3,168 research outputs found
Counts-in-Cylinders in the Sloan Digital Sky Survey with Comparisons to N-body Simulations
Environmental statistics provide a necessary means of comparing the
properties of galaxies in different environments and a vital test of models of
galaxy formation within the prevailing, hierarchical cosmological model. We
explore counts-in-cylinders, a common statistic defined as the number of
companions of a particular galaxy found within a given projected radius and
redshift interval. Galaxy distributions with the same two-point correlation
functions do not necessarily have the same companion count distributions. We
use this statistic to examine the environments of galaxies in the Sloan Digital
Sky Survey, Data Release 4. We also make preliminary comparisons to four models
for the spatial distributions of galaxies, based on N-body simulations, and
data from SDSS DR4 to study the utility of the counts-in-cylinders statistic.
There is a very large scatter between the number of companions a galaxy has and
the mass of its parent dark matter halo and the halo occupation, limiting the
utility of this statistic for certain kinds of environmental studies. We also
show that prevalent, empirical models of galaxy clustering that match observed
two- and three-point clustering statistics well fail to reproduce some aspects
of the observed distribution of counts-in-cylinders on 1, 3 and 6-Mpc/h scales.
All models that we explore underpredict the fraction of galaxies with few or no
companions in 3 and 6-Mpc/h cylinders. Roughly 7% of galaxies in the real
universe are significantly more isolated within a 6 Mpc/h cylinder than the
galaxies in any of the models we use. Simple, phenomenological models that map
galaxies to dark matter halos fail to reproduce high-order clustering
statistics in low-density environments.Comment: 17 pages, 10 figures. Accepted, Ap
Charles S. Blanton to T.L. Treadwell, 8 December 1838
https://egrove.olemiss.edu/aldrichcorr_a/1091/thumbnail.jp
How Common are the Magellanic Clouds?
We introduce a probabilistic approach to the problem of counting dwarf
satellites around host galaxies in databases with limited redshift information.
This technique is used to investigate the occurrence of satellites with
luminosities similar to the Magellanic Clouds around hosts with properties
similar to the Milky Way in the object catalog of the Sloan Digital Sky Survey.
Our analysis uses data from SDSS Data Release 7, selecting candidate
Milky-Way-like hosts from the spectroscopic catalog and candidate analogs of
the Magellanic Clouds from the photometric catalog. Our principal result is the
probability for a Milky-Way-like galaxy to host N_{sat} close satellites with
luminosities similar to the Magellanic Clouds. We find that 81 percent of
galaxies like the Milky Way are have no such satellites within a radius of 150
kpc, 11 percent have one, and only 3.5 percent of hosts have two. The
probabilities are robust to changes in host and satellite selection criteria,
background-estimation technique, and survey depth. These results demonstrate
that the Milky Way has significantly more satellites than a typical galaxy of
its luminosity; this fact is useful for understanding the larger cosmological
context of our home galaxy.Comment: Updated to match published version. Added referenc
The scale-dependence of relative galaxy bias: encouragement for the halo model description
We investigate the relationship between the colors, luminosities, and
environments of galaxies in the Sloan Digital Sky Survey spectroscopic sample,
using environmental measurements on scales ranging from 0.2 to 6 Mpc/h. We
find: (1) that the relationship between color and environment persists even to
the lowest luminosities we probe (absolute magnitude in the r band of about -14
for h=1); (2) at luminosities and colors for which the galaxy correlation
function has a large amplitude, it also has a steep slope; and (3) in regions
of a given overdensity on small scales (1 Mpc/h), the overdensity on large
scales (6 Mpc/h) does not appear to relate to the recent star formation history
of the galaxies. Of these results, the last has the most immediate application
to galaxy formation theory. In particular, it lends support to the notion that
a galaxy's properties are related only to the mass of its host dark matter
halo, and not to the larger scale environment.Comment: submitted to ApJ; full resolution figures and slide material
available at http://cosmo.nyu.edu/blanton/scale_density.htm
Single parameter galaxy classification: The Principal Curve through the multi-dimensional space of galaxy properties
We propose to describe the variety of galaxies from SDSS by using only one
affine parameter. To this aim, we build the Principal Curve (P-curve) passing
through the spine of the data point cloud, considering the eigenspace derived
from Principal Component Analysis of morphological, physical and photometric
galaxy properties. Thus, galaxies can be labeled, ranked and classified by a
single arc length value of the curve, measured at the unique closest projection
of the data points on the P-curve. We find that the P-curve has a "W" letter
shape with 3 turning points, defining 4 branches that represent distinct galaxy
populations. This behavior is controlled mainly by 2 properties, namely u-r and
SFR. We further present the variations of several galaxy properties as a
function of arc length. Luminosity functions variate from steep Schechter fits
at low arc length, to double power law and ending in Log-normal fits at high
arc length. Galaxy clustering shows increasing autocorrelation power at large
scales as arc length increases. PCA analysis allowed to find peculiar galaxy
populations located apart from the main cloud of data points, such as small red
galaxies dominated by a disk, of relatively high stellar mass-to-light ratio
and surface mass density. The P-curve allows not only dimensionality reduction,
but also provides supporting evidence for relevant physical models and
scenarios in extragalactic astronomy: 1) Evidence for the hierarchical merging
scenario in the formation of a selected group of red massive galaxies. These
galaxies present a log-normal r-band luminosity function, which might arise
from multiplicative processes involved in this scenario. 2) Connection between
the onset of AGN activity and star formation quenching, which appears in green
galaxies when transitioning from blue to red populations. (Full abstract in
downloadable version)Comment: Full abstract in downloadable versio
Cooperation, collective action, and the archeology of large-scale societies
Archeologists investigating the emergence of large-scale societies in the past have renewed interest in examining the dynamics of cooperation as a means of understanding societal change and organizational variability within human groups over time. Unlike earlier approaches to these issues, which used models designated voluntaristic or managerial, contemporary research articulates more explicitly with frameworks for cooperation and collective action used in other fields, thereby facilitating empirical testing through better definition of the costs, benefits, and social mechanisms associated with success or failure in coordinated group action. Current scholarship is nevertheless bifurcated along lines of epistemology and scale, which is understandable but problematic for forging a broader, more transdisciplinary field of cooperation studies. Here, we point to some areas of potential overlap by reviewing archeological research that places the dynamics of social cooperation and competition in the foreground of the emergence of large-scale societies, which we define as those having larger populations, greater concentrations of political power, and higher degrees of social inequality. We focus on key issues involving the communal-resource management of subsistence and other economic goods, as well as the revenue flows that undergird political institutions. Drawing on archeological cases from across the globe, with greater detail from our area of expertise in Mesoamerica, we offer suggestions for strengthening analytical methods and generating more transdisciplinary research programs that address human societies across scalar and temporal spectra
Active galactic nucleus feedback in clusters of galaxies
Observations made during the last ten years with the Chandra X-ray
Observatory have shed much light on the cooling gas in the centers of clusters
of galaxies and the role of active galactic nucleus (AGN) heating. Cooling of
the hot intracluster medium in cluster centers can feed the supermassive black
holes found in the nuclei of the dominant cluster galaxies leading to AGN
outbursts which can reheat the gas, suppressing cooling and large amounts of
star formation. AGN heating can come in the form of shocks, buoyantly rising
bubbles that have been inflated by radio lobes, and the dissipation of sound
waves.Comment: Refereed review article published in Chandra's First Decade of
Discovery Special Feature edition of the Proceedings of the National Academy
of Science
A multiscale approach to environment and its influence on the colour distribution of galaxies
We present a multiscale approach to measurements of galaxy density, applied
to a volume-limited sample constructed from SDSS DR5. We populate a rich
parameter space by obtaining independent measurements of density on different
scales for each galaxy, avoiding the implicit assumptions involved, e.g., in
the construction of group catalogues. As the first application of this method,
we study how the bimodality in galaxy colour distribution (u-r) depends on
multiscale density. The u-r galaxy colour distribution is described as the sum
of two gaussians (red and blue) with five parameters: the fraction of red
galaxies (f_r) and the position and width of the red and blue peaks (mu_r,
mu_b, sigma_r and sigma_b). Galaxies mostly react to their smallest scale (<
0.5 Mpc) environments: in denser environments red galaxies are more common
(larger f_r), redder (larger mu_r) and with a narrower distribution (smaller
sigma_r), while blue galaxies are redder (larger mu_b) but with a broader
distribution (larger sigma_b). There are residual correlations of f_r and mu_b
with 0.5 - 1 Mpc scale density, which imply that total or partial truncation of
star formation can relate to a galaxy's environment on these scales. Beyond 1
Mpc (0.5 Mpc for mu_r) there are no positive correlations with density. However
f_r (mu_r) anti-correlates with density on >2 (1) Mpc scales at fixed density
on smaller scales. We examine these trends qualitatively in the context of the
halo model, utilizing the properties of haloes within which the galaxies are
embedded, derived by Yang et al, 2007 and applied to a group catalogue. This
yields an excellent description of the trends with multiscale density,
including the anti-correlations on large scales, which map the region of
accretion onto massive haloes. Thus we conclude that galaxies become red only
once they have been accreted onto haloes of a certain mass.Comment: 22 pages, 14 figures. Accepted for publication in MNRAS
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