3,325 research outputs found
Hubble Space Telescope Planetary Camera Images of NGC 1316
We present HST Planetary Camera V and I~band images of the central region of
the peculiar giant elliptical galaxy NGC 1316. The inner profile is well fit by
a nonisothermal core model with a core radius of 0.41" +/- 0.02" (34 pc). At an
assumed distance of 16.9 Mpc, the deprojected luminosity density reaches \sim
2.0 \times 10^3 L_{\sun} pc.
Outside the inner two or three arcseconds, a constant mass-to-light ratio of
is found to fit the observed line width measurements. The
line width measurements of the center indicate the existence of either a
central dark object of mass 2 \times 10^9 M_{\sun}, an increase in the
stellar mass-to-light ratio by at least a factor of two for the inner few
arcseconds, or perhaps increasing radial orbit anisotropy towards the center.
The mass-to-light ratio run in the center of NGC 1316 resembles that of many
other giant ellipticals, some of which are known from other evidence to harbor
central massive dark objects (MDO's).
We also examine twenty globular clusters associated with NGC 1316 and report
their brightnesses, colors, and limits on tidal radii. The brightest cluster
has a luminosity of 9.9 \times 10^6 L_{\sun} (), and the
faintest detectable cluster has a luminosity of 2.4 \times 10^5 L_{\sun}
(). The globular clusters are just barely resolved, but their core
radii are too small to be measured. The tidal radii in this region appear to be
35 pc. Although this galaxy seems to have undergone a substantial merger
in the recent past, young globular clusters are not detected.Comment: 21 pages, latex, postscript figures available at
ftp://delphi.umd.edu/pub/outgoing/eshaya/fornax
Supermassive Black Holes and the Evolution of Galaxies
Black holes, an extreme consequence of the mathematics of General Relativity,
have long been suspected of being the prime movers of quasars, which emit more
energy than any other objects in the Universe. Recent evidence indicates that
supermassive black holes, which are probably quasar remnants, reside at the
centers of most galaxies. As our knowledge of the demographics of these relics
of a violent earlier Universe improve, we see tantalizing clues that they
participated intimately in the formation of galaxies and have strongly
influenced their present-day structure.Comment: 20 pages, - This is a near-duplicate of the paper in Nature 395, A14,
1998 (Oct 1
Selection Bias in Observing the Cosmological Evolution of the Mbh-sigma and Mbh-L Relationships
Programs to observe evolution in the Mbh-sigma or Mbh-L relations typically
compare black-hole masses, Mbh, in high-redshift galaxies selected by nuclear
activity to Mbh in local galaxies selected by luminosity L, or stellar velocity
dispersion sigma. Because AGN luminosity is likely to depend on Mbh, selection
effects are different for high-redshift and local samples, potentially
producing a false signal of evolution. This bias arises because cosmic scatter
in the Mbh-sigma and Mbh-L relations means that the mean log(L) or log(sigma)
among galaxies that host a black hole of given Mbh, may be substantially
different than the log(L) or log(sigma) obtained from inverting the Mbh-L or
Mbh-sigma relations for the same nominal Mbh. The bias is particularly strong
at high Mbh, where the luminosity and dispersion functions of galaxies are
falling rapidly. The most massive black holes occur more often as rare outliers
in galaxies of modest mass than in the even rarer high-mass galaxies, which
would otherwise be the sole location of such black holes in the absence of
cosmic scatter. Because of this bias, Mbh will typically appear to be too large
in the distant sample for a given L or sigma. For the largest black holes and
the largest plausible cosmic scatter, the bias can reach a factor of 3 in Mbh
for the Mbh-sigma relation and a factor of 9 for the Mbh-L relation.
Unfortunately, the actual cosmic scatter is not known well enough to correct
for the bias. Measuring evolution of the Mbh and galaxy property relations
requires object selection to be precisely defined and exactly the same at all
redshifts.Comment: 28 pages, 6 figures, submitted to the Astrophysical Journa
Blazars in the early Universe
We investigate the relative occurrence of radio--loud and radio-quiet quasars
in the first billion years of the Universe, powered by black holes heavier than
one billion solar masses. We consider the sample of high-redshfit blazars
detected in the hard X-ray band in the 3-years all sky survey performed by the
Burst Alert Telescope (BAT) onboard the Swift satellite. All the black holes
powering these blazars exceed a billion solar mass, with accretion luminosities
close to the Eddington limit. For each blazar pointing at us, there must be
hundreds of similar sources (having black holes of similar masses) pointing
elsewhere. This puts constraints on the density of billion solar masses black
holes at high redshift (z>4), and on the relative importance of (jetted)
radio-loud vs radio-quiet sources. We compare the expected number of high
redshift radio--loud sources with the high luminosity radio-loud quasars
detected in the Sloan Digital Sky Survey (SDSS), finding agreement up to z~3,
but a serious deficit at z>3 of SDSS radio-loud quasars with respect to the
expectations. We suggest that the most likely explanations for this
disagreement are: i) the ratio of blazar to misaligned radio-sources decreases
by an order of magnitude above z=3, possibly as a result of a decrease of the
average bulk Lorentz factor; ii) the SDSS misses a large fraction of radio-loud
sources at high redshifts, iii) the SDSS misses both radio-loud and radio-quiet
quasars at high redshift, possibly because of obscuration or because of
collimation of the optical-UV continuum in systems accreting near Eddington.
These explanations imply very different number density of heavy black holes at
high redshifts, that we discuss in the framework of the current ideas about the
relations of dark matter haloes at high redshifts and the black hole they host.Comment: MNRAS, in pres
Cosmic Background dipole measurements with Planck-High Frequency Instrument
This paper discusses the Cosmic Background (CB) dipoles observations in the
framework of the Planck mission. Dipoles observations can be used in three
ways: (i) It gives a measurement of the peculiar velocity of our Galaxy which
is an important observation in large scale structures formation model. (ii)
Measuring the dipole can give unprecedent information on the monopole (that can
be in some cases hard to obtain due to large foreground contaminations). (iii)
The dipole can be an ideal absolute calibrator, easily detectable in
cosmological experiments. Following the last two objectives, the main goal of
the work presented here is twofold. First, we study the accuracy of the
Planck-HFI calibration using the Cosmic Microwave Background (CMB) dipole
measured by COBE as well as the Earth orbital motion dipole. We show that we
can reach for HFI, a relative calibration between rings of about 1% and an
absolute calibration better than 0.4% for the CMB channels (in the end, the
absolute calibration will be limited by the uncertainties on the CMB
temperature). We also show that Planck will be able to measure the CMB dipole
direction at better than 1.7 arcmin and improve on the amplitude. Second, we
investigate the detection of the Cosmic Far-Infrared Background (FIRB) dipole.
Measuring this dipole could give a new and independent determination of the
FIRB for which a direct determination is quite difficult due to Galactic dust
emission contamination. We show that such a detection would require a Galactic
dust emission removal at better than 1%, which will be very hard to achieve.Comment: 10 pages, 13 figures, submitted to A&A, uses aa.sty V5.
Cosmic Microwave Background Dipole induced by double inflation
The observed CMBR dipole is generally interpreted as the consequence of the
peculiar motion of the Sun with respect to the reference frame of the CMBR.
This article proposes an alternative interpretation in which the observed
dipole is the result of isocurvature perturbations on scales larger than the
present Hubble radius. These perturbations are produced in the simplest model
of double inflation, depending on three parameters. The observed dipole and
quadrupole can be explained in this model, while severely constraining its
parameters.Comment: Latex, 9 pages, no figure, to appear in Phys. Rev.
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