1,915 research outputs found
Dark Matter and Stellar Mass in the Luminous Regions of Disk Galaxies
We investigate the correlations among stellar mass (M_*), disk scale length
(R_d), and rotation velocity at 2.2 disk scale lengths (V_2.2) for a sample of
81 disk-dominated galaxies (disk/total >= 0.9) selected from the SDSS. We
measure V_2.2 from long-slit H-alpha rotation curves and infer M_* from galaxy
i-band luminosities (L_i) and g-r colors. We find logarithmic slopes of
2.60+/-0.13 and 3.05+/-0.12 for the L_i-V_2.2 and M_*-V_2.2 relations, somewhat
shallower than most previous studies, with intrinsic scatter of 0.13 dex and
0.16 dex. Our direct estimates of the total-to-stellar mass ratio within
2.2R_d, assuming a Kroupa IMF, yield a median ratio of 2.4 for M_*>10^10 Msun
and 4.4 for M_*=10^9-10^10 Msun, with large scatter at a given M_* and R_d. The
typical ratio of the rotation speed predicted for the stellar disk alone to the
observed rotation speed at 2.2R_d is ~0.65. The distribution of R_d at fixed
M_* is broad, but we find no correlation between disk size and the residual
from the M_*-V_2.2 relation, implying that this relation is an approximately
edge-on view of the disk galaxy fundamental plane. Independent of the assumed
IMF, this result implies that stellar disks do not, on average, dominate the
mass within 2.2R_d. We discuss our results in the context of infall models of
disk formation in cold dark matter halos. A model with a disk-to-halo mass
ratio m_d=0.05 provides a reasonable match to the R_d-M_* distribution for spin
parameters \lambda ranging from ~0.04-0.08, and it yields a reasonable match to
the mean M_*-V_2.2 relation. A model with m_d=0.1 predicts overly strong
correlations between disk size and M_*-V_2.2 residual. Explaining the wide
range of halo-to-disk mass ratios within 2.2R_d requires significant scatter in
m_d values, with systematically lower m_d for galaxies with lower .Comment: 18 pages, 2 tables, 7 figures, Accepted to ApJ, Table 1 updated,
otherwise minor change
Massive Atropine Eye Drop Ingestion Treated with High-Dose Physostigmine to Avoid Intubation
Case: A 34-year-old male presented after ingesting 150 mg of atropine. He had altered mental status, sinus tachycardia, dry mucosa, flushed skin, and hyperthermia. Sequential doses of physostigmine, totaling 14 mg, were successful in reversing antimuscarinic toxicity and prevented the need to perform airway control with endotracheal intubation. At completion of treatment, heart rate and mental status had improved, and intubation was never performed. Discussion: Atropine causes anticholinergic toxicity; physostigmine reverses this by inhibiting acetylcholinesterase. Atropine eye drop ingestions are rare. The 14 mg of physostigmine administered is much higher than typical dosing. It is likely the physostigmine prevented intubation. Atropine eye drops can be dangerous, and physostigmine should be considered in treatment. [West J Emerg Med. 2012;13(1):77â79.
Observational Constraints on Red and Blue Helium Burning Sequences
We derive the optical luminosity, colors, and ratios of the blue and red
helium burning (HeB) stellar populations from archival Hubble Space Telescope
observations of nineteen starburst dwarf galaxies and compare them with
theoretical isochrones from Padova stellar evolution models across
metallicities from Z=0.001 to 0.009. We find that the observational data and
the theoretical isochrones for both blue and red HeB populations overlap in
optical luminosities and colors and the observed and predicted blue to red HeB
ratios agree for stars older than 50 Myr over the time bins studied. These
findings confirm the usefulness of applying isochrones to interpret
observations of HeB populations. However, there are significant differences,
especially for the red HeB population. Specifically we find: (1) offsets in
color between the observations and theoretical isochrones of order 0.15 mag
(0.5 mag) for the blue (red) HeB populations brighter than M_V ~ -4 mag, which
cannot be solely due to differential extinction; (2) blue HeB stars fainter
than M_V ~ -3 mag are bluer than predicted; (3) the slope of the red HeB
sequence is shallower than predicted by a factor of ~3; and (4) the models
overpredict the ratio of the most luminous blue to red HeB stars corresponding
to ages <50 Myr. Additionally, we find that for the more metal-rich galaxies in
our sample (Z> 0.5 Zsolar) the red HeB stars overlap with the red giant branch
stars in the color magnitude diagrams, thus reducing their usefulness as
indicators of star formation for ages >100 Myr.Comment: 18 pages, 11 figures, 3 table
The Nature of Starbursts : II. The Duration of Starbursts in Dwarf Galaxies
The starburst phenomenon can shape the evolution of the host galaxy and the
surrounding intergalactic medium. The extent of the evolutionary impact is
partly determined by the duration of the starburst, which has a direct
correlation with both the amount of stellar feedback and the development of
galactic winds, particularly for smaller mass dwarf systems. We measure the
duration of starbursts in twenty nearby, ongoing, and "fossil" starbursts in
dwarf galaxies based on the recent star formation histories derived from
resolved stellar population data obtained with the Hubble Space Telescope.
Contrary to the shorter times of 3-10 Myr often cited, the starburst durations
we measure range from 450 - 650 Myr in fifteen of the dwarf galaxies and up to
1.3 Gyr in four galaxies; these longer durations are comparable to or longer
than the dynamical timescales for each system. The same feedback from massive
stars that may quench the flickering SF does not disrupt the overall burst
event in our sample of galaxies. While five galaxies present fossil bursts,
fifteen galaxies show ongoing bursts and thus the final durations may be longer
than we report here for these systems. One galaxy shows a burst that has been
ongoing for only 20 Myr; we are likely seeing the beginning of a burst event in
this system. Using the duration of the starbursts, we calculate that the bursts
deposited 10^(53.9)-10^(57.2) ergs of energy into the interstellar medium
through stellar winds and supernovae and produced 3%-26% of the host galaxy's
mass.Comment: 28 pages, 4 figure
Stellar Populations in Three Outer Fields of the LMC
We present HST photometry for three fields in the outer disk of the LMC
extending approximately four magnitudes below the faintest main sequence
turnoff. We cannot detect any strongly significant differences in the stellar
populations of the three fields based on the morphologies of the
color-magnitude diagrams, the luminosity functions, and the relative numbers of
stars in different evolutionary stages. Our observations therefore suggest
similar star formation histories in these regions, although some variations are
certainly allowed. The fields are located in two regions of the LMC: one is in
the north-east field and two are located in the north-west. Under the
assumption of a common star formation history, we combine the three fields with
ground-based data at the same location as one of the fields to improve
statistics for the brightest stars. We compare this stellar population with
those predicted from several simple star formation histories suggested in the
literature, using a combination of the R-method of Bertelli et al (1992) and
comparisons with the observed luminosity function. The only model which we
consider that is not rejected by the observations is one in which the star
formation rate is roughly constant for most of the LMC's history and then
increases by a factor of three about 2 Gyr ago. Such a model has roughly equal
numbers of stars older and younger than 4 Gyr, and thus is not dominated by
young stars. This star formation history, combined with a closed box chemical
evolution model, is consistent with observations that the metallicity of the
LMC has doubled in the past 2 Gyr.Comment: 30 pages, includes 10 postscript figures. Figure 1 avaiable at
ftp://charon.nmsu.edu/pub/mgeha/LMC. Accepted for publication in Astronomical
Journa
Binary Quasars in the Sloan Digital Sky Survey: Evidence for Excess Clustering on Small Scales
We present a sample of 218 new quasar pairs with proper transverse
separations R_prop < 1 Mpc/h over the redshift range 0.5 < z < 3.0, discovered
from an extensive follow up campaign to find companions around the Sloan
Digital Sky Survey and 2dF Quasar Redshift Survey quasars. This sample includes
26 new binary quasars with separations R_prop < 50 kpc/h (theta < 10
arcseconds), more than doubling the number of such systems known. We define a
statistical sample of binaries selected with homogeneous criteria and compute
its selection function, taking into account sources of incompleteness. The
first measurement of the quasar correlation function on scales 10 kpc/h <
R_prop < 400 kpc/h is presented. For R_prop < 40 kpc/h, we detect an order of
magnitude excess clustering over the expectation from the large scale R_prop >
3 Mpc/h quasar correlation function, extrapolated down as a power law to the
separations probed by our binaries. The excess grows to ~ 30 at R_prop ~ 10
kpc/h, and provides compelling evidence that the quasar autocorrelation
function gets progressively steeper on sub-Mpc scales. This small scale excess
can likely be attributed to dissipative interaction events which trigger quasar
activity in rich environments. Recent small scale measurements of galaxy
clustering and quasar-galaxy clustering are reviewed and discussed in relation
to our measurement of small scale quasar clustering.Comment: 25 pages, 12 figures, 9 tables. Submitted to the Astronomical Journa
The ACS Nearby Galaxy Survey Treasury
The ACS Nearby Galaxy Survey Treasury (ANGST) is a systematic survey to
establish a legacy of uniform multi-color photometry of resolved stars for a
volume-limited sample of nearby galaxies (D<4 Mpc). The survey volume
encompasses 69 galaxies in diverse environments, including close pairs, small &
large groups, filaments, and truly isolated regions. The galaxies include a
nearly complete range of morphological types spanning a factor of ~10^4 in
luminosity and star formation rate. The survey data consists of images taken
with ACS on HST, supplemented with archival data and new WFPC2 imaging taken
after the failure of ACS. Survey images include wide field tilings covering the
full radial extent of each galaxy, and single deep pointings in uncrowded
regions of the most massive galaxies in the volume. The new wide field imaging
in ANGST reaches median 50% completenesses of m_F475W=28.0 mag, m_F606W=27.3
mag, and m_F814W=27.3 mag, several magnitudes below the tip of the red giant
branch (TRGB). The deep fields reach magnitudes sufficient to fully resolve the
structure in the red clump. The resulting photometric catalogs are publicly
accessible and contain over 34 million photometric measurements of >14 million
stars. In this paper we present the details of the sample selection, imaging,
data reduction, and the resulting photometric catalogs, along with an analysis
of the photometric uncertainties (systematic and random), for both the ACS and
WFPC2 imaging. We also present uniformly derived relative distances measured
from the apparent magnitude of the TRGB.Comment: 54 pages, including 24 pages of figures and 16 pages of tables.
Project website and data available at http://www.nearbygalaxies.org/ . Data
is also available through MAST. Scheduled to appear in the Astrophysical
Journal Supplements. (Replaced to fix several figures that were damaged
during compression
How Typical Are The Local Group Dwarf Galaxies?
We compare the cumulative star formation histories (SFHs) of Local Group (LG)
dwarf galaxies with those in the volume-limited ACS Nearby Galaxy Survey
Treasury (ANGST) sample (D < 4 Mpc), in order to understand how typical the LG
dwarf galaxies are relative to those in the nearby universe. The SFHs were
derived in a uniform manner from high quality optical color-magnitude diagrams
constructed from Hubble Space Telescope imaging. We find that the {\it mean}
cumulative SFHs of the LG dwarfs are comparable to the mean cumulative SFHs of
the ANGST sample for the three different morphological types (dwarf
spheroidals/ellipticals: dSph/dE; dwarf irregulars: dI; transition dwarfs:
dTrans). We also discuss effects such as population gradients and systematic
uncertainties in the stellar models that may influence the derived SFHs. Both
the ANGST and Local Group dwarf galaxies show a consistent and strong
morphology-density relationship, emphasizing the importance of environment in
the evolution of dwarf galaxies. Specifically, we confirm that dIs are found at
lower densities and higher luminosities than dSphs, within this large sample.
We also find that dTrans are located in similar environments to those occupied
by dwarf irregular galaxies, but have systematically lower luminosities that
are more comparable to those of dwarf spheroidals. The similarity of the SFHs
and morphology-density relationships of the LG and ANGST dwarf galaxies
suggests that the LG dwarfs are a good representation of dwarf galaxies in the
local universe.Comment: 10 pages, 4 figures. Accepted to Ap
The Nature of Starbursts: I. The Star Formation Histories of Eighteen Nearby Starburst Dwarf Galaxies
We use archival HST observations of resolved stellar populations to derive
the star formation histories (SFHs) of 18 nearby starburst dwarf galaxies. In
this first paper we present the observations, color-magnitude diagrams, and the
SFHs of the 18 starburst galaxies, based on a homogeneous approach to the data
reduction, differential extinction, and treatment of photometric completeness.
We adopt a star formation rate (SFR) threshold normalized to the average SFR of
the individual system as a metric for classifying starbursts in SFHs derived
from resolved stellar populations. This choice facilitates finding not only
currently bursting galaxies but also "fossil" bursts increasing the sample size
of starburst galaxies in the nearby (D<8 Mpc) universe. Thirteen of the
eighteen galaxies are experiencing ongoing bursts and five galaxies show fossil
bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a
burst are sustained for hundreds of Myr with variations on small timescales. A
long >100 Myr temporal baseline is thus fundamental to any starburst definition
or identification method. The longer lived bursts rule out rapid
"self-quenching" of starbursts on global scales. The bursting galaxies' gas
consumption timescales are shorter than the Hubble time for all but one galaxy
confirming the short-lived nature of starbursts based on fuel limitations.
Additionally, we find the strength of the H{\alpha} emission usually correlates
with the CMD based SFR during the last 4-10 Myr. However, in four cases, the
H{\alpha} emission is significantly less than what is expected for models of
starbursts; the discrepancy is due to the SFR changing on timescales of a few
Myr. The inherently short timescale of the H{\alpha} emission limits
identifying galaxies as starbursts based on the current characteristics which
may or may not be representative of the recent SFH of a galaxy.Comment: 53 pages, 11 figure
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