745 research outputs found
Colors, magnitudes and velocity dispersions in early-type galaxies: Implications for galaxy ages and metallicities
We present an analysis of the color-magnitude-velocity dispersion relation
for a sample of 39320 early-type galaxies within the Sloan Digital Sky Survey.
We demonstrate that the color-magnitude relation is entirely a consequence of
the fact that both the luminosities and colors of these galaxies are correlated
with stellar velocity dispersions. Previous studies of the color-magnitude
relation over a range of redshifts suggest that the luminosity of an early-type
galaxy is an indicator of its metallicity, whereas residuals in color from the
relation are indicators of the luminosity-weighted age of its stars. We show
that this, when combined with our finding that velocity dispersion plays a
crucial role, has a number of interesting implications. First, galaxies with
large velocity dispersions tend to be older (i.e., they scatter redward of the
color-magnitude relation). Similarly, galaxies with large dynamical mass
estimates also tend to be older. In addition, at fixed luminosity, galaxies
which are smaller, or have larger velocity dispersions, or are more massive,
tend to be older. Second, models in which galaxies with the largest velocity
dispersions are also the most metal poor are difficult to reconcile with our
data. However, at fixed velocity dispersion, galaxies have a range of ages and
metallicities: the older galaxies have smaller metallicities, and vice-versa.
Finally, a plot of velocity dispersion versus luminosity can be used as an age
indicator: lines of constant age run parallel to the correlation between
velocity dispersion and luminosity.Comment: 12 pages, 9 figures. Accepted by A
The Spectrum and Variability of Circular Polarization in Sagittarius A* from 1.4 to 15 GHz
We report here multi-epoch, multi-frequency observations of the circular
polarization in Sagittarius A*, the compact radio source in the Galactic
Center. Data taken from the VLA archive indicate that the fractional circular
polarization at 4.8 GHz was -0.31% with an rms scatter of 0.13% from 1981 to
1998, in spite of a factor of 2 change in the total intensity. The sign
remained negative over the entire time range, indicating a stable magnetic
field polarity. In the Summer of 1999 we obtained 13 epochs of VLA A-array
observations at 1.4, 4.8, 8.4 and 15 GHz. In May, September and October of 1999
we obtained 11 epochs of Australia Telescope Compact Array observations at 4.8
and 8.5 GHz. In all three of the data sets, we find no evidence for linear
polarization greater than 0.1% in spite of strong circular polarization
detections. Both VLA and ATCA data sets support three conclusions regarding the
fractional circular polarization: the average spectrum is inverted with a
spectral index ~0.5 +/- 0.2; the degree of variability is roughly constant on
timescales of days to years; and, the degree of variability increases with
frequency. We also observed that the largest increase in fractional circular
polarization was coincident with the brightest flare in total intensity.
Significant variability in the total intensity and fractional circular
polarization on a timescale of 1 hour was observed during this flare,
indicating an upper limit to the size of 70 AU at 15 GHz. The fractional
circular polarization at 15 GHz reached -1.1% and the spectral index is
strongly inverted during this flare. We conclude that the spectrum has two
components that match the high and low frequency total intensity components.
(abridged)Comment: Accepted for publication in ApJ, 40 pages, 18 figure
The luminosities, sizes and velocity dispersions of Brightest Cluster Galaxies: Implications for formation history
The size-luminosity relation of early-type Brightest Cluster Galaxies (BCGs),
R_e ~ L^0.88, is steeper than that for the bulk of the early-type galaxy
population, for which R_e ~ L^0.68. In addition, although BCGs are hardly
offset from the Fundamental Plane defined by the bulk of the early-type
population, they show considerably smaller scatter. The larger than expected
sizes of BCGs, and the increased homogeneity, are qualitatively consistent with
models which seek to explain the colors of the most massive galaxies by
invoking dry dissipationless mergers, since dissipation tends to reduce the
sizes of galaxies, and wet mergers which result in star formation would tend to
increase the scatter in luminosity at fixed size and velocity dispersion.
Furthermore, BCGs define the same g-r color-magnitude relation as the bulk of
the early-type population. If BCGs formed from dry mergers, then BCG
progenitors must have been red for their magnitudes, suggesting that they
hosted older stellar populations than typical for their luminosities. Our
findings have two other consequences. First, the R_e-L relation of the
early-type galaxy population as a whole (i.e., normal plus BCG) exhibits some
curvature. Some of this curvature must be a consequence of the fact that an
increasing fraction of the most luminous galaxies are BCGs. The second
consequence is suggested by the fact that, despite following a steeper
size-luminosity relation, BCGs tend to define a tight relation between
dynamical mass R_e sigma^2/G and luminosity. As consequence, we find that BCGs
define a shallower sigma-L relation than the bulk of the early-type galaxy
population.Comment: 16 pages, 16 figures, AJ in pres
The Host Galaxy and Redshift of the Repeating Fast Radio Burst FRB 121102
The precise localization of the repeating fast radio burst (FRB 121102) has
provided the first unambiguous association (chance coincidence probability
) of an FRB with an optical and persistent radio
counterpart. We report on optical imaging and spectroscopy of the counterpart
and find that it is an extended ()
object displaying prominent Balmer and [OIII] emission lines. Based on the
spectrum and emission line ratios, we classify the counterpart as a
low-metallicity, star-forming, AB mag dwarf galaxy at a
redshift of , corresponding to a luminosity distance of 972 Mpc.
From the angular size, the redshift, and luminosity, we estimate the host
galaxy to have a diameter kpc and a stellar mass of
, assuming a mass-to-light ratio between 2 to
3. Based on the H flux, we estimate the star
formation rate of the host to be and a
substantial host dispersion measure depth .
The net dispersion measure contribution of the host galaxy to FRB 121102 is
likely to be lower than this value depending on geometrical factors. We show
that the persistent radio source at FRB 121102's location reported by Marcote
et al (2017) is offset from the galaxy's center of light by 200 mas and
the host galaxy does not show optical signatures for AGN activity. If FRB
121102 is typical of the wider FRB population and if future interferometric
localizations preferentially find them in dwarf galaxies with low metallicities
and prominent emission lines, they would share such a preference with long
gamma ray bursts and superluminous supernovae.Comment: 12 pages, 3 figures, Published in ApJ Letters. V2: Corrected mistake
in author lis
Resolving the inner jet structure of 1924-292 with the EVENT HORIZON TELESCOPE
We present the first 1.3 mm (230 GHz) very long baseline interferometry model
image of an AGN jet using closure phase techniques with a four-element array.
The model image of the quasar 1924-292 was obtained with four telescopes at
three observatories: the James Clerk Maxwell Telescope (JCMT) on Mauna Kea in
Hawaii, the Arizona Radio Observatory's Submillimeter Telescope (SMT) in
Arizona, and two telescopes of the Combined Array for Research in
Millimeterwave Astronomy (CARMA) in California in April 2009. With the greatly
improved resolution compared with previous observations and robust closure
phase measurement, the inner jet structure of 1924-292 was spatially resolved.
The inner jet extends to the northwest along a position angle of at
a distance of 0.38\,mas from the tentatively identified core, in agreement with
the inner jet structure inferred from lower frequencies, and making a position
angle difference of with respect to the cm-jet. The size of
the compact core is 0.15\,pc with a brightness temperature of
\,K. Compared with those measured at lower frequencies, the
low brightness temperature may argue in favor of the decelerating jet model or
particle-cascade models. The successful measurement of closure phase paves the
way for imaging and time resolving Sgr A* and nearby AGN with the Event Horizon
Telescope.Comment: 6 pages, 4 figures, accepted for publication in ApJ
The Cluster-EAGLE project: global properties of simulated clusters with resolved galaxies
We introduce the Cluster-EAGLE (C-EAGLE) simulation project, a set of cosmological hydrodynamical zoom simulations of the formation of 30 galaxy clusters in the mass range of 1014 < M200/M⊙ < 1015.4 that incorporates the Hydrangea sample of Bahé et al. (2017). The simulations adopt the state-of-the-art EAGLE galaxy formation model, with a gas particle mass of 1.8 × 106 M⊙ and physical softening length of 0.7 kpc. In this paper, we introduce the sample and present the low-redshift global properties of the clusters. We calculate the X-ray properties in a manner consistent with observational techniques, demonstrating the bias and scatter introduced by using estimated masses. We find the total stellar content and black hole masses of the clusters to be in good agreement with the observed relations. However, the clusters are too gas rich, suggesting that the active galactic nucleus (AGN) feedback model is not efficient enough at expelling gas from the high-redshift progenitors of the clusters. The X-ray properties, such as the spectroscopic temperature and the soft-band luminosity, and the Sunyaev–Zel'dovich properties are in reasonable agreement with the observed relations. However, the clusters have too high central temperatures and larger-than-observed entropy cores, which is likely driven by the AGN feedback after the cluster core has formed. The total metal content and its distribution throughout the intracluster medium are a good match to the observations
Observations and Theoretical Implications of the Large Separation Lensed Quasar SDSS J1004+4112
We study the recently discovered gravitational lens SDSS J1004+4112, the
first quasar lensed by a cluster of galaxies. It consists of four images with a
maximum separation of 14.62''. The system has been confirmed as a lensed quasar
at z=1.734 on the basis of deep imaging and spectroscopic follow-up
observations. We present color-magnitude relations for galaxies near the lens
plus spectroscopy of three central cluster members, which unambiguously confirm
that a cluster at z=0.68 is responsible for the large image separation. We find
a wide range of lens models consistent with the data, but they suggest four
general conclusions: (1) the brightest cluster galaxy and the center of the
cluster potential well appear to be offset by several kpc; (2) the cluster mass
distribution must be elongated in the North--South direction, which is
consistent with the observed distribution of cluster galaxies; (3) the
inference of a large tidal shear (~0.2) suggests significant substructure in
the cluster; and (4) enormous uncertainty in the predicted time delays between
the images means that measuring the delays would greatly improve constraints on
the models. We also compute the probability of such large separation lensing in
the SDSS quasar sample, on the basis of the CDM model. The lack of large
separation lenses in previous surveys and the discovery of one in SDSS together
imply a mass fluctuation normalization \sigma_8=1.0^{+0.4}_{-0.2} (95% CL), if
cluster dark matter halos have an inner slope -1.5. Shallower profiles would
require higher values of \sigma_8. Although the statistical conclusion might be
somewhat dependent on the degree of the complexity of the lens potential, the
discovery is consistent with the predictions of the abundance of cluster-scale
halos in the CDM scenario. (Abridged)Comment: 21 pages, 24 figures, 5 tables, accepted for publication in Ap
Anatomy of a post-starburst minor merger: a multi-wavelength WFC3 study of NGC 4150
(Abridged) We present a spatially-resolved near-UV/optical study of NGC 4150,
using the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope.
Previous studies of this early-type galaxy (ETG) indicate that it has a large
reservoir of molecular gas, exhibits a kinematically decoupled core (likely
indication of recent merging) and strong, central H_B absorption (indicative of
young stars). The core of NGC 4150 shows ubiquitous near-UV emission and
remarkable dusty substructure. Our analysis shows this galaxy to lie in the
near-UV green valley, and its pixel-by-pixel photometry exhibits a narrow range
of near-UV/optical colours that are similar to those of nearby E+A
(post-starburst) galaxies. We parametrise the properties of the recent star
formation (age, mass fraction, metallicity and internal dust content) in the
NGC 4150 pixels by comparing the observed near-UV/optical photometry to stellar
models. The typical age of the recent star formation (RSF) is around 0.9 Gyrs,
consistent with the similarity of the near-UV colours to post-starburst
systems, while the morphological structure of the young component supports the
proposed merger scenario. The RSF metallicity, representative of the
metallicity of the gas fuelling star formation, is around 0.3 - 0.5 Zsun.
Assuming that this galaxy is a merger and that the gas is sourced mainly from
the infalling companion, these metallicities plausibly indicate the gas-phase
metallicity (GPM) of the accreted satellite. Comparison to the local mass-GPM
relation suggests (crudely) that the mass of the accreted system is around
3x10^8 Msun, making NGC 4150 a 1:20 minor merger. A summation of the pixel RSF
mass fractions indicates that the RSF contributes about 2-3 percent of the
stellar mass. This work reaffirms our hypothesis that minor mergers play a
significant role in the evolution of ETGs at late epochs.Comment: 28 pages, 2 tables, accepted for publication in Ap
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