3,163 research outputs found
The Dark Side of Galaxy Color
We present age distribution matching, a theoretical formalism for predicting
how galaxies of luminosity L and color C occupy dark matter halos. Our model
supposes that there are just two fundamental properties of a halo that
determine the color and brightness of the galaxy it hosts: the maximum circular
velocity Vmax, and the redshift z_starve that correlates with the epoch at
which the star formation in the galaxy ceases. The halo property z_starve is
intended to encompass physical characteristics of halo mass assembly that may
deprive the galaxy of its cold gas supply and, ultimately, quench its star
formation. The new, defining feature of the model is that, at fixed luminosity,
galaxy color is in monotonic correspondence with z_starve, with the larger
values of z_starve being assigned redder colors. We populate an N- body
simulation with a mock galaxy catalog based on age distribution matching, and
show that the resulting mock galaxy distribution accurately describes a variety
of galaxy statistics. Our model suggests that halo and galaxy assembly are
indeed correlated. We make publicly available our low-redshift, SDSS M_r <-19
mock galaxy catalog, and main progenitor histories of all z=0 halos, at
http://logrus.uchicago.edu/~aphearinComment: One new figure; expanded discussion of HOD; conclusions unchanged;
version accepted by MNRA
The Dark Side of Galaxy Color: evidence from new SDSS measurements of galaxy clustering and lensing
The age matching model has recently been shown to predict correctly the
luminosity L and g-r color of galaxies residing within dark matter halos. The
central tenet of the model is intuitive: older halos tend to host galaxies with
older stellar populations. In this paper, we demonstrate that age matching also
correctly predicts the g-r color trends exhibited in a wide variety of
statistics of the galaxy distribution for stellar mass M* threshold samples. In
particular, we present new measurements of the galaxy two-point correlation
function and the galaxy-galaxy lensing signal as a function of M* and g-r color
from the Sloan Digital Sky Survey, and show that age matching exhibits
remarkable agreement with these and other statistics of low-redshift galaxies.
In so doing, we also demonstrate good agreement between the galaxy-galaxy
lensing observed by SDSS and the signal predicted by abundance matching, a new
success of this model. We describe how age matching is a specific example of a
larger class of Conditional Abundance Matching models (CAM), a theoretical
framework we introduce here for the first time. CAM provides a general
formalism to study correlations at fixed mass between any galaxy property and
any halo property. The striking success of our simple implementation of CAM
provides compelling evidence that this technique has the potential to describe
the same set of data as alternative models, but with a dramatic reduction in
the required number of parameters. CAM achieves this reduction by exploiting
the capability of contemporary N-body simulations to determine dark matter halo
properties other than mass alone, which distinguishes our model from
conventional approaches to the galaxy-halo connection.Comment: references added, minor adjustments to text and notatio
The Extreme Small Scales: Do Satellite Galaxies Trace Dark Matter?
We investigate the radial distribution of galaxies within their host dark
matter halos by modeling their small-scale clustering, as measured in the Sloan
Digital Sky Survey. Specifically, we model the Jiang et al. (2011) measurements
of the galaxy two-point correlation function down to very small projected
separations (10 < r < 400 kpc/h), in a wide range of luminosity threshold
samples (absolute r-band magnitudes of -18 up to -23). We use a halo occupation
distribution (HOD) framework with free parameters that specify both the number
and spatial distribution of galaxies within their host dark matter halos. We
assume that the first galaxy in each halo lives at the halo center and that
additional satellite galaxies follow a radial density profile similar to the
dark matter Navarro-Frenk-White (NFW) profile, except that the concentration
and inner slope are allowed to vary. We find that in low luminosity samples,
satellite galaxies have radial profiles that are consistent with NFW. M_r < -20
and brighter satellite galaxies have radial profiles with significantly steeper
inner slopes than NFW (we find inner logarithmic slopes ranging from -1.6 to
-2.1, as opposed to -1 for NFW). We define a useful metric of concentration,
M_(1/10), which is the fraction of satellite galaxies (or mass) that are
enclosed within one tenth of the virial radius of a halo. We find that M_(1/10)
for low luminosity satellite galaxies agrees with NFW, whereas for luminous
galaxies it is 2.5-4 times higher, demonstrating that these galaxies are
substantially more centrally concentrated within their dark matter halos than
the dark matter itself. Our results therefore suggest that the processes that
govern the spatial distribution of galaxies, once they have merged into larger
halos, must be luminosity dependent, such that luminous galaxies become poor
tracers of the underlying dark matter.Comment: 12 pages, 6 figures, Accepted to Ap
Effectiveness of Efavirenz-Based Regimens in Young HIV-Infected Children Treated for Tuberculosis: A Treatment Option for Resource-Limited Settings
Background: Antiretroviral treatment (ART) options for young children co-infected with HIV and tuberculosis are limited in resource-poor settings due to limited data on the use of efavirenz (EFV). Using available pharmacokinetic data, an EFV dosing schedule was developed for young co-infected children and implemented as the standard of care at Macha Hospital in Southern Province, Zambia. Treatment outcomes inchildren younger than 3 years of age or weighing less than 10 kg receiving either EFV-based ART plus anti-tuberculous treatment or nevirapine-based (NVP) ART were compared. Methods: Treatment outcomes were measured in a cohort of HIV-infected children seeking care at Macha Hospital in rural Zambia from 2007 to 2010. Informationon the diagnosis and treatment of tuberculosis was abstracted from medical records. Results: Forty-five children treated for tuberculosis initiated an EFV-based regimen and 69 children initiated a NVP-based regimen, 7 of whom also were treated for tuberculosis. Children receiving both regimens were comparable in age, but children receiving EFV started ART with a lower CD4+ T-cell percentage and weight-for-age z-score. Children receiving EFV experienced increases in both CD4+ T-cell percentage and weight-for-age z-score during follow-up, such that levels were comparable to children receiving NVP after two years of ART. Cumulative survival after 12 months of ART did not differ between groups (NVP:87%;EFV:80%;p = 0.25). Eleven children experienced virologic failure during follow-up.The adjusted hazard ratio of virologic failure comparing EFV to NVP was 0.25 (95% CI:0.05,1.24) and 0.13 (95% CI:0.03,0.62) using thresholds of 5000 and 400 copies/mL, respectively.Five children receiving EFV were reported to have had convulsions after ART initiation compared to only one child receiving NVP (p = 0.04). Conclusions: Despite poorer health at ART initiation, children treated for tuberculosis and receiving EFV-based regimens showed significant improvements comparable to children receiving NVP-based regimens. EFV-based regimens should be considered for young HIV-infected children co-infected with tuberculosis in resource-limited settings
Parity and breast cancer risk among BRCA1 and BRCA2 mutation carriers.
INTRODUCTION: Increasing parity and age at first full-term pregnancy are established risk factors for breast cancer in the general population. However, their effects among BRCA1 and BRCA2 mutation carriers is still under debate. We used retrospective data on BRCA1 and BRCA2 mutation carriers from the UK to assess the effects of parity-related variables on breast cancer risk. METHODS: The data set included 457 mutation carriers who developed breast cancer (cases) and 332 healthy mutation carriers (controls), ascertained through families seen in genetic clinics. Hazard ratios were estimated by using a weighted cohort approach. RESULTS: Parous BRCA1 and BRCA2 mutation carriers were at a significantly lower risk of developing breast cancer (hazard ratio 0.54, 95% confidence interval 0.37 to 0.81; p = 0.002). The protective effect was observed only among carriers who were older than 40 years. Increasing age at first live birth was associated with an increased breast cancer risk among BRCA2 mutation carriers (p trend = 0.002) but not BRCA1 carriers. However, the analysis by age at first live birth was based on small numbers. CONCLUSION: The results suggest that the relative risks of breast cancer associated with parity among BRCA1 and BRCA2 mutation carriers may be similar to those in the general population and that reproductive history may be used to improve risk prediction in carriers.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are
Spitzer IRAC observations of newly-discovered planetary nebulae from the Macquarie-AAO-Strasbourg H-alpha Planetary Nebula Project
We compare H-alpha, radio continuum, and Spitzer Space Telescope (SST) images
of 58 planetary nebulae (PNe) recently discovered by the Macquarie-AAO-Strasbo-
urg H-alpha PN Project (MASH) of the SuperCOSMOS H-alpha Survey. Using InfraRed
Array Camera (IRAC) data we define the IR colors of PNe and demonstrate good
isolation between these colors and those of many other types of astronomical
object. The only substantive contamination of PNe in the color-color plane we
illustrate is due to YSOs. However, this ambiguity is readily resolved by the
unique optical characteristics of PNe and their environs. We also examine the
relationships between optical and MIR morphologies from 3.6 to 8.0um and
explore the ratio of mid-infrared (MIR) to radio nebular fluxes, which is a
valuable discriminant between thermal and nonthermal emission. MASH emphasizes
late evolutionary stages of PNe compared with previous catalogs, enabling study
of the changes in MIR and radio flux that attend the aging process. Spatially
integrated MIR energy distributions were constructed for all MASH PNe observed
by the GLIMPSE Legacy Project, using the H-alpha morphologies to establish the
dimensions for the calculations of the Midcourse Space Experiment (MSX), IRAC,
and radio continuum (from the Molonglo Observatory Synthesis Telescope and the
Very Large Array) flux densities. The ratio of IRAC 8.0-um to MSX 8.3-um flux
densities provides a measure of the absolute diffuse calibration of IRAC at 8.0
um. We independently confirm the aperture correction factor to be applied to
IRAC at 8.0um to align it with the diffuse calibration of MSX. The result
agrees with the recommendations of the Spitzer Science Center and with results
from a parallel study of HII regions. These PNe probe the diffuse calibration
of IRAC on a spatial scale of 9-77 arcsec.Comment: 48 pages, LaTeX (aastex), incl. 18 PostScript (eps) figures and 3
tables. Accepted by Astrophysical Journa
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