40 research outputs found
An Inhomogeneous Model Universe Behaving Homogeneously
We present a new model universe based on the junction of FRW to flat
Lemaitre-Tolman-Bondi (LTB) solutions of Einstein equations along our past
light cone, bringing structures within the FRW models. The model is assumed
globally to be homogeneous, i.e. the cosmological principle is valid. Local
inhomogeneities within the past light cone are modeled as a flat LTB, whereas
those outside the light cone are assumed to be smoothed out and represented by
a FRW model. The model is singularity free, always FRW far from the observer
along the past light cone, gives way to a different luminosity distance
relation as for the CDM/FRW models, a negative deceleration parameter near the
observer, and correct linear and non-linear density contrast. As a whole, the
model behaves like a FRW model on the past light cone with a special behavior
of the scale factor, Hubble and deceleration parameter, mimicking dark energy.Comment: 23 pages, 19 figures, published version in GR
Dwarf Galaxies in the Coma Cluster: I. Velocity Dispersion Measurements
We present the study of a large sample of early-type dwarf galaxies in the
Coma cluster observed with DEIMOS on the Keck II to determine their internal
velocity dispersion. We focus on a subsample of 41 member dwarf elliptical
galaxies for which the velocity dispersion can be reliably measured, 26 of
which were studied for the first time. The magnitude range of our sample is
mag.
This paper (paper I) focuses on the measurement of the velocity dispersion
and their error estimates. The measurements were performed using {\it pPXF
(penalised PiXel Fitting)} and using the Calcium triplet absorption lines. We
use Monte Carlo bootstrapping to study various sources of uncertainty in our
measurements, namely statistical uncertainty, template mismatch and other
systematics. We find that the main source of uncertainty is the template
mismatch effect which is reduced by using templates with a range of spectral
types.
Combining our measurements with those from the literature, we study the
Faber-Jackson relation () and find that the slope of the
relation is for galaxies brighter than mag.
A comprehensive analysis of the results combined with the photometric
properties of these galaxies is reported in paper II.Comment: 17 pages, 13 figures and 9 tables. Accepted for publication in
Monthly Notices of the Royal Astronomical Society Main Journal. (ref.
MN-10-2692-MJ.R2) Accepted 2011 September 27. Received 2011 September 13; in
original form 2010 December 1
Cosmological Backreaction from Perturbations
We reformulate the averaged Einstein equations in a form suitable for use
with Newtonian gauge linear perturbation theory and track the size of the
modifications to standard Robertson-Walker evolution on the largest scales as a
function of redshift for both Einstein de-Sitter and Lambda CDM cosmologies. In
both cases the effective energy density arising from linear perturbations is of
the order of 10^-5 the matter density, as would be expected, with an effective
equation of state w ~ -1/19. Employing a modified Halofit code to extend our
results to quasilinear scales, we find that, while larger, the deviations from
Robertson-Walker behaviour remain of the order of 10^-5.Comment: 15 pages, 8 figures; replaced by version accepted by JCA
Cosmicflows-4
With Cosmicflows-4, distances are compiled for 55,877 galaxies gathered into
38,065 groups. Eight methodologies are employed, with the largest numbers
coming from the correlations between the photometric and kinematic properties
of spiral galaxies (TF) and elliptical galaxies (FP). Supernovae that arise
from degenerate progenitors (SNIa) are an important overlapping component.
Smaller contributions come from distance estimates from the surface brightness
fluctuations of elliptical galaxies (SBF) and the luminosities and expansion
rates of core collapse supernovae (SNII). Cepheid Period-Luminosity Relation
(CPLR) and Tip of the Red Giant Branch (TRGB) observations founded on local
stellar parallax measurements along with the geometric maser distance to NGC
4258 provide the absolute scaling of distances. The assembly of galaxies into
groups is an important feature of the study in facilitating overlaps between
methodologies. Merging between multiple contributions within a methodology and
between methodologies is carried out with Bayesian Markov chain Monte Carlo
procedures. The final assembly of distances is compatible with a value of the
Hubble constant of km s Mpc with the small statistical
error km s Mpc but a large potential systematic error
~3 km s Mpc. Peculiar velocities can be inferred from the
measured distances. The interpretation of the field of peculiar velocities is
complex because of large errors on individual components and invites analyses
beyond the scope of this study.Comment: 38 pages, 24 figures. catalogs available at edd.ifa.hawaii.edu.
Accepted to Ap
Dwarf Galaxies in the Coma Cluster: II. Spectroscopic and Photometric Fundamental Planes
We present a study of the fundamental plane, FP, for a sample of 71 dwarf
galaxies in the core of Coma cluster in magnitude range .
Taking advantage of high resolution DEIMOS spectrograph on Keck II for
measuring the internal velocity dispersion of galaxies and high resolution
imaging of HST/ACS, which allows an accurate surface brightness modeling, we
extend the fundamental plane (FP) of galaxies to 1 magnitude fainter
luminosities than all the previous studies of the FP in Coma cluster. We find
that, the scatter about the FP depends on the faint-end luminosity cutoff, such
that the scatter increases for fainter galaxies. The residual from the FP
correlates with the galaxy colour, with bluer galaxies showing larger residuals
from FP.
We find in F814W-band indicating that in faint
dwarf ellipticals, the ratio is insensitive to the mass. We find that
less massive dwarf ellipticals are bluer than their brighter counterparts,
possibly indicating ongoing star formation activity. Although tidal encounters
and harassment can play a part in removing stars and dark matter from the
galaxy, we believe that the dominant effect will be the stellar wind associated
with the star formation, which will remove material from the galaxy resulting
in larger ratios. We attribute the deviation of a number of faint blue
dwarfs from the FP of brighter ellipticals to this effect.
We also study other scaling relations involving galaxy photometric properties
including the photometric plane. We show that, compared to the FP, the scatter
about the photometric plane is smaller at the faint end.Comment: 19 pages, 12 figures and 4 tables. Accepted for publication in
Monthly Notices of the Royal Astronomical Society Main Journal. (ref.
MN-11-0266-MJ.R1) Accepted 2011 October 10. Received 2011 September 13; in
original form 2011 February
The HST/ACS Coma Cluster Survey. II. Data Description and Source Catalogs
The Coma cluster was the target of a HST-ACS Treasury program designed for deep imaging in the F475W and F814W passbands. Although our survey was interrupted by the ACS instrument failure in 2007, the partially completed survey still covers ~50% of the core high-density region in Coma. Observations were performed for 25 fields that extend over a wide range of cluster-centric radii (~1.75 Mpc) with a total coverage area of 274 arcmin^2. The majority of the fields are located near the core region of Coma (19/25 pointings) with six additional fields in the south-west region of the cluster. In this paper we present reprocessed images and SExtractor source catalogs for our survey fields, including a detailed description of the methodology used for object detection and photometry, the subtraction of bright galaxies to measure faint underlying objects, and the use of simulations to assess the photometric accuracy and completeness of our catalogs. We also use simulations to perform aperture corrections for the SExtractor Kron magnitudes based only on the measured source flux and half-light radius. We have performed photometry for ~73,000 unique objects; one-half of our detections are brighter than the 10-sigma point-source detection limit at F814W=25.8 mag (AB). The slight majority of objects (60%) are unresolved or only marginally resolved by ACS. We estimate that Coma members are 5-10% of all source detections, which consist of a large population of unresolved objects (primarily GCs but also UCDs) and a wide variety of extended galaxies from a cD galaxy to dwarf LSB galaxies. The red sequence of Coma member galaxies has a constant slope and dispersion across 9 magnitudes (-21<M_F814W<-13). The initial data release for the HST-ACS Coma Treasury program was made available to the public in 2008 August. The images and catalogs described in this study relate to our second data release
Apparent and average acceleration of the Universe
In this paper we consider the relation between the volume deceleration
parameter obtained within the Buchert averaging scheme and the deceleration
parameter derived from the supernova observation. This work was motivated by
recent findings that showed that there are models which despite
have volume deceleration parameter . This opens the possibility
that backreaction and averaging effects may be used as an interesting
alternative explanation to the dark energy phenomenon.
We have calculated in some Lema\^itre--Tolman models. For those
models which are chosen to be realistic and which fit the supernova data, we
find that , while those models which we have been able to find
which exhibit turn out to be unrealistic. This indicates that
care must be exercised in relating the deceleration parameter to observations.Comment: 15 pages, 5 figures; matches published versio
Galaxy bulges and their massive black holes: a review
With references to both key and oft-forgotten pioneering works, this article
starts by presenting a review into how we came to believe in the existence of
massive black holes at the centres of galaxies. It then presents the historical
development of the near-linear (black hole)-(host spheroid) mass relation,
before explaining why this has recently been dramatically revised. Past
disagreement over the slope of the (black hole)-(velocity dispersion) relation
is also explained, and the discovery of sub-structure within the (black
hole)-(velocity dispersion) diagram is discussed. As the search for the
fundamental connection between massive black holes and their host galaxies
continues, the competing array of additional black hole mass scaling relations
for samples of predominantly inactive galaxies are presented.Comment: Invited (15 Feb. 2014) review article (submitted 16 Nov. 2014). 590
references, 9 figures, 25 pages in emulateApJ format. To appear in "Galactic
Bulges", E. Laurikainen, R.F. Peletier, and D.A. Gadotti (eds.), Springer
Publishin