, and are on regular disk orbits. Our results overall contradict the findings of previous studies, where region B was thought to be a bound region populated by intermediate-age clusters that formed in an independent, offset starburst episode that commenced 600 Myr-1 Gyr ago. Our findings instead suggest that region B is optically bright because of low extinction patches, and this allows us to view the cluster population of the inner M82 disk, which probably formed as a result of the last encounter with M81. This study forms part of a series of papers aimed at studying the cluster population of M82 using deep optical spectroscopy and multi-band photometry.Comment: 12 pages, 8 figures; accepted for publication in The Astrophysical Journa

Spectroscopic Constraints on the Form of the Stellar Cluster Mass Function

This contribution addresses the question of whether the initial cluster mass function (ICMF) has a fundamental limit (or truncation) at high masses. The shape of the ICMF at high masses can be studied using the most massive young (<10 Myr) clusters, however this has proven difficult due to low-number statistics. In this contribution we use an alternative method based on the luminosities of the brightest clusters, combined with their ages. If a truncation is present, a generic prediction (nearly independent of the cluster disruption law adopted) is that the median age of bright clusters should be younger than that of fainter clusters. In the case of an non-truncated ICMF, the median age should be independent of cluster luminosity. Here, we present optical spectroscopy of twelve young stellar clusters in the face-on spiral galaxy NGC 2997. The spectra are used to estimate the age of each cluster, and the brightness of the clusters is taken from the literature. The observations are compared with the model expectations of Larsen (2009) for various ICMF forms and both mass dependent and mass independent cluster disruption. While there exists some degeneracy between the truncation mass and the amount of mass independent disruption, the observations favour a truncated ICMF. For low or modest amounts of mass independent disruption, a truncation mass of 5-6*10^5 Msun is estimated, consistent with previous determinations. Additionally, we investigate possible truncations in the ICMF in the spiral galaxy M83, the interacting Antennae galaxies, and the collection of spiral and dwarf galaxies present in Larsen (2009) based on photometric catalogues taken from the literature, and find that all catalogues are consistent with having a (environmentally dependent) truncation in the cluster mass functions.Comment: 6 pages, 5 figures, in press, A&A Research Note

The Optical Structure of the Starburst Galaxy M82. II. Nebular Properties of the Disk and Inner-Wind

(Abridged) In this second paper of the series, we present the results from optical Gemini-North GMOS-IFU and WIYN DensePak IFU spectroscopic observations of the starburst and inner wind zones of M82, with a focus on the state of the T~10^4 K ionized interstellar medium. Our electron density maps show peaks of a few 1000 cm-3, local small spatial-scale variations, and a fall-off in the minor axis direction. We discuss the implications of these results with regards to the conditions/locations that may favour the escape of individual cluster winds. Our findings imply that the starburst environment is highly fragmented into a range of clouds from small/dense clumps with low filling factors (<1pc, n_e>10^4 cm-3) to larger filling factor, less dense gas. The near-constant state of the ionization state of the ~10^4 K gas throughout the starburst can be explained as a consequence of the small cloud sizes, which allow the gas conditions to respond quickly to any changes. We have examined in more detail both the broad (FWHM 150-350 km/s) line component found in Paper I that we associated with emission from turbulent mixing layers on the gas clouds, and the discrete outflow channel identified within the inner wind. The channel appears as a coherent, expanding cylindrical structure of length >120 pc and and width 35-50 pc and the walls maintain an approximately constant (but subsonic) expansion velocity of ~60 km/s. We use the channel to examine further the relationship between the narrow and broad component emitting gas within the inner wind. Within the starburst energy injection zone, we find that turbulent motions (as traced by the broad component) appear to play an increasing role with height.Comment: 27 pages, 18 figures (13 in colour), accepted for publication in Ap