668 research outputs found
The Compression of Dark Matter Halos by Baryonic Infall
The initial radial density profiles of dark matter halos are laid down by
gravitational collapse in hierarchical structure formation scenarios and are
subject to further compression as baryons cool and settle to the halo centers.
We here describe an explicit implementation of the algorithm, originally
developed by Young, to calculate changes to the density profile as the result
of adiabatic infall in a spherical halo model. Halos with random motion are
more resistant to compression than are those in which random motions are
neglected, which is a key weakness of the simple method widely employed.
Young's algorithm results in density profiles in excellent agreement with those
from N-body simulations. We show how the algorithm may be applied to determine
the original uncompressed halos of real galaxies, a step which must be computed
with care in order to enable a confrontation with theoretical predictions from
theories such as LCDM.Comment: Revised version for ApJ. 8 pages, 8 figures, latex uses emulateap
A HST study of the stellar populations in the cometary dwarf irregular galaxy NGC 2366
We present V and I photometry of the resolved stars in the cometary dwarf
irregular galaxy NGC 2366, using Wide Field Planetary Camera 2 images obtained
with the Hubble Space Telescope. The resulting color-magnitude diagram reaches
down to I~26.0 mag. It reveals not only a young population of blue
main-sequence stars (age <30 Myr) but also an intermediate-age population of
blue and red supergiants (20 Myr<age<100 Myr), and an older evolved populations
of asymptotic giant branch (AGB) stars (age >100 Myr) and red giant branch
(RGB) stars (age >1 Gyr). The measured magnitude I=23.65+/-0.10 mag of the RGB
tip results in a distance modulus m-M=27.67+/-0.10, which corresponds to a
distance of 3.42+/-0.15 Mpc, in agreement with previous distance
determinations. The youngest stars are associated with the bright complex of
HII regions NGC 2363=Mrk 71 in the southwest extremity of the galaxy. As a
consequence of the diffusion and relaxation processes of stellar ensembles, the
older the stellar population is, the smoother and more extended is its spatial
distribution. An underlying population of older stars is found throughout the
body of NGC 2366. The most notable feature of this older population is the
presence of numerous relatively bright AGB stars. The number ratio of AGB to
RGB stars and the average absolute brightness of AGB stars in NGC 2366 are
appreciably higher than in the BCD VII Zw 403, indicating a younger age of the
AGB stars in NGC 2366. In addition to the present burst of age <100 Myr, there
has been strong star formation activity in the past of NGC 2366, from ~100 Myr
to <3 Gyr ago.Comment: 32 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Discovery of a high state AM CVn binary in the Galactic Bulge Survey
We report on the discovery of a hydrogen-deficient compact binary (CXOGBS
J175107.6-294037) belonging to the AM CVn class in the Galactic Bulge Survey.
Deep archival X-ray observations constrain the X-ray positional uncertainty of
the source to 0.57 arcsec, and allow us to uniquely identify the optical and UV
counterpart. Optical spectroscopic observations reveal the presence of broad,
shallow He i absorption lines while no sign of hydrogen is present, consistent
with a high state system. We present the optical lightcurve from Optical
Gravitational Lensing Experiment monitoring, spanning 15 years. It shows no
evidence for outbursts; variability is present at the 0.2 mag level on
timescales ranging from hours to weeks. A modulation on a timescale of years is
also observed. A Lomb-Scargle analysis of the optical lightcurves shows two
significant periodicities at 22.90 and 23.22 min. Although the physical
interpretation is uncertain, such timescales are in line with expectations for
the orbital and superhump periods. We estimate the distance to the source to be
between 0.5 - 1.1 kpc. Spectroscopic follow-up observations are required to
establish the orbital period, and to determine whether this source can serve as
a verification binary for the eLISA gravitational wave mission.Comment: Accepted for publication in MNRAS Letter
A 180 Kpc Tidal Tail in the Luminous Infrared Merger Arp 299
We present VLA HI observations and UH88 deep optical B- and R-band
observations of the IR luminous merger Arp 299 (= NGC 3690 + IC 694). These
data reveal a gas-rich, optically faint tidal tail with a length of over 180
kpc. The size of this tidal feature necessitates an old interaction age for the
merger (~750 Myr since first periapse), which is currently experiencing a very
young star burst (~20 Myr). The observations reveal a most remarkable structure
within the tidal tail: it appears to be composed of two parallel filaments
separated by ~20 kpc. One of the filaments is gas rich with little if any
starlight, while the other is gas poor. We believe that this bifurcation
results from a warped disk in one of the progenitors. The quantities and
kinematics of the tidal HI suggest that Arp 299 results from the collision of a
retrograde Sab-Sb galaxy (IC 694) and a prograde Sbc-Sc galaxy (NGC 3690) that
occurred 750 Myr ago and which will merge into a single object in ~60 Myr. We
suggest that the present IR luminous phase in this system is due in part to the
retrograde spin of IC 694. Finally, we discuss the apparent lack of tidal dwarf
galaxies within the tail.Comment: LaTex, 14 pages, 11 figures, 4 tables, uses emulateapj.sty. Accepted
to AJ for July 1999. For version with full-resolution images see
http://www.cv.nrao.edu/~jhibbard/a299/HIpaper/a299HI.htm
Constraining the nature of the accreting binary in CXOGBS J174623.5-310550
We report optical and infrared observations of the X-ray source CXOGBS
J174623.5-310550. This Galactic object was identified as a potential quiescent
low-mass X-ray binary accreting from an M-type donor on the basis of optical
spectroscopy and the broad Halpha emission line. The analysis of X-shooter
spectroscopy covering 3 consecutive nights supports an M2/3-type spectral
classification. Neither radial velocity variations nor rotational broadening is
detected in the photospheric lines. No periodic variability is found in I- and
r'-band light curves. We derive r' = 20.8, I = 19.2 and Ks = 16.6 for the
optical and infrared counterparts with the M-type star contributing 90% to the
I-band light. We estimate its distance to be 1.3-1.8 kpc. The lack of radial
velocity variations implies that the M-type star is not the donor star in the
X-ray binary. This could be an interloper or the outer body in a hierarchical
triple. We constrain the accreting binary to be a < 2.2 hr orbital period
eclipsing cataclysmic variable or a low-mass X-ray binary lying in the
foreground of the Galactic Bulge.Comment: (9 pages, 5 figures, accepted for publication in MNRAS
The Tully-Fisher Relation and H_not
The use of the Tully-Fisher (TF) relation for the determination of the Hubble
Constant relies on the availability of an adequate template TF relation and of
reliable primary distances. Here we use a TF template relation with the best
available kinematical zero-point, obtained from a sample of 24 clusters of
galaxies extending to cz ~ 9,000 km/s, and the most recent set of Cepheid
distances for galaxies fit for TF use. The combination of these two ingredients
yields H_not = 69+/-5 km/(s Mpc). The approach is significantly more accurate
than the more common application with single cluster (e.g. Virgo, Coma)
samples.Comment: 10 pages, including 2 figures and 1 table; uses AAS LaTex. Submitted
to ApJ Letter
Compilation and validation of SAR and optical data products for a complete and global map of inland/ocean water tailored to the climate modeling community
Accurate maps of surface water extent are of paramount importance for water management, satellite data processing and climate modeling. Several maps of water bodies based on remote sensing data have been released during the last decade. Nonetheless, none has a truly (90°N/90°S) global coverage while being thoroughly validated. This paper describes a global, spatially-complete (void-free) and accurate mask of inland/ocean water for the 2000–2012 period, built in the framework of the European Space Agency (ESA) Climate Change Initiative (CCI). This map results from the synergistic combination of multiple individual SAR and optical water body and auxiliary datasets. A key aspect of this work is the original and rigorous stratified random sampling designed for the quality assessment of binary classifications where one class is marginally distributed. Input and consolidated products were assessed qualitatively and quantitatively against a reference validation database of 2110 samples spread throughout the globe. Using all samples, overall accuracy was always very high among all products, between 98% and 100%. The CCI global map of open water bodies provided the best water class representation (F-score of 89%) compared to its constitutive inputs. When focusing on the challenging areas for water bodies’ mapping, such as shorelines, lakes and river banks, all products yielded substantially lower accuracy figures with overall accuracies ranging between 74% and 89%. The inland water area of the CCI global map of open water bodies was estimated to be 3.17 million km2 ± 0.24 million km2. The dataset is freely available through the ESA CCI Land Cover viewer
The Chandra Galactic Bulge Survey: optical catalogue and point-source counterparts to X-ray sources
As part of the Chandra Galactic Bulge Survey (GBS), we present a catalogue of
optical sources in the GBS footprint. This consists of two regions centered at
Galactic latitude b = 1.5 degrees above and below the Galactic Centre, spanning
(l x b) = (6x1) degrees. The catalogue consists of 2 or more epochs of
observations for each line of sight in r', i' and H{\alpha} filters. It is
complete down to r' = 20.2 and i' = 19.2 mag; the mean 5{\sigma} depth is r' =
22.5 and i' = 21.1 mag. The mean root-mean-square residuals of the astrometric
solutions is 0.04 arcsec. We cross-correlate this optical catalogue with the
1640 unique X-ray sources detected in Chandra observations of the GBS area, and
find candidate optical counterparts to 1480 X-ray sources. We use a false alarm
probability analysis to estimate the contamination by interlopers, and expect ~
10 per cent of optical counterparts to be chance alignments. To determine the
most likely counterpart for each X-ray source, we compute the likelihood ratio
for all optical sources within the 4{\sigma} X-ray error circle. This analysis
yields 1480 potential counterparts (~ 90 per cent of the sample). 584
counterparts have saturated photometry (r'<17, i'<16), indicating these objects
are likely foreground sources and the real counterparts. 171 candidate
counterparts are detected only in the i'-band. These sources are good qLMXB and
CV candidates as they are X-ray bright and likely located in the Bulge.Comment: 18 pages, 18 figures. Published in MNRAS. 2016MNRAS.458.4530
Spectroscopic classification of X-ray sources in the Galactic Bulge Survey
We present the classification of 26 optical counterparts to X-ray sources
discovered in the Galactic Bulge Survey. We use (time-resolved) photometric and
spectroscopic observations to classify the X-ray sources based on their
multi-wavelength properties. We find a variety of source classes, spanning
different phases of stellar/binary evolution. We classify CX21 as a quiescent
cataclysmic variable (CV) below the period gap, and CX118 as a high accretion
rate (nova-like) CV. CXB12 displays excess UV emission, and could contain a
compact object with a giant star companion, making it a candidate symbiotic
binary or quiescent low mass X-ray binary (although other scenarios cannot be
ruled out). CXB34 is a magnetic CV (polar) that shows photometric evidence for
a change in accretion state. The magnetic classification is based on the
detection of X-ray pulsations with a period of 81 2 min. CXB42 is
identified as a young stellar object, namely a weak-lined T Tauri star
exhibiting (to date unexplained) UX Ori-like photometric variability. The
optical spectrum of CXB43 contains two (resolved) unidentified double-peaked
emission lines. No known scenario, such as an AGN or symbiotic binary, can
easily explain its characteristics. We additionally classify 20 objects as
likely active stars based on optical spectroscopy, their X-ray to optical flux
ratios and photometric variability. In 4 cases we identify the sources as
binary stars.Comment: Accepted for publication in MNRA
H-alpha Imaging of Early-type (Sa-Sab) Spiral Galaxies II. Global Properties
New results, based on one of the most comprehensive H-alpha imaging surveys
of nearby Sa-Sab spirals completed to date, reveals early-type spirals to be a
diverse group of galaxies that span a wide range in massive star formation
rates. While the majority of Sa-Sab galaxies in our sample are forming stars at
a modest rate, a significant fraction (~29%) exhibit star formation rates
greater than 1 M(solar/yr), rivaling the most prolifically star forming
late-type spirals. A similar diversity is apparent in the star formation
history of Sa-Sab spirals as measured by their H-alpha equivalent widths.
Consistent with our preliminary results presented in the first paper in this
series, we find giant HII regions (L(H-alpha)>10^{39}erg/s) in the disks of 37%
of early-type spirals. We suspect that recent minor mergers or past
interactions are responsible for the elevated levels of H-alpha emission and
perhaps, for the presence of giant HII regions in these galaxies.Comment: 42 pages and 17 figures (6 in jpg format, available upon request from
the authors as postscript); to appear in the June issue of A
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