52 research outputs found
Fokker-Planck Models for M15 without a Central Black Hole: The Role of the Mass Function
We have developed a set of dynamically evolving Fokker-Planck models for the
collapsed-core globular star cluster M15, which directly address the issue of
whether a central black hole is required to fit Hubble Space Telescope (HST)
observations of the stellar spatial distribution and kinematics. As in our
previous work reported by Dull et al., we find that a central black hole is not
needed. Using local mass-function data from HST studies, we have also inferred
the global initial stellar mass function. As a consequence of extreme mass
segregation, the local mass functions differs from the global mass function at
every location. In addition to reproducing the observed mass functions, the
models also provide good fits to the star-count and velocity-dispersion
profiles, and to the millisecond pulsar accelerations. We address concerns
about the large neutron star populations adopted in our previous Fokker-Planck
models for M15. We find that good model fits can be obtained with as few as
1600 neutron stars; this corresponds to a retention fraction of 5% of the
initial population for our best fit initial mass function. The models contain a
substantial population of massive white dwarfs, that range in mass up to 1.2
solar masses. The combined contribution by the massive white dwarfs and neutron
stars provides the gravitational potential needed to reproduce HST measurements
of the central velocity dispersion profile.Comment: 10 pages, 7 figure
SHIELD: Neutral Gas Kinematics and Dynamics
We present kinematic analyses of the 12 galaxies in the "Survey of HI in Extremely Low-mass Dwarfs" (SHIELD). We use multi-configuration interferometric observations of the HI 21cm emission line from the Karl G. Jansky Very Large Array (VLA) to produce image cubes at a variety of spatial and spectral resolutions. Both two- and three-dimensional fitting techniques are employed in an attempt to derive inclination-corrected rotation curves for each galaxy. In most cases, the comparable magnitudes of velocity dispersion and projected rotation result in degeneracies that prohibit unambiguous circular velocity solutions. We thus make spatially resolved position-velocity cuts, corrected for inclination using the stellar components, to estimate the circular rotation velocities. We find circular velocities <30 km/s for the entire survey population. Baryonic masses are calculated using single-dish HI fluxes from Arecibo and stellar masses derived from HST and Spitzer imaging. Comparison is made with total dynamical masses estimated from the position-velocity analysis. The SHIELD galaxies are then placed on the baryonic Tully-Fisher relation. There exists an empirical threshold rotational velocity <15 km/s, below which current observations cannot differentiate coherent rotation from pressure support. The SHIELD galaxies are representative of an important population of galaxies whose properties cannot be described by current models of rotationally-dominated galaxy dynamics
SHIELD: Comparing Gas and Star Formation in Low Mass Galaxies
We analyze the relationships between atomic, neutral hydrogen (HI) and star formation (SF) in the 12 low-mass SHIELD galaxies. We compare high spectral (~0.82 km/s/channel) and spatial resolution (physical resolutions of 170 pc - 700 pc) HI imaging from the VLA with H\alpha and far-ultraviolet imaging. We quantify the degree of co-spatiality between star forming regions and regions of high HI column densities. We calculate the global star formation efficiencies (SFE, / ), and examine the relationships among the SFE and HI mass, HI column density, and star formation rate (SFR). The systems are consuming their cold neutral gas on timescales of order a few Gyr. While we derive an index for the Kennicutt-Schmidt relation of N ~ 0.68 0.04 for the SHIELD sample as a whole, the values of N vary considerably from system to system. By supplementing SHIELD results with those from other surveys, we find that HI mass and UV-based SFR are strongly correlated over five orders of magnitude. Identification of patterns within the SHIELD sample allows us to bin the galaxies into three general categories: 1) mainly co-spatial HI and SF regions, found in systems with highest peak HI column densities and highest total HI masses, 2) moderately correlated HI and SF regions, found in systems with moderate HI column densities, and 3) obvious offsets between HI and SF peaks, found in systems with the lowest total HI masses. SF in these galaxies is dominated by stochasticity and random fluctuations in their ISM
F Turnoff Distribution in the Galactic Halo Using Globular Clusters as Proxies
F turnoff stars are important tools for studying Galactic halo substructure
because they are plentiful, luminous, and can be easily selected by their
photometric colors from large surveys such as the Sloan Digital Sky Survey
(SDSS). We describe the absolute magnitude distribution of color-selected F
turnoff stars, as measured from SDSS data, for eleven globular clusters in the
Milky Way halo. We find that the M_g distribution of turnoff stars is
intrinsically the same for all clusters studied, and is well fit by two half
Gaussian functions, centered at mu=4.18, with a bright-side sigma=0.36, and
with a faint-side sigma=0.76. However, the color errors and detection
efficiencies cause the observed sigma of the faint-side Gaussian to change with
magnitude due to contamination from redder main sequence stars (40% at 21st
magnitude). We present a function that will correct for this
magnitude-dependent change in selected stellar populations, when calculating
stellar density from color-selected turnoff stars. We also present a consistent
set of distances, ages and metallicities for eleven clusters in the SDSS Data
Release 7. We calculate a linear correction function to Padova isochrones so
that they are consistent with SDSS globular cluster data from previous papers.
We show that our cluster population falls along the Milky Way Age-Metallicity
Relationship (AMR), and further find that isochrones for stellar populations on
the AMR have very similar turnoffs; increasing metallicity and decreasing age
conspire to produce similar turnoff magnitudes and colors for all old clusters
that lie on the AMR.Comment: Accepted for publication in ApJ, 32 pages, 18 figure
Gas Accretion and Star Formation Rates
Cosmological numerical simulations of galaxy evolution show that accretion of
metal-poor gas from the cosmic web drives the star formation in galaxy disks.
Unfortunately, the observational support for this theoretical prediction is
still indirect, and modeling and analysis are required to identify hints as
actual signs of star-formation feeding from metal-poor gas accretion. Thus, a
meticulous interpretation of the observations is crucial, and this
observational review begins with a simple theoretical description of the
physical process and the key ingredients it involves, including the properties
of the accreted gas and of the star-formation that it induces. A number of
observations pointing out the connection between metal-poor gas accretion and
star-formation are analyzed, specifically, the short gas consumption time-scale
compared to the age of the stellar populations, the fundamental metallicity
relationship, the relationship between disk morphology and gas metallicity, the
existence of metallicity drops in starbursts of star-forming galaxies, the
so-called G dwarf problem, the existence of a minimum metallicity for the
star-forming gas in the local universe, the origin of the alpha-enhanced gas
forming stars in the local universe, the metallicity of the quiescent BCDs, and
the direct measurements of gas accretion onto galaxies. A final section
discusses intrinsic difficulties to obtain direct observational evidence, and
points out alternative observational pathways to further consolidate the
current ideas.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by
Springe
Ultraviolet-bright stellar populations and their evolutionary implications in the collapsed-core cluster M15
We performed deep photometry of the central region of Galactic globular
cluster M15 from archival Hubble Space Telescope data taken on the High
Resolution Channel and Solar Blind Channel of the Advanced Camera for Surveys.
Our data set consists of images in far-UV (FUV; F140LP), near-UV
(NUV; F220W), and blue (B; F435W) filters. The addition of an
optical filter complements previous UV work on M15 by providing an additional
constraint on the UV-bright stellar populations. Using color-magnitude diagrams
(CMDs) we identified several populations that arise from non-canonical
evolution including candidate blue stragglers, extreme horizontal branch stars,
blue hook stars (BHks), cataclysmic variables (CVs), and helium-core white
dwarfs (He WDs). Due to preliminary identification of several He WD and BHk
candidates, we add M15 as a cluster containing a He WD sequence and suggest it
be included among clusters with a BHk population.
We also investigated a subset of CV candidates that appear in the gap between
the main sequence (MS) and WDs in FUVNUV but lie securely on
the MS in NUVB. These stars may represent a magnetic CV or
detached WD-MS binary population. Additionally, we analyze our candidate He WDs
using model cooling sequences to estimate their masses and ages and investigate
the plausibility of thin vs. thick hydrogen envelopes. Finally, we identify a
class of UV-bright stars that lie between the horizontal branch and WD cooling
sequences, a location not usually populated on cluster CMDs. We conclude these
stars may be young, low-mass He WDs.Comment: 26 pages, 11 figure
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