54 research outputs found
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 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
Shocks and Star Formation in Stephan's Quintet. I. Gemini Spectroscopy of H{\alpha}-bright knots
We present a Gemini-GMOS spectroscopic study of HST-selected
H{\alpha}-emitting regions in Stephan's Quintet (HCG 92), a nearby compact
galaxy group, with the aim of disentangling the processes of shock-induced
heating and star formation in its intra-group medium. The 40 sources
are distributed across the system, but most densely concentrated in the
kpc-long shock region. Their spectra neatly divide them into narrow- and
and broad-line emitters, and we decompose the latter into three or more
emission peaks corresponding to spatial elements discernible in HST imaging.
The emission line ratios of the two populations of H{\alpha}-emitters confirm
their nature as H II regions (90% of the sample) or molecular gas heated by a
shock-front propagating at 300 km/s. Their redshift distribution
reveals interesting three-dimensional structure with respect to gas-phase
baryons, with no H II regions associated with shocked gas, no shocked regions
in the intruder galaxy NGC 7318B, and a sharp boundary between shocks and star
formation. We conclude that star formation is inhibited substantially, if not
entirely, in the shock region. Attributing those H II regions projected against
the shock to the intruder, we find a lopsided distribution of star formation in
this galaxy, reminiscent of pile-up regions in models of interacting galaxies.
The H{\alpha} luminosities imply mass outputs, star formation rates, and
efficiencies similar to nearby star-forming regions. Two large knots are an
exception to this, being comparable in stellar output to the prolific 30
Doradus region. We also examine Stephan's Quintet in the context of compact
galaxy group evolution, as a paradigm for intermittent star formation histories
in the presence of a rich, X-ray emitting intra-group medium.Comment: Accepted for publication in the Astrophysical Journal. Decreased
resolution for arXiv version, please contact the lead author for a
full-resolution articl
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
Star Clusters in the Tidal Tails of Interacting Galaxies: Cluster Populations Across a Variety of Tail Environments
We have searched for compact stellar structures within 17 tidal tails in 13
different interacting galaxies using F606W- and F814W- band images from the
Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). The
sample of tidal tails includes a diverse population of optical properties,
merging galaxy mass ratios, HI content, and ages. Combining our tail sample
with Knierman et al. (2003), we find evidence of star clusters formed in situ
with Mv < -8.5 and V-I < 2.0 in 10 of 23 tidal tails; we are able to identify
cluster candidates to Mv = -6.5 in the closest tails. Three tails offer clear
examples of "beads on a string" star formation morphology in V-I color maps.
Two tails present both tidal dwarf galaxy (TDG) candidates and cluster
candidates. Statistical diagnostics indicate that clusters in tidal tails may
be drawn from the same power-law luminosity functions (with logarithmic slopes
~ -2 - -2.5) found in quiescent spiral galaxies and the interiors of
interacting systems. We find that the tail regions with the largest number of
observable clusters are relatively young (< 250 Myr old) and bright (V < 24 mag
arcsec^(-2)), probably attributed to the strong bursts of star formation in
interacting systems soon after periapse. Otherwise, we find no statistical
difference between cluster-rich and cluster-poor tails in terms of many
observable characteristics, though this analysis suffers from complex,
unresolved gas dynamics and projection effects.Comment: Accepted for publication in the Astrophysical Journal. 27 pages, 8
figure
The Early Expansion of Cluster Cores
The observed properties of young star clusters, such as the core radius and
luminosity profile, change rapidly during the early evolution of the clusters.
Here we present observations of 6 young clusters in M51 where we derive their
sizes using HST imaging and ages using deep Gemini-North spectroscopy. We find
evidence for a rapid expansion of the cluster cores during the first 20 Myr of
their evolution. We confirm this trend by including data from the literature of
both Galactic and extra-galactic embedded and young clusters, and possible
mechanisms (rapid gas removal, stellar evolutionary mass-loss, and internal
dynamical heating) are discussed. We explore the implications of this result,
focussing on the fact that clusters were more concentrated in the past,
implying that their stellar densities were much higher and relaxation times
correspondingly shorter. Thus, when estimating if a particular cluster is
dynamically relaxed, (i.e. when determining if a cluster's mass segregation is
due to primordial or dynamical processes), the current relaxation time is only
an upper-limit, with the relaxation time likely being significantly shorter in
the past.Comment: 9 pages, 5 figures, accepted MNRA
Evidence for Environmentally Dependent Cluster Disruption in M83
Using multi-wavelength imaging from the Wide Field Camera 3 on the Hubble
Space Telescope we study the stellar cluster populations of two adjacent fields
in the nearby face-on spiral galaxy, M83. The observations cover the galactic
centre and reach out to ~6 kpc, thereby spanning a large range of environmental
conditions, ideal for testing empirical laws of cluster disruption. The
clusters are selected by visual inspection to be centrally concentrated,
symmetric, and resolved on the images. We find that a large fraction of objects
detected by automated algorithms (e.g. SExtractor or Daofind) are not clusters,
but rather are associations. These are likely to disperse into the field on
timescales of tens of Myr due to their lower stellar densities and not due to
gas expulsion (i.e. they were never gravitationally bound). We split the sample
into two discrete fields (inner and outer regions of the galaxy) and search for
evidence of environmentally dependent cluster disruption. Colour-colour
diagrams of the clusters, when compared to simple stellar population models,
already indicate that a much larger fraction of the clusters in the outer field
are older by tens of Myr than in the inner field. This impression is quantified
by estimating each cluster's properties (age, mass, and extinction) and
comparing the age/mass distributions between the two fields. Our results are
inconsistent with "universal" age and mass distributions of clusters, and
instead show that the ambient environment strongly affects the observed
populations.Comment: 6 pages, 3 figures, MNRAS in pres
P/2010A2 LINEAR - I: An impact in the Asteroid Main Belt
Comet P/2010A2 LINEAR is a good candidate for membership with the Main Belt
Comet family. It was observed with several telescopes (ESO NTT, La Silla;
Gemini North, Mauna Kea; UH 2.2m, Mauna Kea) from 14 Jan. until 19 Feb. 2010 in
order to characterize and monitor it and its very unusual dust tail, which
appears almost fully detached from the nucleus; the head of the tail includes
two narrow arcs forming a cross. The immediate surroundings of the nucleus were
found dust-free, which allowed an estimate of the nucleus radius of 80-90m. A
model of the thermal evolution indicates that such a small nucleus could not
maintain any ice content for more than a few million years on its current
orbit, ruling out ice sublimation dust ejection mechanism. Rotational spin-up
and electrostatic dust levitations were also rejected, leaving an impact with a
smaller body as the favoured hypothesis, and ruling out the cometary nature of
the object.
The impact is further supported by the analysis of the tail structure.
Finston-Probstein dynamical dust modelling indicates the tail was produced by a
single burst of dust emission. More advanced models, independently indicate
that this burst populated a hollow cone with a half-opening angle alpha~40degr
and with an ejection velocity v_max ~ 0.2m/s, where the small dust grains fill
the observed tail, while the arcs are foreshortened sections of the burst cone.
The dust grains in the tail are measured to have radii between a=1-20mm, with a
differential size distribution proportional to a^(-3.44 +/- 0.08). The dust
contained in the tail is estimated to at least 8x10^8kg, which would form a
sphere of 40m radius. Analysing these results in the framework of crater
physics, we conclude that a gravity-controlled crater would have grown up to
~100m radius, i.e. comparable to the size of the body. The non-disruption of
the body suggest this was an oblique impact.Comment: 15 pages, 11 figures, in pres
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