2,725 research outputs found
Galaxies as Fluctuations in the Ionizing Background Radiation at Low Redshift
Some Lyman continuum photons are likely to escape from most galaxies, and
these can play an important role in ionizing gas around and between galaxies,
including gas that gives rise to Lyman alpha absorption. Thus the gas
surrounding galaxies and in the intergalactic medium will be exposed to varying
amounts of ionizing radiation depending upon the distances, orientations, and
luminosities of any nearby galaxies. The ionizing background can be
recalculated at any point within a simulation by adding the flux from the
galaxies to a uniform quasar contribution. Normal galaxies are found to almost
always make some contribution to the ionizing background radiation at redshift
zero, as seen by absorbers and at random points in space. Assuming that about 2
percent of ionizing photons escape from a galaxy like the Milky Way, we find
that normal galaxies make a contribution of at least 30 to 40 percent of the
assumed quasar background. Lyman alpha absorbers with a wide range of neutral
column densities are found to be exposed to a wide range of ionization rates,
although the distribution of photoionization rates for absorbers is found to be
strongly peaked. On average, less highly ionized absorbers are found to arise
farther from luminous galaxies, while local fluctuations in the ionization rate
are seen around galaxies having a wide range of properties.Comment: 10 pages, 8 figures, references added, clarified explanation of first
two equation
A multi-beam HI survey of the Virgo Cluster - two isolated HI clouds ?
We have carried out a fully sampled large area ()
21cm \HI line survey of part of the Virgo cluster using the Jodrell Bank
multi-beam instrument. The survey has a sensitivity some 3 times better than
the standard HIJASS and HIPASS surveys. We detect 31 galaxies, 27 of which are
well known cluster members. The four new detections have been confirmed in the
HIPASS data and by follow up Jodrell Bank pointed observations. One object lies
behind M86, but the other 3 have no obvious optical counter parts upon
inspection of the digital sky survey fields. These 3 objects were mapped at
Arecibo with a smaller \am{3}{6} HPBW and a 4 times better sensitivity than the
Jodrell Bank data, which allow an improved determination of the dimensions and
location of two of the objects, but surprisingly failed to detect the third.
The two objects are resolved by the Arecibo beam giving them a size far larger
than any optical images in the nearby field. To our mass limit of and column density limit of
atoms cm these new
detections represent only about 2% of the cluster atomic hydrogen mass. Our
observations indicate that the \HI mass function of the cluster turns down at
the low mass end making it very different to the field galaxy \HI mass
function. This is quite different to the Virgo cluster optical luminosity
function which is much steeper than that in the general field. Many of the
sample galaxies are relatively gas poor compared to \HI selected samples of
field galaxies, confirming the 'anaemic spirals' view of Virgo cluster late
type galaxies.Comment: Accepted for publication in MNRA
A PHABULOSA/cytokinin feedback loop controls root growth in arabidopsis
The hormone cytokinin (CK) controls root length in Arabidopsis thaliana by defining where dividing cells, derived from stem cells of the root meristem, start to differentiate [ [1], [2], [3], [4], [5] and [6]]. However, the regulatory inputs directing CK to promote differentiation remain poorly understood. Here, we show that the HD-ZIPIII transcription factor PHABULOSA (PHB) directly activates the CK biosynthesis gene ISOPENTENYL TRANSFERASE 7 (IPT7), thus promoting cell differentiation and regulating root length. We further demonstrate that CK feeds back to repress both PHB and microRNA165, a negative regulator of PHB. These interactions comprise an incoherent regulatory loop in which CK represses both its activator and a repressor of its activator. We propose that this regulatory circuit determines the balance of cell division and differentiation during root development and may provide robustness against CK fluctuations
Radiative transfer in disc galaxies -- III. The observed kinematics of dusty disc galaxies
We present SKIRT (Stellar Kinematics Including Radiative Transfer), a new
Monte Carlo radiative transfer code that allows the calculation of the observed
stellar kinematics of a dusty galaxy. The code incorporates the effects of both
absorption and scattering by interstellar dust grains, and calculates the
Doppler shift of the emerging radiation exactly by taking into account the
velocities of the emitting stars and the individual scattering dust grains. The
code supports arbitrary distributions of dust through a cellular approach,
whereby the integration through the dust is optimized by means of a novel
efficient trilinear interpolation technique.
We apply our modelling technique to calculate the observed kinematics of
realistic models for dusty disc galaxies. We find that the effects of dust on
the mean projected velocity and projected velocity dispersion are severe for
edge-on galaxies. For galaxies which deviate more than a few degrees from
exactly edge-on, the effects are already strongly reduced. As a consequence,
dust attenuation cannot serve as a possible way to reconcile the discrepancy
between the observed shallow slopes of the inner rotation curves of LSB
galaxies and the predictions of CDM cosmological models. For face-on galaxies,
the velocity dispersion increases with increasing dust mass due to scattering,
but the effects are limited, even for extended dust distributions. Finally, we
show that serious errors can be made when the individual velocities of the dust
grains are neglected in the calculations.Comment: 14 pages, 6 figures, accepted for publication in MNRA
The far-infrared view of M87 as seen by the Herschel Space Observatory
The origin of the far-infrared emission from the nearby radio galaxy M87
remains a matter of debate. Some studies find evidence of a far-infrared excess
due to thermal dust emission, whereas others propose that the far-infrared
emission can be explained by synchrotron emission without the need for an
additional dust emission component. We observed M87 with PACS and SPIRE as part
of the Herschel Virgo Cluster Survey (HeViCS). We compare the new Herschel data
with a synchrotron model based on infrared, submm and radio data to investigate
the origin of the far-infrared emission. We find that both the integrated SED
and the Herschel surface brightness maps are adequately explained by
synchrotron emission. At odds with previous claims, we find no evidence of a
diffuse dust component in M87.Comment: 4 pages, 2 figures, proceedings IAU Symposium 275 (Jets at all
scales
The Herschel Virgo Cluster Survey: I. Luminosity functions
We describe the Herschel Virgo Cluster Survey (HeViCS) and the first data
obtained as part of the Science Demonstration Phase (SDP). The data cover a
central 4x4 sq deg region of the cluster. We use SPIRE and PACS photometry data
to produce 100, 160, 250, 350 and 500 micron luminosity functions (LFs) for
optically bright galaxies that are selected at 500 micron and detected in all
bands. We compare these LFs with those previously derived using IRAS, BLAST and
Herschel-ATLAS data. The Virgo Cluster LFs do not have the large numbers of
faint galaxies or examples of very luminous galaxies seen previously in surveys
covering less dense environments.Comment: Letter accepted for publication in A&A (Herschel special issue
The dwarf galaxy population in Abell 2218
We present results from a deep photometric study of the rich galaxy cluster
Abell 2218 (z=0.18) based on archival HST WFPC2 F606W images. These have been
used to derive the luminosity function to extremely faint limits
(M_{F606W}=-13.2 mag, mu_{0}=24.7 mag arcsec^{-2}) over a wide field of view
(1.3 h^{-2} Mpc^2). We find the faint-end slope of the luminosity function to
vary with environment within the cluster, going from alpha=-1.23\pm0.13 within
the projected central core of the cluster (100 < r < 300 h^{-1} kpc) to
alpha=-1.49\pm 0.06 outside this radius (300 < r < 750 h^{-1} kpc). We infer
that the core is 'dwarf depleted', and further quantify this by studying the
ratio of 'dwarf' to 'giant' galaxies and its dependency as a function of
cluster-centric radius and local galaxy density. We find that this ratio varies
strongly with both quantities, and that the dwarf galaxy population in A2218
has a more extended distribution than the giant galaxy population.Comment: Accepted for publication in MNRAS; 11 pages, 12 figure
The Herschel Virgo Cluster Survey: IV. Resolved dust analysis of spiral galaxies
We present a resolved dust analysis of three of the largest angular size
spiral galaxies, NGC 4501 and NGC 4567/8, in the Herschel Virgo Cluster Survey
(HeViCS) Science Demonstration field. Herschel has unprecedented spatial
resolution at far-infrared wavelengths and with the PACS and SPIRE instruments
samples both sides of the peak in the far infrared spectral energy distribution
(SED).We present maps of dust temperature, dust mass, and gas-to-dust ratio,
produced by fitting modified black bodies to the SED for each pixel. We find
that the distribution of dust temperature in both systems is in the range ~19 -
22 K and peaks away from the centres of the galaxies. The distribution of dust
mass in both systems is symmetrical and exhibits a single peak coincident with
the galaxy centres. This Letter provides a first insight into the future
analysis possible with a large sample of resolved galaxies to be observed by
Herschel.Comment: Letter accepted for publication in A&A (Herschel special issue
Evolutionary properties of the low-luminosity galaxy population in the NGC5044 Group
With this third paper of a series we present Johnson-Gunn B,g,V,r,i,z
multicolour photometry for 79 objects, including a significant fraction of the
faintest galaxies around NGC5044, assessing group membership on the basis of
apparent morphology and low-resolution optical spectroscopy to estimate
redshift for 21 objects. Together, dE's and Im's provide the bulk of the galaxy
luminosity function, around M(g)\sim-18.0, while the S0 and dSph components
dominate, respectively, the bright and faint-end tails of the distribution.
This special mix places the NGC 5044 group just "midway" between the
high-density cosmic aggregation scale typical of galaxy clusters, and the
low-density environment of looser galaxy clumps like our Local Group. The
bright mass of the 136 member galaxies with available photometry and
morphological classification, amounts to a total of 2.3x10^{12}M_sun while
current SFR within the group turns to be about or higher than 23M_sun/yr. In
this regard, a drift toward bluer integrated colours is found to be an issue
for dE's pointing to a moderate but pervasive star-formation activity even
among nominally "quiescent" stellar systems. Through Lick narrow-band index
analysis, dwarf ellipticals are found to share a sub-solar metallicity (-1.0 <
[Fe/H] <-0.5) with a clear decoupling between Iron and alpha elements, as
already established also for high-mass systems. Both dE's and dS0's are
consistent with an old age, about one Hubble time, although a possible bias,
toward higher values of age, may be induced by the gas emission affecting the
Hbeta strength.Comment: 25 pages with 19 figure & 8 tables - To appear in the Monthly Notices
of the Royal Astronomical Society (in press) See
http://www.bo.astro.it/~eps/buz10602/10602.html for a complete overview of
the projec
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