382 research outputs found
The Strongly Polarized Afterglow of GRB 020405
We report polarization measurements and photometry for the optical afterglow
of the gamma-ray burst GRB 020405. We measured a highly significant 9.9%
polarization (in V band) 1.3 days after the burst and argue that it is
intrinsic to the GRB. The light curve decay is well fitted by a
power-law; we do not see any evidence for a break between 1.24 and 4.3 days
after the burst. We discuss these measurements in the light of several models
of GRB afterglows.Comment: submitted to ApJ
Morphological Composition of z~0.4 groups: The site of S0 formation
The low redshift Universe (z<~0.5) is not a dull place. Processes leading to
the suppression of star formation and morphological transformation are
prevalent: this is particularly evident in the dramatic upturn in the fraction
of S0-type galaxies in clusters. However, until now, the process and
environment of formation has remained unidentified. We present a HST-based
morphological analysis of galaxies in the redshift-space selected group and
field environments at z~0.4. Groups contain a much higher fraction of S0s at
fixed luminosity than the lower density field, with >99.999% confidence. Indeed
the S0 fraction in groups is at least as high as in z~0.4 clusters and X-ray
selected groups, which have more luminous Intra Group Medium (IGM). An 97%
confident excess of S0s at >=0.3Mpc from the group centre at fixed luminosity,
tells us that formation is not restricted to, and possibly even avoids, the
group cores. Interactions with a bright X-ray emitting IGM cannot be important
for the formation of the majority of S0s in the Universe. In contrast to S0s,
the fraction of elliptical galaxies in groups at fixed luminosity is similar to
the field, whilst the brightest ellipticals are strongly enhanced towards the
group centres (>99.999% confidence within 0.3Mpc). We conclude that the group
and sub-group environments must be dominant for the formation of S0 galaxies,
and that minor mergers, galaxy harassment and tidal interactions are the most
likely responsible mechanisms. This has implications not only for the inferred
pre-processing of cluster galaxies, but also for the global morphological and
star formation budget of galaxies: as hierarchical clustering progresses, more
galaxies will be subject to these transformations as they enter the group
environment.Comment: 13 pages, 6 figures. Accepted for publication in Ap
The obscured growth of massive black holes
The mass density of massive black holes observed locally is consistent with
the hard X-ray Background provided that most of the radiation produced during
their growth was absorbed by surrounding gas. A simple model is proposed here
for the formation of galaxy bulges and central black holes in which young
spheroidal galaxies have a significant distributed component of cold dusty
clouds which accounts for the absorption. The central accreting black hole is
assumed to emit both a quasar-like spectrum, which is absorbed by the
surrounding gas, and a slow wind. The power in both is less than the Eddington
limit for the black hole. The wind however exerts the most force on the gas
and, as earlier suggested by Silk & Rees, when the black hole reaches a
critical mass, it is powerful enough to eject the cold gas from the galaxy, so
terminating the growth of both black hole and galaxy. In the present model this
point occurs when the Thomson depth in the surrounding gas has dropped to about
unity and results in the mass of the black hole being proportional to the mass
of the spheroid, with the normalization agreeing with that found for local
galaxies by Magorrian et al. for reasonable wind parameters. The model predicts
a new population of hard X-ray and sub-mm sources at redshifts above one which
are powered by black holes in their main growth phase.Comment: 5 pages, no figures, MN LATEX style, accepted for publication in the
MNRA
Evolution in the Disks and Bulges of Group Galaxies since z=0.4
We present quantitative morphology measurements of a sample of optically
selected group galaxies at 0.3 < z < 0.55 using the Hubble Space Telescope
(HST) Advanced Camera for Surveys (ACS) and the GIM2D surface
brightness--fitting software package. The group sample is derived from the
Canadian Network for Observational Cosmology Field Redshift survey (CNOC2) and
follow-up Magellan spectroscopy. We compare these measurements to a similarly
selected group sample from the Millennium Galaxy Catalogue (MGC) at 0.05 < z <
0.12. We find that, at both epochs, the group and field fractional bulge
luminosity (B/T) distributions differ significantly, with the dominant
difference being a deficit of disk--dominated (B/T < 0.2) galaxies in the group
samples. At fixed luminosity, z=0.4 groups have ~ 5.5 +/- 2 % fewer
disk--dominated galaxies than the field, while by z=0.1 this difference has
increased to ~ 19 +/- 6 %. Despite the morphological evolution we see no
evidence that the group environment is actively perturbing or otherwise
affecting the entire existing disk population. At both redshifts, the disks of
group galaxies have similar scaling relations and show similar median
asymmetries as the disks of field galaxies. We do find evidence that the
fraction of highly asymmetric, bulge--dominated galaxies is 6 +/- 3 % higher in
groups than in the field, suggesting there may be enhanced merging in group
environments. We replicate our group samples at z=0.4 and z=0 using the
semi-analytic galaxy catalogues of Bower et al (2006). This model accurately
reproduces the B/T distributions of the group and field at z=0.1. However, the
model does not reproduce our finding that the deficit of disks in groups has
increased significantly since z=0.4.Comment: Accepted for publication in MNRAS. 20 pages, 17 figure
The KMOS^3D Survey: design, first results, and the evolution of galaxy kinematics from 0.7<z<2.7
We present the KMOS^3D survey, a new integral field survey of over 600
galaxies at 0.7<z<2.7 using KMOS at the Very Large Telescope (VLT). The KMOS^3D
survey utilizes synergies with multi-wavelength ground and space-based surveys
to trace the evolution of spatially-resolved kinematics and star formation from
a homogeneous sample over 5 Gyrs of cosmic history. Targets, drawn from a
mass-selected parent sample from the 3D-HST survey, cover the star
formation-stellar mass () and rest-frame planes uniformly. We
describe the selection of targets, the observations, and the data reduction. In
the first year of data we detect Halpha emission in 191
Msun galaxies at z=0.7-1.1 and z=1.9-2.7. In
the current sample 83% of the resolved galaxies are rotation-dominated,
determined from a continuous velocity gradient and , implying
that the star-forming 'main sequence' (MS) is primarily composed of rotating
galaxies at both redshift regimes. When considering additional stricter
criteria, the Halpha kinematic maps indicate at least ~70% of the resolved
galaxies are disk-like systems. Our high-quality KMOS data confirm the elevated
velocity dispersions reported in previous IFS studies at z>0.7. For
rotation-dominated disks, the average intrinsic velocity dispersion decreases
by a factor of two from 50 km/s at z~2.3 to 25 km/s at z~0.9 while the
rotational velocities at the two redshifts are comparable. Combined with
existing results spanning z~0-3, disk velocity dispersions follow an
approximate (1+z) evolution that is consistent with the dependence of velocity
dispersion on gas fractions predicted by marginally-stable disk theory.Comment: 20 pages, 11 figures, 1 Appendix; Accepted to ApJ November 2
Synchronous Evolution of Galaxies in Groups: NGC 524 Group
By means of panoramic spectroscopy at the SAO RAS BTA telescope, we
investigated the properties of stellar populations in the central regions of
five early-type galaxies -- the NGC 524 group members. The evolution of the
central regions of galaxies looks synchronized: the average age of stars in the
bulges of all the five galaxies lies in the range of 3--6 Gyr. Four of the five
galaxies revealed synchronized bursts of star formation in the nuclei 1--2 Gyr
ago. The only galaxy, in which the ages of stellar population in the nucleus
and in the bulge coincide (i.e. the nuclear burst of star formation did not
take place) is NGC 502, the farthest from the center of the group of all the
galaxies studied.Comment: Slightly edited version of the paper to appear in the Astrophysical
Bulletin, 67(3); 24 pages including 8 figure
A multiscale approach to environment and its influence on the colour distribution of galaxies
We present a multiscale approach to measurements of galaxy density, applied
to a volume-limited sample constructed from SDSS DR5. We populate a rich
parameter space by obtaining independent measurements of density on different
scales for each galaxy, avoiding the implicit assumptions involved, e.g., in
the construction of group catalogues. As the first application of this method,
we study how the bimodality in galaxy colour distribution (u-r) depends on
multiscale density. The u-r galaxy colour distribution is described as the sum
of two gaussians (red and blue) with five parameters: the fraction of red
galaxies (f_r) and the position and width of the red and blue peaks (mu_r,
mu_b, sigma_r and sigma_b). Galaxies mostly react to their smallest scale (<
0.5 Mpc) environments: in denser environments red galaxies are more common
(larger f_r), redder (larger mu_r) and with a narrower distribution (smaller
sigma_r), while blue galaxies are redder (larger mu_b) but with a broader
distribution (larger sigma_b). There are residual correlations of f_r and mu_b
with 0.5 - 1 Mpc scale density, which imply that total or partial truncation of
star formation can relate to a galaxy's environment on these scales. Beyond 1
Mpc (0.5 Mpc for mu_r) there are no positive correlations with density. However
f_r (mu_r) anti-correlates with density on >2 (1) Mpc scales at fixed density
on smaller scales. We examine these trends qualitatively in the context of the
halo model, utilizing the properties of haloes within which the galaxies are
embedded, derived by Yang et al, 2007 and applied to a group catalogue. This
yields an excellent description of the trends with multiscale density,
including the anti-correlations on large scales, which map the region of
accretion onto massive haloes. Thus we conclude that galaxies become red only
once they have been accreted onto haloes of a certain mass.Comment: 22 pages, 14 figures. Accepted for publication in MNRAS
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