306 research outputs found
The Distribution and Condition of the Warm Molecular Gas in Abell 2597 and Sersic 159-03
We have used the SINFONI integral field spectrograph to map the near-infrared
K-band emission lines of molecular and ionised hydrogen in the central regions
of two cool core galaxy clusters, Abell 2597 and Sersic 159-03. Gas is detected
out to 20 kpc from the nuclei of the brightest cluster galaxies and found to be
distributed in clumps and filaments around it. The ionised and molecular gas
phases trace each other closely in extent and dynamical state. Both gas phases
show signs of interaction with the active nucleus. Within the nuclear regions
the kinetic luminosity of this gas is found to be somewhat smaller than the
current radio luminosity. Outside the nuclear region the gas has a low velocity
dispersion and shows smooth velocity gradients. There is no strong correlation
between the intensity of the molecular and ionised gas emission and either the
radio or X-ray emission. The molecular gas in Abell 2597 and Sersic 159-03 is
well described by a gas in local thermal equilibrium (LTE) with a single
excitation temperature T_exc ~ 2300 K. The emission line ratios do not vary
strongly as function of position, with the exception of the nuclear regions
where the ionised to molecular gas ratio is found decrease. These constant line
ratios imply a single source of heating and excitation for both gas phases.Comment: 44 pages, 32 figures, accepted for publication in MNRA
The location of the broad HI absorption in 3C305: clear evidence for a jet-accelerated neutral outflow
We present high-spatial resolution 21-cm HI VLA observations of the radio
galaxy 3C305 (z=0.041). These new high-resolution data show that the ~1000 km/s
broad HI absorption, earlier detected in low-resolution WSRT observations, is
occurring against the bright, eastern radio lobe, about 1.6 kpc from the
nucleus. We use new optical spectra taken with the WHT to make a detailed
comparison of the kinematics of the neutral hydrogen with that of the ionised
gas. The striking similarity between the complex kinematics of the two gas
phases suggests that both the ionised gas and the neutral gas are part of the
same outflow. Earlier studies of the ionised gas had already found evidence for
a strong interaction between the radio jet and the interstellar medium at the
location of the eastern radio lobe. Our results show that the fast outflow
produced by this interaction also contains a component of neutral atomic
hydrogen. The most likely interpretation is that the radio jet ionises the ISM
and accelerates it to the high outflow velocities observed. Our observations
demonstrate that, following this strong jet-cloud interaction, not all gas
clouds are destroyed and that part of the gas can cool and become neutral. The
mass outflow rate measured in 3C~305 is comparable, although at the lower end
of the distribution, to that found in Ultra-Luminous IR galaxies. This suggests
that AGN-driven outflows, and in particular jet-driven outflows, can have a
similar impact on the evolution of a galaxy as starburst-driven superwinds.Comment: Accepted for publication in A&A. 7 pages, 4 figure
Induced Anisotropies in NiCo Obliquely Deposited Films and Their effect on Magnetic Domains
English Article: Oblique and in-plane anisotropies in obliquely evaporated NiCo thin films were investigated in order to understand their origin. All the compositions studied clearly show the effect of columnar grain morphology coupled with some intrinsic factors such as magnetostriction and crystallinity. Energy calculations are undertaken to explain the effect of
Far Ultraviolet Emission in the A2597 and A2204 Brightest Cluster Galaxies
We use the Hubble Space Telescope ACS/SBC and Very Large Telescope FORS
cameras to observe the Brightest Cluster Galaxies in Abell 2597 and Abell 2204
in the far-ultraviolet (FUV) F150LP and optical U, B, V, R, I Bessel filters.
The FUV and U band emission is enhanced in bright, filamentary structures
surrounding the BCG nuclei. These filaments can be traced out to 20 kpc from
the nuclei in the FUV. Excess FUV and U band light is determined by removing
emission due to the underlying old stellar population and mapped with 1 arcsec
spatial resolution over the central 20 kpc regions of both galaxies. We find
the FUV and U excess emission to be spatially coincident and a stellar
interpretation requires the existence of a significant amount of 10000-50000 K
stars. Correcting for nebular continuum emission and dust intrinsic to the BCG
further increases the FUV to U band emission ratio and implies that stars alone
may not suffice to explain the observations. However, lack of detailed
information on the gas and dust distribution and extinction law in these
systems prevents us from ruling out a purely stellar origin. Non-stellar
processes, such as the central AGN, Scattering, Synchrotron and Bremsstrahlung
emission are investigated and found to not be able to explain the FUV and U
band measurements in A2597. Contributions from non-thermal processes not
treated here should be investigated. Comparing the FUV emission to the optical
H-alpha line emitting nebula shows good agreement on kpc-scales in both A2597
and A2204. In concordance with an earlier investigation by O'Dea et al. (2004)
we find that O-stars can account for the ionising photons necessary to explain
the observed H-alpha line emission.Comment: accepted by mnra
Riding the wake of a merging galaxy cluster
Using WHT OASIS integral field unit observations, we report the discovery of
a thin plume of ionised gas extending from the brightest cluster galaxy in
Abell 2146 to the sub-cluster X-ray cool core which is offset from the BCG by
~37 kpc. The plume is greater than 15 kpc long and less than 3 kpc wide. This
plume is unique in that the cluster it is situated in is currently undergoing a
major galaxy cluster merger. The brightest cluster galaxy is unusually located
behind the X-ray shock front and in the wake of the ram pressure stripped X-ray
cool core and evidence for recent disruption to the BCG is observed. We examine
the gas and stellar morphology, the gas kinematics of the BCG and their
relation to the X-ray gas. We propose that a causal link between the ionised
gas plume and the offset X-ray cool core provides the simplest explanation for
the formation of the plume. An interaction or merger between the BCG and
another cluster galaxy is probably the cause of the offset.Comment: 14 pages, 18 figures, accepted for publication in MNRA
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
The evolution of galaxy star formation activity in massive halos
There is now a large consensus that the current epoch of the Cosmic Star
Formation History (CSFH) is dominated by low mass galaxies while the most
active phase at 1<z<2 is dominated by more massive galaxies, which undergo a
faster evolution. Massive galaxies tend to inhabit very massive halos such as
galaxy groups and clusters. We aim to understand whether the observed "galaxy
downsizing" could be interpreted as a "halo downsizing", whereas the most
massive halos, and their galaxy populations, evolve more rapidly than the halos
of lower mass. Thus, we study the contribution to the CSFH of galaxies
inhabiting group-sized halos. This is done through the study of the evolution
of the Infra-Red (IR) luminosity function of group galaxies from redshift 0 to
~1.6. We use a sample of 39 X-ray selected groups in the Extended Chandra Deep
Field South (ECDFS), the Chandra Deep Field North (CDFN), and the COSMOS field,
where the deepest available mid- and far-IR surveys have been conducted with
Spitzer MIPS and Hersche PACS. Groups at low redshift lack the brightest,
rarest, and most star forming IR-emitting galaxies observed in the field. Their
IR-emitting galaxies contribute <10% of the comoving volume density of the
whole IR galaxy population in the local Universe. At redshift >~1, the most
IR-luminous galaxies (LIRGs and ULIRGs) are preferentially located in groups,
and this is consistent with a reversal of the star-formation rate vs .density
anti-correlation observed in the nearby Universe. At these redshifts, group
galaxies contribute 60-80% of the CSFH, i.e. much more than at lower redshifts.
Below z~1, the comoving number and SFR densities of IR-emitting galaxies in
groups decline significantly faster than those of all IR-emitting galaxies. Our
results are consistent with a "halo downsizing" scenario and highlight the
significant role of "environment" quenching in shaping the CSFH.Comment: 14 pages, 10 figures, accepted for publication by A&
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
The role of massive halos in the Star Formation History of the Universe
The most striking feature of the Cosmic Star Formation History (CSFH) of the
Universe is a dramatic drop of the star formation (SF) activity, since z~1. In
this work we investigate if the very same process of assembly and growth of
structures is one of the major drivers of the observed decline. We study the
contribution to the CSFH of galaxies in halos of different masses. This is done
by studying the total SFR-halo mass-redshift plane from redshift 0 to redshift
z~1.6 in a sample of 57 groups and clusters by using the deepest available mid-
and far-infrared surveys conducted with Spitzer MIPS and Herschel PACS and
SPIRE. Our results show that low mass groups provide a 60-80% contribution to
the CSFH at z~1. Such contribution declines faster than the CSFH in the last 8
billion years to less than 10% at z<0.3, where the overall SF activity is
sustained by lower mass halos. More massive systems provide only a marginal
contribution (<10%) at any epoch. A simplified abundance matching method shows
that the large contribution of low mass groups at z~1 is due to a large
fraction (>50%) of very massive, highly star forming Main Sequence galaxies.
Below z~1 a quenching process must take place in massive halos to cause the
observed faster suppression of their SF activity. Such process must be a slow
one though, as most of the models implementing a rapid quenching of the SF
activity in accreting satellites significantly underpredicts the observed SF
level in massive halos at any redshift. Starvation or the transition from cold
to hot accretion would provide a quenching timescale of 1 Gyrs more consistent
with the observations. Our results suggest a scenario in which, due to the
structure formation process, more and more galaxies experience the group
environment and, thus, the associated quenching process. This leads to the
progressive suppression of their SF activity shaping the CSFH below z~1.Comment: 18 pages, 21 figures, accepted for publication by A&
The discovery of a typical radio galaxy at z = 4.88
‘The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical Society. DOI: 10.1111/j.1745-3933.2009.00715.xIn this Letter, we report the discovery of a z= 4.88 radio galaxy discovered with a new technique which does not rely on pre-selection of a sample based on radio properties such as steep-spectral index or small angular size. This radio galaxy was discovered in the Elais-N2 field and has a spectral index of α= 0.75 , i.e. not ultra-steep spectrum. It also has a luminosity consistent with being drawn from the break of the radio luminosity function and can therefore be considered as a typical radio galaxy. Using the Spitzer Wide-Area Infrared Extragalactic Survey (SWIRE) data over this field, we find that the host galaxy is consistent with being similarly massive to the lower redshift powerful radio galaxies (∼1–3L★) . However, we note that at z= 4.88, the Hα line is redshifted into the IRAC 3.6 μm filter, and some of the flux in this band may be due to this fact rather than the stellar continuum emission. The discovery of such a distant radio source from our initial spectroscopic observations demonstrates the promise of our survey for finding the most distant radio sources.Peer reviewe
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