1,056 research outputs found
Dynamical evidence for a strong tidal interaction between the Milky Way and its satellite, Leo V
We present a chemodynamical analysis of the Leo~V dwarf galaxy, based on Keck
II DEIMOS spectra of 8 member stars. We find a systemic velocity for the system
of kms, and barely resolve a
velocity dispersion for the system, with kms, consistent with previous studies of Leo~V. The
poorly resolved dispersion means we are unable to adequately constrain the dark
matter content of Leo~V. We find an average metallicity for the dwarf of
[Fe/H], and measure a significant spread in the iron abundance
of its member stars, with [Fe/H] dex, which cleanly
identifies Leo~V as a dwarf galaxy that has been able to self-enrich its
stellar population through extended star formation. Owing to the tentative
photometric evidence for tidal substructure around Leo~V, we also investigate
whether there is any evidence for tidal stripping or shocking of the system
within its dynamics. We measure a significant velocity gradient across the
system, of kms per
arcmin (or kms~kpc), which points almost directly
toward the Galactic centre. We argue that Leo~V is likely a dwarf on the brink
of dissolution, having just barely survived a past encounter with the centre of
the Milky Way.Comment: 14 pages, 12 figures, accepted for publication in MNRAS. Updated to
include minor revisions from referee proces
An HST/WFPC2 Survey of Bright Young Clusters in M31 III. Structural Parameters
Surface brightness profiles for 23 M31 star clusters were measured using
images from the Wide Field Planetary Camera 2 on the Hubble Space Telescope,
and fit to two types of models to determine the clusters' structural
properties. The clusters are primarily young (~10^8 yr) and massive (~10^4.5
solar masses), with median half-light radius 7 pc and dissolution times of a
few Gyr. The properties of the M31 clusters are comparable to those of clusters
of similar age in the Magellanic Clouds. Simulated star clusters are used to
derive a conversion from statistical measures of cluster size to half-light
radius so that the extragalactic clusters can be compared to young massive
clusters in the Milky Way. All three sets of star clusters fall approximately
on the same age-size relation. The young M31 clusters are expected to dissolve
within a few Gyr and will not survive to become old, globular clusters.
However, they do appear to follow the same fundamental plane relations as old
clusters; if confirmed with velocity dispersion measurements, this would be a
strong indication that the star cluster fundamental plane reflects universal
cluster formation conditions.Comment: AJ in press; 37 pages, 12 figure
An optical/NIR survey of globular clusters in early-type galaxies III. On the colour bimodality of GC systems
The interpretation that bimodal colour distributions of globular clusters
(GCs) reflect bimodal metallicity distributions has been challenged.
Non-linearities in the colour to metallicity conversions caused by the
horizontal branch (HB) stars may be responsible for transforming a unimodal
metallicity distribution into a bimodal (optical) colour distribution. We study
optical/near-infrared (NIR) colour distributions of the GC systems in 14 E/S0
galaxies. We test whether the bimodal feature, generally present in optical
colour distributions, remains in the optical/NIR ones. The latter colour
combination is a better metallicity proxy than the former. We use KMM and GMM
tests to quantify the probability that different colour distributions are
better described by a bimodal, as opposed to a unimodal distribution. We find
that double-peaked colour distributions are more commonly seen in optical than
in optical/NIR colours. For some of the galaxies where the optical (g-z)
distribution is clearly bimodal, the (g-K) and (z-K) distributions are better
described by a unimodal distribution. The two most cluster-rich galaxies in our
sample, NGC4486 and NGC4649, show some interesting differences. The (g-K)
distribution of NGC4649 is better described by a bimodal distribution, while
this is true for the (g-K) distribution of NGC4486 GCs only if restricted to a
brighter sub-sample with small K-band errors (< 0.05 mag). Formally, the K-band
photometric errors cannot be responsible for blurring bimodal metallicity
distributions to unimodal (g-K) colour distributions. However, simulations
including the extra scatter in the colour-colour diagrams (not fully accounted
for in the photometric errors) show that such scatter may contribute to the
disappearance of bimodality in (g-K) for the full NGC4486 sample. For the less
cluster-rich galaxies results are inconclusive due to poorer statistics.
[Abridged]Comment: A&A accepted, 15 pages, 10 figures, 4 table
An HST/WFPC2 survey of bright young clusters in M31. I. VdB0, a massive star cluster seen at t β 25 Myr
Aims. We introduce our imaging survey of possible young massive globular clusters in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). We obtained shallow (to B ~ 25) photometry of individual stars in 20 candidate clusters. We present here details of the data reduction pipeline that is being applied to all the survey data and describe its application to the brightest among our targets, van den Bergh 0 (VdB0), taken as a test case.
Methods. Point spread function fitting photometry of individual stars was obtained for all the WFPC2 images of VdB0 and the completeness of the final samples was estimated using an extensive set of artificial stars experiments. The reddening, the age and the metallicity of the cluster were estimated by comparing the observed color magnitude diagram (CMD) with theoretical isochrones. Structural parameters were obtained from model-fitting to the intensity profiles measured within circular apertures on the WFPC2 images.
Results. Under the most conservative assumptions, the stellar mass of VdB0 is M> 2.4 x 10^4 M_β , but our best estimates lie in the range β4-9 x 10^4 M_β. The CMD of VdB0 is best reproduced by models having solar metallicity and age β25 Myr. Ages less than β12 Myr and greater than β60 Myr are clearly ruled out by the available data. The cluster has a remarkable number of red super giants (β³18) and a CMD very similar to Large Magellanic Cloud clusters usually classified as young globulars such as NGC 1850, for example.
Conclusions. VdB0 is significantly brighter (β³1 mag) than Galactic open clusters of similar age. Its present-day mass and half-light radius ((r_h = 7.4 pc) are more typical of faint globular clusters than of open clusters. However, given its position within the disk of M31, it is expected to be destroyed by dynamical effects, in particular by encounters with giant molecular clouds, within the next ~4 Gyr
- β¦