253 research outputs found
A faint red stellar halo around an edge-on disc galaxy in the Hubble Ultra Deep Field
We analyse the detailed structure of a highly-inclined (i>~80 degrees) disc
galaxy which lies within the Hubble Ultra Deep Field (UDF). The unprecedented
depth of the UDF data allow disc and extraplanar emission to be reliably traced
to surface brightness levels of mu_{V,i,z}~29-30 mag/arcsec^2 (corresponding to
rest-frame equivalents of mu_{g,r,i}~28-29 mag/arcsec^2) in this redshift
z=0.32 system. We detect excess emission above the disc which is characterised
by a moderately-flattened (b/a~0.6) power-law (I proportional to R^(-2.6)). The
structure and colour of this component are very similar to the stellar halo
detected in an SDSS stacking analysis of local disc galaxies (Zibetti, White
and Brinkmann 2004) and lend support to the idea that we have detected a
stellar halo in this distant system. Although the peculiar colours of the halo
are difficult to understand in terms of normal stellar populations, the
consistency found between the UDF and SDSS analyses suggests that they cannot
be easily discounted.Comment: 5 pages, 5 figures, accepted for publication in MNRAS Letters. Figure
1 substantially degraded, full resolution version available at
http://www.mpa-garching.mpg.de/~zibetti/UDFhalo.pd
An Old Cluster in NGC 6822
We present spectroscopy of two clusters in the dwarf irregular galaxy NGC
6822. From these we deduce an age for Cluster VII of 11 Gyr and [Fe/H] = -1.95
+/- 0.15 dex. Cluster VII appears to be an analog of the metal-poor galactic
globular clusters. Cluster VI is found to be much younger and more metal rich,
with an age of approximately 2 Gyr. Its derived metallicity, [Fe/H], of
approximately -1.0 dex is comparable to that of the gas seen today in NGC 6822.
The existence of a metal-poor old cluster in NGC 6822 rules out models for the
chemical evolution of this galaxy with significant prompt initial enhancement.
We find that a star formation rate which is constant with time and is within a
factor of two of the present star formation rate can reproduce the two points
on the age-metallicity relationship for NGC 6822 over the past 10 Gyr defined
by these two clusters.Comment: 8 pages; accepted for publication in A
A Quantitative Comparison of SMC, LMC, and Milky Way UV to NIR Extinction Curves
We present an exhaustive, quantitative comparison of all of the known
extinction curves in the Small and Large Magellanic Clouds (SMC and LMC) with
our understanding of the general behavior of Milky Way extinction curves. The
R_V dependent CCM relationship and the sample of extinction curves used to
derive this relationship is used to describe the general behavior of Milky Way
extinction curves. The ultraviolet portion of the SMC and LMC extinction curves
are derived from archival IUE data, except for one new SMC extinction curve
which was measured using HST/STIS observations. The optical extinction curves
are derived from new (for the SMC) and literature UBVRI photometry (for the
LMC). The near-infrared extinction curves are calculated mainly from 2MASS
photometry supplemented with DENIS and new JHK photometry. For each extinction
curve, we give R_V = A(V)/E(B-V) and N(HI) values which probe the same dust
column as the extinction curve. We compare the properties of the SMC and LMC
extinction curves with the CCM relationship three different ways: each curve by
itself, the behavior of extinction at different wavelengths with R_V, and
behavior of the extinction curve FM fit parameters with R_V. As has been found
previously, we find that a small number of LMC extinction curves are consistent
with the CCM relationship, but majority of the LMC and all of the SMC curves do
not follow the CCM relationship. For the first time, we find that the CCM
relationship seems to form a bound on the properties of all of the LMC and SMC
extinction curves. This result strengthens the picture of dust extinction
curves exhibit a continuum of properties between those found in the Milky Way
and the SMC Bar. (abridged)Comment: 18 pages, 10 figures, ApJ in pres
The DEEP2 Galaxy Redshift Survey: Discovery of Luminous, Metal-poor, Sta r-forming Galaxies at Redshifts z~0.7
We have discovered a sample of 17 metal-poor, yet luminous, star-forming
galaxies at redshifts z~0.7. They were selected from the initial phase of the
DEEP2 survey of 3900 galaxies and the Team Keck Redshift Survey (TKRS) of 1536
galaxies as those showing the temperature-sensitive [OIII]l4363 auroral line.
These rare galaxies have blue luminosities close to L*, high star formation
rates of 5 to 12 solar masses per year, and oxygen abundances of 1/3 to 1/10
solar. They thus lie significantly off the luminosity-metallicity relation
found previously for field galaxies with strong emission lines at redshifts
z~0.7. The prior surveys relied on indirect, empirical calibrations of the R23
diagnostic and the assumption that luminous galaxies are not metal-poor. Our
discovery suggests that this assumption is sometimes invalid. As a class, these
newly-discovered galaxies are: (1) more metal-poor than common classes of
bright emission-line galaxies at z~0.7 or at the present epoch; (2) comparable
in metallicity to z~3 Lyman Break Galaxies but less luminous; and (3)
comparable in metallicity to local metal-poor eXtreme Blue Compact Galaxies
(XBCGs), but more luminous. Together, the three samples suggest that the
most-luminous, metal-poor, compact galaxies become fainter over time.Comment: This is a .tgz file. It should create the following files: texto.tex,
tab1.tex, f1.eps and f2.eps. The LaTeX style used is emulateapj.cls, version
November 26, 2004. This submission is 5 pages long, one table and two
figures. To appear in ApJ
Scaling Relations of Dwarf Galaxies without Supernova-Driven Winds
Nearby dwarf galaxies exhibit tight correlations between their global stellar
and dynamical properties, such as circular velocity, mass-to-light ratio,
stellar mass, surface brightness, and metallicity. Such correlations have often
been attributed to gas or metal-rich outflows driven by supernova energy
feedback to the interstellar medium. We use high-resolution cosmological
simulations of high-redshift galaxies with and without energy feedback, as well
as analytic modeling, to investigate whether the observed correlations can
arise without supernova-driven outflows. We find that the simulated dwarf
galaxies exhibit correlations similar to those observed as early as z~10,
regardless of whether supernova feedback is included. We also show that the
correlations can be well reproduced by our analytic model that accounts for
realistic gas inflow but assumes no outflows, and star formation rate obeying
the Kennicutt-Schmidt law with a critical density threshold. We argue that
correlations in simulated galaxies arise due to the increasingly inefficient
conversion of gas into stars in low-mass dwarf galaxies rather than
supernova-driven outflows. We also show that the decrease of the observed
effective yield in low-mass objects, often used as an indicator of gas and
metal outflows, can be reasonably reproduced in our simulations without
outflows. We show that this trend can arise if a significant fraction of metals
in small galaxies is spread to the outer regions of the halo outside the
stellar extent via mixing. In this case the effective yield can be
significantly underestimated if only metals within the stellar radius are taken
into account. Measurements of gas metallicity in the outskirts of gaseous disks
of dwarfs would thus provide a key test of such explanation.Comment: accepted for publication in ApJ, 19 pages, 12 figures, uses
emulateapj
Constraints from Al Measurements on the Galaxy's Recent Global Star Formation Rate and Core Collapse Supernovae Rate
Gamma-rays from the decay of Al offer a stringent constraint on the
Galaxy's global star formation rate over the past million years, supplementing
other methods for quantifying the recent Galactic star formation rate, such as
equivalent widths of H emission. Advantages and disadvantages of using
Al gamma-ray measurements as a tracer of the massive star formation rate
are analyzed. Estimates of the Galactic Al mass derived from COMPTEL
measurements are coupled with a simple, analytical model of the Al
injection rate from massive stars and restrict the Galaxy's recent star
formation rate to \hbox{5 4 M\sun yr}. In addition, we show that
the derived Al mass implies a present day \hbox{Type II + Ib} supernovae
rate of 3.4 2.8 per century, which seems consistent with other
independent estimates of the Galactic core collapse supernova rate. If some
independent measure of the massive star initial mass function or star formation
rate or \hbox{Type II + Ib} supernovae rate were to become available (perhaps
through estimates of the Galactic Fe mass), then a convenient way to
restrain, or possibly determine, the other parameters is presented.Comment: 11 pages including 1 figure, ApJ in pres
Warm Molecular Hydrogen Emission in Normal Edge-On Galaxies NGC 4565 and NGC 5907
We have observed warm molecular hydrogen in two nearby edge-on disk galaxies,
NGC 4565 and NGC 5907, using the Spitzer high-resolution infrared spectrograph.
The 0-0 S(0) 28.2 micron and 0-0 S(1) 17.0 micron pure rotational lines were
detected out to 10 kpc from the center of each galaxy on both sides of the
major axis, and in NGC 4565 the S(0) line was detected at r = 15 kpc on one
side. This location lies beyond a steep drop in the radio continuum emission
from cosmic rays in the disk. Despite indications that star formation activity
decreases with radius, the H2 excitation temperature and the ratio of the H2
line and the far-IR luminosity surface densities, Sigma_L(H2}/Sigma_L(TIR},
change very little as a function of radius, even into the diffuse outer region
of the disk of NGC 4565. This suggests that the source of excitation of the H2
operates over a large range of radii, and is broadly independent of the
strength and relative location of UV emission from young stars. Although
excitation in photodissociation regions is the most common explanation for the
widespread H2 emission, cosmic ray heating or shocks cannot be ruled out. The
inferred mass surface densities of warm molecular hydrogen in both edge-on
galaxies differ substantially, being 4(-60) M_solar/pc^2 and 3(-50)
M_solar/pc^2 at r = 10 kpc for NGC 4565 and NGC 5907, respectively. The higher
values represent very unlikely point-source upper limits. The point source case
is not supported by the observed emission distribution in the spectral slits.
These mass surface densities cannot support the observed rotation velocities in
excess of 200 km/s. Therefore, warm molecular hydrogen cannot account for dark
matter in these disk galaxies, contrary to what was implied by a previous ISO
study of the nearby edge-on galaxy NGC 891.Comment: Accepted for publication in the Astronomical Journal (20 pages, 17
figures, 7 tables
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