3,776 research outputs found
A Stellar Mass Threshold for Quenching of Field Galaxies
We demonstrate that dwarf galaxies (10^7 < M_stellar < 10^9 Msun) with no
active star formation are extremely rare (<0.06%) in the field. Our sample is
based on the NASA-Sloan Atlas which is a re-analysis of the Sloan Digital Sky
Survey Data Release 8. We examine the relative number of quenched versus star
forming dwarf galaxies, defining quenched galaxies as having no Halpha emission
(EW_Halpha < 2 AA) and a strong 4000AA-break. The fraction of quenched dwarf
galaxies decreases rapidly with increasing distance from a massive host,
leveling off for distances beyond 1.5 Mpc. We define galaxies beyond 1.5 Mpc of
a massive host galaxy to be in the field. We demonstrate that there is a
stellar mass threshold of M_stellar < 1.0x10^9 Msun below which quenched
galaxies do not exist in the field. Below this threshold, we find that none of
the 2951 field dwarf galaxies are quenched; all field dwarf galaxies show
evidence for recent star formation. Correcting for volume effects, this
corresponds to a 1-sigma upper limit on the quenched fraction of 0.06%. In more
dense environments, quenched galaxies account for 23% of the dwarf population
over the same stellar mass range. The majority of quenched dwarf galaxies
(often classified as dwarf elliptical galaxies) are within 2 virial radii of a
massive galaxy, and only a few percent of quenched dwarf galaxies exist beyond
4 virial radii. Thus, for galaxies with stellar mass less than 1.0x10^9 Msun,
ending star-formation requires the presence of a more massive neighbor,
providing a stringent constraint on models of star formation feedback.Comment: 9 pages, 6 figures, accepted to Ap
Galaxies in SDSS and DEEP2: a quiet life on the blue sequence?
In the six billion years between redshifts z=1 and z=0.1, galaxies change due
to the aging of their stellar populations, the formation of new stars, and
mergers with other galaxies. Here I explore the relative importance of these
various effects, finding that while mergers are likely to be important for the
red galaxy sequence they are unlikely to affect more than 10% of the blue
galaxy sequence. I compare the galaxy population at redshift z=0.1 from the
Sloan Digital Sky Survey to that at z=1 from the Deep Extragalactic
Evolutionary Probe 2. Galaxies are bluer at z=1: the blue sequence by about 0.3
mag and the red sequence by about 0.1 mag, in redshift z=0.1 (u-g) color. I
evaluate the change in color and in the luminosity functions of the two
sequences using some simplistic stellar population synthesis models. These
models indicate that the luminous end of the red sequence fades less than
passive evolution allows by about 0.2 mag. Due to a lack of luminous blue
progenitors, ``dry'' mergers betweeen red galaxies then must create the
luminous red population at z=0.1, if stellar population models are correct. The
blue sequence colors and luminosity function are consistent with a reduction in
the star-formation rate since redshift z=1 by a factor of about three, with no
change in the number density to within 10%. These results restrict the number
of blue galaxies that can fall onto the red sequence by any process, and in
particular suggest that if mergers are catastrophic events they must be rare
for blue galaxies.Comment: submitted to ApJ, summary and viewgraphs available at
http://cosmo.nyu.edu/blanton/deep2sdss
An investigation of rooftop STOL port aerodynamics
An investigation into aerodynamic problems associated with large building rooftop STOLports was performed. Initially, a qualitative flow visualization study indicated two essential problems: (1) the establishment of smooth, steady, attached flow over the rooftop, and (2) the generation of acceptable crosswind profile once (1) has been achieved. This study indicated that (1) could be achieved by attaching circular-arc rounded edge extensions to the upper edges of the building and that crosswind profiles could be modified by the addition of porous vertical fences to the lateral edges of the rooftop. Important fence parameters associated with crosswind alteration were found to be solidity, fence element number and spacing. Large scale building induced velocity fluctuations were discovered for most configurations tested and a possible explanation for their occurrence was postulated. Finally, a simple equation relating fence solidity to the resulting velocity profile was developed and tested for non-uniform single element fences with 30 percent maximum solidity
Who Gets A Dead Man\u27s Gold? The Dilemma of Lottery Winnings Payable to a Decedent\u27s Estate
This article addresses the federal estate tax and federal income tax consequences of lottery winnings which flow to the estate of a decedent or, alternatively, directly to the decedent\u27s beneficiaries. State income tax and state death tax considerations must also be taken into account. With respect to these secondary implications, this article draws largely upon the Code of Virginia and the relevant sections of Virginia\u27s income tax and estate tax statutes. Some references will be made to the possibility of contrary statutory treatment in other states, but primary reliance will be upon Virginia law
Bill of Sale of an Enslaved Person Named Malvina
Bill of sale of an enslaved woman named Malvina.https://egrove.olemiss.edu/lanternproject/1044/thumbnail.jp
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
Autocorrelations of stellar light and mass in the low-redshift Universe
The final data release of the Sloan Digital Sky Survey (SDSS) provides
reliable photometry and spectroscopy for about half a million galaxies with
median redshift 0.09. Here we use these data to estimate projected
autocorrelation functions w_p(r_p) for the light of galaxies in the five SDSS
photometric bands. Comparison with the analogous stellar mass autocorrelation,
estimated in a previous paper, shows that stellar luminosity is less strongly
clustered than stellar mass in all bands and on all scales. Over the full
nonlinear range 10 kpc/h < r_p < 10 Mpc/h our autocorrelation estimates are
extremely well represented by power laws. The parameters of the corresponding
spatial functions \xi(r) = (r/r_0)^\gamma vary systematically from r_0=4.5
Mpc/h and \gamma=-1.74 for the bluest band (the u band) to r_0=5.8 Mpc/h and
\gamma=-1.83 for the reddest one (the z band). These may be compared with
r_0=6.1 Mpc/h and \gamma=-1.84 for the stellar mass. Ratios of w_p(r_p) between
two given wavebands are proportional to the mean colour of correlated stars at
projected distance r_p from a randomly chosen star. The ratio of the stellar
mass and luminosity autocorrelations measures an analogous mean stellar
mass-to-light ratio (M*/L). All colours get redder and all mass-to-light ratios
get larger with decreasing r_p, with the amplitude of the effects decreasing
strongly to redder passbands. Even for the u-band the effects are quite modest,
with maximum shifts of about 0.1 in u-g and about 25% in M*/L_u. These trends
provide a precise characterisation of the well-known dependence of stellar
populations on environment.Comment: 6 pages, 4 figures, accepted to MNRAS; three new paragraphs added:
two at the end of Sec. 2 concerning cross-correlations between different
bands and possible biases due to photometry errors, and one at the end of the
paper discussing marked correlation function
The Environments of Low and High Luminosity Radio Galaxies at Moderate Redshifts
In the local Universe, high-power radio galaxies live in lower density
environments than low-luminosity radio galaxies. If this trend continues to
higher redshifts, powerful radio galaxies would serve as efficient probes of
moderate redshift groups and poor clusters. Photometric studies of radio
galaxies at 0.3 < z < 0.5 suggest that the radio luminosity-environment
correlation disappears at moderate redshifts, though this could be the result
of foreground/background contamination affecting the photometric measures of
environment. We have obtained multi-object spectroscopy of in the fields of 14
lower luminosity (L_1.4GHz
1.2x10^25 W/Hz) radio galaxies at z ~ 0.3 to spectroscopically investigate the
link between the environment and the radio luminosity of radio galaxies at
moderate redshifts. Our results support the photometric analyses; there does
not appear to be a correlation between the luminosity of a radio galaxy and its
environment at moderate redshifts. Hence, radio galaxies are not efficient
signposts for group environments at moderate redshifts.Comment: 7 pages, 9 figures, Accepted for publication in A
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