1,312 research outputs found
Imaging of the Stellar Population of IC10 with Laser Guide Star Adaptive Optics and the Hubble Space Telescope
We present adaptive optics (AO) images of the central starburst region of the
dwarf irregular galaxy IC10. The Keck 2 telescope laser guide star was used to
achieve near diffraction-limited performance at H and K' (Strehls of 18% and
32%, respectively). The images are centered on the putative Wolf-Rayet (W-R)
object [MAC92]24. We combine our AO images with F814W data from HST. By
comparing the K' vs. [F814W]-K' color-magnitude diagram (CMD) with theoretical
isochrones, we find that the stellar population is best represented by at least
two bursts of star formation, one ~ 10 Myr ago and one much older (150-500
Myr). Young, blue stars are concentrated in the vicinity of [MAC92]24. This
population represents an OB association with a half-light radius of about 3 pc.
We resolve the W-R object [MAC92]24 into at least six blue stars. Four of these
components have near-IR colors and luminosities that make them robust WN star
candidates. By matching the location of C-stars in the CMD with those in the
SMC we derive a distance modulus for IC10 of about 24.5 mag. and a foreground
reddening of E(B-V) = 0.95. We find a more precise distance by locating the tip
of the giant branch in the F814W, H, and K' luminosity functions. We find a
weighted mean distance modulus of 24.48 +/- 0.08. The systematic error in this
measurement, due to a possible difference in the properties of the RGB
populations in IC10 and the SMC, is +/- 0.16 mag.Comment: 13 pages, 13 figures, ApJ in pres
Intrinsic Axis Ratio Distribution of Early-type Galaxies From Sloan Digital Sky Survey
Using Sloan Digital Sky Survey Data Release 5, we have investigated the
intrinsic axis ratio distribution (ARD) for early-type galaxies. We have
constructed a volume-limited sample of 3,922 visually-inspected early-type
galaxies at carefully considering sampling biases
caused by the galaxy isophotal size and luminosity. We attempt to de-project
the observed ARD into three-dimensional types (oblate, prolate, and triaxial),
which are classified in terms of triaxiality. We confirm that no linear
combination of -distributed axis ratios of the three types can
reproduce the observed ARD. However, using Gaussian intrinsic distributions, we
have found reasonable fits to the data with preferred mean axis ratios for
oblate, prolate, and triaxial (triaxials in two axis ratios), where the fractions of
oblate, prolate and triaxial types are
\textrm{O:P:T}=0.29^{\pm0.09}:0.26^{\pm0.11}:0.45^{\pm0.13}-23.3 < M_r \leq -21.2-21.2 < M_r <-19.3$) sample does. Oblate is
relatively more abundant among the less luminous galaxies. Interestingly, the
preferences of axis ratios for triaxial types in the two luminosity classes are
remarkably similar. We have not found any significant influence of the local
galaxy number density on ARD. We show that the results can be seriously
affected by the details in the data selection and type classification scheme.
Caveats and implications on galaxy formation are discussed.Comment: 9 pages, 11 figures, Accepted for publication in Ap
Application of the Gaussian mixture model in pulsar astronomy -- pulsar classification and candidates ranking for {\it Fermi} 2FGL catalog
Machine learning, algorithms to extract empirical knowledge from data, can be
used to classify data, which is one of the most common tasks in observational
astronomy. In this paper, we focus on Bayesian data classification algorithms
using the Gaussian mixture model and show two applications in pulsar astronomy.
After reviewing the Gaussian mixture model and the related
Expectation-Maximization algorithm, we present a data classification method
using the Neyman-Pearson test. To demonstrate the method, we apply the
algorithm to two classification problems. Firstly, it is applied to the well
known period-period derivative diagram, where we find that the pulsar
distribution can be modeled with six Gaussian clusters, with two clusters for
millisecond pulsars (recycled pulsars) and the rest for normal pulsars. From
this distribution, we derive an empirical definition for millisecond pulsars as
. The two
millisecond pulsar clusters may have different evolutionary origins, since the
companion stars to these pulsars in the two clusters show different chemical
composition. Four clusters are found for normal pulsars. Possible implications
for these clusters are also discussed. Our second example is to calculate the
likelihood of unidentified \textit{Fermi} point sources being pulsars and rank
them accordingly. In the ranked point source list, the top 5% sources contain
50% known pulsars, the top 50% contain 99% known pulsars, and no known active
galaxy (the other major population) appears in the top 6%. Such a ranked list
can be used to help the future follow-up observations for finding pulsars in
unidentified \textit{Fermi} point sources.Comment: 9 pages, 4 figures, accepted by MNRA
Twisting of X-ray isophotes in triaxial galaxies
We investigate X-ray isophote twists created by triaxiality differences
between the luminous stellar distributions and the dark halos in elliptical
galaxies. For a typically oblate luminous galaxy embedded in a more prolate
halo formed by dissipationless collapse, the triaxiality difference of \Delta T
= ~0.7 leads to typical isophote twists of = ~16 deg +/- 19 deg
at 3 stellar effective radii. In a model which includes baryonic dissipation
the effect is smaller, with \Delta T = ~0.3 and = ~5 deg +/- 8
deg. Thus, accurate measurements of X-ray isophote twists may be able to set
constraints on the interactions between baryons and dissipationless dark matter
during galaxy formation. The 30-deg X-ray isophote twist in the E4 galaxy NGC
720 cannot be reproduced by our model, suggesting an intrinsic misalignment
between the halo and the stars rather than a projection effect.Comment: 18 pages, with 9 inline Postscript figures, LaTeX, aaspp4.sty,
submitted to ApJ; postscript paper w/figs (200 kb) also avaliable at
http://cfa-www.harvard.edu/~romanow/int.email.v2.ps.g
The Dependence of Galaxy Shape on Luminosity and Surface Brightness Profile
For a sample of 96,951 galaxies from the Sloan Digital Sky Survey Data
Release 3, we study the distribution of apparent axis ratios as a function of
r-band absolute magnitude and surface brightness profile type. We use the
parameter fracDeV to quantify the profile type (fracDeV = 1 for a de
Vaucouleurs profile; fracDeV = 0 for an exponential profile). When the apparent
axis ratio q_{am} is estimated from the moments of the light distribution, the
roundest galaxies are very bright (M_r \sim -23) de Vaucouleurs galaxies and
the flattest are modestly bright (M_r \sim -18) exponential galaxies. When the
apparent axis ratio q_{25} is estimated from the axis ratio of the 25
mag/arcsec^2 isophote, we find that de Vaucouleurs galaxies are flatter than
exponential galaxies of the same absolute magnitude. For a given surface
brightness profile type, very bright galaxies are rounder, on average, than
fainter galaxies. We deconvolve the distributions of apparent axis ratios to
find the distribution of the intrinsic short-to-long axis ratio gamma, assuming
constant triaxiality T. For all profile types and luminosities, the
distribution of apparent axis ratios is inconsistent with a population of
oblate spheroids, but is usually consistent with a population of prolate
spheroids. Bright galaxies with a de Vaucouleurs profile (M_r < -21.84, fracDeV
> 0.9) have a distribution of q_{am} that is consistent with triaxiality in the
range 0.4 < T < 0.8, with mean intrinsic axis ratio 0.66 < gamma < 0.69. The
fainter de Vaucouleurs galaxies are best fit with prolate spheroids (T = 1)
with mean axis ratio gamma = 0.51.Comment: 32 pages, 12 figures, to appear in Ap
The Low- and Intermediate-Mass Stellar Population in the Small Magellanic Cloud: The Central Stars of Planetary Nebulae
We present a study on the central stars (CSs) of Planetary Nebulae (PNe)
observed in the Small Magellanic Cloud (SMC) with the Space Telescope Imaging
Spectrograph instrument on-board the HST. The stellar magnitudes have been
measured using broad-band photometry, and Zanstra analysis of the nebulae
provided the stellar temperatures. From the location of the CSs on the HR
diagram, and by comparing the observed CSs with current models of stellar
evolution, we infer the CSs masses. We examine closely the possibility of light
contamination in the bandpass from an unrecognized stellar companion, and we
establish strong constraints on the existence and nature of any binary
companion. We find an average mass of 0.63 Msun, which is similar to the mass
obtained for a sample of CSs in the LMC (0.65 Msun). However, the SMC and LMC
CS mass distributions differ slightly, the SMC sample lacking an
intermediate-mass stellar population (0.65 to 0.75 Msun). We discuss the
significance and possible reasons for the difference between the two mass
distributions. In particular, we consider the differences in the star formation
history between the clouds and the mass-loss rate dependence on metallicity.Comment: 30 pages, 6 figures, 5 tables. To be published in ApJ (October 20
GASP II. A MUSE view of extreme ram-pressure stripping along the line of sight: kinematics of the jellyfish galaxy JO201
This paper presents a spatially-resolved kinematic study of the jellyfish
galaxy JO201, one of the most spectacular cases of ram-pressure stripping (RPS)
in the GASP (GAs Stripping Phenomena in Galaxies with MUSE) survey. By studying
the environment of JO201, we find that it is moving through the dense
intra-cluster medium of Abell 85 at supersonic speeds along our line of sight,
and that it is likely accompanied by a small group of galaxies. Given the
density of the intra-cluster medium and the galaxy's mass, projected position
and velocity within the cluster, we estimate that JO201 must so far have lost
~50% of its gas during infall via RPS. The MUSE data indeed reveal a smooth
stellar disk, accompanied by large projected tails of ionised (Halpha) gas,
composed of kinematically cold (velocity dispersion <40km/s) star-forming knots
and very warm (>100km/s) diffuse emission which extend out to at least ~50 kpc
from the galaxy centre. The ionised Halpha-emitting gas in the disk rotates
with the stars out to ~6 kpc but in the disk outskirts becomes increasingly
redshifted with respect to the (undisturbed) stellar disk. The observed
disturbances are consistent with the presence of gas trailing behind the
stellar component, resulting from intense face-on RPS happening along the line
of sight. Our kinematic analysis is consistent with the estimated fraction of
lost gas, and reveals that stripping of the disk happens outside-in, causing
shock heating and gas compression in the stripped tails.Comment: ApJ, revised version after referee comments, 15 pages, 16 figures.
The interactive version of Figure 9 can be viewed at
web.oapd.inaf.it/gasp/publications.htm
Continuum and Emission-Line Properties of Broad Absorption Line Quasars
We investigate the continuum and emission-line properties of 224 broad
absorption line quasars (BALQSOs) with 0.9<z<4.4 drawn from the Sloan Digital
Sky Survey (SDSS) Early Data Release (EDR), which contains 3814 bona fide
quasars. We find that low-ionization BALQSOs (LoBALs) are significantly
reddened as compared to normal quasars, in agreement with previous work.
High-ionization BALQSOs (HiBALs) are also more reddened than the average
nonBALQSO. Assuming SMC-like dust reddening at the quasar redshift, the amount
of reddening needed to explain HiBALs is E(B-V)~0.023 and LoBALs is
E(B-V)~0.077 (compared to the ensemble average of the entire quasar sample). We
find that there are differences in the emission-line properties between the
average HiBAL, LoBAL, and nonBAL quasar. These differences, along with
differences in the absorption line troughs, may be related to intrinsic quasar
properties such as the slope of the intrinsic (unreddened) continuum; more
extreme absorption properties are correlated with bluer intrinsic continua.
Despite the differences among BALQSO sub-types and nonBALQSOs, BALQSOs appear
to be drawn from the same parent population as nonBALQSOs when both are
selected by their UV/optical properties. We find that the overall fraction of
traditionally defined BALQSOs, after correcting for color-dependent selection
effects due to different SEDs of BALQSO and nonBALQSOs, is 13.4+/-1.2% and
shows no significant redshift dependence for 1.7<z<3.45. After a rough
completeness correction for the effects of dust extinction, we find that
approximately one in every six quasars is a BALQSO.Comment: 35 pages, 11 figures (1 color), 1 table; accepted by A
WINGS: a WIde-field Nearby Galaxy-cluster Survey. I - Optical imaging
This is the first paper of a series that will present data and scientific
results from the WINGS project, a wide-field, multiwavelength imaging and
spectroscopic survey of galaxies in 77 nearby clusters. The sample was
extracted from the ROSAT catalogs with constraints on the redshift
(0.0420). The global goal of
the WINGS project is the systematic study of the local cosmic variance of the
cluster population and of the properties of cluster galaxies as a function of
cluster properties and local environment. This data collection will allow to
define a local 'Zero-Point' reference against which to gauge the cosmic
evolution when compared to more distant clusters. The core of the project
consists of wide-field optical imaging of the selected clusters in the B and V
bands. We have also completed a multi-fiber, medium resolution spectroscopic
survey for 51 of the clusters in the master sample. In addition, a NIR (JK)
survey of ~50 clusters and an H_alpha + UV survey of some 10 clusters are
presently ongoing, while a very-wide-field optical survey has also been
programmed. In this paper we briefly outline the global objectives and the main
characteristics of the WINGS project. Moreover, the observing strategy and the
data reduction of the optical imaging survey (WINGS-OPT) are presented. We have
achieved a photometric accuracy of ~0.025mag, reaching completeness to V~23.5.
Field size and resolution (FWHM) span the absolute intervals (1.6-2.7)Mpc and
(0.7-1.7)kpc, respectively, depending on the redshift and on the seeing. This
allows the planned studies to get a valuable description of the local
properties of clusters and galaxies in clusters.Comment: 24 pages, 15 figures, Accepted by Astronomy and Astrophysic
Molecular Gas in Elliptical Galaxies: Distribution and Kinematics
I present interferometric images (approx. 7" resolution) of CO emission in
five elliptical galaxies and nondetections in two others. These data double the
number of elliptical galaxies whose CO emission has been fully mapped. The
sample galaxies have 10^8 to 5x10^9 solar masses of molecular gas distributed
in mostly symmetric rotating disks with diameters of 2 to 12 kpc. Four out of
the five molecular disks show remarkable alignment with the optical major axes
of their host galaxies. The molecular masses are a few percent of the total
dynamical masses which are implied if the gas is on circular orbits. If the
molecular gas forms stars, it will make rotationally supported stellar disks
which will be very similar in character to the stellar disks now known to be
present in many ellipticals. Comparison of stellar kinematics to gas kinematics
in NGC 4476 implies that the molecular gas did not come from internal stellar
mass loss because the specific angular momentum of the gas is about three times
larger than that of the stars.Comment: 47 pages, 6 tables, 27 figures. Accepted by AJ, scheduled for August
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