7,955 research outputs found
Is the far border of the Local Void expanding?
According to models of evolution in the hierarchical structure formation
scenarios, voids of galaxies are expected to expand. The Local Void (LV) is the
closest large void, and it provides a unique opportunity to test
observationally such an expansion. It has been found that the Local Group,
which is on the border of the LV, is running away from the void center at ~260
km/s. In this study we investigate the motion of the galaxies at the far-side
border of the LV to examine the presence of a possible expansion. We selected
late-type, edge-on spiral galaxies with radial velocities between 3000 km/s and
5000 km/s, and carried out HI 21 cm line and H-band imaging observations. The
near-infrared Tully-Fisher relation was calibrated with a large sample of
galaxies and carefully corrected for Malmquist bias. It was used to compute the
distances and the peculiar velocities of the LV sample galaxies. Among the 36
sample LV galaxies with good quality HI line width measurements, only 15
galaxies were selected for measuring their distances and peculiar velocities,
in order to avoid the effect of Malmquist bias. The average peculiar velocity
of these 15 galaxies is found to be -419+208-251 km/s, which is not
significantly different from zero. Due to the intrinsically large scatter of
Tully-Fisher relation, we cannot conclude whether there is a systematic motion
against the center of the LV for the galaxies at the far-side boundary of the
void. However, our result is consistent with the hypothesis that those galaxies
at the far-side boundary have an average velocity of ~260 km/s equivalent to
what is found at the position of the Local Group.Comment: 15 pages, 6 figures, and 4 tables. Accepted for publication in A&
The observed infall of galaxies towards the Virgo cluster
We examine the velocity field of galaxies around the Virgo cluster induced by
its overdensity. A sample of 1792 galaxies with distances from the Tip of the
Red Giant Branch, the Cepheid luminosity, the SNIa luminosity, the surface
brightness fluctuation method, and the Tully-Fisher relation has been used to
study the velocity-distance relation in the Virgocentric coordinates. Attention
was paid to some observational biases affected the Hubble flow around Virgo.
We estimate the radius of the zero-velocity surface for the Virgo cluster to
be within (5.0 - 7.5) Mpc corresponding to (17 - 26)^\circ at the mean cluster
distance of 17.0 Mpc. In the case of spherical symmetry with cosmological
parameter \Omega_m=0.24 and the age of the Universe T_0= 13.7 Gyr, it yields
the total mass of the Virgo cluster to be within M_T=(2.7 - 8.9) * 10^{14}
M_\sun in reasonable agreement with the existing virial mass estimates for the
cluster.Comment: 22 pages, 11 figures, 2 tables. Accepted for publication in MNRA
Filaments from the galaxy distribution and from the velocity field in the local universe
The cosmic web that characterizes the large-scale structure of the Universe
can be quantified by a variety of methods. For example, large redshift surveys
can be used in combination with point process algorithms to extract long
curvilinear filaments in the galaxy distribution. Alternatively, given a full
3D reconstruction of the velocity field, kinematic techniques can be used to
decompose the web into voids, sheets, filaments and knots. In this paper we
look at how two such algorithms - the Bisous model and the velocity shear web -
compare with each other in the local Universe (within 100 Mpc), finding good
agreement. This is both remarkable and comforting, given that the two methods
are radically different in ideology and applied to completely independent and
different data sets. Unsurprisingly, the methods are in better agreement when
applied to unbiased and complete data sets, like cosmological simulations, than
when applied to observational samples. We conclude that more observational data
is needed to improve on these methods, but that both methods are most likely
properly tracing the underlying distribution of matter in the Universe.Comment: 6 Pages, 2 figures, Submitted to MNRAS Letter
The LCO/Palomar 10,000 km/sec Cluster Survey. I. Properties of the Tully-Fisher Relation
The first results from a Tully-Fisher (TF) survey of cluster galaxies are
presented. The galaxies are drawn from fifteen Abell clusters that lie in the
redshift range 9000-12,000 km/sec and are distributed uniformly around the
celestial sky. The data set consists of R-band CCD photometry and long- slit
H-alpha spectroscopy. The rotation curves (RCs) are characterized by a turnover
radius (r_t) and an asymptotic velocity v_a, while the surface brightness
profiles are characterized in terms of an effective exponential surface
brightness I_e and a scale length r_e. The TF scatter is minimized when the
rotation velocity is measured at 2.0 +/- 0.2 r_e; a significantly larger
scatter results when the rotation velocity is measured at > 3 or < 1.5 scale
lengths. This effect demonstrates that RCs do not have a universal form, as has
been suggested by Persic, Salucci, and Stel. In contrast to previous studies, a
modest but statistically significant surface-brightness dependence of the TF
relation is found, log v = const + 0.28*log L + 0.14*log I_e. This indicates a
stronger parallel between the TF relation and the FP relations of elliptical
galaxies than has previously been recognized. Future papers in this series will
consider the implications of this cluster sample for deviations from Hubble
flow on 100-200 Mpc scales.Comment: 35 pages, 8 figures, uses aaspp4.sty. Submitted to ApJ. Also
available at http://astro.stanford.edu/jeff
The Extragalactic Distance Database: Color-Magnitude Diagrams
The CMDs/TRGB (Color-Magnitude Diagrams/Tip of the Red Giant Branch) section
of the Extragalactic Distance Database contains a compilation of observations
of nearby galaxies from the Hubble Space Telescope. Approximately 250 (and
increasing) galaxies in the Local Volume have CMDs and the stellar photometry
tables used to produce them available through the web. Various stellar
populations that make up a galaxy are visible in the CMDs, but our primary
purpose for collecting and analyzing these galaxy images is to measure the TRGB
in each. We can estimate the distance to a galaxy by using stars at the TRGB as
standard candles. In this paper we describe the process of constructing the
CMDs and make the results available to the public.Comment: 8 pages, 5 figures, 1 long table, submitted to Astronomical Journa
Nonlocal Gravity: Modified Poisson's Equation
The recent nonlocal generalization of Einstein's theory of gravitation
reduces in the Newtonian regime to a nonlocal and nonlinear modification of
Poisson's equation of Newtonian gravity. The nonlocally modified Poisson
equation implies that nonlocality can simulate dark matter. Observational data
regarding dark matter provide limited information about the functional form of
the reciprocal kernel, from which the original nonlocal kernel of the theory
must be determined. We study this inverse problem of nonlocal gravity in the
linear domain, where the applicability of the Fourier transform method is
critically examined and the conditions for the existence of the nonlocal kernel
are discussed. This approach is illustrated via simple explicit examples for
which the kernels are numerically evaluated. We then turn to a general
discussion of the modified Poisson equation and present a formal solution of
this equation via a successive approximation scheme. The treatment is
specialized to the gravitational potential of a point mass, where in the linear
regime we recover the Tohline-Kuhn approach to modified gravity.Comment: 27 pages, 4 figures; v2: minor improvements, accepted for publication
in J. Math. Phy
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