71 research outputs found
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 ()
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 , where , 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 times the rate of the western tail and 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
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