1,559 research outputs found
The Herschel Virgo Cluster Survey - I. Luminosity function
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 4Ă4 sqâdeg region of the cluster. We use SPIRE and PACS photometry data to produce 100, 160, 250, 350 and 500 ÎŒm luminosity functions (LFs) for optically bright galaxies that are selected at 500 ÎŒm 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
Further Discoveries of 12CO in Low Surface Brightness Galaxies
Using the IRAM 30m telescope we have obtained seven new, deep CO J(1-0) and
J(2-1) observations of low surface brightness (LSB) galaxies. Five of the
galaxies have no CO detected to extremely low limits (0.1-0.4 K km/s at
J(1-0)), while two of the galaxies, UGC 01922 and UGC 12289, have clear
detections in both line transitions. When these observations are combined with
all previous CO observations taken of LSB systems, we compile a total of 34
observations, in which only 3 galaxies have had detections of their molecular
gas. Comparing the LSB galaxies with and without CO detections to a sample of
high surface brightness (HSB) galaxies with CO observations indicates that it
is primarily the low density of baryonic matter within LSB galaxies which is
causing their low CO fluxes. Finally, we note that one of the massive LSB
galaxies studied in this project, UGC 06968 (a Malin-1 `cousin'), has upper
limits placed on both M_H2 and M_H2/M_HI which are 10-20 times lower than the
lowest values found for any galaxy (LSB or HSB) with similar global properties.
This may be due to an extremely low temperature and metallicity within UGC
06968, or simply due to the CO distribution within the galaxy being too diffuse
to be detected by the IRAM beam.Comment: 18 pages, 8 figures, 4 tables. Accepted by Ap
Spectrophotometric Observations of Blue Compact Dwarf Galaxies: Mrk 370
We present results from a detailed spectrophotometric analysis of the blue
compact dwarf galaxy (BCD) Mrk 370, based on deep UBVRI broad-band and Halpha
narrow-band observations, and long-slit and two-dimensional spectroscopy of its
brightest knots. The spectroscopic data are used to derive the internal
extinction, and to compute metallicities, electronic density and temperature in
the knots. By subtracting the contribution of the underlying older stellar
population, modeled by an exponential function, removing the contribution from
emission lines, and correcting for extinction, we can measure the true colors
of the young star-forming knots. We show that the colors obtained this way
differ significantly from those derived without the above corrections, and lead
to different estimates of the ages and star-forming history of the knots. Using
predictions of evolutionary synthesis models, we estimate the ages of both the
starburst regions and the underlying stellar component. We found that we can
reproduce the colors of all the knots with an instantaneous burst of star
formation and the Salpeter initial mass function with an upper mass limit of
100 solar masses. The resulting ages range between 3 and 6 Myrs. The colors of
the low surface brightness component are consistent with ages larger than 5
Gyr. The kinematic results suggest ordered motion around the major axis of the
galaxy.Comment: 26 pages with 14 figures; accepted for publication in Ap
A Deep Multicolor Survey. VI. Near-Infrared Observations, Selection Effects, and Number Counts
I present near-infrared J (1.25um), H (1.65um), and K (2.2um) imaging
observations of 185 square arcminutes in 21 high galactic latitude fields.
These observations reach limiting magnitudes of J ~ 21 mag, H ~ 20 mag and K ~
18.5 mag. The detection efficiency, photometric accuracy and selection biases
as a function of integrated object brightness, size, and profile shape are
quantified in detail. I evaluate several popular methods for measuring the
integrated light of faint galaxies and show that only aperture magnitudes
provide an unbiased measure of the integrated light that is independent of
apparent magnitude. These J, H, and K counts and near-infrared colors are in
best agreement with passive galaxy formation models with at most a small amount
of merging (for Omega_M = 0.3, Omega_Lambda = 0.7).Comment: AJ Accepted (Feb 2001). 28 pages, 7 embedded ps figures, AASTEX5.
Minor changes to submitted version. Also available at
http://www.astronomy.ohio-state.edu/~martini/pubs
Empirical Models for Dark Matter Halos. III. The Kormendy relation and the log(rho_e)-log(R_e) relation
We have recently shown that the 3-parameter density-profile model from
Prugniel & Simien provides a better fit to simulated, galaxy- and
cluster-sized, dark matter halos than an NFW-like model with arbitrary inner
profile slope gamma (Paper I). By construction, the parameters of the
Prugniel-Simien model equate to those of the Sersic R^{1/n} function fitted to
the projected distribution. Using the Prugniel-Simien model, we are therefore
able to show that the location of simulated (10^{12} M_sun) galaxy-sized dark
matter halos in the _e-log(R_e) diagram coincides with that of brightest
cluster galaxies, i.e., the dark matter halos appear consistent with the
Kormendy relation defined by luminous elliptical galaxies. These objects are
also seen to define the new, and equally strong, relation log(rho_e) = 0.5 -
2.5log(R_e), in which rho_e is the internal density at r=R_e. Simulated
(10^{14.5} M_sun) cluster-sized dark matter halos and the gas component of real
galaxy clusters follow the relation log(rho_e) = 2.5[1 - log(R_e)]. Given the
shapes of the various density profiles, we are able to conclude that while
dwarf elliptical galaxies and galaxy clusters can have dark matter halos with
effective radii of comparable size to the effective radii of their baryonic
component, luminous elliptical galaxies can not. For increasingly large
elliptical galaxies, with increasingly large profile shapes `n', to be dark
matter dominated at large radii requires dark matter halos with increasingly
large effective radii compared to the effective radii of their stellar
component.Comment: AJ, in press. (Paper I can be found at astro-ph/0509417
How effective is harassment on infalling late-type dwarfs?
A new harassment model is presented that models the complex, and dynamical
tidal field of a Virgo like galaxy cluster. The model is applied to small,
late-type dwarf disc galaxies (of substantially lower mass than in previous
harassment simulations) as they infall into the cluster from the outskirts.
These dwarf galaxies are only mildly affected by high speed tidal encounters
with little or no observable consequences; typical stellar losses are ,
producing very low surface brightness streams ( mag arcsec),
and a factor of two drop in dynamical mass-to-light ratio. Final stellar discs
remain disc-like, and dominated by rotation although often with tidally induced
spiral structure. By means of Monte-Carlo simulations, the statistically likely
influences of harassment on infalling dwarf galaxies are determined. The
effects of harassment are found to be highly dependent on the orbit of the
galaxy within the cluster, such that newly accreted dwarf galaxies typically
suffer only mild harassment. Strong tidal encounters, that can morphologically
transform discs into spheroidals, are rare occurring in of dwarf
galaxy infalls for typical orbits of sub-structure within CDM cluster
mass halos. For orbits with small apocentric distances (250 kpc), harassment
is significantly stronger resulting in complete disruption or heavy mass loss
( dark matter and stellar), however, such orbits are expected
to be highly improbable for newly infalling galaxies due to the deep potential
well of the cluster.Comment: 15 pages, 11 figures, 4 table
The Low Surface Brightness Extent of the Fornax Cluster
We have used a large format CCD camera to survey the nearby Fornax cluster
and its immediate environment for low luminosity low surface brightness
galaxies. Recent observations indicate that these are the most dark matter
dominated galaxies known and so they are likely to be a good tracer of the dark
matter in clusters. We have identified large numbers of these galaxies
consistent with a steep faint end slope of the luminosity function (alpha~ -2)
down to MB ~ -12. These galaxies contribute almost the same amount to the total
cluster light as the brighter galaxies and they have a spatial extent that is
some four times larger. They satisfy two of the important predictions of N-body
hierarchical simulations of structure formation using dark halos. The
luminosity (mass ?) function is steep and the mass distribution is more
extended than that defined by the brighter galaxies. We also find a large
concentration of low surface brightness galaxies around the nearby galaxy
NGC1291.Comment: 16 pages, 6 figure
Monte Carlo Predictions of Far-Infrared Emission from Spiral Galaxies
We present simulations of Far Infrared (FIR) emission by dust in spiral
galaxies, based on the Monte Carlo radiative transfer code of Bianchi, Ferrara
& Giovanardi (1996). The radiative transfer is carried out at several
wavelength in the Ultraviolet, optical and Near Infrared, to cover the range of
the stellar Spectral Energy Distribution (SED). Together with the images of the
galactic model, a map of the energy absorbed by dust is produced. Using
Galactic dust properties, the spatial distribution of dust temperature is
derived under the assumption of thermal equilibrium. A correction is applied
for non-equilibrium emission in the Mid Infrared. Images of dust emission can
then be produced at any wavelength in the FIR.
We show the application of the model to the spiral galaxy NGC 6946. The
observed stellar SED is used as input and models are produced for different
star-dust geometries. It is found that only optically thick dust disks can
reproduce the observed amount of FIR radiation. However, it is not possible to
reproduce the large FIR scalelength suggested by recent observation of spirals
at 200 um, even when the scalelength of the dust disk is larger than that for
stars. Optically thin models have ratios of optical/FIR scalelengths closer to
the 200um observations, but with smaller absolute scalelengths than optically
thick cases. The modelled temperature distributions are compatible with
observations of the Galaxy and other spirals. We finally discuss the
approximations of the model and the impact of a clumpy stellar and dust
structure on the FIR simulations.Comment: 19 pages, 6 figures, accepted by A&
Dynamical Friction in dE Globular Cluster Systems
The dynamical friction timescale for globular clusters to sink to the center
of a dwarf elliptical galaxy (dE) is significantly less than a Hubble time if
the halos have King-model or isothermal profiles and the globular clusters
formed with the same radial density profile as the underlying stellar
population. We examine the summed radial distribution of the entire globular
cluster systems and the bright globular cluster candidates in 51 Virgo and
Fornax Cluster dEs for evidence of dynamical friction processes. We find that
the summed distribution of the entire globular cluster population closely
follows the exponential profile of the underlying stellar population. However,
there is a deficit of bright clusters within the central regions of dEs
(excluding the nuclei), perhaps due to the orbital decay of these massive
clusters into the dE cores. We also predict the magnitude of each dE's nucleus
assuming the nuclei form via dynamical friction. The observed trend of
decreasing nuclear luminosity with decreasing dE luminosity is much stronger
than predicted if the nuclei formed via simple dynamical friction processes. We
find that the bright dE nuclei could have been formed from the merger of
orbitally decayed massive clusters, but the faint nuclei are several magnitudes
fainter than expected. These faint nuclei are found primarily in M_V > -14 dEs
which have high globular cluster specific frequencies and extended globular
cluster systems. In these galaxies, supernovae-driven winds, high central dark
matter densities, extended dark matter halos, the formation of new star
clusters, or tidal interactions may act to prevent dynamical friction from
collapsing the entire globular cluster population into a single bright nucleus.Comment: 15 pages, 2 tables, 7 figures; to appear in the Astrophysical
Journal, April 20, 200
A Morphological-type dependence in the mu_0-log(h) plane of Spiral galaxy disks
We present observational evidence for a galaxy `Type' dependence to the
location of a spiral galaxy's disk parameters in the mu_0-log(h) (central disk
surface-brightness - disk scale-length) plane. With a sample of ~40 Low Surface
Brightness galaxies (both bulge- and disk-dominated) and ~80 High Surface
Brightness galaxies, the early-type disk galaxies (<=Sc) tend to define a
bright envelope in the mu_0-log(h) plane, while the late-type (>=Scd) spiral
galaxies have, in general, smaller and fainter disks. Below the defining
surface brightness threshold for a Low Surface Brightness galaxy (i.e. more
than 1 mag fainter than the 21.65 B-mag arcsec^(-2) Freeman value), the
early-type spiral galaxies have scale-lengths greater than 8-9 kpc, while the
late-type spiral galaxies have smaller scale-lengths. All galaxies have been
modelled with a seeing-convolved Sersic r^(1/n) bulge and exponential disk
model. We show that the trend of decreasing bulge shape parameter (n) with
increasing Hubble type and decreasing bulge-to-disk luminosity ratio, which has
been observed amongst the High Surface Brightness galaxies, extends to the Low
Surface Brightness galaxies, revealing a continuous range of structural
parameters.Comment: To be published in ApJ. Inc. three two-part figure
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