515 research outputs found
The Reionization of Carbon
Observations suggest that CII was more abundant than CIV in the intergalactic
medium towards the end of the hydrogen reionization epoch. This transition
provides a unique opportunity to study the enrichment history of intergalactic
gas and the growth of the ionizing background (UVB) at early times. We study
how carbon absorption evolves from z=10-5 using a cosmological hydrodynamic
simulation that includes a self-consistent multifrequency UVB as well as a
well-constrained model for galactic outflows to disperse metals. Our predicted
UVB is within 2-4 times that of Haardt & Madau (2012), which is fair agreement
given the uncertainties. Nonetheless, we use a calibration in post-processing
to account for Lyman-alpha forest measurements while preserving the predicted
spectral slope and inhomogeneity. The UVB fluctuates spatially in such a way
that it always exceeds the volume average in regions where metals are found.
This implies both that a spatially-uniform UVB is a poor approximation and that
metal absorption is not sensitive to the epoch when HII regions overlap
globally even at column densites of 10^{12} cm^{-2}. We find, consistent with
observations, that the CII mass fraction drops to low redshift while CIV rises
owing the combined effects of a growing UVB and continued addition of carbon in
low-density regions. This is mimicked in absorption statistics, which broadly
agree with observations at z=6-3 while predicting that the absorber column
density distributions rise steeply to the lowest observable columns. Our model
reproduces the large observed scatter in the number of low-ionization absorbers
per sightline, implying that the scatter does not indicate a partially-neutral
Universe at z=6.Comment: 16 pages, 14 figures, accepted to MNRA
The Snapshot Hubble U-Band Cluster Survey (SHUCS) II. Star Cluster Population of NGC 2997
We study the star cluster population of NGC 2997, a giant spiral galaxy
located at 9.5 Mpc and targeted by the Snapshot Hubble U-band Cluster Survey
(SHUCS). Combining our U-band imaging from SHUCS with archival BVI imaging from
HST, we select a high confidence sample of clusters in the circumnuclear ring
and disk through a combination of automatic detection procedures and visual
inspection. The cluster luminosity functions in all four filters can be
approximated by power-laws with indices of to . Some deviations
from pure power-law shape are observed, hinting at the presence of a high-mass
truncation in the cluster mass function. However, upon inspection of the
cluster mass function, we find it is consistent with a pure power-law of index
despite a slight bend at M. No
statistically significant truncation is observed. From the cluster age
distributions, we find a low rate of disruption () in both the
disk and circumnuclear ring. Finally, we estimate the cluster formation
efficiency () over the last 100 Myr in each region, finding %
for the disk, % for the circumnuclear ring, and % for the
entire UBVI footprint. This study highlights the need for wide-field UBVI
coverage of galaxies to study cluster populations in detail, though a small
sample of clusters can provide significant insight into the characteristics of
the population.Comment: 31 pages, 9 figures, accepted to the A
On the Detection of Supermassive Primordial Stars. II. Blue Supergiants
Supermassive primordial stars in hot, atomically-cooling haloes at
15 - 20 may have given birth to the first quasars in the universe. Most
simulations of these rapidly accreting stars suggest that they are red, cool
hypergiants, but more recent models indicate that some may have been bluer and
hotter, with surface temperatures of 20,000 - 40,000 K. These stars have
spectral features that are quite distinct from those of cooler stars and may
have different detection limits in the near infrared (NIR) today. Here, we
present spectra and AB magnitudes for hot, blue supermassive primordial stars
calculated with the TLUSTY and CLOUDY codes. We find that photometric
detections of these stars by the James Webb Space Telescope (JWST) will be
limited to 10 - 12, lower redshifts than those at which red stars
can be found, because of quenching by their accretion envelopes. With moderate
gravitational lensing, Euclid and the Wide-Field Infrared Space Telescope
(WFIRST) could detect blue supermassive stars out to similar redshifts in
wide-field surveys.Comment: 9 pages, 5 figures, accepted by MNRA
Unlocking the secrets of stellar haloes using combined star counts and surface photometry
The stellar haloes of galaxies can currently be studied either through
observations of resolved halo stars or through surface photometry. Curiously,
the two methods appear to give conflicting results, as a number of surface
photometry measurements have revealed integrated colours that are too red to be
reconciled with the halo properties inferred from the study of resolved stars.
Several explanations for this anomaly have been proposed - including dust
photoluminescence, extinction of extragalactic background light and a
bottom-heavy stellar initial mass function. A decisive test is, however, still
lacking. Here, we explain how observations of the halo of a nearby galaxy,
involving a combination of both surface photometry and bright star counts, can
be used to distinguish between the proposed explanations. We derive the
observational requirements for this endeavour and find that star counts in
filters VI and surface photometry in filters VIJ appears to be the optimal
strategy. Since the required halo star counts are already available for many
nearby galaxies, the most challenging part of this test is likely to be the
optical surface photometry, which requires several nights of exposure time on a
4-8 m telescope, and the near-IR surface photometry, which is most readily
carried out using the upcoming James Webb Space Telescope.Comment: 14 pages, 4 figures; v.2 matches published version (minor changes
only
Revealing a Ring-like Cluster Complex in a Tidal Tail of the Starburst Galaxy NGC 2146
We report the discovery of a ring-like cluster complex in the starburst
galaxy NGC 2146. The Ruby Ring, so named due to its appearance, shows a clear
ring-like distribution of star clusters around a central object. It is located
in one of the tidal streams which surround the galaxy. NGC 2146 is part of the
Snapshot Hubble U-band Cluster Survey (SHUCS). The WFC3/F336W data has added
critical information to the available archival Hubble Space Telescope imaging
set of NGC 2146, allowing us to determine ages, masses, and extinctions of the
clusters in the Ruby Ring. These properties have then been used to investigate
the formation of this extraordinary system. We find evidence of a spatial and
temporal correlation between the central cluster and the clusters in the ring.
The latter are about 4 Myr younger than the central cluster, which has an age
of 7 Myr. This result is supported by the H alpha emission which is strongly
coincident with the ring, and weaker at the position of the central cluster.
From the derived total H alpha luminosity of the system we constrain the star
formation rate density to be quite high, e.g. ~ 0.47 Msun/yr/kpc^2. The Ruby
Ring is the product of an intense and localised burst of star formation,
similar to the extended cluster complexes observed in M51 and the Antennae, but
more impressive because is quite isolated. The central cluster contains only 5
% of the total stellar mass in the clusters that are determined within the
complex. The ring-like morphology, the age spread, and the mass ratio support a
triggering formation scenario for this complex. We discuss the formation of the
Ruby Ring in a "collect & collapse" framework. The predictions made by this
model agree quite well with the estimated bubble radius and expansion velocity
produced by the feedback from the central cluster, making the Ruby Ring an
interesting case of triggered star formation.Comment: 11 pages, 7 figures, 1 table; Accepted for publication in MNRA
A Comprehensive Comparative Test of Seven Widely-Used Spectral Synthesis Models Against Multi-Band Photometry of Young Massive Star Clusters
We test the predictions of spectral synthesis models based on seven different
massive-star prescriptions against Legacy ExtraGalactic UV Survey (LEGUS)
observations of eight young massive clusters in two local galaxies, NGC 1566
and NGC 5253, chosen because predictions of all seven models are available at
the published galactic metallicities. The high angular resolution, extensive
cluster inventory and full near-ultraviolet to near-infrared photometric
coverage make the LEGUS dataset excellent for this study. We account for both
stellar and nebular emission in the models and try two different prescriptions
for attenuation by dust. From Bayesian fits of model libraries to the
observations, we find remarkably low dispersion in the median E(B-V) (~0.03
mag), stellar masses (~10^4 M_\odot) and ages (~1 Myr) derived for individual
clusters using different models, although maximum discrepancies in these
quantities can reach 0.09 mag and factors of 2.8 and 2.5, respectively. This is
for ranges in median properties of 0.05-0.54 mag, 1.8-10x10^4 M_\odot and
1.6-40 Myr spanned by the clusters in our sample. In terms of best fit, the
observations are slightly better reproduced by models with interacting binaries
and least well reproduced by models with single rotating stars. Our study
provides a first quantitative estimate of the accuracies and uncertainties of
the most recent spectral synthesis models of young stellar populations,
demonstrates the good progress of models in fitting high-quality observations,
and highlights the needs for a larger cluster sample and more extensive tests
of the model parameter space.Comment: Accepted for publication in MNRAS (14 Jan. 2016). 30 pages, 16
figures, 9 table
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
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