56 research outputs found
Testing the Peculiar Velocity Field predicted from Redshift Surveys
The reconstruction of the peculiar velocity field from the 1.936~Jy iras
selected sample of galaxies is compared to a similar reconstruction from an
optically selected sample. A general method for combining different samples to
reconstruct a self-consistent density and peculiar velocity field is presented.
The method is applied to determine how sensitive the derived peculiar velocity
field is to the characteristics of the sample used. The possibility that the
iras galaxies do not trace the general galaxy population is explored adopting a
simple model of linear biasing between the iras and optical samples. We find
that the velocity fields derived from the two samples are consistent, within
the estimated shot noise error, for the case of no relative bias. This result
suggests that the predicted peculiar velocity field based on iras samples is
not sensitive to the sampling properties of iras galaxies. Combined with
previous suggestion of a relative biasing of iras galaxies on small scales
(about 5 h^-1Mpc), this result suggests scale dependent biasing.Comment: tar-compressed and uudecoded postscript files, 12 pages+8 figure
Dust in high-z radio-loud AGN
We present continuum observations of a small sample of high-redshift,
radio-loud AGN (radio galaxies and quasars) aimed at the detection of thermal
emission from dust. Seven AGN were observed with IRAM and SEST at 1.25mm; two
of them, the radio galaxies 1243+036 () and MG1019+0535 () were also observed at 0.8mm with the JCMT submillimetre telescope.
Additional VLA observations were obtained in order to derive the spectral shape
of the synchrotron radiation of MG1019+0535 at high radio frequencies.
MG1019+0535 and TX0211122 were expected to contain a large amount of dust
based on their depleted Ly emission. The observations suggest a clear
1.25-mm flux density excess over the synchrotron radiation spectrum of
MG1019+0535, suggesting the presence of thermal emission from dust in this
radio galaxy, whereas the observations of TX0211122 were not sensitive
enough to meaningfully constrain its dust content. On the other hand, our
observations of 1243+036 provide a stringent upper limit on the total dust mass
of M. Finally, we find that the spectra of the radio-loud
quasars in our sample () steepen between rest-frame radio and the
far-infrared. We discuss the main implications of our results, concentrating on
the dusty radio galaxy, MG1019+0535.Comment: 11 pages, A&A LaTeX, 4 figure
On the mass-metallicity relation, velocity dispersion and gravitational well depth of GRB host galaxies
We analyze a sample of 16 absorption systems intrinsic to long duration GRB
host galaxies at for which the metallicities are known. We
compare the relation between the metallicity and cold gas velocity width for
this sample to that of the QSO-DLAs, and find complete agreement. We then
compare the redshift evolution of the mass-metallicity relation of our sample
to that of QSO-DLAs and find that also GRB hosts favour a late onset of this
evolution, around a redshift of . We compute predicted stellar
masses for the GRB host galaxies using the prescription determined from QSO-DLA
samples and compare the measured stellar masses for the four hosts where
stellar masses have been determined from SED fits. We find excellent agreement
and conclude that, on basis of all available data and tests, long duration
GRB-DLA hosts and intervening QSO-DLAs are consistent with being drawn from the
same underlying population. GRB host galaxies and QSO-DLAs are found to have
different impact parameter distributions and we briefly discuss how this may
affect statistical samples. The impact parameter distribution has two effects.
First any metallicity gradient will shift the measured metallicity away from
the metallicity in the centre of the galaxy, second the path of the sightline
through different parts of the potential well of the dark matter halo will
cause different velocity fields to be sampled. We report evidence suggesting
that this second effect may have been detected.Comment: 11 pages, 6 figures, 6 tables. Accepted for publication in MNRAS Main
Journal. For the definitive version visit http://mnras.oxfordjournals.org
The Mass Distribution in the Nearby Universe
We present a new reconstruction of the mass density and the peculiar velocity fields in the nearby universe using recent measurements of Tully-Fisher distances for a sample of late spirals. We find significant differences between our reconstructed fields and those obtained in earlier work: overdensities tend to be more compact while underdense regions, consisting of individual voids, are more abundant. Our results suggest that voids observed in redshift surveys of galaxies represent real voids in the underlying matter distribution. While we detect a bulk velocity of ~300 km s-1, within a top-hat window 6000 km s-1 in radius, the flow is less coherent than previously claimed, exhibiting a bifurcation toward the Perseus-Pisces and the Great Attractor complexes. This is the first time that this feature is seen from peculiar velocity measurements. The observed velocity field resembles, more closely than any previous reconstruction, the velocity field predicted from self-consistent reconstructions based on all-sky redshift surveys. This better match is likely to affect estimates of the parameter β = Ω0.6/b and its uncertainty based on velocity-velocity comparisons
Determination of Malmquist Bias and Selection Effects from Monte Carlo Simulations
Maps of the peculiar velocity field derived from distance relations are affected by Malmquist type bias and selection effects. Because of the large number of interdependent effects, they are in most cases difficult to treat analytically. Monte Carlo simulations are used to understand and evaluate these effects. In these simulations the true spatial distribution and relevant properties of galaxies as well as selection effects and observational uncertainties are realistically modeled. The results of the simulation can be directly applied to correct observed peculiar velocity maps. The simulation is used to investigate biases in samples of measured peculiar velocities by Lynden-Bell et al. (ApJ, 326, 19, 1988). Willick (ApJ, 351, L5, 1990) and the new sample of spiral galaxies by Haynes et al. (BAAS, 25, 1403, 1993). Based on the results obtained from the application of our method to toy models we find that the method is a useful tool to estimate the bias induced both by inhomogeneities and selection effects. This is a crucial step for the analysis of the Haynes et al. sample which was selected with a redshift dependent criterion
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