8,003 research outputs found
The Impact of Distance Uncertainties on Local Luminosity and Mass Functions
In order to investigate discrepancies between recent published estimates of
the the HI mass function (HIMF), we explore the impact of distance
uncertainties on the derivation of the faint end slope of mass and luminosity
functions of galaxies in the local volume by deriving HIMFs from mock HI
surveys. We consider various survey geometries and depths and compare the HIMFs
measured when using ``real'' distances, distances derived by assuming pure
Hubble flow and distances assigned from parametric models of the local velocity
field. The effect is variable and dependent on the exact survey geometry, but
can easily lead to incorrect estimates of the HIMF, particularly at the low
mass end. We show that at least part of the discrepancies among recent
derivations of the HIMF can be accounted for by the use of different methods to
assign distances. We conclude that a better understanding of the local velocity
field will be necessary for accurate determinations of the local galaxy
luminosity and mass functions.Comment: 4 pages, accepted to ApJ
SFI++ I: A New I-band Tully-Fisher Template, the Cluster Peculiar Velocity Dispersion and H0
The SFI++ consists of ~5000 spiral galaxies which have measurements suitable
for the application of the I-band Tully-Fisher (TF) relation. This sample
builds on the SCI and SFI samples published in the 1990s but includes
significant amounts of new data as well as improved methods for parameter
determination. We derive a new I-band TF relation from a subset of this sample
which consists of 807 galaxies in the fields of 31 nearby clusters and groups.
This sample constitutes the largest ever available for the calibration of the
TF template and extends the range of line-widths over which the template is
reliably measured. Careful accounting is made of observational and sample
biases such as incompleteness, finite cluster size, galaxy morphology and
environment. We find evidence for a type-dependent TF slope which is shallower
for early type than for late type spirals. The line-of-sight cluster peculiar
velocity dispersion is measured for the sample of 31 clusters. This value is
directly related to the spectrum of initial density fluctuations and thus
provides an independent verification of the best fit WMAP cosmology and an
estimate of Omega^0.6 sigma_8 = 0.52+/-0.06. We also provide an independent
measure of the TF zeropoint using 17 galaxies in the SFI++ sample for which
Cepheid distances are available. In combination with the ``basket of clusters''
template relation these calibrator galaxies provide a measure of H0 = 74+/-2
(random) +/-6 (systematic) km/s/Mpc.Comment: Accepted by ApJ (scheduled for 20 Dec 2006, issue 653). 21 pages (2
column emulateapj) including 12 figures. Version 2 corrects typos and other
small errors noticed in proof
SFI++ II: A New I-band Tully-Fisher Catalog, Derivation of Peculiar Velocities and Dataset Properties
We present the SFI++ dataset, a homogeneously derived catalog of photometric
and rotational properties and the Tully-Fisher distances and peculiar
velocities derived from them. We make use of digital optical images, optical
long-slit spectra, and global HI line profiles to extract parameters of
relevance to disk scaling relations, incorporating several previously published
datasets as well as a new photometric sample of some 2000 objects. According to
the completeness of available redshift samples over the sky area, we exploit
both a modified percolation algorithm and the Voronoi-Delaunay method to assign
individual galaxies to groups as well as clusters, thereby reducing scatter
introduced by local orbital motions. We also provide corrections to the
peculiar velocities for both homogeneous and inhomogeneous Malmquist bias,
making use of the 2MASS Redshift Survey density field to approximate large
scale structure. We summarize the sample selection criteria, corrections made
to raw observational parameters, the grouping techniques, and our procedure for
deriving peculiar velocities. The final SFI++ peculiar velocity catalog of 4861
field and cluster galaxies is large enough to permit the study not just of the
global statistics of large scale flows but also of the {\it details} of the
local velocity field.Comment: 14 pages, 6 figures, 4 external online tables, accepted for
publication in ApJ
NGC 4254: An Act of Harassment Uncovered by the Arecibo Legacy Fast ALFA Survey
We present an HI map constructed from the Arecibo Legacy Fast ALFA (ALFALFA)
survey of the surroundings of the strongly asymmetric Virgo cluster Sc galaxy
NGC 4254. Noted previously for its lopsided appearance, rich interstellar
medium, and extradisk HI emission, NGC 4254 is believed to be entering the
Virgo environment for the first time and at high speed. The ALFALFA map clearly
shows a long HI tail extending ~250 kpc northward from the galaxy. Embedded as
one condensation within this HI structure is the object previously identified
as a "dark galaxy": Virgo HI21 (Davies et al. 2004). A body of evidence
including its location within and velocity with respect to the cluster and the
appearance and kinematics of its strong spiral pattern, extra-disk HI and
lengthy HI tail is consistent with a picture of "galaxy harassment" as proposed
by Moore et al. (1996a,b; 1998). The smoothly varying radial velocity field
along the tail as it emerges from NGC 4254 can be used as a timing tool, if
interpreted as resulting from the coupling of the rotation of the disk and the
collective gravitational forces associated with the harassment mechanism.Comment: accepted for publication in Ap.J.(Lett.). higher resolution figure
available at http://egg.astro.cornell.edu/alfalfa/pubs/figs/n4254_f1.ep
The appearance, motion, and disappearance of three-dimensional magnetic null points
N.A.M. acknowledges support from NASA grants NNX11AB61G, NNX12AB25G, and NNX15AF43G; NASA contract NNM07AB07C; and NSF SHINE grants AGS-1156076 and AGS-1358342 to SAO. C.E.P. acknowledges support from the St Andrews 2013 STFC Consolidated grant.While theoretical models and simulations of magnetic reconnection often assume symmetry such that the magnetic null point when present is co-located with a flow stagnation point, the introduction of asymmetry typically leads to non-ideal flows across the null point. To understand this behavior, we present exact expressions for the motion of three-dimensional linear null points. The most general expression shows that linear null points move in the direction along which the magnetic field and its time derivative are antiparallel. Null point motion in resistive magnetohydrodynamics results from advection by the bulk plasma flow and resistive diffusion of the magnetic field, which allows non-ideal flows across topological boundaries. Null point motion is described intrinsically by parameters evaluated locally; however, global dynamics help set the local conditions at the null point. During a bifurcation of a degenerate null point into a null-null pair or the reverse, the instantaneous velocity of separation or convergence of the null-null pair will typically be infinite along the null space of the Jacobian matrix of the magnetic field, but with finite components in the directions orthogonal to the null space. Not all bifurcating null-null pairs are connected by a separator. Furthermore, except under special circumstances, there will not exist a straight line separator connecting a bifurcating null-null pair. The motion of separators cannot be described using solely local parameters because the identification of a particular field line as a separator may change as a result of non-ideal behavior elsewhere along the field line.Publisher PDFPeer reviewe
Seeking the Local Convergence Depth. The Abell Cluster Dipole Flow to 200 Mpc/h
We have obtained new Tully-Fisher (TF) peculiar velocity measurements for 52
Abell galaxy clusters distributed throughout the sky between ~ 50 and 200
Mpc/h.The measurements are based on I band photometry and optical rotation
curves for a sample of 522 spiral galaxies, from which an accurate TF template
relation has been constructed. Individual cluster TF relations are referred to
the template to compute cluster peculiar motions.
The reflex motion of the Local Group of galaxies is measured with respect to
the reference frame defined by our cluster sample and the distant portion of
the Giovanelli et al. (1998) cluster set. We find the Local Group motion in
this frame to be 565+/-113 km/s in the direction (l,b)=(267,26)+/-10 when
peculiar velocities are weighted according to their errors. After optimizing
the dipole calculation to sample equal volumes equally, the vector is 509+/-195
km/s towards (255,33)+/-22. Both solutions agree, to within 1-sigma or better,
with the Local Group motion as inferred from the cosmic microwave background
(CMB) dipole. Thus, the cluster sample as a whole moves slowly in the CMB
reference frame, its bulk flow being at most 200 km/s.Comment: 11 pages, uses AAS LaTeX; to appear in the Astrophysical Journal
Letter
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