Comparison of the ENEAR peculiar velocities with the PSCz gravity field
Adi Nusser
2
w Luiz N. da Costa
0
1
Enzo Branchini
6
Mariangela Bernardi
5
Alonso
4
Gary Wegner
3
C. N. A. Willmer
0
7
P. S. Pellegrini
0
0
Observatorio Nacional, Rua General Jose Cristino 77,
Rio de Janeiro
, RJ, 20921,
Brazil
1
European Southern Observatory, Karl-Schwarzschild Strasse 2, D-85748 Garching bei Munchen,
Germany
2
Physics Department, Technion-Israel Institute of Technology
, Haifa 32000,
Israel
3
Department of Physics and Astronomy, Dartmouth College
, Hanover,
NH 03755, USA
4
Observatorio Astronomico de Cordoba, Laprida 854, Cordoba, 5000,
Argentina
5
The University of Chicago
, 5640 South Ellis Avenue,
Chicago, IL 60637, USA
6
Kapteyn Institute, University of Groningen
, Landleven 12, 9700 AV, Groningen,
The Netherlands
7
UCO/Lick Observatory,
University of California
, 1156 High Street, Santa Cruz,
CA 95064, USA
A B S T R A C T We present a comparison between the peculiar velocity field measured from the ENEAR allsky Dn-s catalogue and that derived from the galaxy distribution of the IRAS Point Source Catalog Redshift Survey (PSCz). The analysis is based on a modal expansion of these data in redshift space by means of spherical harmonics and Bessel functions. The effective smoothing scale of the expansion is almost linear with redshift reaching 1500 km s21 at 3000 km s21. The general flow patterns in the filtered ENEAR and PSCz velocity fields agree well within 6000 km s21, assuming a linear biasing relation between the mass and the PSCz galaxies. The comparison allows us to determine the parameter b V0:6=b; where V is the cosmological density parameter and b is the linear biasing factor. A likelihood analysis of the ENEAR and PSCz modes yields b 0:5 ^ 0:1; in good agreement with values obtained from Tully-Fisher surveys.
I N T R O D U C T I O N
In the standard picture for the formation of cosmic structures via
gravitational instability, the peculiar velocity of a galaxy is
generated by fluctuations in the mass distribution. For galaxies
outside virialized systems, linear perturbation theory predicts
where V is the mass density parameter, H0 is the Hubble constant
and d m is the mass density fluctuation field. If the relationship
between the galaxy distribution, d g, and d m is approximately
linear, dg bdm; then the parameter b V0:6=b can be derived
from the comparison between the observed peculiar velocity field
and that predicted from the galaxy distribution. A particularly
useful method for performing a velocityvelocity comparison is
the modal expansion method developed by Nusser & Davis (1995,
hereafter ND95). This method expands the velocity fields by
means of smooth functions defined in redshift space, thus
alleviating the Malmquist biases inherent in real-space analysis.
Furthermore, the modal expansion filters the observed and
predicted velocities in the same way, so that the smoothed fields
can be compared directly. Because the number of modes is
substantially smaller than the number of data points, the method
also provides the means of estimating b from a likelihood analysis
carried out on a mode-by-mode basis, instead of galaxy-by-galaxy.
The similar smoothing and the mode-by-mode comparison
substantially simplify the error analysis. The modal expansion
method has previously been used in comparisons between the
1.2-Jy IRAS predicted velocities and observed velocities inferred
from TullyFisher (TF) measurements (Davis, Nusser & Willick
1996, hereafter DNW; da Costa et al. 1998). In this paper, we
perform a similar analysis using the recently completed redshift
distance survey of early-type galaxies (hereafter ENEAR: da
Costa et al. 2000) and the IRAS Point Source Catalog Redshift
Survey (PSCz) (Saunders et al. 2000). Because of differences in
the nature of the data sets considered, some slight changes in the
method are required and are described below. Our goal is to
investigate how well the velocity field mapped by early-type
galaxies matches the velocity field inferred from the PSCz, and to
obtain the parameter b yielding the best match.
In Section 2, we briefly describe the ENEAR redshiftdistance
catalogue. In Sections 3 and 4, we describe the modal expansion
method as used here, present maps of the ENEAR and PSCz radial
peculiar velocity field and perform a likelihood analysis to derive
b . A brief summary of our conclusions is presented in Section 5.
We use a subsample extracted from the all-sky ENEAR redshift
distance survey (da Costa et al. 2000) comprising 578 objects
within cz < 6000 km s21 355 field galaxies and 223 groups/
clusters. Galaxies have been objectively assigned to groups and
clusters using redshifts taken from complete redshift surveys
sampling the same volume. Individual galaxy distances were
estimated from an inverse Dns template relation derived by
combining cluster data (e.g. Bernardi et al., in preparation). The
cluster sample consists of 569 galaxies in 28 clusters. Over 80 per
cent of the galaxies in the magnitude-limited sample and roughly
60 p (...truncated)