155 research outputs found
Time-series spectroscopy of the rapidly oscillating Ap star HR 3831
We present time-series spectroscopy of the rapidly oscillating Ap star HR
3831. This star has a dominant pulsation period of 11.7 minutes and a rotation
period of 2.85 days. We have analysed 1400 intermediate-resolution spectra of
the wavelength region 6100--7100 AA obtained over one week, using techniques
similar to those we applied to another roAp star, Alpha Cir.
We confirm that the H-alpha velocity amplitude of HR 3831 is modulated with
rotation phase. Such a modulation was predicted by the oblique pulsator model,
and rules out the spotted pulsator model. However, further analysis of H-alpha
and other lines reveal rotational modulations that cannot easily be explained
using the oblique pulsator model. In particular, the phase of the pulsation as
measured by the width of the H-alpha line varies with height in the line.
The variation of the H-alpha bisector shows a very similar pattern to that
observed in Alpha Cir, which we have previously attributed to a radial node in
the stellar atmosphere. However, the striking similarities between the two
stars despite the much shorter period of Alpha Cir (6.8 min) argues against
this interpretation unless the structure of the atmosphere is somewhat
different between the two stars. Alternatively, the bisector variation is a
signature of the degree l of the mode and not the overtone value n.
High-resolution studies of the metal lines in roAp stars are needed to
understand fully the form of the pulsation in the atmosphere.Comment: 13 pages, 20 figures, accepted by MNRA
Environment from cross-correlations: connecting hot gas and the quenching of galaxies
The observable properties of galaxies depend on both internal processes and
the external environment. In terms of the environmental role, we still do not
have a clear picture of the processes driving the transformation of galaxies.
The use of proxies for environment (e.g., host halo mass, distance to the N^th
nearest neighbour, etc.), as opposed to the real physical conditions (e.g., hot
gas density) may bear some responsibility for this. Here we propose a new
method that directly links galaxies to their local environments, by using
spatial cross-correlations of galaxy catalogues with maps from large-scale
structure surveys (e.g., thermal Sunyaev-Zel'dovich [tSZ] effect, diffuse X-ray
emission, weak lensing of galaxies or the CMB). We focus here on the quenching
of galaxies and its link to local hot gas properties. Maps of galaxy
overdensity and quenched fraction excess are constructed from volume-limited
SDSS catalogs, which are cross-correlated with tSZ effect and X-ray maps from
Planck and ROSAT, respectively. Strong signals out to Mpc scales are detected
for most cross-correlations and are compared to predictions from the EAGLE and
BAHAMAS cosmological hydrodynamical simulations. The simulations successfully
reproduce many, but not all, of the observed power spectra, with an indication
that environmental quenching may be too efficient in the simulations. We
demonstrate that the cross-correlations are sensitive to both the internal
(e.g., AGN and stellar feedback) and external processes (e.g., ram pressure
stripping, harassment, strangulation, etc.) responsible for quenching. The
methods outlined in this paper can be adapted to other observables and, with
upcoming surveys, will provide a stringent test of physical models for
environmental transformation.Comment: 23 pages, 11 figures, MNRAS, in pres
Red riding on hood: Exploring how galaxy colour depends on environment
Galaxy populations are known to exhibit a strong colour bimodality,
corresponding to blue star-forming and red quiescent subpopulations. The
relative abundance of the two populations has been found to vary with stellar
mass and environment. In this paper, we explore the effect of environment
considering different types of measurements. We choose a sample of
galaxies with from the Galaxy And Mass Assembly survey. We
study the dependence of the fraction of red galaxies on different measures of
the local environment as well as the large-scale "geometric" environment
defined by density gradients in the surround- ing cosmic web. We find that the
red galaxy fraction varies with the environment at fixed stellar mass. The red
fraction depends more strongly on local environmental measures than on
large-scale geometric environment measures. By comparing the different
environmental densities, we show that no density measurement fully explains the
observed environmental red fraction variation, suggesting the different
measures of environmental density contain different information. We test
whether the local environmental measures, when combined together, can explain
all the observed environmental red fraction variation. The geometric
environment has a small residual effect, and this effect is larger for voids
than any other type of geometric environment. This could provide a test of the
physics applied to cosmological-scale galaxy evolution simulations as it
combines large-scale effects with local environmental impact.Comment: Accepted for publication in MNRAS; 16 pages; 10 figures; 2 tables
The local star-formation rate density: assessing calibrations using [OII], Ha and UV luminosities
We explore the use of simple star-formation rate (SFR) indicators (such as
may be used in high-redshift galaxy surveys) in the local Universe using [OII],
Ha, and u-band luminosities from the deeper 275 deg^2 Stripe 82 subsample of
the Sloan Digital Sky Survey (SDSS) coupled with UV data from the Galaxy
Evolution EXplorer satellite (GALEX). We examine the consistency of such
methods using the star-formation rate density (SFRD) as a function of stellar
mass in this local volume, and quantify the accuracy of corrections for dust
and metallicity on the various indicators. Rest-frame u-band promises to be a
particularly good SFR estimator for high redshift studies since it does not
require a particularly large or sensitive extinction correction, yet yields
results broadly consistent with more observationally expensive methods. We
suggest that the [OII]-derived SFR, commonly used at higher redshifts (z~1),
can be used to reliably estimate SFRs for ensembles of galaxies, but for high
mass galaxies (log(M*/Msun)>10), a larger correction than is typically used is
required to compensate for the effects of metallicity dependence and dust
extinction. We provide a new empirical mass-dependent correction for the
[OII]-SFR.Comment: 22 pages, 16 figures. This version corrects typos in equations 2, 7,
and 9 of the published version, as described in the MNRAS Erratum. Published
results are unaffected. A simple piece of IDL Code for applying the
mass-dependent correction to [OII] SFR available from
http://astro.uwaterloo.ca/~dgilbank/data/corroii.pr
An Empirical Calibration of the Completeness of the SDSS Quasar Survey
Spectra of nearly 20000 point-like objects to a Galactic reddening corrected
magnitude of i=19.1 have been obtained to test the completeness of the SDSS
quasar survey. The spatially-unresolved objects were selected from all regions
of color space, sparsely sampled from within a 278 sq. deg. area of sky covered
by this study. Only ten quasars were identified that were not targeted as
candidates by the SDSS quasar survey (including both color and radio source
selection). The inferred density of unresolved quasars on the sky that are
missed by the SDSS algorithm is 0.44 per sq. deg, compared to 8.28 per sq. deg.
for the selected quasar density, giving a completeness of 94.9(+2.6,-3.8) to
the limiting magnitude. Omitting radio selection reduces the color-only
selection completeness by about 1%. Of the ten newly identified quasars, three
have detected broad absorption line systems, six are significantly redder than
other quasars at the same redshift, and four have redshifts between 2.7 and 3.0
(the redshift range where the SDSS colors of quasars intersect the stellar
locus). The fraction of quasars missed due to image defects and blends is
approximately 4%, but this number varies by a few percent with magnitude.
Quasars with extended images comprise about 6% of the SDSS sample, and the
completeness of the selection algorithm for extended quasars is approximately
81%, based on the SDSS galaxy survey. The combined end-to-end completeness for
the SDSS quasar survey is approximately 89%. The total corrected density of
quasars on the sky to i=19.1 is estimated to be 10.2 per sq. deg.Comment: 37 pages, 10 figures, accepted for publication in A
- …