56 research outputs found
Ultraluminous Quasars At High Redshift Show Evolution In Their Radio-Loudness Fraction In Both Redshift And Ultraviolet Luminosity
We take a sample of 94 ultraluminous, optical quasars from the search of over
14,486 deg^2 by Onken et al. 2022 in the range 4.4<redshift<5.2 and match them
against the Rapid ASKAP Continuum Survey (RACS) observed on the Australian
Square Kilometre Array Pathfinder (ASKAP). From this most complete sample of
the bright end of the redshift ~5 quasar luminosity function, there are 10
radio continuum detections of which 8 are considered radio-loud quasars. The
radio-loud fraction for this sample is 8.5 \pm 2.9 per cent. Jiang et al. 2007
found that there is a decrease in the radio-loud fraction of quasars with
increasing redshift and an increase with increasing absolute magnitude at rest
frame 2500 Angstroms. We show that the radio-loud fraction of our quasar sample
is consistent with that predicted by Jiang et al. 2007, extending their result
to higher redshifts.Comment: Accepted by MNRA
The 6dF Galaxy Survey: a low-redshift benchmark for bulge-dominated galaxies
The 6dF Galaxy Survey provides a very large sample of galaxies with reliable
measurements of Lick line indices and velocity dispersions. This sample can be
used to explore the correlations between mass and stellar population parameters
such as age, metallicity and [alpha/Fe]. Preliminary results from such an
analysis are presented here, and show that age and metallicity are
significantly anti-correlated for both passive and star-forming galaxies.
Passive galaxies have strong correlations between mass and metallicity and
between age and alpha-element over-abundance, which combine to produce a
downsizing relation between age and mass. For old passive galaxies, the
different trends of M/L with mass and luminosity in different passbands result
from the differential effect of the mass-metallicity relation on the
luminosities in each passband. Future work with this sample will examine the
Fundamental Plane of bulge-dominated galaxies and the influence of environment
on relations between stellar population parameters and mass.Comment: 4 pages, to appear in "Formation and Evolution of Galaxy Bulges",
Proceedings of IAU Symposium No. 245 (Oxford, 16-20 July 2007), eds Martin
Bureau, Lia Athanassoula, Beatriz Barbu
The HI gas content of galaxies around Abell 370, a galaxy cluster at z = 0.37
We used observations from the Giant Metrewave Radio Telescope to measure the
atomic hydrogen gas content of 324 galaxies around the galaxy cluster Abell 370
at a redshift of z = 0.37 (a look-back time of ~4 billion years). The HI 21-cm
emission from these galaxies was measured by coadding their signals using
precise optical redshifts obtained with the Anglo-Australian Telescope. The
average HI mass measured for all 324 galaxies is (6.6 +- 3.5)x10^9 solar
masses, while the average HI mass measured for the 105 optically blue galaxies
is (19.0 +- 6.5)x10^9 solar masses. The significant quantities of gas found
around Abell 370, suggest that there has been substantial evolution in the gas
content of galaxy clusters since redshift z = 0.37. The total amount of HI gas
found around Abell 370 is up to ~8 times more than that seen around the Coma
cluster, a nearby galaxy cluster of similar size. Despite this higher gas
content, Abell 370 shows the same trend as nearby clusters, that galaxies close
to the cluster core have lower HI gas content than galaxies further away. The
Abell 370 galaxies have HI mass to optical light ratios similar to local galaxy
samples and have the same correlation between their star formation rate and HI
mass as found in nearby galaxies. The average star formation rate derived from
[OII] emission and from de-redshifted 1.4 GHz radio continuum for the Abell 370
galaxies also follows the correlation found in the local universe. The large
amounts of HI gas found around the cluster can easily be consumed by the
observed star formation rate in the galaxies over the ~4 billion years (from z
= 0.37) to the present day.Comment: accepted by MNRA
Comparison of the Stellar Populations of Bulges and Discs using the MaNGA Survey
We use the MaNGA integral-field spectroscopic survey of low-redshift galaxies
to compare the stellar populations of the bulge and disc components, identified
from their Sersic profiles, for various samples of galaxies. Bulge dominated
regions tend to be more metal-rich and have slightly older stellar ages than
their associated disc dominated regions. The metallicity difference is
consistent with the deeper gravitational potential in bulges relative to discs,
which allows bulges to retain more of the metals produced by stars. The age
difference is due to star formation persisting longer in discs relative to
bulges. Relative to galaxies with lower stellar masses, galaxies with higher
stellar masses tend to have bulge dominated regions that are more metal-rich
and older (in light-weighted measurements) than their disc dominated regions.
This suggests high-mass galaxies quench from the inside out, while lower-mass
galaxies quench across the whole galaxy simultaneously. Early-type galaxies
tend to have bulge dominated regions the same age as their disc dominated
regions, while late-type galaxies tend to have disc dominated regions
significantly younger than their bulge dominated regions. Central galaxies tend
to have a greater metallicity difference between their bulge dominated regions
and disc dominated regions than satellite galaxies at similar stellar mass.
This difference may be explained by central galaxies being subject to mergers
or extended gas accretion bringing new, lower-metallicity gas to the disc,
thereby reducing the average metallicity and age of the stars; quenching of
satellite discs may also play a role.Comment: Accepted by PAS
The HI content of star-forming galaxies at z = 0.24
We use observations from the Giant Metrewave Radio Telescope (GMRT) to
measure the atomic hydrogen gas content of star-forming galaxies at z = 0.24
(i.e. a look-backtime of ~3 Gyr). The sample of galaxies studied were selected
from Halpha-emitting field galaxies detected in a narrow-band imaging survey
with the Subaru Telescope. The Anglo-Australian Telescope was used to obtain
precise optical redshifts for these galaxies. We then coadded the HI 21 cm
emission signal for all the galaxies within the GMRT spectral line data cube.
From the coadded signal of 121 galaxies, we measure an average atomic
hydrogen gas mass of (2.26 +- 0.90)*10^9 solar masses. We translate this HI
signal into a cosmic density of neutral gas at z = 0.24 of Omega_gas = (0.91 +-
0.42)*10^-3. This is the current highest redshift at which Omega_gas has been
constrained from 21 cm emission and our value is consistent with that estimated
from damped Lyman-alpha systems around this redshift. We also find that the
correlations between the Halpha luminosity and the radio continuum luminosity
and between the star formation rate and the HI gas content in star-forming
galaxies at z = 0.24 are consistent with the correlations found at z = 0. These
two results suggest that the star formation mechanisms in field galaxies ~3 Gyr
ago were not substantially different from the present, even though the star
formation rate is 3 times higher.Comment: 11 pages, contains 9 figures and 1 table. Accepted for publishing in
MNRAS 2007 January 22. Received 2007 January 22; in original form 2006
November 3
Neutral atomic hydrogen (H i) gas evolution in field galaxies at z similar to 0.1 and similar to 0.2
<p>We measure the neutral atomic hydrogen (H i) gas content of field galaxies at intermediate redshifts of z similar to 0.1 and similar to 0.2 using hydrogen 21-cm emission lines observed with the Westerbork Synthesis Radio Telescope. In order to make high signal-to-noise ratio detections, an H i signal stacking technique is applied: H i emission spectra from multiple galaxies, optically selected by the second Canadian Network for Observational Cosmology redshift survey project, are co-added to measure the average H i mass of galaxies in the two redshift bins. We calculate the cosmic H i gas densities ((Hi)) at the two redshift regimes and compare those with measurements at other redshifts to investigate the global evolution of the H i gas density over cosmic time. From a total of 59 galaxies at z similar to 0.1 we find (Hi) = (0.33 +/- 0.05) x 10(-3), and at z similar to 0.2 we find (Hi) = (0.34 +/- 0.09) x 10(-3), based on 96 galaxies. These measurements help bridge the gap between high-z damped Lyman alpha observations and blind 21-cm surveys at z = 0. We find that our measurements of (Hi) at z similar to 0.1 and similar to 0.2 are consistent with the H i gas density at z similar to 0 and that all measurements of (Hi) from 21-cm emission observations at z less than or similar to 0.2 are in agreement with no evolution of the H i gas content in galaxies during the last 2.4 Gyr.</p>
Voltammetric Characterization of Redox-Inactive Guest Binding to Ln III [15-Metallacrown-5] Hosts Based on Competition with a Redox Probe
A novel competitive binding assay was implemented to monitor the binding of a redox inactive substrate to a redox inactive metallacrown host based on its competition with ferrocene carboxylate (FcC − ) using cyclic voltammetry (CV). First, the binding of FcC − to Ln III [15-MC Cu II ,N,L-pheHA -5] (LnMC) hosts was characterized by cyclic voltammetry. It was shown that the voltammetric half wave potentials, E 1/2 , shifted to more positive potentials upon the addition of LnMC. The explicit dependence of E 1/2 with the concentration of LnMC was used to determine the association constants for the complex. The FcC − binding strength decreased with larger central lanthanide metals in the LnMC hosts, and substantially weaker binding was observed with La III . X-ray crystallography revealed that the hydrophobic host cavity incompletely encapsulated FcC − when the guest was bound to the nine-coordinate La III , suggesting the LnMC’s ligand side chains play a substantial role in guest recognition. With knowledge of the MC-FcC − solution thermodynamics, the binding affinity of a redox inactive guest was then assessed. Addition of sodium benzoate to a LnMC and FcC − mixture resulted in E 1/2 shifting back to the value observed for FcC − in the absence of LnMC. The association constants between benzoate and LnMC’s were calculated via the competitive binding approach. Comparison with literature values suggests this novel assay is a viable method for determining association constants for host–guest systems that exhibit the proper electrochemical behavior. Notably, this CV competitive binding approach does not require the preparation of a modified electrode or a tethered guest, and thus can be generalized to a number of host–guest systems.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77442/1/chem_200903015_sm_miscellaneous_information.pd
The 6dF galaxy survey: Fundamental Plane data
We report the 6dFGS Fundamental Plane (6dFGSv) catalogue that is used to estimate distances and peculiar velocities for nearly 9000 early-type galaxies in the local (z < 0.055) universe. Velocity dispersions are derived by cross-correlation from 6dF V
Stellar population gradients in the cores of nearby field E+A galaxies
We have selected a sample of local E+A galaxies from the Sloan Digital Sky
Survey (SDSS) Data Release 7 for follow up integral field spectroscopy with the
Wide Field Spectrograph (WiFeS) on the ANU 2.3-m telescope. The sample was
selected using the Halpha line in place of the [OII]3727 line as the indicator
of on-going star formation (or lack thereof). This allowed us to select a lower
redshift sample of galaxies than available in the literature since the
[OII]3727 falls off the blue end of the wavelength coverage in the SDSS for the
very lowest redshift objects. This low redshift selection means that the
galaxies have a large angular to physical scale which allows us to resolve the
central ~1kpc region of the galaxies; the region where stellar population
gradients are expected. Such observations have been difficult to make using
other higher redshift samples because even at redshifts z~0.1 the angular to
physical scale is similar to the resolution provided by ground based seeing.
Our integral field spectroscopy has enabled us to make the first robust
detections of Balmer line gradients in the centres of E+A galaxies. Six out of
our sample of seven, and all the galaxies with regular morphologies, are
observed to have compact and centrally-concentrated Balmer line absorption.
This is evidence for compact young cores and stellar population gradients which
are predicted from models of mergers and tidal interactions which funnel gas
into the galaxy core. Given the generally isolated nature of our sample this
argues for the galaxies being seen in the late stage of a merger where the
progenitors have already coalesced.Comment: accepted to MNRA
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