897 research outputs found
Spectral candles to measure the Extragalactic Background Light
Extragalactic Background Light (EBL) is the integrated light from all stars that have ever formed, and spans in a range of Infrared (IR) to Ultraviolet (UV). The interaction of very-high-energy (VHE: E > 100 GeV) γ-rays emitted by
Active Galactic Nuclei (AGN) at cosmological distances with the EBL results in electron-positron pair production that leads to an energy-dependent attenuation of the observed VHE flux. Here we introduce a method based on the attenuation to measure the EBL photon number density. We then apply this method on simultaneous blazar data—PKS 2155-304—to determine the optical density at z = 0.12 and compare it with the optical densities predicted by popular EBL models
Ejection of Supernova-Enriched Gas From Dwarf Disk Galaxies
We examine the efficiency with which supernova-enriched gas may be ejected
from dwarf disk galaxies, using a methodology previously employed to study the
self-enrichment efficiency of dwarf spheroidal systems. Unlike previous studies
that focused on highly concentrated starbursts, in the current work we consider
discrete supernova events spread throughout various fractions of the disk. We
model disk systems having gas masses of 10^8 and 10^9 solar masses with
supernova rates of 30, 300, and 3000 per Myr. The supernova events are confined
to the midplane of the disk, but distributed over radii of 0, 30, and 80% of
the disk radius, consistent with expectations for Type II supernovae. In
agreement with earlier studies, we find that the enriched material from
supernovae is largely lost when the supernovae are concentrated near the
nucleus, as expected for a starburst event. In contrast, however, we find the
loss of enriched material to be much less efficient when the supernovae occur
over even a relatively small fraction of the disk. The difference is due to the
ability of the system to relax following supernova events that occur over more
extended regions. Larger physical separations also reduce the likelihood of
supernovae going off within low-density "chimneys" swept out by previous
supernovae. We also find that, for the most distributed systems, significant
metal loss is more likely to be accompanied by significant mass loss. A
comparison with theoretical predications indicates that, when undergoing
self-regulated star formation, galaxies in the mass range considered shall
efficiently retain the products of Type II supernovae.Comment: 16 pages, 14 figures, to appear in Astrophysical Journal; higher
resolution figures available through Ap
Semi-Analytical Models for the Formation of Disk Galaxies II. Dark Matter versus Modified Newtonian Dynamics
We present detailed semi-analytical models for the formation of disk galaxies
both in a Universe dominated by dark matter (DM), and in one for which the
force law is given by modified Newtonian dynamics (MOND). We tune the models to
fit the observed near-infrared Tully-Fisher (TF) relation, and compare numerous
predictions of the resulting models with observations. The DM and MOND models
are almost indistinguishable. They both yield gas mass fractions and dynamical
mass-to-light ratios which are in good agreement with observations. Both models
reproduce the narrow relation between global mass-to-light ratio and central
surface brightness, and reveal a characteristic acceleration, contrary to
claims that these relations are not predicted by DM models. Both models require
SN feedback in order to reproduce the lack of high surface brightness dwarf
galaxies. However, the introduction of feedback to the MOND models steepens the
TF relation and increases the scatter, making MOND only marginally consistent
with observations. The most serious problem for the DM models is their
prediction of steep central rotation curves. However, the DM rotation curves
are only slightly steeper than those of MOND, and are only marginally
inconsistent with the poor resolution data on LSB galaxies.Comment: 26 pages, 11 figures. Accepted for publication in Ap
High-Resolution Measurements of the Dark Matter Halo of NGC 2976: Evidence for a Shallow Density Profile
We have obtained two-dimensional velocity fields of the dwarf spiral galaxy
NGC 2976 in Halpha and CO. The high spatial (~75 pc) and spectral (13 km/s and
2 km/s, respectively) resolution of these observations, along with our
multicolor optical and near-infrared imaging, allow us to measure the shape of
the density profile of the dark matter halo with good precision. We find that
the total (baryonic plus dark matter) mass distribution of NGC 2976 follows a
rho_tot ~ r^(-0.27 +/- 0.09) power law out to a radius of 1.8 kpc, assuming
that the observed radial motions provide no support. The density profile
attributed to the dark halo is even shallower, consistent with a nearly
constant density of dark matter over the entire observed region. A maximal disk
fit yields an upper limit to the K-band stellar mass-to-light ratio (M*/L_K) of
0.09^{+0.15}_{-0.08} M_sun/L_sun,K (including systematic uncertainties), with
the caveat that for M*/L_K > 0.19 M_sun/L_sun,K the dark matter density
increases with radius, which is unphysical. Assuming 0.10 M_sun/L_sun,K <
M*/L_K < 0.19 M_sun/L_sun,K, the dark matter density profile lies between
rho_dm ~ r^-0.17 and rho_dm ~ r^-0.01. Therefore, independent of any
assumptions about the stellar disk or the functional form of the density
profile, NGC 2976 does not contain a cuspy dark matter halo. We also
investigate some of the systematic effects that can hamper rotation curve
studies, and show that 1) longslit rotation curves are far more vulnerable to
systematic errors than two-dimensional velocity fields, 2) NGC 2976 contains
large radial motions at small radii, and 3) the Halpha and CO velocity fields
of NGC 2976 agree within their uncertainties. [slightly abridged]Comment: 30 pages, 4 tables, 13 figures (7 in color; Figures 1 and 3 are
low-resolution to save space). Accepted for publication in ApJ. Version with
full-resolution figures available at
http://astro.berkeley.edu/~bolatto/ngc2976rotation.ps (46 MB
The rotation curve and mass-distribution in highly flattened galaxies
A new method is developed which permits the reconstruction of the
surface-density distribution in the galactic disk of finite radius from an
arbitrary smooth distribution of the angular velocity via two simple
quadratures. The existence of upper limits for disk's mass and radius during
the analytic continuation of rotation curves into the hidden (non-radiating)
part of the disk is demonstrated.Comment: 13 pages, 2 figure
On the relation between circular velocity and central velocity dispersion in high and low surface brightness galaxies
In order to investigate the correlation between the circular velocity Vc and
the central velocity dispersion of the spheroidal component sigma_c, we
analyzed these quantities for a sample of 40 high surface brightness disc
galaxies (hereafter HSB), 8 giant low surface brightness spiral galaxies
(hereafter LSB), and 24 elliptical galaxies characterized by flat rotation
curves. We find that the Vc-sigma_c relation is descri ed by a linear law out
to velocity dispersions as low as sigma_c~50km/s, while in previous works a
power law was adopted for galaxies with sigma_c>80k/ms.
Elliptical galaxies with Vc based on dynamical models or directly derived
from the HI rotation curves follow the same relation as the HSB galaxies in the
Vc-sigma_c plane. On the contrary, the LSB galaxies follow a different
relation, since most of them show either higher Vc (or lower sigma_c) with
respect to the HSB galaxies. This argues against the relevance of baryon
collapse in the radial density profile of the dark matter haloes of LSB
galaxies. (abridged)Comment: 18 pages, 4 figures, ApJ in pres
Cluster vs. Field Elliptical Galaxies and Clues on their Formation
Using new observations for a sample of 931 early-type galaxies we investigate
whether the \mg2--\so relation shows any dependence on the local environment.
The galaxies have been assigned to three different environments depending on
the local overdensity: clusters, groups, and field, having used our
completeredshift database to guide the assignment of galaxies. It is found that
cluster, group and field early-type galaxies follow almost identical \mg2--\so\
relations, with the largest \mg2 zero-point difference (clusters minus field)
being only mag. No correlation of the residuals is found with
the morphological type or the bulge to disk ratio. Using stellar population
models in a differential fashion, this small zero-point difference implies a
luminosity-weighted age difference of only Gyr between the
corresponding stellar populations, with field galaxies being younger. The
mass-weighted age difference could be significantly smaller, if minor events of
late star formation took place preferentially in field galaxies. We combine
these results with the existing evidence for the bulk of stars in cluster
early-type galaxies having formed at very high redshift, and conclude that the
bulk of stars in galactic spheroids had to form at high redshifts (z\gsim 3),
no matter whether such spheroids now reside in low or high density regions. The
cosmological implications of these findings are briefly discussed.Comment: 16 pages, 2 figures, accepted for publication in the ApJ.
Quantum Phase Shift in Chern-Simons Modified Gravity
Using a unified approach of optical-mechanical analogy in a semiclassical
formula, we evaluate the effect of Chern-Simons modified gravity on the quantum
phase shift of de Broglie waves in neutron interferometry. The phase shift
calculated here reveals, in a single equation, a combination of effects coming
from Newtonian gravity, inertial forces, Schwarzschild and Chern-Simons
modified gravity. However the last two effects, though new, turn out to be too
tiny to be observed, and hence only of academic interest at present. The
approximations, wherever used, as well as the drawbacks of the non-dynamical
approach are clearly indicated.Comment: 16 pages, minor errors corrected. Accepted for publication in Phys.
Rev.
Geometrical Effects of Baryon Density Inhomogeneities on Primordial Nucleosynthesis
We discuss effects of fluctuation geometry on primordial nucleosynthesis. For
the first time we consider condensed cylinder and cylindrical-shell fluctuation
geometries in addition to condensed spheres and spherical shells. We find that
a cylindrical shell geometry allows for an appreciably higher baryonic
contribution to be the closure density (\Omega_b h_{50}^2 \la 0.2) than that
allowed in spherical inhomogeneous or standard homogeneous big bang models.
This result, which is contrary to some other recent studies, is due to both
geometry and recently revised estimates of the uncertainties in the
observationally inferred primordial light-element abundances. We also find that
inhomogeneous primordial nucleosynthesis in the cylindrical shell geometry can
lead to significant Be and B production. In particular, a primordial beryllium
abundance as high as [Be] = 12 + log(Be/H) is possible while still
satisfying all of the light-element abundance constraints.Comment: Latex, 20 pages + 11 figures(not included). Entire ps file with
embedded figures available via anonymous ftp at
ftp://genova.mtk.nao.ac.jp/pub/prepri/bbgeomet.ps.g
Properties of mm galaxies: Constraints from K-band blank fields
We have used the IRAM Plateau de Bure mm interferometer to locate with
subarcsecond accuracy the dust emission of three of the brightest 1.2mm sources
in the NTT Deep Field (NDF) selected from our 1.2mm MAMBO survey at the IRAM
30m telescope. We combine these results with deep B to K imaging and VLA
interferometry. Strikingly, none of the three accurately located mm galaxies
MMJ120546-0741.5, MMJ120539-0745.4, and MMJ120517-0743.1 has a K-band
counterpart down to the faint limit of K>21.9. This implies that these three
galaxies are either extremely obscured and/or are at very high redshifts
(z>~4). We combine our results with literature data for 11 more (sub)mm
galaxies that are identified with similar reliability. In terms of their K-band
properties, the sample divides into three roughly equal groups: (i) undetected
to K~22, (ii) detected in the near-infrared but not the optical and (iii)
detected in the optical with the possibility of optical follow-up spectroscopy.
We find a trend in this sample between near-infrared to submm and submm to
radio spectral indices, which in comparison to spectral energy distributions
(SEDs) of low redshift infrared luminous galaxies suggests that the most
plausible primary factor causing the extreme near-infrared faintness of our
objects is their high redshift. We show that the near-infrared to radio SEDs of
the sample are inconsistent with SEDs that resemble local far-infrared cool
galaxies with moderate luminosities, which were proposed in some models of the
submm sky. We briefly discuss the implications of the results for our
understanding of galaxy formation.Comment: aastex, 5 figures. Accepted by Ap
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