37 research outputs found
Radial distributions of sub-populations in the globular cluster M15: a more centrally concentrated primordial population
We examine the radial distributions of stellar populations in the globular
cluster (GC) M15, using HST/WFC3 photometry of red giants in the
nitrogen-sensitive F343N-F555W color. Surprisingly, we find that giants with
"primordial" composition (i.e., N abundances similar to those in field stars)
are the most centrally concentrated within the WFC3 field. We then combine our
WFC3 data with SDSS u, g photometry and find that the trend reverses for radii
>1' (3 pc) where the ratio of primordial to N-enhanced giants increases
outwards, as already found by Lardo et al. The ratio of primordial to enriched
stars thus has a U-shaped dependency on radius with a minimum near the
half-light radius. N-body simulations show that mass segregation might produce
a trend resembling the observed one, but only if the N-enhanced giants are
~0.25 Mo less massive than the primordial giants, which requires extreme He
enhancement (Y~0.40). However, such a large difference in Y is incompatible
with the negligible optical color differences between primordial and enriched
giants which suggest Delta Y < 0.03 and thus a difference in turn-off mass of
Delta M < 0.04 Mo between the different populations. The radial trends in M15
are thus unlikely to be of dynamical origin and presumably reflect initial
conditions, a result that challenges all current GC formation scenarios. We
note that population gradients in the central regions of GCs remain poorly
investigated and may show a more diverse behavior than hitherto thought.Comment: 16 pages, accepted for Ap
Ogden Valley Development Analysis and Plan
A team of four Huntsman Scholars undertook an intensive, semester-long project, analyzing how the Ogden Valley Planning Commission could best ensure sustainable, wise development in its pristine Valley, preventing unwise overdevelopment down the road. The final presentation, with analysis and recommendations, was made on May 6, 2014, in Ogden before the Ogden Valley Planning Commission and members of the public. The results were praised highly by Planning Commissioner Pen Hollist
Sizes and Shapes of Young Star Cluster Light Profiles in M83
We measure the radii and two-dimensional light profiles of a large sample of
young, massive star clusters in M83 using archival HST/WFC3 imaging of seven
adjacent fields. We use GALFIT to fit the two-dimensional light profiles of the
clusters, from which we find effective (half-light) radii, core radii, and
slopes of the power-law (EFF) profile (). We find lognormal distributions
of effective radius and core radius, with medians of 2.5 pc and
1.3 pc, respectively. Our results provide strong evidence for a
characteristic size of young, massive clusters. The average effective radius
and core radius increase somewhat with cluster age. Little to no change in
effective radius is observed with increasing galactocentric distance, except
perhaps for clusters younger than 100 Myr. We find a shallow correlation
between effective radius and mass for the full cluster sample, but a stronger
correlation is present for clusters 200-300 Myr in age. Finally, the majority
of the clusters are best fit by an EFF model with index . There is
no strong evidence for change in with cluster age, mass, or
galactocentric distance. Our results suggest that clusters emerge from early
evolution with similar radii and are not strongly affected by the tidal field
of M83. Mass loss due to stellar evolution and/or GMC interactions appear to
dominate cluster expansion in the age range we study.Comment: 34 pages, 11 figures, 3 tables, accepted by MNRAS. Machine-readable
table attached (full version of Table 3). To obtain, download the source file
from the "Other formats" link abov
Transmission and Transport of Energy in the Western U.S. and Canada: A Law and Policy Road Map
This collection of short essays arose from the inaugural meeting of the Idaho Symposium on Energy in the West, which was held in November, 2014. The topic for this first Symposium was Transmission and Transport of Energy in the Western U.S. and Canada: A Law and Policy Road Map. The essays in this collection provide a notable introduction to the major energy issues facing the West today. Topics include: building a resilient legal architecture for western energy production; natural gas flaring; transmission planning for wind energy; utilities and rooftop solar; special considerations for western states and the Clean Power Plan; the Clean Power Plan\u27s implications for the western grid; siting renewable energy on public lands; and implications of utility reform in New York and Hawaii for the Northwest
Searching for Multiple Populations in the Integrated Light of the Young and Extremely Massive Clusters in the Merger Remnant NGC~7252
Recent work has shown that the properties of multiple populations within
massive stellar clusters (i.e., in the extent of their abundance variations as
well as the fraction of stars that show the anomalous chemistry) depend on the
mass as well as the age of the host cluster. Such correlations are largely
unexpected in current models for the formation of multiple populations and
hence provide essential insight into their origin. Here we extend or previous
study into the presence or absence of multiple populations using integrated
light spectroscopy of the ~Myr, massive (~\msun)
clusters, W3 and W30, in the galactic merger remnant, NGC 7252. Due to the
extreme mass of both clusters, the expectation is that they should host rather
extreme abundance spreads, manifested through, e.g., high mean [Na/Fe]
abundances. However, we do not find evidence for a strong [Na/Fe] enhancement,
with the observations being consistent with the solar value. This suggests that
age is playing a key role, or alternatively that multiple populations only
manifest below a certain stellar mass, as the integrated light at all ages
above ~Myr is dominated by stars near or above the main sequence
turn-off.Comment: 7 pages, 7 figures, accepted for publication in MNRA
Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies. IV. Stephan's Quintet
We present a spectroscopic survey of 21 young massive clusters and complexes
and one tidal dwarf galaxy candidate (TDG) in Stephan's Quintet, an interacting
compact group of galaxies. All of the selected targets lie outside the main
galaxies of the system and are associated with tidal debris. We find clusters
with ages between a few and 125 Myr and confirm the ages estimated through HST
photometry by Fedotov et al. (2011), as well as their modelled interaction
history of the Quintet. Many of the clusters are found to be relatively
long-lived, given their spectrosopically derived ages, while their high masses
suggest that they will likely evolve to eventually become intergalactic
clusters. One cluster, T118, is particularly interesting, given its age (\sim
125 Myr), high mass (\sim 2\times10^6 M\odot) and position in the extreme outer
end of the young tidal tail. This cluster appears to be quite extended (Reff
\sim 12 - 15 pc) compared to clusters observed in galaxy disks (Reff \sim 3 - 4
pc), which confirms an effect we previously found in the tidal tails of NGC
3256, where clusters are similarly extended. We find that star and cluster
formation can proceed at a continuous pace for at least \sim 150 Myr within the
tidal debris of interacting galaxies. The spectrum of the TDG candidate is
dominated by a young population (\sim 7 Myr), and assuming a single age for the
entire region, has a mass of at least 10^6 M\odot.Comment: 37 pages, 10 Figures, 7 Tabl
GLIMPSE-CO1: the most massive intermediate-age stellar cluster in the Galaxy
The stellar cluster GLIMPSE-C01 is a dense stellar system located in the
Galactic Plane. Though often referred to in the literature as an old globular
cluster traversing the Galactic disk, previous observations do not rule out
that it is an intermediate age (less than a few Gyr) disk-borne cluster. Here,
we present high-resolution near-infrared spectroscopy of over 50 stars in the
cluster. We find an average radial velocity is consistent with being part of
the disk, and determine the cluster's dynamical mass to be (8 \pm 3)x10^4 Msun.
Analysis of the cluster's M/L ratio, the location of the Red Clump, and an
extremely high stellar density, all suggest an age of 400-800Myr for
GLIMPSE-C01, much lower than for a typical globular cluster. This evidence
therefore leads us to conclude that GLIMPSE-C01 is part of the disk population,
and is the most massive Galactic intermediate-age cluster discovered to date.Comment: 10 pages, 9 figures, accepted for publication in MNRA
A Young Super Star Cluster in the Nuclear Region of NGC 253
We present observations of a massive star cluster near the nuclear region of
the nearby starburst galaxy NGC 253. The peak of near-infrared emission, which
is spatially separated by 4" from the kinematic center of the galaxy, is
coincident with a super star cluster whose properties we examine with
low-resolution (R ~ 1,200) infrared CTIO spectroscopy and optical/near-infrared
HST imaging. Extinction, measured from [FeII] lines, is estimated at Av = 17.7
+/- 2.6. The age of the cluster is estimated at 5.7 Myr, based on Bry
equivalent width for an instantaneous burst using Starburst99 modeling.
However, a complex star formation history is inferred from the presence of both
recombination emission and photospheric CO absorption. The ionizing photon flux
has a lower limit of 7.3 +/- 2.5 x 10^53 inverse seconds, corrected for
extinction. Assuming a Kroupa IMF, we estimate a cluster mass of 1.4 +/- 0.4 x
10^7 solar masses. We observe a strong Wolf-Rayet signature at 2.06 microns and
report a weak feature at 2.19 microns which may be due to a massive stellar
population, consistent with the derived mass and age of this cluster.Comment: 8 pages, 5 figures, accepted for publication in Ap
Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies. III. The Antennae
We present optical spectroscopy of 16 star clusters in the merging galaxies
NGC 4038/39 ("The Antennae") and supplement this dataset with HST imaging. The
age and metallicity of each cluster is derived through a comparison between the
observed Balmer and metal line strengths with simple stellar population models.
We then estimate extinctions and masses using the photometry. We find that all
but three clusters have ages between ~3-200 Myr, consistent with the expected
increase in the star-formation rate due to the merger. Most of the clusters
have velocities in agreement with nearby molecular and HI gas that has been
previously shown to be rotating within the progenitor galaxies, hence
star/cluster formation is still taking place within the galactic disks.
However, three clusters have radial velocities that are inconsistent with being
part of the rotating gas disks, which is surprising given their young
(200-500Myr) ages. Interestingly, we find a stellar association with the same
colors (V-I) near one of these three clusters, suggesting that the cluster and
association were formed concurrently and have remained spatially correlated. We
find evidence for spatially distributed cluster formation throughout the
duration of the merger. The impact of various assumptions about the
star/cluster formation rate on the interpretation of the cluster age
distribution are explored, and we do not find evidence for long term "infant
mortality" as has been previously suggested. Models of galaxy mergers that
include a prescription for star formation can provide an overall good fit to
the observed cluster age distribution.Comment: 15 pages, 11 figures, ApJ in pres