1,115 research outputs found
The Rich Globular Cluster System of Abell 1689 and the Radial Dependence of the Globular Cluster Formation Efficiency
We study the rich globular cluster (GC) system in the center of the massive
cluster of galaxies Abell 1689 (z=0.18), one of the most powerful gravitational
lenses known. With 28 HST/ACS orbits in the F814W bandpass, we reach magnitude
I_814=29 with >90% completeness and sample the brightest ~5% of the GC system.
Assuming the well-known Gaussian form of the GC luminosity function (GCLF), we
estimate a total population of N(GC_total) = 162,850 GCs within a projected
radius of 400kpc. As many as half may comprise an intracluster component. Even
with the sizable uncertainties, which mainly result from the uncertain GCLF
parameters, this is by far the largest GC system studied to date. The specific
frequency S_N is high, but not uncommon for central galaxies in massive
clusters, rising from S_N~5 near the center to ~12 at large radii. Passive
galaxy fading would increase S_N by ~20% at z=0. We construct the radial mass
profiles of the GCs, stars, intracluster gas, and lensing-derived total mass,
and we compare the mass fractions as a function of radius. The estimated mass
in GCs, M(GC_total)=3.9x10^10 Msun, is comparable to ~80% of the total stellar
mass of the Milky Way. The shape of the GC mass profile appears intermediate
between those of the stellar light and total cluster mass. Despite the extreme
nature of this system, the ratios of the GC mass to the baryonic and total
masses, and thus the GC formation efficiency, are typical of those in other
rich clusters when comparing at the same physical radii. The GC formation
efficiency is not constant, but varies with radius, in a manner that appears
similar for different clusters; we speculate on the reasons for this similarity
in profile.Comment: 13 pages, 11 figures; accepted for publication in Ap
The Metal-Enriched Outer Disk of NGC 2915
We present optical emission-line spectra for outlying HII regions in the
extended neutral gas disk surrounding the blue compact dwarf galaxy NGC 2915.
Using a combination of strong-line R23 and direct oxygen abundance
measurements, we report a flat, possibly increasing, metallicity gradient out
to 1.2 times the Holmberg radius. We find the outer-disk of NGC 2915 to be
enriched to a metallicity of 0.4 Z_solar. An analysis of the metal yields shows
that the outer disk of NGC 2915 is overabundant for its gas fraction, while the
central star-foming core is similarly under-abundant for its gas fraction. Star
formation rates derived from very deep ~14 ks GALEX FUV exposures indicate that
the low-level of star formation observed at large radii is not sufficient to
have produced the measured oxygen abundances at these galactocentric distances.
We consider 3 plausible mechanisms that may explain the metal-enriched outer
gaseous disk of NGC 2915: radial redistribution of centrally generated metals,
strong galactic winds with subsequent fallback, and galaxy accretion. Our
results have implications for the physical origin of the mass-metallicity
relation for gas-rich dwarf galaxies.Comment: 11 pages, 4 figures, accepted to ApJ April 8th, 201
Imaging of Ultraluminous Infrared Galaxies in the Near-UV
We present the first ground-based U' (3410 angstroms) images of Ultraluminous
Infrared Galaxies (ULIGs). Strong U' emission (median total M_U' = -20.8) is
seen in all systems and in some cases the extended tidal features (both the
smooth stellar distribution and compact star-forming features) contribute up to
60-80% of the total flux. The star-forming regions in both samples are found to
have ages based on spectral synthesis models in the range 10-100 Myrs, and most
differences in color between them can be attributed to the effects of dust
reddening. Additionally, it is found that star-formation in compact knots in
the tidal tails is most prominent in those ULIGs which have double nuclei,
suggesting that the star-formation rate in the tails peaks prior to the actual
coalescence of the galaxy nuclei and diminishes quickly thereafter. Similar to
results at other wavelengths, the observed star formation at U' can only
account for a small fraction of the known bolometric luminosity of the ULIGs.
Azimuthally averaged radial light profiles at U' are characterized by a sersic
law with index n=2, which is intermediate between an exponential disk and an
r^(-1/4) law and closely resembles the latter at large radii. The implications
of this near-ultraviolet imaging for optical/near-infrared observations of high
redshift counterparts of ULIGs are discussed.Comment: 30 pages, 4 tables, and 9 figures, 2 of which are JPEGs. To appear in
the August, 2000 edition of the Astronomical Journa
Isolated OB Associations in Stripped HI Gas Clouds
HST ACS/HRC images in UV (F250W), V (F555W), and I (F814W) resolve three
isolated OB associations that lie up to 30 kpc from the stellar disk of the S0
galaxy NGC 1533. Previous narrow-band Halpha imaging and optical spectroscopy
showed these objects as unresolved intergalactic HII regions having Halpha
luminosities consistent with single early-type O stars. These young stars lie
in stripped HI gas with column densities ranging from 1.5 - 2.5 * 10^20 cm^-2
and velocity dispersions near 30 km s^-1. Using the HST broadband colors and
magnitudes along with previously-determined Halpha luminosities, we place
limits on the masses and ages of each association, considering the importance
of stochastic effects for faint (M_V >-8) stellar populations. The upper limits
to their stellar masses range from 600 M_sun to 7000 M_sun, and ages range from
2 - 6 Myrs. This analysis includes an updated calculation of the conversion
factor between the ionizing luminosity and the total number of main sequence O
stars contained within an HII region. The photometric properties and sizes of
the isolated associations and other objects in the HRC fields are consistent
with those of Galactic stellar associations, open clusters and/or single O and
B stars. We interpret the age-size sequence of associations and clustered field
objects as an indication that these isolated associations are most likely
rapidly dispersing. Furthermore, we consider the possibility that these
isolated associations represent the first generation of stars in the HI ring
surrounding NGC 1533. This work suggests star formation in the unique
environment of a galaxy's outermost gaseous regions proceeds similarly to that
within the Galactic disk and that star formation in tidal debris may be
responsible for building up a younger halo component.Comment: 21 pages, 9 figures, 6 tables; accepted for publication in Ap
Outlying HII Regions in HI-Selected Galaxies
We present results from the first systematic search for outlying HII regions,
as part of a sample of 96 emission-line point sources (referred to as ELdots -
emission-line dots) derived from the NOAO Survey for Ionization in Neutral Gas
Galaxies (SINGG). Our automated ELdot-finder searches SINGG narrow-band and
continuum images for high equivalent width point sources outside the optical
radius of the target galaxy (> 2 X r25 in the R-band). Follow-up longslit
spectroscopy and deep GALEX images (exposure time > 1000 s) distinguish
outlying HII regions from background galaxies whose strong emission lines
([OIII], Hbeta or [OII]) have been redshifted into the SINGG bandpass. We find
that these deep GALEX images can serve as a substitute for spectroscopic
follow-up because outlying HII regions separate cleanly from background
galaxies in color-color space. We identify seven SINGG systems with outlying
massive star formation that span a large range in Halpha luminosities
corresponding to a few O stars in the most nearby cases, and unresolved dwarf
satellite companion galaxies in the most distant cases. Six of these seven
systems feature galaxies with nearby companions or interacting galaxies.
Furthermore, our results indicate that some outlying HII regions are linked to
the extended-UV disks discovered by GALEX, representing emission from the most
massive O stars among a more abundant population of lower mass (or older) star
clusters. The overall frequency of outlying HII regions in this sample of
gas-rich galaxies is 8 - 11% when we correct for background emission-line
galaxy contamination (~75% of ELdots).Comment: 20 pages, 14 Figures, Accepted by A
The Shape and Figure Rotation of NGC 2915's Dark Halo
NGC 2915 is a blue compact dwarf galaxy with a very extended HI disk showing
a short central bar and extended spiral arms, both reaching far beyond the
optical component. We use Tremaine & Weinberg (1984) method to measure the
pattern speed of the bar from HI radio synthesis data. Our measurements yield a
pattern speed of 0.21+/-0.06 km/s/arcsec (8.0+/-2.4 km/s/kpc for D=5.3 Mpc), in
disagreement with the general view that corotation in barred disks lies just
outside the end of the bar, but consistent with recent models of barred
galaxies with dense dark matter halos. Our adopted bar semi-length puts
corotation at more than 1.7 bar radii. The existence of the pattern is also
problematic. Because NGC 2915 is isolated, interactions cannot account for the
structure observed in the HI disk. We also demonstrate that the low observed
disk surface density and the location of the pseudo-rings make it unlikely that
swing amplification or bar-driven spiral arms could explain the bar and spiral
pattern.
Based on the similarity of the dark matter and HI surface density profiles,
we discuss the possibility of dark matter distributed in a disk and following
closely the HI distribution. The disk then becomes unstable and can naturally
form a bar and spiral pattern. However, this explanation is hard to reconcile
with some properties of NGC 2915. We also consider the effect of a massive and
extended triaxial dark matter halo with a rotating figure. The existence of
such halos is supported by CDM simulations showing strongly triaxial dark halos
with slow figure rotation. The observed structure of the HI disk can then arise
through forcing by the rotating triaxial figure. We associate the measured
pattern speed in NGC 2915 with the figure rotation of its dark halo.Comment: 37 pages, including 8 figures and 2 tables (AASTeX, aaspp4.sty).
Fig.1 and 2 available as jpg. Accepted for publication in The Astronomical
Journal. Online manuscript with PostScript figures available at:
http://www.strw.leidenuniv.nl/~bureau/pub_list.htm
Few-anyon systems in a parabolic dot
The energy levels of two and three anyons in a two-dimensional parabolic
quantum dot and a perpendicular magnetic field are computed as power series in
1/|J|, where J is the angular momentum. The particles interact repulsively
through a coulombic (1/r) potential. In the two-anyon problem, the reached
accuracy is better than one part in 10^5. For three anyons, we study the
combined effects of anyon statistics and coulomb repulsion in the ``linear''
anyonic states.Comment: LaTeX, 6 pages, 4 postscript figure
Local Starbursts in a Cosmological Context
In this contribution I introduce some of the major issues that motivate the
conference, with an emphasis on how starbursts fit into the ``big picture''. I
begin by defining starbursts in several different ways, and discuss the merits
and limitations of these definitions. I will argue that the most physically
useful definition of a starburst is its ``intensity'' (star formation rate per
unit area). This is the most natural parameter to compare local starbursts with
physically similar galaxies at high redshift, and indeed I will argue that
local starbursts are unique laboratories to study the processes at work in the
early universe. I will describe how NASA's GALEX mission has uncovered a rare
population of close analogs to Lyman Break Galaxies in the local universe. I
will then compare local starbursts to the Lyman-Break and sub-mm galaxies high
redshift populations, and speculate that the multidimensional ``manifold'' of
starbursts near and far can be understood largely in terms of the
Schmidt/Kennicutt law and galaxy mass-metallicity relation. I will briefly
summarize he properties of starburst-driven galactic superwinds and their
possible implications for the evolution of galaxies and the IGM. These complex
multiphase flows are best studied in nearby starbursts, where we can study the
the hot X-ray gas that contains the bulk of the energy and newly produced
metals.Comment: Proceedings of the Conference "Starbursts: Fropm 30 Doradus to Lyman
Break Galaxies
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