122 research outputs found
The Spectral Types of White Dwarfs in Messier 4
We present the spectra of 24 white dwarfs in the direction of the globular
cluster Messier 4 obtained with the Keck/LRIS and Gemini/GMOS spectrographs.
Determining the spectral types of the stars in this sample, we find 24 type DA
and 0 type DB (i.e., atmospheres dominated by hydrogen and helium
respectively). Assuming the ratio of DA/DB observed in the field with effective
temperature between 15,000 - 25,000 K, i.e., 4.2:1, holds for the cluster
environment, the chance of finding no DBs in our sample due simply to
statistical fluctuations is only 6 X 10^(-3). The spectral types of the ~100
white dwarfs previously identified in open clusters indicate that DB formation
is strongly suppressed in that environment. Furthermore, all the ~10 white
dwarfs previously identified in other globular clusters are exclusively type
DA. In the context of these two facts, this finding suggests that DB formation
is suppressed in the cluster environment in general. Though no satisfactory
explanation for this phenomenon exists, we discuss several possibilities.Comment: Accepted for Publication in Astrophys. J. 11 pages including 4
figures and 2 tables (journal format
Constraints on growth index parameters from current and future observations
We use current and future simulated data of the growth rate of large scale
structure in combination with data from supernova, BAO, and CMB surface
measurements, in order to put constraints on the growth index parameters. We
use a recently proposed parameterization of the growth index that interpolates
between a constant value at high redshifts and a form that accounts for
redshift dependencies at small redshifts. We also suggest here another
exponential parameterization with a similar behaviour. The redshift dependent
parametrizations provide a sub-percent precision level to the numerical growth
function, for the full redshift range. Using these redshift parameterizations
or a constant growth index, we find that current available data from galaxy
redshift distortions and Lyman-alpha forests is unable to put significant
constraints on any of the growth parameters. For example both CDM and
flat DGP are allowed by current growth data. We use an MCMC analysis to study
constraints from future growth data, and simulate pessimistic and moderate
scenarios for the uncertainties. In both scenarios, the redshift
parameterizations discussed are able to provide significant constraints and
rule out models when incorrectly assumed in the analysis. The values taken by
the constant part of the parameterizations as well as the redshift slopes are
all found to significantly rule out an incorrect background. We also find that,
for our pessimistic scenario, an assumed constant growth index over the full
redshift range is unable to rule out incorrect models in all cases. This is due
to the fact that the slope acts as a second discriminator at smaller redshifts
and therefore provide a significant test to identify the underlying gravity
theory.Comment: 13 pages, 5 figures, matches JCAP accepted versio
The Masses of Population II White Dwarfs
Globular star clusters are among the first stellar populations to have formed
in the Milky Way, and thus only a small sliver of their initial spectrum of
stellar types are still burning hydrogen on the main-sequence today. Almost all
of the stars born with more mass than 0.8 M_sun have evolved to form the white
dwarf cooling sequence of these systems, and the distribution and properties of
these remnants uniquely holds clues related to the nature of the now evolved
progenitor stars. With ultra-deep HST imaging observations, rich white dwarf
populations of four nearby Milky Way globular clusters have recently been
uncovered, and are found to extend an impressive 5 - 8 magnitudes in the
faint-blue region of the H-R diagram. In this paper, we characterize the
properties of these population II remnants by presenting the first direct mass
measurements of individual white dwarfs near the tip of the cooling sequence in
the nearest of the Milky Way globulars, M4. Based on Gemini/GMOS and Keck/LRIS
multiobject spectroscopic observations, our results indicate that 0.8 M_sun
population II main-sequence stars evolving today form 0.53 +/- 0.01 M_sun white
dwarfs. We discuss the implications of this result as it relates to our
understanding of stellar structure and evolution of population II stars and for
the age of the Galactic halo, as measured with white dwarf cooling theory.Comment: Accepted for Publication in Astrophys. J. on Aug. 05th, 2009. 19
pages including 9 figures and 2 tables (journal format
Steep Faint-end Slopes of Galaxy Mass and Luminosity Functions at z>=6 and the Implications for Reionisation
We present the results of a numerical study comparing photometric and
physical properties of simulated z=6-9 galaxies to the observations taken by
the WFC3 instrument aboard the Hubble Space Telescope. Using cosmological
hydrodynamical simulations we find good agreement with observations in
color-color space at all studied redshifts. We also find good agreement between
observations and our Schechter luminosity function fit in the observable range,
Muv<= -18, provided that a moderate dust extinction effect exists for massive
galaxies. However beyond what currently can be observed, simulations predict a
very large number of low-mass galaxies and evolving steep faint-end slopes from
alpha_L = -2.15 at z=6 to alpha_L = -2.64 at z=9, with a dependence of
|alpha_L| \propto (1+z)^0.59. During the same epoch, the normalization phi*
increases and the characteristic magnitude Muv* becomes moderately brighter
with decreasing redshift. We find similar trends for galaxy stellar mass
function with evolving low-mass end slope from alpha_M = - 2.26 at z=6 to
alpha_M = -2.87 at z=9, with a dependence of |alpha_M| \propto (1+z)^0.65.
Together with our recent result on the high escape fraction of ionizing photons
for low-mass galaxies, our results suggest that the low-mass galaxies are
important contributor of ionizing photons for the reionisation of the Universe
at z>=6.Comment: Revised metadata, 16 pages, 5 tables, 17 figures. MNRAS, in pres
Imaging Electronic Correlations in Twisted Bilayer Graphene near the Magic Angle
Twisted bilayer graphene with a twist angle of around 1.1{\deg} features a
pair of isolated flat electronic bands and forms a strongly correlated
electronic platform. Here, we use scanning tunneling microscopy to probe local
properties of highly tunable twisted bilayer graphene devices and show that the
flat bands strongly deform when aligned with the Fermi level. At half filling
of the bands, we observe the development of gaps originating from correlated
insulating states. Near charge neutrality, we find a previously unidentified
correlated regime featuring a substantially enhanced flat band splitting that
we describe within a microscopic model predicting a strong tendency towards
nematic ordering. Our results provide insights into symmetry breaking
correlation effects and highlight the importance of electronic interactions for
all filling factors in twisted bilayer graphene.Comment: Main text 9 pages, 4 figures; Supplementary Information 25 page
Ubiquitous outflows in DEEP2 spectra of star-forming galaxies at z=1.4
Galactic winds are a prime suspect for the metal enrichment of the
intergalactic medium and may have a strong influence on the chemical evolution
of galaxies and the nature of QSO absorption line systems. We use a sample of
1406 galaxy spectra at z~1.4 from the DEEP2 redshift survey to show that
blueshifted Mg II 2796, 2803 A absorption is ubiquitous in starforming galaxies
at this epoch. This is the first detection of frequent outflowing galactic
winds at z~1. The presence and depth of absorption are independent of AGN
spectral signatures or galaxy morphology; major mergers are not a prerequisite
for driving a galactic wind from massive galaxies. Outflows are found in
coadded spectra of galaxies spanning a range of 30x in stellar mass and 10x in
star formation rate (SFR), calibrated from K-band and from MIPS IR fluxes. The
outflows have column densities of order N_H ~ 10^20 cm^-2 and characteristic
velocities of ~ 300-500 km/sec, with absorption seen out to 1000 km/sec in the
most massive, highest SFR galaxies. The velocities suggest that the outflowing
gas can escape into the IGM and that massive galaxies can produce
cosmologically and chemically significant outflows. Both the Mg II equivalent
width and the outflow velocity are larger for galaxies of higher stellar mass
and SFR, with V_wind ~ SFR^0.3, similar to the scaling in low redshift
IR-luminous galaxies. The high frequency of outflows in the star-forming galaxy
population at z~1 indicates that galactic winds occur in the progenitors of
massive spirals as well as those of ellipticals. The increase of outflow
velocity with mass and SFR constrains theoretical models of galaxy evolution
that include feedback from galactic winds, and may favor momentum-driven models
for the wind physics.Comment: Accepted by ApJ. 25 pages, 17 figures. Revised to add discussions of
intervening absorbers and AGN-driven outflows; conclusions unchange
Very Low-Mass Stellar and Substellar Companions to Solar-Like Stars from MARVELS I: A Low Mass Ratio Stellar Companion to TYC 4110-01037-1 in a 79-day Orbit
TYC 4110-01037-1 has a low-mass stellar companion, whose small mass ratio and
short orbital period are atypical amongst solar-like (Teff ~< 6000 K) binary
systems. Our analysis of TYC 4110-01037-1 reveals it to be a moderately aged
(~<5 Gyr) solar-like star having a mass of 1.07 +/- 0.08 MSun and radius of
0.99 +/- 0.18 RSun. We analyze 32 radial velocity measurements from the
SDSS-III MARVELS survey as well as 6 supporting radial velocity measurements
from the SARG spectrograph on the 3.6m TNG telescope obtained over a period of
~2 years. The best Keplerian orbital fit parameters were found to have a period
of 78.994 +/- 0.012 days, an eccentricity of 0.1095 +/- 0.0023, and a
semi-amplitude of 4199 +/- 11 m/s. We determine the minimum companion mass (if
sin i = 1) to be 97.7 +/- 5.8 MJup. The system's companion to host star mass
ratio, >0.087 +/- 0.003, places it at the lowest end of observed values for
short period stellar companions to solar-like (Teff ~< 6000 K) stars. One
possible way to create such a system would be if a triple-component stellar
multiple broke up into a short period, low q binary during the cluster
dispersal phase of its lifetime. A candidate tertiary body has been identified
in the system via single-epoch, high contrast imagery. If this object is
confirmed to be co-moving, we estimate it would be a dM4 star. We present these
results in the context of our larger-scale effort to constrain the statistics
of low mass stellar and brown dwarf companions to FGK-type stars via the
MARVELS survey.Comment: 22 pages; accepted in A
Duty Cycle and the Increasing Star Formation History of z>=6 Galaxies
We examine the duty cycle and the history of star formation (SFH) for
high-redshift galaxies at z>=6 using cosmological hydrodynamic simulations. We
find that, even though individual galaxies have bursty SFH, the averaged SFH
between z~15 to z=6 can be characterized well by either an exponentially
increasing functional form with characteristic time-scales of 70 Myr to 200 Myr
for galaxies with stellar masses Ms~10^6 Msun to >10^10 Msun respectively, or
by a simple power-law form which exhibits a similar mass dependent time-scales.
Using the SFH of individual galaxies, we measure the duty cycle of star
formation (DC_SFH); i.e., the fraction of time a galaxy of a particular mass
spends above a star formation rate (SFR) threshold which would make it
observable to the Hubble Space Telescope (HST) during a given epoch. We also
examine the fraction of galaxies at a given redshift that are brighter than a
rest-frame UV magnitude (Muv ~ -18), which is sufficient enough to make them
observable (DC_Muv). We find that both DC_SFH and DC_Muv make a sharp
transition from zero (for galaxies with Ms
10^9 Msun). The measured duty cycle is also manifested in the intrinsic scatter
in the Ms-SFR relationship (~ 1 dex) and Ms-Muv relationship (\Delta Muv ~ +-1
mag). We provide analytic fits to the DC as a function of Ms using a sigmoid
function, which can be used to correct for catalogue incompleteness. We
consider the effects of duty cycle to the observational estimate of galaxy
stellar mass functions (GSMF) and the star formation rate density (SFRD), and
find that it results in a much shallower low-mass end slopes of the GSMF and a
reduction of >~ 70% of our intrinsic SFRD, making our simulation results more
compatible with observational estimates.Comment: 13 pages, 11 figures, 5 tables. Accepted for publication MNRA
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