22 research outputs found
Keck Imaging of the Globular Cluster Systems in the Early--type Galaxies NGC 1052 and NGC 7332
The presence of two globular cluster subpopulations in early-type galaxies is
now the norm rather than the exception. Here we present two more examples for
which the host galaxy appears to have undergone a recent merger. Using
multi-colour Keck imaging of NGC 1052 and NGC 7332 we find evidence for a
bimodal globular cluster colour distribution in both galaxies, with roughly
equal numbers of blue and red globular clusters. The blue ones have similar
colours to those in the Milky Way halo and are thus probably very old and
metal-poor. If the red GC subpopulations are at least solar metallicity, then
stellar population models indicate young ages. We discuss the origin of
globular clusters within the framework of formation models. We conclude that
recent merger events in these two galaxies have had little effect on their
overall GC systems. We also derive globular cluster density profiles, global
specific frequencies and in the case of NGC 1052, radial colour gradients and
azimuthal distribution. In general these globular cluster properties are normal
for early-type galaxies.Comment: 11 pages, Latex, 15 figures, 2 tables, accepted by MNRA
The Phoenix Deep Survey: Extremely Red Galaxies and Cluster Candidates
We present the results of a study of a sample of 375 Extremely Red Galaxies
(ERGs) in the Phoenix Deep Survey, 273 of which constitute a subsample which is
80% complete to K_s = 18.5 over an area of 1160 arcmin^2. The angular
correlation function for ERGs is estimated, and the association of ERGs with
faint radio sources explored. We find tentative evidence that ERGs and faint
radio sources are associated at z > 0.5. A new overdensity-mapping algorithm
has been used to characterize the ERG distribution, and identify a number of
cluster candidates, including a likely cluster containing ERGs at 0.5 < z < 1.
Our algorithm is also used in an attempt to probe the environments in which
faint radio sources and ERGs are associated. We find limited evidence that the
I - K_s > 4 criterion is more efficient than R - K_s > 5 at selecting dusty
star-forming galaxies, rather than passively evolving ERGs.Comment: 14 emulateapj pages, 15 figures, 1 table, accepted for publication in
Astronomical Journal. A version with full resolution figures is available at
http://www.physics.usyd.edu.au/~asmith/research/ERGpaper.pd
Now You See It, Now You Don't: The Disappearing Central Engine of the Quasar J1011+5442
We report the discovery of a new "changing-look" quasar, SDSS
J101152.98+544206.4, through repeat spectroscopy from the Time Domain
Spectroscopic Survey. This is an addition to a small but growing set of quasars
whose blue continua and broad optical emission lines have been observed to
decline by a large factor on a time scale of approximately a decade. The 5100
Angstrom monochromatic continuum luminosity of this quasar drops by a factor of
> 9.8 in a rest-frame time interval of < 9.7 years, while the broad H-alpha
luminosity drops by a factor of 55 in the same amount of time. The width of the
broad H-alpha line increases in the dim state such that the black hole mass
derived from the appropriate single-epoch scaling relation agrees between the
two epochs within a factor of 3. The fluxes of the narrow emission lines do not
appear to change between epochs. The light curve obtained by the Catalina Sky
Survey suggests that the transition occurs within a rest-frame time interval of
approximately 500 days. We examine three possible mechanisms for this
transition suggested in the recent literature. An abrupt change in the
reddening towards the central engine is disfavored by the substantial
difference between the timescale to obscure the central engine and the observed
timescale of the transition. A decaying tidal disruption flare is consistent
with the decay rate of the light curve but not with the prolonged bright state
preceding the decay, nor can this scenario provide the power required by the
luminosities of the emission lines. An abrupt drop in the accretion rate onto
the supermassive black hole appears to be the most plausible explanation for
the rapid dimming.Comment: Submitted to MNRA
Towards an Understanding of Changing-Look Quasars: An Archival Spectroscopic Search in SDSS
The uncertain origin of the recently-discovered `changing-looking' quasar
phenomenon -- in which a luminous quasar dims significantly to a quiescent
state in repeat spectroscopy over ~10 year timescales -- may present unexpected
challenges to our understanding of quasar accretion. To better understand this
phenomenon, we take a first step to building a sample of changing-look quasars
with a systematic but simple archival search for these objects in the Sloan
Digital Sky Survey Data Release 12. By leveraging the >10 year baselines for
objects with repeat spectroscopy, we uncover two new changing-look quasars, and
a third discovered previously. Decomposition of the multi-epoch spectra and
analysis of the broad emission lines suggest that the quasar accretion disk
emission dims due to rapidly decreasing accretion rates (by factors of >2.5),
while disfavoring changes in intrinsic dust extinction for the two objects
where these analyses are possible. Broad emission line energetics also support
intrinsic dimming of quasar emission as the origin for this phenomenon rather
than transient tidal disruption events or supernovae. Although our search
criteria included quasars at all redshifts and transitions from either
quasar-like to galaxy-like states or the reverse, all of the clear cases of
changing-look quasars discovered were at relatively low-redshift (z ~ 0.2 -
0.3) and only exhibit quasar-like to galaxy-like transitions.Comment: 15 pages, 8 figures. Updated to accepted versio
The eROSITA extragalactic CalPV serendipitous catalog
Context. The eROSITA X-ray telescope on board the Spectrum-Roentgen-Gamma (SRG) observatory performed calibration and performance verification (CalPV) observations between September 2019 and December 2019, ahead of the planned 4-yr all-sky surveys. Most of them were deep, pointing-mode observations. Aims. We present here the X-ray catalog detected from the set of extra-galactic CalPV observations released to the public by the German eROSITA consortium, and the multiband counterparts of these X-ray sources. Methods. We developed a source detection method optimized for point-like X-ray sources by including extended X-ray emission in the background measurement. The multiband counterparts were identified using a Bayesian method from the CatWISE catalog. Results. Combining 11 CalPV fields, we present a catalog containing 9515 X-ray sources, whose X-ray fluxes were measured through spectral fitting. CatWISE counterparts are presented for 77% of the sources. Significant variabilities are found in 99 of the sources, which are also presented with this paper. Most of these fields show similar number counts of point sources as typical extragalactic fields, and a few harbor particular stellar populations