32 research outputs found
Discovery of two M32 twins in Abell 1689
The M31 satellite galaxy M32 has long been considered an object of unique
properties, being the most extreme example of the very rare compact elliptical
galaxy class. Here we present the spectroscopic discovery of two M32 twins in
the massive galaxy cluster Abell 1689. As these objects are so rare, this is an
important step towards a better understanding of the nature of compact
galaxies. The two M32 twins had first been detected within our photometric
search for ultra compact dwarf galaxy (UCDs) candidates in A1689 (Mieske et al.
2004) with the Advanced Camera for Surveys (ACS). Their luminosities (M_V ~ -17
mag) are very similar to M32 and their surface brightness profiles are
consistent with that of M32 projected to A1689's distance. From our ACS imaging
we detect several fainter compact galaxy candidates with luminosities
intermediate between M32 and the Fornax UCDs. If spectroscopically confirmed as
cluster members, this would almost close the gap in the magnitude-surface
brightness plane between the region of UCDs and the compact ellipticals,
implying a sequence of faint compact galaxies well separated from that of dwarf
ellipticals.Comment: 5 pages, 5 figures, accepted for publication in A&A letter
The Galaxy Content of SDSS Clusters and Groups
Imaging data from the Sloan Digital Sky Survey are used to characterize the
population of galaxies in groups and clusters detected with the MaxBCG
algorithm. We investigate the dependence of Brightest Cluster Galaxy (BCG)
luminosity, and the distributions of satellite galaxy luminosity and satellite
color, on cluster properties over the redshift range 0.1 < z < 0.3. The size of
the dataset allows us to make measurements in many bins of cluster richness,
radius and redshift. We find that, within r_200 of clusters with mass above
3e13 h-1 M_sun, the luminosity function of both red and blue satellites is only
weakly dependent on richness. We further find that the shape of the satellite
luminosity function does not depend on cluster-centric distance for magnitudes
brighter than ^{0.25}M_i - 5log(h) < -19. However, the mix of faint red and
blue galaxies changes dramatically. The satellite red fraction is dependent on
cluster-centric distance, galaxy luminosity and cluster mass, and also
increases by ~5% between redshifts 0.28 and 0.2, independent of richness. We
find that BCG luminosity is tightly correlated with cluster richness, scaling
as L_{BCG} ~ M_{200}^{0.3}, and has a Gaussian distribution at fixed richness,
with sigma_{log L} ~ 0.17 for massive clusters. The ratios of BCG luminosity to
total cluster luminosity and characteristic satellite luminosity scale strongly
with cluster richness: in richer systems, BCGs contribute a smaller fraction of
the total light, but are brighter compared to typical satellites. This study
demonstrates the power of cross-correlation techniques for measuring galaxy
populations in purely photometric data.Comment: 22 pages, 14 figures, submitted to Ap
Evolution of the Color-Magnitude Relation in Galaxy Clusters at z ~1 from the ACS Intermediate Redshift Cluster Survey
We apply detailed observations of the Color-Magnitude Relation (CMR) with the
ACS/HST to study galaxy evolution in eight clusters at z~1. The early-type red
sequence is well defined and elliptical and lenticular galaxies lie on similar
CMRs. We analyze CMR parameters as a function of redshift, galaxy properties
and cluster mass. For bright galaxies (M_B < -21mag), the CMR scatter of the
elliptical population in cluster cores is smaller than that of the S0
population, although the two become similar at faint magnitudes. While the
bright S0 population consistently shows larger scatter than the ellipticals,
the scatter of the latter increases in the peripheral cluster regions. If we
interpret these results as due to age differences, bright elliptical galaxies
in cluster cores are on average older than S0 galaxies and peripheral
elliptical galaxies (by about 0.5Gyr). CMR zero point, slope, and scatter in
the (U-B)_z=0 rest-frame show no significant evolution out to redshift z~1.3
nor significant dependence on cluster mass. Two of our clusters display CMR
zero points that are redder (by ~2sigma) than the average (U-B)_z=0 of our
sample. We also analyze the fraction of morphological early-type and late-type
galaxies on the red sequence. We find that, while in the majority of the
clusters most (80% to 90%) of the CMR population is composed of early-type
galaxies, in the highest redshift, low mass cluster of our sample, the CMR
late-type/early-type fractions are similar (~50%), with most of the late-type
population composed of galaxies classified as S0/a. This trend is not
correlated with the cluster's X-ray luminosity, nor with its velocity
dispersion, and could be a real evolution with redshift.Comment: ApJ, in press, 27 pages, 22 figure
Keck Spectroscopy of Faint 3<z<7 Lyman Break Galaxies: - I. New constraints on cosmic reionisation from the luminosity and redshift-dependent fraction of Lyman-alpha emission
We present results from a new Keck spectroscopic survey of UV-faint LBGs in
the redshift range 3<z<7. Combined with earlier Keck and published ESO VLT
data, our sample contains more than 600 dropouts, offering new insight into the
nature of sub-L* sources typical of those likely to dominate the cosmic
reionisation process. Here we use this sample to characterise the fraction of
strong Lya emitters within the continuum-selected dropouts. By quantifying how
the "Lya fraction" varies with redshift, we seek to constrain changes in Lya
transmission associated with reionisation. In order to distinguish the effects
of reionisation from other factors which affect the Lya fraction (e.g. dust,
ISM kinematics), we study the luminosity and redshift-dependence of the Lya
fraction over 3<z<6, when the IGM is known to be ionised. These results reveal
that low luminosity galaxies show strong Lya emission much more frequently than
luminous systems, and that at fixed luminosity, the prevalence of strong Lya
emission increases moderately with redshift over 3 < z < 6. Based on the
correlation between blue UV slopes and strong Lya emitting galaxies in our
dataset, we argue that the Lya fraction trends are governed by redshift and
luminosity-dependent variations in the dust obscuration, with likely additional
contributions from trends in the kinematics and covering fraction of neutral
hydrogen. We find a tentative decrease in the Lya fraction at z~7 based on the
limited IR spectra for candidate z~7 lensed LBGs, a result which, if confirmed
with future surveys, would suggest an increase in the neutral fraction by this
epoch. Given the supply of z and Y-drops now available from Hubble WFC3/IR
surveys, we show it will soon be possible to significantly improve estimates of
the Lya fraction using optical and near-IR spectrographs, thereby extending the
study conducted in this paper to 7<z<8.Comment: 23 pages, 15 figures, submitted to MNRA
Clustering properties of galaxies selected in stellar mass: Breaking down the link between luminous and dark matter in massive galaxies from z=0 to z=2
We present a study on the clustering of a stellar mass selected sample of
18,482 galaxies with stellar masses M*>10^10M(sun) at redshifts 0.4<z<2.0,
taken from the Palomar Observatory Wide-field Infrared Survey. We examine the
clustering properties of these stellar mass selected samples as a function of
redshift and stellar mass, and discuss the implications of measured clustering
strengths in terms of their likely halo masses. We find that galaxies with high
stellar masses have a progressively higher clustering strength, and amplitude,
than galaxies with lower stellar masses. We also find that galaxies within a
fixed stellar mass range have a higher clustering strength at higher redshifts.
We furthermore use our measured clustering strengths, combined with models from
Mo & White (2002), to determine the average total masses of the dark matter
haloes hosting these galaxies. We conclude that for all galaxies in our sample
the stellar-mass-to-total-mass ratio is always lower than the universal
baryonic mass fraction. Using our results, and a compilation from the
literature, we furthermore show that there is a strong correlation between
stellar-mass-to-total-mass ratio and derived halo masses for central galaxies,
such that more massive haloes contain a lower fraction of their mass in the
form of stars over our entire redshift range. For central galaxies in haloes
with masses M(halo)>10^13M(sun) we find that this ratio is <0.02, much lower
than the universal baryonic mass fraction. We show that the remaining baryonic
mass is included partially in stars within satellite galaxies in these haloes,
and as diffuse hot and warm gas. We also find that, at a fixed stellar mass,
the stellar-to-total-mass ratio increases at lower redshifts. This suggests
that galaxies at a fixed stellar mass form later in lower mass dark matter
haloes, and earlier in massive haloes. We interpret this as a "halo downsizing"
effect, however some of this evolution could be attributed to halo assembly
bias.Comment: Accepted for publication in MNRAS. 19 pages, 8 figures and 3 tables
Fitting the integrated Spectral Energy Distributions of Galaxies
Fitting the spectral energy distributions (SEDs) of galaxies is an almost
universally used technique that has matured significantly in the last decade.
Model predictions and fitting procedures have improved significantly over this
time, attempting to keep up with the vastly increased volume and quality of
available data. We review here the field of SED fitting, describing the
modelling of ultraviolet to infrared galaxy SEDs, the creation of
multiwavelength data sets, and the methods used to fit model SEDs to observed
galaxy data sets. We touch upon the achievements and challenges in the major
ingredients of SED fitting, with a special emphasis on describing the interplay
between the quality of the available data, the quality of the available models,
and the best fitting technique to use in order to obtain a realistic
measurement as well as realistic uncertainties. We conclude that SED fitting
can be used effectively to derive a range of physical properties of galaxies,
such as redshift, stellar masses, star formation rates, dust masses, and
metallicities, with care taken not to over-interpret the available data. Yet
there still exist many issues such as estimating the age of the oldest stars in
a galaxy, finer details ofdust properties and dust-star geometry, and the
influences of poorly understood, luminous stellar types and phases. The
challenge for the coming years will be to improve both the models and the
observational data sets to resolve these uncertainties. The present review will
be made available on an interactive, moderated web page (sedfitting.org), where
the community can access and change the text. The intention is to expand the
text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics &
Space Scienc
A search for flaring Very-High-Energy cosmic-ray sources with the L3+C muon spectrometer
The L3+C muon detector at the Cern electron-position collider, LEP, is used for the detection of very-high-energy cosmic \gamma-ray sources through the observation of muons of energies above 20, 30, 50 and 100 GeV. Daily or monthly excesses in the rate of single-muon events pointing to some particular direction in the sky are searched for. The periods from mid July to November 1999, and April to November 2000 are considered. Special attention is also given to a selection of known \gamma-ray sources. No statistically significant excess is observed for any direction or any particular source
Shedding Light on the Galaxy Luminosity Function
From as early as the 1930s, astronomers have tried to quantify the
statistical nature of the evolution and large-scale structure of galaxies by
studying their luminosity distribution as a function of redshift - known as the
galaxy luminosity function (LF). Accurately constructing the LF remains a
popular and yet tricky pursuit in modern observational cosmology where the
presence of observational selection effects due to e.g. detection thresholds in
apparent magnitude, colour, surface brightness or some combination thereof can
render any given galaxy survey incomplete and thus introduce bias into the LF.
Over the last seventy years there have been numerous sophisticated
statistical approaches devised to tackle these issues; all have advantages --
but not one is perfect. This review takes a broad historical look at the key
statistical tools that have been developed over this period, discussing their
relative merits and highlighting any significant extensions and modifications.
In addition, the more generalised methods that have emerged within the last few
years are examined. These methods propose a more rigorous statistical framework
within which to determine the LF compared to some of the more traditional
methods. I also look at how photometric redshift estimations are being
incorporated into the LF methodology as well as considering the construction of
bivariate LFs. Finally, I review the ongoing development of completeness
estimators which test some of the fundamental assumptions going into LF
estimators and can be powerful probes of any residual systematic effects
inherent magnitude-redshift data.Comment: 95 pages, 23 figures, 3 tables. Now published in The Astronomy &
Astrophysics Review. This version: bring in line with A&AR format
requirements, also minor typo corrections made, additional citations and
higher rez images adde
XX. CoRoT-20b: A very high density, high eccentricity transiting giant planet
We report the discovery by the CoRoT space mission of a new giant planet, CoRoT-20b. The planet has a mass of 4.24 +/- 0.23 MJ and a radius of 0.84 +/- 0.04 RJ. With a mean density of 8.87 +/- 1.10 g/cm^3, it is among the most compact planets known so far. Evolution models for the planet suggest a mass of heavy elements of the order of 800 ME if embedded in a central core, requiring a revision either of the planet formation models or of planet evolution and structure models. We note however that smaller amounts of heavy elements are expected from more realistic models in which they are mixed throughout the envelope. The planet orbits a G-type star with an orbital period of 9.24 days and an eccentricity of 0.56. The star's projected rotational velocity is vsini = 4.5 +/- 1.0 km/s, corresponding to a spin period of 11.5 +/- 3.1 days if its axis of rotation is perpendicular to the orbital plane. In the framework of Darwinian theories and neglecting stellar magnetic breaking, we calculate the tidal evolution of the system and show that CoRoT-20b is presently one of the very few Darwin-stable planets that is evolving towards a triple synchronous state with equality of the orbital, planetary and stellar spin periods