1,985 research outputs found
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
Wind Canyon Sand Theodore Roosevelt National Park
The lenticular body of sandstone in Wind Canyon Theodore Roosevelt National Park, North Dakota, displays a variety of conditions that lend themselves to immaturity. It is these conditions and their genetic relations that the author pursued
How BAO measurements can fail to detect quintessence
We model the nonlinear growth of cosmic structure in different dark energy
models, using large volume N-body simulations. We consider a range of
quintessence models which feature both rapidly and slowly varying dark energy
equations of state, and compare the growth of structure to that in a universe
with a cosmological constant. The adoption of a quintessence model changes the
expansion history of the universe, the form of the linear theory power spectrum
and can alter key observables, such as the horizon scale and the distance to
last scattering. The difference in structure formation can be explained to
first order by the difference in growth factor at a given epoch; this scaling
also accounts for the nonlinear growth at the 15% level. We find that
quintessence models which feature late , rapid transitions towards
in the equation of state, can have identical baryonic acoustic
oscillation (BAO) peak positions to those in CDM, despite being very
different from CDM both today and at high redshifts .
We find that a second class of models which feature non-negligible amounts of
dark energy at early times cannot be distinguished from CDM using
measurements of the mass function or the BAO. These results highlight the need
to accurately model quintessence dark energy in N-body simulations when testing
cosmological probes of dynamical dark energy.Comment: 10 pages, 7 figures, to appear in the Invisible Univers International
Conference AIP proceedings serie
A Spectroscopic Analysis of the Eclipsing Short-Period Binary v505 Per and the Origin of the Lithium Dip
As a test of rotationally-induced mixing causing the well-known Li dip in
older mid-F dwarfs in the local Galactic disk, we utilize high-resolution and
-S/N Keck/HIRESspectroscopy to measure the Li abundance in the components of
the1 Gyr, [Fe/H]=-0.15 eclipsing short-period binary V505 Per. We find
A(Li)=2.7+/-0.1 and 2.4+/-0.2 in the Teff=6500 and 6450 K primary and secondary
components, respectively. Previous Teff determinations and uncertainties
suggest that each component is located in the midst of the Li dip. If so, their
A(Li) are >=2-5 times larger than A(Li) detections and upper limits observed in
the similar metallicity and intermediate-age open clusters NGC 752 and 3680, as
well as the more metal-rich and younger Hyades and Praesepe. These differences
are even larger if the consistent estimates of the scaling ofinitial Li with
metallicity inferred from nearby disk stars, open clusters, and recent Galactic
chemical evolution models are correct. Our results suggest, independently of
complementary evidence based on Li/Be ratios, Be/B ratios, and Li in subgiants
evolving out of the Li dip, that main-sequence angular momentum evolution is
the origin of the Li dip. Specifically, our stars' A(Li) indicates tidal
synchronization can be sufficiently efficient and occur early enough in
short-period binary mid-F stars to reduce the effects of rotationally-induced
mixing and destruction of Li occuring during the main-sequence in otherwise
similar stars that are not short-period tidally-locked binaries.Comment: Accepted for publication in Publications of the Astronomical Society
of the Pacific (July 2013 volume
The Influence of Middle School Teacher-Student Relationships on Future Academic Decisions of African American Males
The purpose of this study was to investigate the possible effects of middle school teacher-student relationships on students’ future academic decisions after dropping out of high school. With a national dropout rate of 30% and in some urban areas of 50%, among poor black minority youths, it is crucial that educators strongly consider the possibility that teacher-student relationships may play a vital role in the future decision-making or planning of middle school students (Kafele, 2012). Middle school research suggests that indicators of student disengagement can be traced back as early as elementary school when student reading and math scores begin to decline and disruptive classroom behavior begins to rise (Kafele, 2012; Kunjufu, 2005). The primary focus of this paper was student disengagement and dropout at the middle school level. Do positive teacher-student relationships, at the middle school level, have any impact on students’ future decisions to graduate from high school or return to school, such as a technical college after dropping out of high school. Five African American males between the ages of 23 and 46 were asked to share their personal experiences with their teachers in middle school, whether positive or negative. The participants were asked to give their own personal definitions of characteristics of positive and negative teacher-student relationships, and to disclose whether or not those relationships had any influence on their decisions to enroll into the local technical college. None of these males was influenced to enroll in school by a relationship with a teacher, but they shared information that is invaluable to all educators today as it relates to improving student engagement and increasing the graduation rates – Rules without relationships equal rebellion
Assumptions of the primordial spectrum and cosmological parameter estimation
The observables of the perturbed universe, CMB anisotropy and large
structures, depend on a set of cosmological parameters, as well as, the assumed
nature of primordial perturbations. In particular, the shape of the primordial
power spectrum (PPS) is, at best, a well motivated assumption. It is known that
the assumed functional form of the PPS in cosmological parameter estimation can
affect the best fit parameters and their relative confidence limits. In this
paper, we demonstrate that a specific assumed form actually drives the best fit
parameters into distinct basins of likelihood in the space of cosmological
parameters where the likelihood resists improvement via modifications to the
PPS. The regions where considerably better likelihoods are obtained allowing
free form PPS lie outside these basins. In the absence of a preferred model of
inflation, this raises a concern that current cosmological parameters estimates
are strongly prejudiced by the assumed form of PPS. Our results strongly
motivate approaches toward simultaneous estimation of the cosmological
parameters and the shape of the primordial spectrum from upcoming cosmological
data. It is equally important for theorists to keep an open mind towards early
universe scenarios that produce features in the PPS.Comment: 11 pages, 2 figures, discussions extended, main results unchanged,
matches published versio
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