17,209 research outputs found
The loss of earnings capability from disability/health limitations: Toward a new social indicator
Asteroid flux and impact cratering rate on Venus
By the end of 1990, 65 Venus-crossing asteroids were recognized; these represent 59 percent of the known Earth-crossing asteroids. Further studies, chiefly numerical integrations of orbit evolution, may reveal one or two more Venus crossers among the set of discovered asteroids. A Venus crosser was defined as an asteroid whose orbit can intersect the orbit of Venus as a result of secular (long range) perturbations. Venus crossers revolving on orbits that currently overlap the orbit of Venus are called Venapol asteroids, and those on orbit that don't overlap are called Venamor asteroids; 42 Venapols and 23 Venamors were recognized. Collision probabilities with Venus for 60 of the known Venus crossers were determined
Measurement of the Spatial Cross-Correlation Function of Damped Lyman Alpha Systems and Lyman Break Galaxies
We present the first spectroscopic measurement of the spatial
cross-correlation function between damped Lyman alpha systems (DLAs) and Lyman
break galaxies (LBGs). We obtained deep u'BVRI images of nine QSO fields with
11 known z ~ 3 DLAs and spectroscopically confirmed 211 R < 25.5
photometrically selected z > 2 LBGs. We find strong evidence for an overdensity
of LBGs near DLAs versus random, the results of which are similar to that of
LBGs near other LBGs. A maximum likelihood cross-correlation analysis found the
best fit correlation length value of r_0 = 2.9^(+1.4)_(-1.5) h^(-1)Mpc using a
fixed value of gamma = 1.6. The implications of the DLA-LBG clustering
amplitude on the average dark matter halo mass of DLAs are discussed.Comment: 12 pages, 2 figures, accepted for publication in Astrophysical
Journal Letter
The Evolution of Luminous Compact Blue Galaxies: Disks or Spheroids?
Luminous compact blue galaxies (LCBGs) are a diverse class of galaxies
characterized by high luminosities, blue colors, and high surface brightnesses.
Residing at the high luminosity, high mass end of the blue sequence, LCBGs sit
at the critical juncture of galaxies that are evolving from the blue to the red
sequence. Yet we do not understand what drives the evolution of LCBGs, nor how
they will evolve. Based on single-dish HI observations, we know that they have
a diverse range of properties. LCBGs are HI-rich with M(HI)=10^{9-10.5} M(sun),
have moderate M(dyn)=10^{10-12} M(sun), and 80% have gas depletion timescales
less than 3 Gyr. These properties are consistent with LCBGs evolving into
low-mass spirals or high mass dwarf ellipticals or dwarf irregulars. However,
LCBGs do not follow the Tully-Fisher relation, nor can most evolve onto it,
implying that many LCBGs are not smoothly rotating, virialized systems. GMRT
and VLA HI maps confirm this conclusion revealing signatures of recent
interactions and dynamically hot components in some local LCBGs, consistent
with the formation of a thick disk or spheroid. Such signatures and the high
incidence of close companions around LCBGs suggest that star formation in local
LCBGs is likely triggered by interactions. The dynamical masses and apparent
spheroid formation in LCBGs combined with previous results from optical
spectroscopy are consistent with virial heating being the primary mechanism for
quenching star formation in these galaxies.Comment: 4 pages, 1 figure, to appear in "Hunting for the Dark: The Hidden
Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P. Debattista &
C.C. Popescu, AIP Conf. Se
Search for cold gas in z>2 damped Lyman-alpha systems: 21-cm and H_2 absorption
(Abridged) We present the results of a systematic GBT and GMRT survey for
21-cm absorption in a sample of 10 DLAs at 2<z_abs<3.4. Analysis of L-band VLBA
images of the background QSOs are also presented. We detect 21-cm absorption in
only one DLA (at z_abs = 3.1745 towards J1337+3152). Combining our data with
the data from the literature (a sample of 28 DLAs) and assuming the measured
core fraction at milliarcsecond scale to represent the gas covering factor, we
find that the HI gas in DLAs at z> 2 is predominantly constituted by WNM. The
detection rate of 21-cm absorption seems to be higher for systems with higher
N(HI) or metallicity. However, no clear correlation is found between the
integrated 21-cm optical depth (or spin temperature) and either N(HI),
metallicity or velocity spread of the low ionization species. There are 13 DLAs
in our sample for which high resolution optical spectra covering the expected
wavelength range of H_2 absorption are available. We report the detection of
H_2 molecules in the z_abs = 3.3871 21-cm absorber towards J0203+1134 (PKS
0201+113). In 8 cases, neither H_2 nor 21-cm absorption are detected. The lack
of 21-cm and H_2 absorption in these systems can be explained if most of the HI
in these DLAs originate from low density high temperature gas. In one case we
have a DLA with 21-cm absorption not showing H_2 absorption. In two cases, both
species are detected but do not originate from the same velocity component. In
the remaining 2 cases 21-cm absorption is not detected despite the presence of
H_2 with evidence for the presence of cold gas. All this is consistent with the
idea that the H_2 components seen in DLAs are compact (with sizes of < 15 pc)
and contain only a small fraction (i.e typically <10%) of the total N(HI)
measured in the DLAs.Comment: Accepted for publication in MNRA
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