828 research outputs found
European ALMA operations: the interaction with and support to the users
The Atacama Large Millimetre/submillimetre Array (ALMA) is one of the largest
and most complicated observatories ever built. Constructing and operating an
observatory at high altitude (5000m) in a cost effective and safe manner, with
minimal effect on the environment creates interesting challenges. Since the
array will have to adapt quickly to prevailing weather conditions, ALMA will be
operated exclusively in service mode. By the time of full science operations,
the fundamental ALMA data product shall be calibrated, deconvolved data cubes
and images, but raw data and data reduction software will be made available to
users as well. User support is provided by the ALMA Regional Centres (ARCs)
located in Europe, North America and Japan. These ARCs constitute the interface
between the user community and the ALMA observatory in Chile. For European
users the European ARC is being set up as a cluster of nodes located throughout
Europe, with the main centre at the ESO Headquarters in Garching. The main
centre serves as the access portal and in synergy with the distributed network
of ARC nodes, the main aim of the ARC is to optimize the ALMA science output
and to fully exploit this unique and powerful facility. The aim of this article
is to introduce the process of proposing for observing time, subsequent
execution of the observations, obtaining and processing of the data in the ALMA
epoch. The complete end-to-end process of the ALMA data flow from the proposal
submission to the data delivery is described.Comment: 7 pages, three figure
Hydrogen 21-Centimeter Emission from a Galaxy at Cosmological Distance
We have detected the neutral atomic hydrogen (HI) emission line at a
cosmologically significant distance (z=0.18) in the rich galaxy cluster Abell
2218 with the Westerbork Synthesis Radio Telescope. The HI emission originates
in a spiral galaxy 2.0 megaparsecs from the cluster core. No other significant
detections have been made in the cluster, suggesting that the mechanisms that
remove neutral gas from cluster galaxies are efficient. We infer that fewer
than three gas-rich galaxies were accreted by Abell 2218 over the past 10^9
years. This low accretion rate is qualitatively consistent with low-density
cosmological models in which clusters are largely assembled at z>1.Comment: 7 pages, 2 figures. Science 2001 September 7; 293, 1800 (in reports).
Also available from
http://www.sciencemag.org/cgi/content/abstract/293/5536/180
Constraints on the Space Density of Extragalactic HVCs
High Velocity Clouds (HVCs) have recently attracted renewed attention as
being long lived, massive dark matter dominated clouds of primordial
composition distributed throughout the Local Group. In this picture the HVCs
would contain a few times 10^7 M_sun of HI and would be at distances of a few
hundred kpc to 1.5 Mpc from the Local Group barycenter. If this extragalactic
interpretation of HVCs is true, similar clouds are expected in other galaxy
groups and around galaxies. We discuss the limits blind HI surveys and QSO
absorption line studies put on this proposed population of clouds.Comment: 8 pages, 2 figures, to appear in proceedings of "Mapping the Hidden
Universe", held in Guanajuato, Mexico, Feb 200
First measurement of HI 21cm emission from a GRB host galaxy indicates a post-merger system
We report the detection and mapping of atomic hydrogen in HI 21cm emission
from ESO 184-G82, the host galaxy of the gamma ray burst 980425. This is the
first instance where HI in emission has been detected from a galaxy hosting a
gamma ray burst. ESO 184-G82 is an isolated galaxy and contains a Wolf-Rayet
region close to the location of the gamma ray burst and the associated
supernova, SN 1998bw. This is one of the most luminous HII regions identified
in the local Universe, with a very high inferred density of star formation. The
HI 21cm observations reveal a high HI mass for the galaxy, twice as large as
the stellar mass. The spatial and velocity distribution of the HI 21cm emission
reveals a disturbed rotating gas disk, which suggests that the galaxy has
undergone a recent minor merger that disrupted its rotation. We find that the
Wolf-Rayet region and the gamma ray burst are both located in the highest HI
column density region of the galaxy. We speculate that the merger event has
resulted in shock compression of the gas, triggering extreme star formation
activity, and resulting in the formation of both the Wolf-Rayet region and the
gamma ray burst. The high HI column density environment of the GRB is
consistent with the high HI column densities seen in absorption in the host
galaxies of high redshift gamma ray bursts.Comment: Accepted for publication in MNRAS Letters. 5 pages, 5 figures, 2
tables. For the definitive version visit http://mnrasl.oxfordjournals.org
Where is the Molecular Hydrogen in Damped Lyman-Alpha Absorbers?
We show in this paper why molecular millimeter absorption line searches in
DLAs have been unsuccessful. We use CO emission line maps of local galaxies to
derive the H2 column density distribution function f(N_H2) at z=0. We show that
it forms a natural extension to f(N_HI): the H2 distribution exceeds f(N_HI) at
N_H ~ 10^22 cm^-2 and exhibits a power law drop-off with slope ~ -2.5.
Approximately 97% of the H2 mass density rho_H2 is in systems above N_H2=10^21
cm^-2. We derive a value rho_H2 = 1.1 x 10^7 h_70 M_sun Mpc^-3, which is ~25%
the mass density of atomic hydrogen. Yet, the redshift number density of H2
above this N_H2 limit is only ~3 x 10^-4, a factor 150 lower than that for HI
in DLAs at z=0. Furthermore, we show that the median impact parameter between a
N_H2>10^21 cm^-2 absorber and the centre of the galaxy hosting the H2 gas is
only 2.5 kpc. Based on arguments related to the Schmidt law, we argue that H2
gas above this column density limit is associated with a large fraction of the
integral star formation rate density. Even allowing for an increased molecular
mass density at higher redshifts, the derived cross-sections indicate that it
is very unlikely to identify the bulk of the molecular gas in present quasar
absorption lines samples. We discuss the prospects for identifying this
molecular mass in future surveys.Comment: 6 pages, 4 figures. Accepted for publication in the Astrophysical
Journa
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