340 research outputs found
Conference Summary: HI Science in the Next Decade
The atomic hydrogen (HI) 21cm line measures the gas content within and around
galaxies, traces the dark matter potential and probes volumes and objects that
other surveys do not. Over the next decade, 21cm line science will exploit new
technologies, especially focal plane and aperture arrays, and will see the
deployment of Epoch of Reionization/Dark Age detection experiments and Square
Kilometer Array (SKA) precursor instruments. Several experiments designed to
detect and eventually to characterize the reionization history of the
intergalactic medium should deliver first results within two-three years time.
Although "precision cosmology" surveys of HI in galaxies at z ~ 1 to 3 require
the full collecting area of the SKA, a coherent program of HI line science
making use of the unique capabilities of both the existing facilities and the
novel ones demonstrated by the SKA precursors will teach us how many gas rich
galaxies there really are and where they reside and will yield fundamental
insight into how galaxies accrete gas, form stars and interact with their
environment.Comment: To appear in AIP Conference Proceedings, "The Evolution of Galaxies
through the Neutral Hydrogen Window", Feb 1-3 2008, Arecibo, Puerto Rico,
eds. R. Minchin & E. Momjian. 8 page
The ALFALFA Search for (Almost) Dark Galaxies across the HI Mass Function
The Arecibo Legacy Fast ALFA (ALFALFA) survey is a second generation blind
extragalactic HI survey currently in progess which is exploiting Arecibo's
superior sensitivity, angular resolution and digital technology to derive a
census of the local HI universe over a cosmologically significant volume. As of
the time of this meeting, some 4500 good quality extragalactic HI line sources
have been identified in about 15% of the final survey area. ALFALFA is
detecting HI masses as low as 10**6 solar masses and as large as 10**10.8 solar
masses with positional accuracies typically better than 20", allowing immediate
identification of the most probable optical counterparts. Only 3% of all
extragalactic HI sources and less than 1% of detections with HI mass 10**9.5
solar masses cannot be identified with a stellar component. Because ALFALFA is
far from complete, the discussion here focuses on limitations of past surveys
that ALFALFA will overcome because of its greater volume, sensitivity and
reduced susceptibility to source confusion and on a sampling of illustrative
preliminary results. First ALFALFA results already suggest, in agreement with
previous studies, that there does not appear to be a cosmologically significant
population of optically dark but HI rich galaxies. ALFALFA promises a wealthy
dataset for the exploration of many issues in near-field cosmology and galaxy
evolution studies, setting the stage for their extension to higher redshifts in
the future with the Square Kilometer Array (SKA).Comment: To appear in Proceedings of IAU Symp #244, "Dark Galaxies and Lost
Baryons", June 2007, 10 pages including 5 figure
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
Is there a "too big to fail" problem in the field?
We use the Arecibo legacy fast ALFA (ALFALFA) 21cm survey to measure the
number density of galaxies as a function of their rotational velocity,
(as inferred from the width of their 21cm emission line).
Based on the measured velocity function we statistically connect galaxies with
their host halo, via abundance matching. In a lambda cold dark matter
(CDM) cosmology, dwarf galaxies are expected to be hosted by halos
that are significantly more massive than indicated by the measured galactic
velocity; if smaller halos were allowed to host galaxies, then ALFALFA would
measure a much higher galactic number density. We then seek observational
verification of this predicted trend by analyzing the kinematics of a
literature sample of gas-rich dwarf galaxies. We find that galaxies with
are
kinematically incompatible with their predicted CDM host halos, in the
sense that hosts are too massive to be accommodated within the measured
galactic rotation curves. This issue is analogous to the "too big to fail"
problem faced by the bright satellites of the Milky Way, but here it concerns
extreme dwarf galaxies in the field. Consequently, solutions based on
satellite-specific processes are not applicable in this context. Our result
confirms the findings of previous studies based on optical survey data and
addresses a number of observational systematics present in these works.
Furthermore, we point out the assumptions and uncertainties that could strongly
affect our conclusions. We show that the two most important among them -namely
baryonic effects on the abundances of halos and on the rotation curves of
halos- do not seem capable of resolving the reported discrepancy.Comment: v3 matches the version published in A&A. Main differences with v2 are
in Secs 3.2 & 4.4 and the addition of Appendix B. 11 figures, 14 pages (+2
appendices
The Clustering Characteristics of HI-Selected Galaxies from the 40% ALFALFA Survey
The 40% Arecibo Legacy Fast ALFA (ALFALFA) survey catalog (\alpha.40) of
approximately 10,150 HI-selected galaxies is used to analyze the clustering
properties of gas-rich galaxies. By employing the Landy-Szalay estimator and a
full covariance analysis for the two-point galaxy-galaxy correlation function,
we obtain the real-space correlation function and model it as a power law,
\xi(r) = (r/r_0)^(-\gamma), on scales less than 10 h^{-1} Mpc. As the largest
sample of blindly HI-selected galaxies to date, \alpha.40 provides detailed
understanding of the clustering of this population. We find \gamma = 1.51 +/-
0.09 and r_0 = 3.3 +0.3, -0.2 h^{-1} Mpc, reinforcing the understanding that
gas-rich galaxies represent the most weakly clustered galaxy population known;
we also observe a departure from a pure power law shape at intermediate scales,
as predicted in \Lambda CDM halo occupation distribution models. Furthermore,
we measure the bias parameter for the \alpha.40 galaxy sample and find that HI
galaxies are severely antibiased on small scales, but only weakly antibiased on
large scales. The robust measurement of the correlation function for gas-rich
galaxies obtained via the \alpha.40 sample constrains models of the
distribution of HI in simulated galaxies, and will be employed to better
understand the role of gas in environmentally-dependent galaxy evolution.Comment: 30 pages, 10 figures, accepted by Ap
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