[Show abstract][Hide abstract] ABSTRACT: We investigate the effects of magnetic fields and turbulence on ram pressure
stripping in elliptical galaxies using ideal magnetohydrodynamics simulations.
We consider weakly-magnetised interstellar medium (ISM) characterised by
subsonic turbulence, and two orientations of the magnetic fields in the
intracluster medium (ICM) - parallel and perpendicular to the direction of the
galaxy motion through the ICM. While the stronger turbulence enhances the ram
pressure stripping mass loss, the magnetic fields tend to suppress the
stripping rates, and the suppression is stronger for parallel fields. However,
the effect of magnetic fields on the mass stripping rate is mild. Nevertheless,
the morphology of the stripping tails depends significantly on the direction of
the ICM magnetic field. The effect of the magnetic field geometry on the tail
morphology is much stronger than that of the level of the ISM turbulence. The
tail has a highly collimated shape for parallel fields, while it has a
sheet-like morphology in the plane of the ICM magnetic field for perpendicular
fields. The magnetic field in the tail is amplified irrespectively of the
orientation of the ICM field. More strongly magnetised regions in the ram
pressure stripping tails are expected to have systematically higher metallicity
due to the strong concentration of the stripped ISM than the less magnetised
regions. Strong dependence of the morphology of the stripped ISM on the
magnetic field could potentially be used to constrain the relative orientation
of the ram pressure direction and the dominant component of the ICM magnetic
Preview · Article · Sep 2014 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: The EPOCH (EROS-2 periodic variable star classification using machine
learning) project aims to detect periodic variable stars in the EROS-2 light
curve database. In this paper, we present the first result of the
classification of periodic variable stars in the EROS-2 LMC database. In order
to classify these variables, we first build a training set by compiling known
variables in the Large Magellanic Could area from the OGLE and MACHO surveys.
We crossmatch these variables with the EROS-2 sources and extract 22
variability features from 28,392 light curves of the corresponding EROS-2
sources. We then use Random Forests to classify the EROS-2 sources in the
training set. We design the model to separate not only $\delta$ Scuti stars, RR
Lyraes, Cepheids, eclipsing binaries and long-period variables, the
"superclasses", but also their subclasses, such as RRab, RRc, RRd and RRe for
RR Lyraes, and similarly for the other variable types. The model trained using
only the superclasses shows 99% recall and precision while the model trained on
all subclasses shows 87% recall and precision. We apply the trained model to
the entire EROS-2 LMC database containing about 29 million sources and find
117,234 periodic variable candidates. Out of these 117,234 periodic variables,
55,285 have not been discovered by either OGLE or MACHO variability studies.
This set comprises 1906 $\delta$ Scuti stars, 6,607 RR Lyraes, 638 Cepheids,
178 Type II Cepheids, 34,562 eclipsing binaries and 11,394 long-period
variables. A catalog of these EROS-2 LMC periodic variable stars will be
available online at http://stardb.yonsei.ac.kr and at the CDS website
Full-text · Article · Mar 2014 · Astronomy and Astrophysics
[Show abstract][Hide abstract] ABSTRACT: Elliptical galaxies contain X-ray emitting gas that is subject to continuous
ram pressure stripping over timescales comparable to cluster ages. The gas in
these galaxies is not in perfect hydrostatic equilibrium. Supernova feedback,
stellar winds, or active galactic nuclei (AGN) feedback can significantly
perturb the interstellar medium (ISM). Using hydrodynamical simulations, we
investigate the effect of subsonic turbulence in the hot ISM on the ram
pressure stripping process in early-type galaxies. We find that galaxies with
more turbulent ISM produce longer, wider, and more smoothly distributed tails
of the stripped ISM than those characterised by weaker ISM turbulence. Our main
conclusion is that even very weak internal turbulence, at the level of <15% of
the average ISM sound speed, can significantly accelerate the gas removal from
galaxies via ram pressure stripping. The magnitude of this effect increases
sharply with the strength of turbulence. As most of the gas stripping takes
place near the boundary between the ISM and the intraclustermedium (ICM), the
boost in the ISM stripping rate is due to the "random walk" of the ISM from the
central regions of the galactic potential well to larger distances, where the
ram pressure is able to permanently remove the gas from galaxies. The ICM can
be temporarily trapped inside the galactic potential well due to the mixing of
the turbulent ISM with the ICM. The galaxies with more turbulent ISM, yet still
characterised by very weak turbulence, can hold larger amounts of the ICM.
Preview · Article · Jun 2012 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We report the discovery of a pair of quasars at z = 1.487, with a separation of 8585 ± 0002. Subaru Telescope infrared imaging reveals the presence of an elliptical and a disk-like galaxy located almost symmetrically between the quasars, in a cross-like configuration. Based on absorption lines in the quasar spectra and the colors of the galaxies, we estimate that both galaxies are located at redshift z = 0.899. This, as well as the similarity of the quasar spectra, suggests that the system is a single quasar multiply imaged by a galaxy group or cluster acting as a gravitational lens, although the possibility of a binary quasar cannot be fully excluded. We show that the gravitational lensing hypothesis implies that these galaxies are not isolated, but must be embedded in a dark matter halo of virial mass ~4 × 1014h
☉ assuming a Navarro-Frenk-White model with a concentration parameter of c
vir = 6, or a singular isothermal sphere profile with a velocity dispersion of ~670 km s–1. We place constraints on the location of the dark matter halo, as well as the velocity dispersions of the galaxies. In addition, we discuss the influence of differential reddening, microlensing, and intrinsic variability on the quasar spectra and broadband photometry.
Full-text · Article · Jun 2012 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: We present a statistical analysis of the final lens sample from the Sloan Digital Sky Survey Quasar Lens Search (SQLS). The number distribution of a complete subsample of 19 lensed quasars selected from 50,836 source quasars is compared with theoretical expectations, with particular attention given to the selection function. Assuming that the velocity function of galaxies does not evolve with redshift, the SQLS sample constrains the cosmological constant to ΩΛ = 0.79+0.06 –0.07(stat.)+0.06 –0.06(syst.) for a flat universe. The dark energy equation of state is found to be consistent with w = –1 when the SQLS is combined with constraints from baryon acoustic oscillation (BAO) measurements or results from the Wilkinson Microwave Anisotropy Probe (WMAP). We also obtain simultaneous constraints on cosmological parameters and redshift evolution of the galaxy velocity function, finding no evidence for redshift evolution at z 1 in any combinations of constraints. For instance, number density evolution quantified as ν n ≡ dln */dln (1 + z) and the velocity dispersion evolution νσ ≡ dln σ*/dln (1 + z) are constrained to ν n = 1.06+1.36 –1.39(stat.)+0.33 –0.64(syst.) and νσ = –0.05+0.19 –0.16(stat.)+0.03 –0.03(syst.), respectively, when the SQLS result is combined with BAO and WMAP for flat models with a cosmological constant. We find that a significant amount of dark energy is preferred even after fully marginalizing over the galaxy evolution parameters. Thus, the statistics of lensed quasars robustly confirm the accelerated cosmic expansion.
Preview · Article · Apr 2012 · The Astronomical Journal
[Show abstract][Hide abstract] ABSTRACT: We present the final statistical sample of lensed quasars from the Sloan Digital Sky Survey (SDSS) Quasar Lens Search (SQLS). The well-defined statistical lens sample consists of 26 lensed quasars brighter than i = 19.1 and in the redshift range of 0.6 < z < 2.2 selected from 50,826 spectroscopically confirmed quasars in the SDSS Data Release 7 (DR7), where we restrict the image separation range to 1'' < θ < 20'' and the i-band magnitude differences in two images to be smaller than 1.25 mag. The SDSS DR7 quasar catalog also contains 36 additional lenses identified with various techniques. In addition to these lensed quasars, we have identified 81 pairs of quasars from follow-up spectroscopy, 26 of which are physically associated binary quasars. The statistical lens sample covers a wide range of image separations, redshifts, and magnitudes, and therefore is suitable for systematic studies of cosmological parameters and surveys of the structure and evolution of galaxies and quasars.
Preview · Article · Apr 2012 · The Astronomical Journal
[Show abstract][Hide abstract] ABSTRACT: We present variability analysis of data from the Northern Sky Variability
Survey (NSVS). Using the clustering method which defines variable candidates as
outliers from large clusters, we cluster 16,189,040 light curves, having data
points at more than 15 epochs, as variable and non-variable candidates in 638
NSVS fields. Variable candidates are selected depending on how strongly they
are separated from the largest cluster and how rarely they are grouped together
in eight dimensional space spanned by variability indices. All NSVS light
curves are also cross-correlated to the Infrared Astronomical Satellite, AKARI,
Two Micron All Sky Survey, Sloan Digital Sky Survey (SDSS), and Galaxy
Evolution Explorer objects as well as known objects in the SIMBAD database. The
variability analysis and cross-correlation results are provided in a public
online database which can be used to select interesting objects for further
investigation. Adopting conservative selection criteria for variable
candidates, we find about 1.8 million light curves as possible variable
candidates in the NSVS data, corresponding to about 10% of our entire NSVS
samples. Multi-wavelength colors help us find specific types of variability
among the variable candidates. Moreover, we also use morphological
classification from other surveys such as SDSS to suppress spurious cases
caused by blending objects or extended sources due to the low angular
resolution of the NSVS.
Full-text · Article · Nov 2011 · The Astronomical Journal
[Show abstract][Hide abstract] ABSTRACT: Gravitational lens systems containing lensed quasars are important as
cosmological probes, as diagnostics of structural properties of the lensing
galaxies and as tools to study the quasars themselves. The largest lensed
quasar sample is the SDSS Quasar Lens Search, drawn from the Sloan Digital Sky
Survey (SDSS). We are attempting to extend this survey using observations of
lens candidates selected from a combination of the quasar sample from the SDSS
and the UKIRT Infrared Deep Sky Survey (UKIDSS). This adds somewhat higher
image quality together with a wider range of wavelength for the selection
process. In previous pilot surveys we observed 5 objects, finding 2 lenses;
here we present further observations of 20 objects in which we find 4 lenses,
of which 2 are independently discovered in SQLS (in preparation). Following
earlier work on the combination of these two surveys, we have refined our
method and find that use of a colour-separation diagnostic, where we select for
separations between components which appear to decrease in wavelength, is an
efficient method to find lensed quasars and may be useful in ongoing and future
large-scale strong lensing surveys with instruments such as Pan-STARRS and
LSST. The new lenses have mostly high flux ratios, with faint secondaries
buried in the lensing galaxy and typically 6-10 times less bright than the
primary. Our survey brings the total number of lenses discovered in the SDSS
quasar sample to 46, plus 13 lenses already known. This is likely to be up to
60-70% of the total number of lensed quasars; we briefly discuss strategies by
which the rest might be found.
Preview · Article · Sep 2011 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We investigate radio-mode AGN activity among post-starburst galaxies from the Sloan Digital Sky Survey (SDSS) to determine
whether AGN feedback may be responsible for the cessation of star formation. Based on radio morphology and radio loudness
from the FIRST and NVSS data, we separate objects with radio activity due to an AGN from ongoing residual star formation.
Of 513 SDSS galaxies with strong A-star spectra, 12 objects have 21-cm flux density above 1 mJy. These galaxies do not show
optical AGN emission lines. Considering that the lifetime of radio emission is much shorter than the typical time-scale of
the spectroscopic features of post-starburst galaxies, we conclude that the radio-emitting AGN activity in these objects was
triggered after the end of the recent starburst, and thus cannot be an important feedback process to explain the post-starburst
phase. The radio luminosities show a positive correlation with total galaxy stellar mass, but not with the mass of recently
formed stars. Thus the mechanical power of AGN feedback derived from the radio luminosity is related to old stellar populations
dominating the stellar mass, which in turn are related to the masses of central supermassive black holes.
Preview · Article · Oct 2010 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We present low-resolution, rest-frame ~5-12 μm Spitzer/IRS spectra of two lensed z ~ 2 UV-bright star-forming galaxies, SDSS J120602.09+514229.5 and SDSS J090122.37+181432.3. Using the magnification boost from lensing, we are able to study the physical properties of these objects in greater detail than is possible for unlensed systems. In both targets, we detect strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2, 7.7, and 11.3 μm, indicating the presence of vigorous star formation. For J1206, we find a steeply rising continuum and significant [S IV] emission, suggesting that a moderately hard radiation field is powering continuum emission from small dust grains. The strength of the [S IV] emission also implies a sub-solar metallicity of ~0.5 Z
☉, confirming published rest-frame optical measurements. In J0901, the PAH lines have large rest-frame equivalent widths (>1 μm) and the continuum rises slowly with wavelength, suggesting that any active galactic nucleus (AGN) contribution to L
IR is insignificant, in contrast to the implications of optical emission-line diagnostics. Using [O III] line flux as a proxy for AGN strength, we estimate that the AGN in J0901 provides only a small fraction of its mid-infrared continuum flux. By combining the detection of [Ar II] with an upper limit on [Ar III] emission, we infer a metallicity of 1.3 Z
☉. This work highlights the importance of combining rest-frame optical and mid-IR spectroscopy in order to understand the detailed properties of star-forming galaxies at high redshift.
Full-text · Article · Sep 2010 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: We investigate radio-mode AGN activity among post-starburst galaxies from the Sloan Digital Sky Survey to determine whether AGN feedback may be responsible for the cessation of star formation. Based on radio morphology and radio-loudness from the FIRST and NVSS data, we separate objects with radio activity due to an AGN from ongoing residual star formation. Of 513 SDSS galaxies with strong A-star spectra, 12 objects have 21-cm flux density above 1 mJy. These galaxies do not show optical AGN emission lines. Considering that the lifetime of radio emission is much shorter than the typical time-scale of the spectroscopic features of post-starburst galaxies, we conclude that the radio-emitting AGN activity in these objects was triggered after the end of the recent starburst, and thus cannot be an important feedback process to explain the post-starburst phase. The radio luminosities show a positive correlation with total galaxy stellar mass, but not with the mass of recently formed stars. Thus the mechanical power of AGN feedback derived from the radio luminosity is related to old stellar populations dominating the stellar mass, which in turn are related to the masses of central supermassive black holes. Comment: 11 pages, 6 figures, accepted for publication in MNRAS
[Show abstract][Hide abstract] ABSTRACT: We present the second report of our systematic search for strongly lensed quasars from the data of the Sloan Digital Sky Survey (SDSS). From extensive follow-up observations of 136 candidate objects, we find 36 lenses in the full sample of 77,429 spectroscopically confirmed quasars in the SDSS Data Release 5. We then define a complete sample of 19 lenses, including 11 from our previous search in the SDSS Data Release 3, from the sample of 36,287 quasars with i < 19.1 in the redshift range 0.6 < z < 2.2, where we require the lenses to have image separations of 1'' < θ < 20'' and i-band magnitude differences between the two images smaller than 1.25 mag. Among the 19 lensed quasars, three have quadruple-image configurations, while the remaining 16 show double images. This lens sample constrains the cosmological constant to be ΩΛ = 0.84+0.06 –0.08(stat.)+0.09 –0.07(syst.) assuming a flat universe, which is in good agreement with other cosmological observations. We also report the discoveries of seven binary quasars with separations ranging from 11 to 166, which are identified in the course of our lens survey. This study concludes the construction of our statistical lens sample in the full SDSS-I data set.
Full-text · Article · Jun 2010 · The Astronomical Journal
[Show abstract][Hide abstract] ABSTRACT: We present a new framework to detect various types of variable objects within massive astronomical time series data. Assuming
that the dominant population of objects is non-variable, we find outliers from this population by using a non-parametric Bayesian
clustering algorithm based on an infinite Gaussian mixture model (GMM) and the Dirichlet process. The algorithm extracts information
from a given data set, which is described by six variability indices. The GMM uses those variability indices to recover clusters
that are described by six-dimensional multivariate Gaussian distributions, allowing our approach to consider the sampling
pattern of time series data, systematic biases, the number of data points for each light curve and photometric quality. Using
the Northern Sky Variability Survey data, we test our approach and prove that the infinite GMM is useful at detecting variable
objects, while providing statistical inference estimation that suppresses false detection. The proposed approach will be effective
in the exploration of future surveys such as Gaia, Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) and Large
Synoptic Survey Telescope (LSST), which will produce massive time series data.
Preview · Article · Aug 2009 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We report the discovery of five gravitationally lensed quasars from the Sloan Digital Sky Survey (SDSS). All five systems are selected as two-image lensed quasar candidates from a sample of high-redshift (z > 2.2) SDSS quasars. We confirmed their lensing nature with additional imaging and spectroscopic observations. The new systems are SDSS J0819+5356 (source redshift zs = 2.237, lens redshift zl = 0.294, and image separation θ = 404), SDSS J1254+2235 (zs = 3.626, θ = 156), SDSS J1258+1657 (zs = 2.702, θ = 128), SDSS J1339+1310 (zs = 2.243, θ = 169), and SDSS J1400+3134 (zs = 3.317, θ = 174). We estimate the lens redshifts of the latter four systems to be zl = 0.2-0.8 from the colors and magnitudes of the lensing galaxies. We find that the image configurations of all systems are well reproduced by standard mass models. Although these lenses will not be included in our statistical sample of zs < 2.2 lenses, they expand the number of lensed quasars which can be used for high-redshift galaxy and quasar studies.
Preview · Article · Mar 2009 · The Astronomical Journal
[Show abstract][Hide abstract] ABSTRACT: We report the discovery of a cluster-scale lensed quasar, SDSS J1029+2623, selected from the Sloan Digital Sky Survey. The lens system exhibits two lensed images of a quasar at zs = 2.197. The image separation of 225 makes it the largest separation lensed quasar discovered to date. The similarity of the optical spectra and the radio loudnesses of the two components support the lensing hypothesis. Images of the field show a cluster of galaxies at zl ~ 0.55 that is responsible for the large image separation. The lensed images and the cluster light center are not collinear, which implies that the lensing cluster has a complex structure.
Preview · Article · Dec 2008 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: We present the discovery of a new quadruply lensed quasar. The lens system, SDSS J1330+1810 at zs= 1.393, was identified as a lens candidate from the spectroscopic sample of the Sloan Digital Sky Survey. Optical and near-infrared
images clearly show four quasar images with a maximum image separation of 1.76 arcsec, as well as a bright lensing galaxy.
We measure a redshift of the lensing galaxy of zl= 0.373 from absorption features in the spectrum. We find a foreground group of galaxies at z= 0.31, centred at ∼120 arcsec south-west of the lens system. Simple mass models fit the data quite well, including the flux
ratios between images, although the lens galaxy appears to be ∼1 mag brighter than expected by the Faber–Jackson relation.
Our mass modelling suggests that shear from nearby structure is affecting the lens potential.
Preview · Article · Sep 2008 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We present the first result of a survey for strong galaxy-galaxy lenses in Sloan Digital Sky Survey (SDSS) images. SDSS J082728.70+223256.4 was selected as a lensing candidate using selection criteria based on the color and positions of objects in the SDSS photometric catalog. Follow-up imaging and spectroscopy showed this object to be a lensing system. The lensing galaxy is elliptical at z = 0.349 in a galaxy cluster. The lensed galaxy has the spectrum of a post-starburst galaxy at z = 0.766. The lensing galaxy has an estimated mass of ~1.2 × 1012M☉, and the corresponding mass-to-light ratio in the B-band is ~26M
☉ inside 1.1 effective radii of the lensing galaxy. Our study shows how catalogs drawn from multi-band surveys can be used to find strong galaxy-galaxy lenses having multiple lens images. Our strong lensing candidate selection based on photometry-only catalogs will be useful in future multi-band imaging surveys such as SNAP and LSST.
Full-text · Article · May 2008 · The Astronomical Journal
[Show abstract][Hide abstract] ABSTRACT: Observations of galaxies at z=2-3 probe the epoch when star formation in galaxies peaked and the morphologies of galaxies were being set. UV-selected populations like the Lyman break galaxies (LBGs) have been extensively studied at short wavelengths, but in most cases are too faint to permit observations (particularly spectroscopy) in the mid-infrared. We propose new Spitzer observations of five recently discovered LBG-like objects whose apparent fluxes have been strongly boosted by gravitational lensing. By combining IRAC, MIPS, and (irreplaceable) IRS data with our extensive ancillary datasets, we will constrain our targets' star formation histories and stellar masses, bolometric dust luminosities, starburst/AGN bolometric energetics, and degree of similarity with various low-redshift templates. The proposed observations would more than double the number of lensed UV-selected galaxies at high redshift that Spitzer will have observed by the end of its cryogenic mission.
[Show abstract][Hide abstract] ABSTRACT: We present the discovery of four gravitationally lensed quasars selected from the spectroscopic quasar catalog of the Sloan Digital Sky Survey (SDSS). We describe imaging and spectroscopic follow-up observations that support the lensing interpretation of the following four quasars: SDSS J0832+0404 (image separation θ = 198, source redshift zs = 1.115, lens redshift zl = 0.659); SDSS J1216+3529 (θ = 149, zs = 2.012); SDSS J1322+1052 (θ = 200, zs = 1.716); and SDSS J1524+4409 (θ = 167, zs = 1.210, zl = 0.320). Each system has two lensed images. We find that the fainter image component of SDSS J0832+0404 is significantly redder than the brighter component, perhaps because of differential reddening by the lensing galaxy. The lens potential of SDSS J1216+3529 might be complicated by the presence of a secondary galaxy near the main lensing galaxy.
Preview · Article · Jan 2008 · The Astronomical Journal
[Show abstract][Hide abstract] ABSTRACT: The 28 cataclysmic variables (CVs) found in 2005 in the Sloan Digital Sky Survey are presented with their coordinates, magnitudes, and spectra. Five of these systems are previously known CVs (HH Cnc, SX LMi, QZ Ser, AP CrB, and HS 1016+3412), and the rest are new discoveries. Additional spectroscopic, photometric, and/or polarimetric observations of 10 systems were carried out, resulting in estimates of the orbital periods for seven of the new binaries. The 23 new CVs include one eclipsing system, one new Polar, and five systems whose spectra clearly reveal atmospheric absorption lines from the underlying white dwarf.
Preview · Article · Dec 2007 · The Astronomical Journal