1,380 research outputs found
Binary Black Hole Mergers from Planet-like Migrations
If supermassive black holes (BHs) are generically present in galaxy centers,
and if galaxies are built up through hierarchical merging, BH binaries are at
least temporary features of most galactic bulges. Observations suggest,
however, that binary BHs are rare, pointing towards a binary lifetime far
shorter than the Hubble time. We show that, regardless of the detailed
mechanism, all stellar-dynamical processes are insufficient to reduce
significantly the orbital separation once orbital velocities in the binary
exceed the virial velocity of the system. We propose that a massive gas disk
surrounding a BH binary can effect its merger rapidly, in a scenario analogous
to the orbital decay of super-jovian planets due to a proto-planetary disk. As
in the case of planets, gas accretion onto the secondary (here a supermassive
BH) is integrally connected with its inward migration. Such accretion would
give rise to quasar activity. BH binary mergers could therefore be responsible
for many or most quasars.Comment: 8 pages, submitted to ApJ Letter
Large stellar disks in small elliptical galaxies
We present the rotation velocities V and velocity dispersions sigma along the
principal axes of seven elliptical galaxies less luminous than M_B= -19.5.
These kinematics extend beyond the half-light radii for all systems in this
photometrically selected sample. At large radii the kinematics not only confirm
that rotation and "diskiness" are important in faint ellipticals, as was
previously known, but also demonstrate that in most sample galaxies the stars
at large galactocentric distances have (V/sigma)_max of about 2, similar to the
disks in bona-fide S0 galaxies. Comparing this high degree of ordered stellar
motion in all sample galaxies with numerical simulations of dissipationless
mergers argues against mergers with mass ratios <=3:1 as an important mechanism
in the final shaping of low-luminosity ellipticals, and favors instead the
dissipative formation of a disk.Comment: 11 pages LaTex with 4 Postscript figure
Precise Ages of Field Stars from White Dwarf Companions
Observational tests of stellar and Galactic chemical evolution call for the
joint knowledge of a star's physical parameters, detailed element abundances,
and precise age. For cool main-sequence (MS) stars the abundances of many
elements can be measured from spectroscopy, but ages are very hard to
determine. The situation is different if the MS star has a white dwarf (WD)
companion and a known distance, as the age of such a binary system can then be
determined precisely from the photometric properties of the cooling WD. As a
pilot study for obtaining precise age determinations of field MS stars, we
identify nearly one hundred candidates for such wide binary systems: a faint WD
whose GPS1 proper motion matches that of a brighter MS star in Gaia/TGAS with a
good parallax (). We model the WD's multi-band
photometry with the BASE-9 code using this precise distance (assumed to be
common for the pair) and infer ages for each binary system. The resulting age
estimates are precise to () for () MS-WD systems.
Our analysis more than doubles the number of MS-WD systems with precise
distances known to date, and it boosts the number of such systems with precise
age determination by an order of magnitude. With the advent of the Gaia DR2
data, this approach will be applicable to a far larger sample, providing ages
for many MS stars (that can yield detailed abundances for over 20 elements),
especially in the age range 2 to 8\,\Gyr, where there are only few known star
clusters.Comment: 9 pages, 5 figures, 1 catalog; Submitted to Ap
A comprehensive Maximum Likelihood analysis of the structural properties of faint Milky Way satellites
We derive the structural parameters of the recently discovered very low
luminosity Milky Way satellites through a Maximum Likelihood algorithm applied
to SDSS data. For each satellite, even when only a few tens of stars are
available down to the SDSS flux limit, the algorithm yields robust estimates
and errors for the centroid, position angle, ellipticity, exponential
half-light radius and number of member stars. This latter parameter is then
used in conjunction with stellar population models of the satellites to derive
their absolute magnitudes and stellar masses, accounting for `CMD shot-noise'.
We find that faint systems are somewhat more elliptical than initially found
and ascribe that to the previous use of smoothed maps which can be dominated by
the smoothing kernel. As a result, the faintest half of the Milky Way dwarf
galaxies (M_V>-7.5) is significantly (4-sigma) flatter (e=0.47+/-0.03) than its
brightest half (M_V<-7.5, e=0.32+/-0.02). From our best models, we also
investigate whether the seemingly distorted shape of the satellites, often
taken to be a sign of tidal distortion, can be quantified. We find that, except
for tentative evidence of distortion in CVnI and UMaII, these can be completely
accounted for by Poisson scatter in the sparsely sampled systems. We consider
three scenarios that could explain the rather elongated shape of faint
satellites: rotation supported systems, stars following the shape of more
triaxial dark matter subhalos, or elongation due to tidal interaction with the
Milky Way. Although none of these is entirely satisfactory, the last one
appears the least problematic, but warrants much deeper observations to track
evidence of such tidal interaction.Comment: 20 pages, 11 figures, ApJ in press; some typos corrected, magnitude
of BooII corrected (thanks go to Shane Walsh for spotting the erroneous
original value
Mapping low-latitude stellar substructure with SEGUE photometry
Encircling the Milky Way at low latitudes, the Low Latitude Stream is a large
stellar structure, the origin of which is as yet unknown. As part of the SEGUE
survey, several photometric scans have been obtained that cross the Galactic
plane, spread over a longitude range of 50 to 203 degrees. These data allow a
systematic study of the structure of the Galaxy at low latitudes, where the Low
Latitude Stream resides. We apply colour-magnitude diagram fitting techniques
to map the stellar (sub)structure in these regions, enabling the detection of
overdensities with respect to smooth models. These detections can be used to
distinguish between different models of the Low Latitude Stream, and help to
shed light on the nature of the system.Comment: To appear in the proceedings of IAU Symposium 254 "The Galaxy disk in
a cosmological context", Copenhagen, June 200
Discovery of a Fifth Image of the Large Separation Gravitationally Lensed Quasar SDSS J1004+4112
We report the discovery of a fifth image in the large separation lensed
quasar system SDSS J1004+4112. A faint point source located 0.2'' from the
center of the brightest galaxy in the lensing cluster is detected in images
taken with the Advanced Camera for Surveys (ACS) and the Near Infrared Camera
and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope. The flux
ratio between the point source and the brightest lensed component in the ACS
image is similar to that in the NICMOS image. The location and brightness of
the point source are consistent with lens model predictions for a lensed image.
We therefore conclude that the point source is likely to be a fifth image of
the source quasar. In addition, the NICMOS image reveals the lensed host galaxy
of the source quasar, which can strongly constrain the structure of the lensing
critical curves and thereby the mass distribution of the lensing cluster.Comment: 5 pages, 5 figures, accepted for publication in PAS
Translating Cultural Safety to the UK
Disproportional morbidity and mortality experienced by ethnic minorities in the UK have been highlighted by the COVID-19 pandemic. The âBlack Lives Matterâ movement has exposed structural racismâs contribution to these health inequities. âCultural Safetyâ, an antiracist, decolonising and educational innovation originating in New Zealand, has been adopted in Australia. Cultural Safety aims to dismantle barriers faced by colonised Indigenous peoples in mainstream healthcare by addressing systemic racism.
This paper explores what it means to be âculturally safeâ. The ways in which New Zealand and Australia are incorporating Cultural Safety into educating healthcare professionals and in day-to-day practice in medicine are highlighted. We consider the ânuts and boltsâ of translating Cultural Safety into the UK to reduce racism within healthcare. Listening to the voices of black, Asian and minority ethnic National Health Service (NHS) consumers, education in reflexivity, both personal and organisational within the NHS are key. By listening to Indigenous colonised peoples, the ex-Empire may find solutions to health inequity. A decolonising feedback loop is required; however, we should take care not to culturally appropriate this valuable reverse innovation
Constraints on the Space Density of Methane Dwarfs and the Substellar Mass Function from a Deep Near-Infrared Survey
We report preliminary results of a deep near-infrared search for
methane-absorbing brown dwarfs; almost five years after the discovery of Gl
229b, there are only a few confirmed examples of this type of object. New J
band, wide-field images, combined with pre-existing R band observations, allow
efficient identification of candidates by their extreme (R-J) colours.
Follow-up measurements with custom filters can then confirm objects with
methane absorption. To date, we have surveyed a total of 11.4 square degrees to
J~20.5 and R~25. Follow-up CH_4 filter observations of promising candidates in
1/4 of these fields have turned up no methane absorbing brown dwarfs. With 90%
confidence, this implies that the space density of objects similar to Gl 229b
is less than 0.012 per cubic parsec. These calculations account for the
vertical structure of the Galaxy, which can be important for sensitive
measurements. Combining published theoretical atmospheric models with our
observations sets an upper limit of alpha <= 0.8 for the exponent of the
initial mass function power law in this domain.Comment: 11 pages + 2 figures To be published in Astrophysical Journal Letter
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