1,281 research outputs found
C IV BAL disappearance in a large SDSS QSO sample
Broad absorption lines (BALs) in the spectra of quasi-stellar objects (QSOs)
originate from outflowing winds along our line of sight; winds are thought to
originate from the inner regions of the QSO accretion disk, close to the
central supermassive black hole (SMBH). Winds likely play a role in galaxy
evolution and aid the accretion mechanism onto the SMBH. BAL equivalent widths
can change on typical timescales from months to years; such variability is
generally attributed to changes in the covering factor and/or in the ionization
level of the gas. We investigate BAL variability, focusing on BAL
disappearance. We analyze multi-epoch spectra of more than 1500 QSOs -the
largest sample ever used for such a study- observed by different programs from
the Sloan Digital Sky Survey-I/II/III (SDSS), and search for disappearing C IV
BALs. The spectra rest-frame time baseline ranges from 0.28 to 4.9 yr; the
source redshifts range from 1.68 to 4.27. We detect 73 disappearing BALs in the
spectra of 67 sources. This corresponds to 3.9% of disappearing BALs, and 5.1%
of our BAL QSOs exhibit at least one disappearing BAL. We estimate the average
lifetime of a BAL along our line of sight (~ 80-100 yr), which appears
consistent with the accretion disk orbital time at distances where winds are
thought to originate. We inspect properties of the disappearing BALs and
compare them to the properties of our main sample. We also investigate the
existence of a correlation in the variability of multiple troughs in the same
spectrum, and find it persistent at large velocity offsets between BAL pairs,
suggesting that a mechanism extending on a global scale is necessary to explain
the phenomenon. We select a more reliable sample of disappearing BALs following
Filiz Ak et al. (2012), where a subset of our sample was analyzed, and compare
the findings from the two works, obtaining generally consistent results.Comment: 22 pages, 9 figures. Accepted for publication in A&
From Starburst to Quiescence: Testing AGN feedback in Rapidly Quenching Post-Starburst Galaxies
Post-starbursts are galaxies in transition from the blue cloud to the red
sequence. Although they are rare today, integrated over time they may be an
important pathway to the red sequence. This work uses SDSS, GALEX, and WISE
observations to identify the evolutionary sequence from starbursts to fully
quenched post-starbursts in the narrow mass range , and identifies "transiting" post-starbursts which are intermediate
between these two populations. In this mass range, of galaxies are
starbursts, are quenched post-starbursts, and are the
transiting types in between. The transiting post-starbursts have stellar
properties that are predicted for fast-quenching starbursts and morphological
characteristics that are already typical of early-type galaxies. The AGN
fraction, as estimated from optical line ratios, of these post-starbursts is
about 3 times higher () than that of normal star-forming
galaxies of the same mass, but there is a significant delay between the
starburst phase and the peak of nuclear optical AGN activity (median age
difference of Myr), in agreement with previous studies.
The time delay is inferred by comparing the broad-band near NUV-to-optical
photometry with stellar population synthesis models. We also find that
starbursts and post-starbursts are significantly more dust-obscured than normal
star-forming galaxies in the same mass range. About of the starbursts
and of the transiting post-starbursts can be classified as the
"Dust-Obscured Galaxies" (DOGs), while only of normal galaxies are
DOGs.The time delay between the starburst phase and AGN activity suggests that
AGN do not play a primary role in the original quenching of starbursts but may
be responsible for quenching later low-level star formation during the
post-starburst phase.Comment: 30 pages, 18 figures,accepted to Ap
Linking black-hole growth with host galaxies: The accretion-stellar mass relation and its cosmic evolution
Previous studies suggest that the growth of supermassive black holes (SMBHs)
may be fundamentally related to host-galaxy stellar mass (). To
investigate this SMBH growth- relation in detail, we calculate
long-term SMBH accretion rate as a function of and redshift
[] over ranges of
and . Our
is constrained by high-quality survey data
(GOODS-South, GOODS-North, and COSMOS), and by the stellar mass function and
the X-ray luminosity function. At a given , is
higher at high redshift. This redshift dependence is stronger in more massive
systems (for , is
three decades higher at than at ), possibly due to AGN feedback.
Our results indicate that the ratio between and average
star formation rate () rises toward high at a
given redshift. This dependence on
does not support the scenario that SMBH and galaxy growth are in
lockstep. We calculate SMBH mass history [] based on our
and the from the literature, and
find that the - relation has weak redshift evolution since
. The ratio is higher toward massive galaxies:
it rises from at to at . Our predicted ratio
at high is similar to that observed in local giant ellipticals,
suggesting that SMBH growth from mergers is unlikely to dominate over growth
from accretion.Comment: 27 pages, 21 figures, 2 tables; MNRAS accepte
z~2: An Epoch of Disk Assembly
We explore the evolution of the internal gas kinematics of star-forming
galaxies from the peak of cosmic star-formation at to today.
Measurements of galaxy rotation velocity , which quantify ordered
motions, and gas velocity dispersion , which quantify disordered
motions, are adopted from the DEEP2 and SIGMA surveys. This sample covers a
continuous baseline in redshift from to , spanning 10 Gyrs. At
low redshift, nearly all sufficiently massive star-forming galaxies are
rotationally supported (). By , the percentage of
galaxies with rotational support has declined to 50 at low stellar mass
() and 70 at high stellar mass
(). For , the percentage
drops below 35 for all masses. From to now, galaxies exhibit
remarkably smooth kinematic evolution on average. All galaxies tend towards
rotational support with time, and it is reached earlier in higher mass systems.
This is mostly due to an average decline in by a factor of 3 since a
redshift of 2, which is independent of mass. Over the same time period,
increases by a factor of 1.5 for low mass systems, but does not
evolve for high mass systems. These trends in and with
time are at a fixed stellar mass and should not be interpreted as evolutionary
tracks for galaxy populations. When galaxy populations are linked in time with
abundance matching, not only does decline with time as before, but
strongly increases with time for all galaxy masses. This enhances the
evolution in . These results indicate that is a
period of disk assembly, during which the strong rotational support present in
today's massive disk galaxies is only just beginning to emerge.Comment: 12 pages, 8 figures, submitted to Ap
Incommensurate magnetism near quantum criticality in CeNiAsO
Two phase transitions in the tetragonal strongly correlated electron system
CeNiAsO were probed by neutron scattering and zero field muon spin rotation.
For = 8.7(3) K, a second order phase transition yields an
incommensurate spin density wave with wave vector . For = 7.6(3) K, we find co-planar commensurate order with a
moment of , reduced to of the saturation moment of the
Kramers doublet ground state, which we establish by
inelastic neutron scattering. Muon spin rotation in
shows the commensurate order only exists for x 0.1 so the transition at
= 0.4(1) is from an incommensurate longitudinal spin density wave to a
paramagnetic Fermi liquid
A wide angle tail radio galaxy in the COSMOS field: evidence for cluster formation
We have identified a complex galaxy cluster system in the COSMOS field via a
wide angle tail (WAT) radio galaxy consistent with the idea that WAT galaxies
can be used as tracers of clusters. The WAT galaxy, CWAT-01, is coincident with
an elliptical galaxy resolved in the HST-ACS image. Using the COSMOS
multiwavelength data set, we derive the radio properties of CWAT-01 and use the
optical and X-ray data to investigate its host environment. The cluster hosting
CWAT-01 is part of a larger assembly consisting of a minimum of four X-ray
luminous clusters within ~2 Mpc distance. We apply hydrodynamical models that
combine ram pressure and buoyancy forces on CWAT-01. These models explain the
shape of the radio jets only if the galaxy's velocity relative to the
intra-cluster medium (ICM) is in the range of about 300-550 km/s which is
higher than expected for brightest cluster galaxies (BCGs) in relaxed systems.
This indicates that the CWAT-01 host cluster is not relaxed, but is possibly
dynamically young. We argue that such a velocity could have been induced
through subcluster merger within the CWAT-01 parent cluster and/or
cluster-cluster interactions. Our results strongly indicate that we are
witnessing the formation of a large cluster from an assembly of multiple
clusters, consistent with the hierarchical scenario of structure formation. We
estimate the total mass of the final cluster to be approximately 20% of the
mass of the Coma cluster.Comment: 18 pages, 13 figures; accepted for publication in ApJS, COSMOS
special issue; added color figure (Fig. 13) which was previously unavailabl
The Mean Star-Forming Properties of QSO Host Galaxies
Quasi-stellar objects (QSOs) occur in galaxies in which supermassive black
holes (SMBHs) are growing substantially through rapid accretion of gas. Many
popular models of the co-evolutionary growth of galaxies and SMBHs predict that
QSOs are also sites of substantial recent star formation, mediated by important
processes, such as major mergers, which rapidly transform the nature of
galaxies. A detailed study of the star-forming properties of QSOs is a critical
test of such models. We present a far-infrared Herschel/PACS study of the mean
star formation rate (SFR) of a sample of spectroscopically observed QSOs to z~2
from the COSMOS extragalactic survey. This is the largest sample to date of
moderately luminous AGNs studied using uniform, deep far-infrared photometry.
We study trends of the mean SFR with redshift, black hole mass, nuclear
bolometric luminosity and specific accretion rate (Eddington ratio). To
minimize systematics, we have undertaken a uniform determination of SMBH
properties, as well as an analysis of important selection effects within
spectroscopic QSO samples that influence the interpretation of SFR trends. We
find that the mean SFRs of these QSOs are consistent with those of normal
massive star-forming galaxies with a fixed scaling between SMBH and galaxy mass
at all redshifts. No strong enhancement in SFR is found even among the most
rapidly accreting systems, at odds with several co-evolutionary models.
Finally, we consider the qualitative effects on mean SFR trends from different
assumptions about the star-forming properties of QSO hosts and redshift
evolution of the SMBH-galaxy relationship. While limited currently by
uncertainties, valuable constraints on AGN-galaxy co-evolution can emerge from
our approach.Comment: 10 figures, 1 table; accepted for publication in Astronomy &
Astrophysic
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