360 research outputs found
Morphology-dependent trends of galaxy age with environment in Abell 901/902 seen with COMBO-17
We investigate correlations between galaxy age and environment in the Abell
901/2 supercluster for separate morphologies. Using COMBO-17 data, we define a
sample of 530 galaxies, complete at on an area of (Mpc/). We explore several age indicators including an
extinction-corrected residual from the colour-magnitude relation (CMR). As a
result, we find a clear trend of age with density for galaxies of all
morphologies that include a spheroidal component, in the sense that galaxies in
denser environments are older. This trend is not seen among Scd/Irr galaxies
since they all have young ages. However, the trend among the other types is
stronger for fainter galaxies. While we also see an expected age-morphology
relation, we find no evidence for a morphology-density relation at fixed age.Comment: Accepted for publication in MNRAS (Letters
Environmental dependence of AGN activity in the supercluster A901/2
We present XMM data for the supercluster A901/2, at z ~ 0.17, which is
combined with deep imaging and 17-band photometric redshifts (from the COMBO-17
survey), 2dF spectra and Spitzer 24um data, to identify AGN in the
supercluster. The 90ksec XMM image contains 139 point sources, of which 11 are
identified as supercluster AGN with L_X(0.5-7.5keV) > 1.7x10^41 erg/cm2/s. The
host galaxies have M_R < -20 and only 2 of 8 sources with spectra could have
been identified as AGN by the detected optical emission lines. Using a large
sample of 795 supercluster galaxies we define control samples of massive
galaxies with no detected AGN. The local environments of the AGN and control
samples differ at >98 per cent significance. The AGN host galaxies lie
predominantly in areas of moderate projected galaxy density and with more local
blue galaxies than the control sample, with the exception of one very bright
Type I AGN very near the centre of a cluster. These environments are similar
to, but not limited to, cluster outskirts and blue groups. Despite the large
number of potential host galaxies, no AGN are found in regions with the highest
galaxy density (excluding some cluster cores where emission from the ICM
obscures moderate luminosity AGN). AGN are also absent from the areas with
lowest galaxy density. We conclude that the prevalence of cluster AGN is linked
to their environment.Comment: 20 pages, 15 figures. MNRAS accepted. Version with full resolution
figures, including Figure 14, is available at
http://www.sc.eso.org/~rgilmour
The merger rate of massive galaxies
We calculate the projected two point correlation function for samples of
luminous and massive galaxies in the COMBO-17 photometric redshift survey,
focusing particularly on the amplitude of the correlation function at small
projected radii and exploring the constraints such measurements can place on
the galaxy merger rate. For nearly volume-limited samples with 0.4<z<0.8, we
find that 4+/-1% of luminous M_B<-20 galaxies are in close physical pairs (with
real space separation of <30 proper kpc). The corresponding fraction for
massive galaxies with M_*>2.5e10 M_sun is 5+/-1%. Incorporating close pair
fractions from the literature, the 2dFGRS and the SDSS, we find a fairly rapid
evolution of the merger fraction of massive galaxies between z=0.8 and the
present day. Assuming that the major merger timescale is of order the dynamical
timescale for close massive galaxy pairs, we tentatively infer that ~50% (70%)
of all galaxies with present-day masses M_*>5e10 M_sun (remnants of mergers
between galaxies with M_*>2.5e10 M_sun) have undergone a major merger since
z=0.8(1): major mergers between massive galaxies are a significant driver of
galaxy evolution over the last eight billion years.Comment: ApJ, in press. 8 pages, 3 figures. Expanded discussion section with
explicit discussion of merger fraction vs. close pair fraction. Change of
typical close pair timescale results in increased inferred merger rat
Stochastic Acceleration in Relativistic Parallel Shocks
(abridged) We present results of test-particle simulations on both the first
and the second order Fermi acceleration at relativistic parallel shock waves.
We consider two scenarios for particle injection: (i) particles injected at the
shock front, then accelerated at the shock by the first order mechanism and
subsequently by the stochastic process in the downstream region; and (ii)
particles injected uniformly throughout the downstream region to the stochastic
process. We show that regardless of the injection scenario, depending on the
magnetic field strength, plasma composition, and the employed turbulence model,
the stochastic mechanism can have considerable effects on the particle spectrum
on temporal and spatial scales too short to be resolved in extragalactic jets.
Stochastic acceleration is shown to be able to produce spectra that are
significantly flatter than the limiting case of particle energy spectral index
-1 of the first order mechanism. Our study also reveals a possibility of
re-acceleration of the stochastically accelerated spectrum at the shock, as
particles at high energies become more and more mobile as their mean free path
increases with energy. Our findings suggest that the role of the second order
mechanism in the turbulent downstream of a relativistic shock with respect to
the first order mechanism at the shock front has been underestimated in the
past, and that the second order mechanism may have significant effects on the
form of the particle spectra and its evolution.Comment: 14 pages, 11 figures (9 black/white and 2 color postscripts). To be
published in the ApJ (accepted 6 Nov 2004
The origins of X-ray emission from the hotspots of FRII radio sources
We use new and archival Chandra data to investigate the X-ray emission from a
large sample of compact hotspots of FRII radio galaxies and quasars from the 3C
catalogue. We find that only the most luminous hotspots tend to be in good
agreement with the predictions of a synchrotron self-Compton model with
equipartition magnetic fields. At low hotspot luminosities inverse-Compton
predictions are routinely exceeded by several orders of magnitude, but this is
never seen in more luminous hotspots. We argue that an additional synchrotron
component of the X-ray emission is present in low-luminosity hotspots, and that
the hotspot luminosity controls the ability of a given hotspot to produce
synchrotron X-rays, probably by determining the high-energy cutoff of the
electron energy spectrum. It remains plausible that all hotspots are close to
the equipartition condition.Comment: 49 pages, 16 figures. ApJ accepted. Revised version fixes a typo in
one of the Tables and corrects a statement about 3C27
Particle Diffusion and Acceleration by Shock Wave in Magnetized Filamentary Turbulence
We expand the off-resonant scattering theory for particle diffusion in
magnetized current filaments that can be typically compared to astrophysical
jets, including active galactic nucleus jets. In a high plasma beta region
where the directional bulk flow is a free-energy source for establishing
turbulent magnetic fields via current filamentation instabilities, a novel
version of quasi-linear theory to describe the diffusion of test particles is
proposed. The theory relies on the proviso that the injected energetic
particles are not trapped in the small-scale structure of magnetic fields
wrapping around and permeating a filament but deflected by the filaments, to
open a new regime of the energy hierarchy mediated by a transition compared to
the particle injection. The diffusion coefficient derived from a quasi-linear
type equation is applied to estimating the timescale for the stochastic
acceleration of particles by the shock wave propagating through the jet. The
generic scalings of the achievable highest energy of an accelerated ion and
electron, as well as of the characteristic time for conceivable energy
restrictions, are systematically presented. We also discuss a feasible method
of verifying the theoretical predictions. The strong, anisotropic turbulence
reflecting cosmic filaments might be the key to the problem of the acceleration
mechanism of the highest energy cosmic rays exceeding 100 EeV (10^{20} eV),
detected in recent air shower experiments.Comment: 39 pages, 2 figures, accepted for publication in Ap
Sub-Arcsecond Imaging of 3C123:108-GHz Continuum Observations of the Radio Hotspots
We present the results of sub-arcsecond 108 GHz continuum interferometric
observations toward the radio luminous galaxy 3C123. Using multi-array
observations, we utilize the high u,v dynamic range of the BIMA millimeter
array to sample fully spatial scales ranging from 0.5" to 50". This allows us
to make one-to-one comparisons of millimeter-wavelength emission in the radio
lobes and hotspots to VLA centimeter observations at 1.4, 4.9, 8.4, and 15 GHz.
At 108 GHz, the bright, eastern double hotspot in the southern lobe is
resolved. This is only the second time that a multiple hotspot region has been
resolved in the millimeter regime. We model the synchrotron spectra of the
hotspots and radio lobes using simple broken power-law models with high energy
cutoffs, and discuss the hotspot spectra and their implications for models of
multiple hotspot formation.Comment: 16 pages, 3 Figures, ApJ Accepte
The Electron Energy Distribution in the Hotspots of Cygnus A: Filling the Gap with the Spitzer Space Telescope
Here we present Spitzer Space Telescope imaging of Cyg A with the Infrared
Array Camera, resulting in the detection of the high-energy tails or cut-offs
in the synchrotron spectra for all four hotspots of this archetype radio
galaxy. When combined with the other data collected from the literature, our
observations allow for detailed modeling of the broad-band emission for the
brightest spots A and D. We confirm that the X-ray flux detected previously
from these features is consistent with the synchrotron self-Compton radiation
for the magnetic field intensity 170 muG in spot A, and 270 muG in spot D. We
also find that the energy density of the emitting electrons is most likely
larger by a factor of a few than the energy density of the hotspots' magnetic
field. We construct energy spectra of the radiating ultrarelativistic
electrons. We find that for both hotspots A and D these spectra are consistent
with a broken power-law extending from at least 100 MeV up to 100 GeV, and that
the spectral break corresponds almost exactly to the proton rest energy of 1
GeV. We argue that the shape of the electron continuum reflects two different
regimes of the electron acceleration process at mildly relativistic shocks,
rather than resulting from radiative cooling and/or absorption effects. In this
picture the protons' inertia defines the critical energy for the hotspot
electrons above which Fermi-type acceleration processes may play a major role,
but below which the operating acceleration mechanism has to be of a different
type. At energies >100 GeV, the electron spectra cut-off/steepen again, most
likely as a result of spectral aging due to radiative loss effects. We discuss
several implications of the presented analysis for the physics of extragalactic
jets.Comment: 29 pages, 8 figures and 2 tables included. Accepted for publication
in Ap
Star Formation and the Growth of Stellar Mass
Recent observations have demonstrated a significant growth in the integrated
stellar mass of the red sequence since z=1, dominated by a steadily increasing
number of galaxies with stellar masses M* < 10^11 M_sun. In this paper, we use
the COMBO-17 photometric redshift survey in conjunction with deep Spitzer 24
micron data to explore the relationship between star formation and the growth
of stellar mass. We calculate `star formation rate functions' in four different
redshift slices, splitting also into contributions from the red sequence and
blue cloud for the first time. We find that the growth of stellar mass since
z=1 is consistent with the integrated star formation rate. Yet, most of the
stars formed are in blue cloud galaxies. If the stellar mass already in, and
formed in, z<1 blue cloud galaxies were to stay in the blue cloud the total
stellar mass in blue galaxies would be dramatically overproduced. We explore
the expected evolution of stellar mass functions, finding that in this picture
the number of massive M* > 3x10^10 M_sun blue galaxies would also be
overproduced; i.e., most of the new stars formed in blue cloud galaxies are in
the massive galaxies. We explore a simple truncation scenario in which these
`extra' blue galaxies have their star formation suppressed by an unspecified
mechanism or mechanisms; simple cessation of star formation in these extra blue
galaxies is approximately sufficient to build up the red sequence at M*<10^11
M_sun.Comment: 9 Pages; ApJ in pres
X-ray Emission Properties of Large Scale Jets, Hotspots and Lobes in Active Galactic Nuclei
We examine a systematic comparison of jet-knots, hotspots and radio lobes
recently observed with Chandra and ASCA. This report will discuss the origin of
their X-ray emissions and investigate the dynamics of the jets. The data was
compiled at well sampled radio (5GHz) and X-ray frequencies (1keV) for more
than 40 radio galaxies. We examined three models for the X-ray production:
synchrotron (SYN), synchrotron self-Compton (SSC) and external Compton on CMB
photons (EC). For the SYN sources -- mostly jet-knots in nearby low-luminosity
radio galaxies -- X-ray photons are produced by ultrarelativistic electrons
with energies 10-100 TeV that must be accelerated in situ. For the other
objects, conservatively classified as SSC or EC sources, a simple formulation
of calculating the ``expected'' X-ray fluxes under an equipartition hypothesis
is presented. We confirmed that the observed X-ray fluxes are close to the
expected ones for non-relativistic emitting plasma velocities in the case of
radio lobes and majority of hotspots, whereas considerable fraction of
jet-knots is too bright at X-rays to be explained in this way. We examined two
possibilities to account for the discrepancy in a framework of the
inverse-Compton model: (1) magnetic field is much smaller than the
equipartition value, and (2) the jets are highly relativistic on kpc/Mpc
scales. We concluded, that if the inverse-Compton model is the case, the X-ray
bright jet-knots are most likely far from the minimum-power condition. We also
briefly discuss the other possibility, namely that the observed X-ray emission
from all of the jet-knots is synchrotron in origin.Comment: 20 pages, 10 figures, accepted for publication in the Astrophysical
Journal, vol.62
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