2,070 research outputs found
Searching for Stars in Compact High-Velocity Clouds. II
We address the hypothesis that High Velocity Clouds correspond to the
"missing" dwarf galaxies of the Local Group predicted by cosmological
simulations. To this end, we present optical and near-infrared photometry of
five additional High Velocity Clouds, one of which produces Lyman series
absorption on the sight line towards the Quasar Ton S210, with sufficient
resolution and sensitivity to enable the detection of an associated stellar
content. We do not detect significant stellar populations intrinsic to any of
the five clouds. In combination with the results from our paper I, which had
yielded non detections of stellar content in another five cases, we find that
there is a 50% chance of getting a null result in ten trials if fewer than 7%
of all High Velocity Clouds contain stars. We conclude that the population of
High Velocity Clouds is an unlikely repository for the "missing" dwarfs of the
Local Group.Comment: 6 pages, 3 figures. submitted to MNRA
Searching for Stars in Compact High-Velocity Clouds. I First Results from VLT and 2MASS
We investigate the hypothesis that compact high-velocity clouds (CHVC) are
the "missing" dwarf galaxies of the Local Group, by searching them for
populations of resolved stars. To this end we conducted two distinct tests
based on optical and near-infrared single-star photometry. The optical and the
near-infrared experiments complement one another; the optical data help us to
rule out distant populations but they are restricted to the central regions of
the gas distributions, whereas the near-infrared photometry allows us to set
limits on nearby populations spread over the typical cloud size. First, we
discuss deep optical single-star photometry of five CHVCs in the V and I
filters, obtained with the FORS instrument at the Very Large Telecope (VLT). We
find that their optical colour-magnitude diagrams are indistinguishable from
that of a population of Galactic stars, and attribute all of the resolved stars
to Galactic foreground. We present simulations which address the question of
how much of a "normal" dwarf-galaxy type population we might hide in the data.
A Kolmogorov-Smirnov test allows us to set very stringent limits on the absence
of a resolved stellar population in CHVCs. Second, we also culled near infrared
single-star photometry in the J, H, and K_S bands for four of the CHVCs from
the Two Micron All Sky Survey (2MASS). The infrared data do not reveal any
stellar contents in the CHVCs which resembles that of nearby dwarf galaxies
either, and are explained with Galactic foreground as well. We interpret our
null detections to indicate that the five CHVCs investigated by us do not host
an associated stellar content which is similar to that of the known dwarf
galaxies of the Local Group. These CHVCs are very likely pure hydrogen clouds
in which no star formation has taken place over cosmic time.Comment: 13 pages, 10 figures, accepted for publication by MNRA
The Munich Near-Infrared Cluster Survey -- IV. Biases in the Completeness of Near-Infrared Imaging Data
We present the results of completeness simulations for the detection of point
sources as well as redshifted elliptical and spiral galaxies in the K'-band
images of the Munich Near-Infrared Cluster Survey (MUNICS). The main focus of
this work is to quantify the selection effects introduced by threshold-based
object detection algorithms used in deep imaging surveys. Therefore, we
simulate objects obeying the well-known scaling relations between effective
radius and central surface brightness, both for de Vaucouleurs and exponential
profiles. The results of these simulations, while presented for the MUNICS
project, are applicable in a much wider context to deep optical and
near-infrared selected samples. We investigate the detection probability as
well as the reliability for recovering the true total magnitude with Kron-like
(adaptive) aperture photometry. The results are compared to the predictions of
the visibility theory of Disney and Phillipps in terms of the detection rate
and the lost-light fraction. Additionally, the effects attributable to seeing
are explored. The results show a bias against detecting high-redshifted massive
elliptical galaxies in comparison to disk galaxies with exponential profiles,
and that the measurements of the total magnitudes for intrinsically bright
elliptical galaxies are systematically too faint. Disk galaxies, in contrast,
show no significant offset in the magnitude measurement of luminous objects.
Finally we present an analytic formula to predict the completeness of
point-sources using only basic image parameters.Comment: 13 pages, 11 figures, accepted for publication in MNRA
Phenology satellite experiment
The detection of a phenological event (the brown wave-vegetation senescence) for specific forest and crop types using ERTS-1 imagery is described. Data handling techniques included computer analysis and photo interpretation procedures. Computer analysis of ERTS-1 multispectral scanner digital tapes in all bands was used to give the relative changes of spectral reflectance with time of forests and specified crops. These data were obtained for a number of the study's twenty-four sites located within four north-south corridors across the United States. Analysis of ground observation photography and ERTS-1 imagery for sites in the Appalachian Corridor and Mississippi Valley Corridor indicates that the recession of vegetation development can be detected very well. Tentative conclusions are that specific phenological events such as crop maturity or leaf fall can be mapped for specific sites and possibly for entire regions
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