75 research outputs found
Galaxy Formation In The Reionization Epoch As Hinted By Wide Field Camera 3 Observations Of The Hubble Ultra Deep Field
We present a large sample of candidate galaxies at z~7--10, selected in the
HUDF using the new observations made by the HST/WFC3. Our sample is composed of
20 z-dropouts, 15 Y-dropouts, and 20 J-dropouts. The surface densities of the
z-dropouts are close to what predicted by earlier studies, however, those of
the Y- and J-dropouts are quite unexpected. While no Y- or J-dropouts have been
found at AB < 28.0 mag, their surface densities seem to increase sharply at
fainter levels. While some of these candidates seem to be close to foreground
galaxies and thus could possibly be gravitationally lensed, the overall surface
densities after excluding such cases are still much higher than what would be
expected if the luminosity function does not evolve from z~7 to 10. Motivated
by such steep increases, we tentatively propose a set of Schechter function
parameters to describe the LFs at z~8 and 10. As compared to their counterpart
at z~7, here L* decreases by ~ 6.5x and Phi* increases by 17--90x. Although
such parameters are not yet demanded by the existing observations, they are
allowed and seem to agree with the data better than other alternatives. If
these LFs are still valid beyond our current detection limit, this would imply
a sudden emergence of a large number of low-luminosity galaxies when looking
back in time to z~10, which, while seemingly exotic, would naturally fit in the
picture of the cosmic hydrogen reionization. These early galaxies could easily
account for the ionizing photon budget required by the reionization, and they
would imply that the global star formation rate density might start from a very
high value at z~10, rapidly reach the minimum at z~7, and start to rise again
towards z~6. In this scenario, the majority of the stellar mass that the
universe assembled through the reionization epoch seems still undetected by
current observations at z~6. [Abridged]Comment: accepted for publication in Research in Astronomy and Astrophysic
Improved Photometric Redshifts with Surface Luminosity Priors
We apply Bayesian statistics with prior probabilities of galaxy surface
luminosity (SL) to improve photometric redshifts. We apply the method to a
sample of 1266 galaxies with spectroscopic redshifts in the GOODS North and
South fields at 0.1 < z < 2.0. We start with spectrophotometric redshifts
(SPZs) based on Probing Evolution and Reionization Spectroscopically grism
spectra, which cover a wavelength range of 6000-9000A, combined with
(U)BViz(JHK) broadband photometry in the GOODS fields. The accuracy of SPZ
redshifts is estimated to be \sigma (\Delta(z))=0.035 with an systematic offset
of -0.026, where \Delta(z)=\Delta z / (1+z), for galaxies in redshift range of
0.5 < z < 1.25. The addition of the SL prior probability helps break the
degeneracy of SPZ redshifts between low redshift 4000 A break galaxies and
high-redshift Lyman break galaxies which are mostly catastrophic outliers. For
the 1138 galaxies at z < 1.6, the fraction of galaxies with redshift deviation
\Delta (z) > 0.2 is reduced from 15.0% to 10.4%, while the rms scatter of the
fractional redshift error does not change much.Comment: 7 pages, 7 figures, published in A
The road to the red sequence: A detailed view of the formation of a massive galaxy at z~2
(Abridged) We present here a detailed analysis of the star formation history
(SFH) of FW4871, a massive galaxy at z=1.893+-0.002. We compare rest-frame
optical and NUV slitless grism spectra from the Hubble Space Telescope with a
large set of composite stellar populations to constrain the underlying star
formation history. Even though the morphology features prominent tidal tails,
indicative of a recent merger, there is no sign of on-going star formation
within an aperture encircling one effective radius, which corresponds to a
physical extent of 2.6 kpc. A model assuming truncation of an otherwise
constant SFH gives a formation epoch zF~10, with a truncation after 2.7 Gyr,
giving a mass-weighted age of 1.5 Gyr and a stellar mass of 0.8-3E11Msun,
implying star formation rates of 30-110 Msun/yr. A more complex model including
a recent burst of star formation places the age of the youngest component at
145 Myr, with a mass contribution lower than 20%, and a maximum amount of dust
reddening of E(B-V)<0.4 mag (95% confidence levels). This low level of dust
reddening is consistent with the low emission observed at 24 micron,
corresponding to rest-frame 8 micron, where PAH emission should contribute
significantly if a strong formation episode were present. The colour profile of
FW4871 does not suggest a significant radial trend in the properties of the
stellar populations out to 3Re. We suggest that the recent merger that formed
FW4871 is responsible for the quenching of its star formation.Comment: 11 pages, 8 figures, 4 tables. In press (Astronomical Journal
Spectroscopic Confirmation of Faint Lyman Break Galaxies at Redshifts Four and Five in the Hubble Ultra Deep Field
We present the faintest spectroscopically confirmed sample of redshift four
and five Lyman break galaxies to date. The sample is based on slitless grism
spectra of the Hubble Ultra Deep Field region from the GRAPES (Grism ACS
Program for Extragalactic Science) and PEARS (Probing Evolution and
Reionization Spectroscopically) projects, using the G800L grism on the HST
Advanced Camera for Surveys. We report here confirmations of 39 galaxies,
pre-selected as candidate Lyman break galaxies using photometric selection
criteria. We compare a "traditional" V-dropout selection to a more liberal one
(with V-i > 0.9), and find that the traditional criteria are about 64% complete
and 81% reliable. We also study the Lyman alpha emission properties of our
sample. We find that Lyman alpha emission is detected in about 1/4 of the
sample, and that our broad-band color selected sample includes 55% of
previously published line-selected Lyman alpha sources. Finally, we examine our
stacked 2D spectra. We demonstrate that strong, spatially extended (arcsecond
scale) Lyman alpha emission is not a generic property of these Lyman break
galaxies, but that a modest extension of the Lyman alpha photosphere (compared
to the starlight) may be present in those galaxies with prominent Lyman alpha
emission.Comment: Submitted to The Astrophysical Journal. Reduced spectra from both
GRAPES and PEARS are available from STScI, at
http://www.stsci.edu/science/grapes/ and at
http://archive.stsci.edu/prepds/pears
Emission-Line Galaxies from the Hubble Space Telescope Probing Evolution and Reionization Spectroscopically (PEARS) Grism Survey. II: The Complete Sample
We present a full analysis of the Probing Evolution And Reionization
Spectroscopically (PEARS) slitess grism spectroscopic data obtained with the
Advanced Camera for Surveys on HST. PEARS covers fields within both the Great
Observatories Origins Deep Survey (GOODS) North and South fields, making it
ideal as a random survey of galaxies, as well as the availability of a wide
variety of ancillary observations to support the spectroscopic results. Using
the PEARS data we are able to identify star forming galaxies within the
redshift volume 0< z<1.5. Star forming regions in the PEARS survey are
pinpointed independently of the host galaxy. This method allows us to detect
the presence of multiple emission line regions (ELRs) within a single galaxy.
1162 Ha, [OIII] and/or [OII] emission lines have been identified in the PEARS
sample of ~906 galaxies down to a limiting flux of ~1e-18 erg/s/cm^2. The ELRs
have also been compared to the properties of the host galaxy, including
morphology, luminosity, and mass. From this analysis we find three key results:
1) The computed line luminosities show evidence of a flattening in the
luminosity function with increasing redshift; 2) The star forming systems show
evidence of disturbed morphologies, with star formation occurring predominantly
within one effective (half-light) radius. However, the morphologies show no
correlation with host stellar mass; and 3) The number density of star forming
galaxies with M_* > 1e9} M_sun decreases by an order of magnitude at z<0.5
relative to the number at 0.5<z<0.9 in support of the argument for galaxy
downsizing.Comment: Submitted. 48 pages. 19 figures. Accepted to Ap
A Critical Assessment of Stellar Mass Measurement Methods
In this paper we perform a comprehensive study of the main sources of random
and systematic errors in stellar mass measurement for galaxies using their
Spectral Energy Distributions (SEDs). We use mock galaxy catalogs with
simulated multi-waveband photometry (from U-band to mid-infrared) and known
redshift, stellar mass, age and extinction for individual galaxies. Given
different parameters affecting stellar mass measurement (photometric S/N
ratios, SED fitting errors, systematic effects, the inherent degeneracies and
correlated errors), we formulated different simulated galaxy catalogs to
quantify these effects individually. We studied the sensitivity of stellar mass
estimates to the codes/methods used, population synthesis models, star
formation histories, nebular emission line contributions, photometric
uncertainties, extinction and age. For each simulated galaxy, the difference
between the input stellar masses and those estimated using different simulation
catalogs, , was calculated and used to identify the most
fundamental parameters affecting stellar masses. We measured different
components of the error budget, with the results listed as follows: (1). no
significant bias was found among different codes/methods, with all having
comparable scatter; (2). A source of error is found to be due to photometric
uncertainties and low resolution in age and extinction grids; (3). The median
of stellar masses among different methods provides a stable measure of the mass
associated with any given galaxy; (4). The deviations in stellar mass strongly
correlate with those in age, with a weaker correlation with extinction; (5).
the scatter in the stellar masses due to free parameters are quantified, with
the sensitivity of the stellar mass to both the population synthesis codes and
inclusion of nebular emission lines studied.Comment: 33 pages, 20 Figures, Accepted for publication in Astrophysical
Journa
The Lyman Continuum Escape Fraction of Galaxies and AGN in the GOODS Fields
We present our analysis of the LyC emission and escape fraction of 111
spectroscopically verified galaxies with and without AGN from . We
extended our ERS sample from Smith et al. (2018; arXiv:1602.01555) with 64
galaxies in the GOODS North and South fields using WFC3/UVIS F225W, F275W, and
F336W mosaics we independently drizzled using the HDUV, CANDELS, and UVUDF
data. Among the 17 AGN from the 111 galaxies, one provided a LyC detection in
F275W at mag (S/N 133) and NUV at
mag (S/N 13). We simultaneously fit and
spectra of this AGN to an accretion disk and Comptonization model and
find values of % and
%. For the remaining 110 galaxies, we stack
image cutouts that capture their LyC emission using the F225W, F275W, and F336W
data of the GOODS and ERS samples, and both combined, as well as subsamples of
galaxies with and without AGN, and galaxies. We find the stack of 17 AGN
dominate the LyC production from by a factor
of 10 compared to all 94 galaxies without AGN. While the IGM of the early
universe may have been reionized mostly by massive stars, there is evidence
that a significant portion of the ionizing energy came from AGN.Comment: 36 pages, 17 figures, 5 tables. Accepted for publication in The
Astrophysical Journal. v1 abstract latex errors corrected, minor changes to
table 5, orcid ID corrected for one autho
Structural Competency: Curriculum for Medical Students, Residents, and Interprofessional Teams on the Structural Factors That Produce Health Disparities
Introduction: Research on disparities in health and health care has demonstrated that social, economic, and political factors are key drivers of poor health outcomes. Yet the role of such structural forces on health and health care has been incorporated unevenly into medical training. The framework of structural competency offers a paradigm for training health professionals to recognize and respond to the impact of upstream, structural factors on patient health and health care. Methods: We report on a brief, interprofessional structural competency curriculum implemented in 32 distinct instances between 2015 and 2017 throughout the San Francisco Bay Area. In consultation with medical and interprofessional education experts, we developed open-ended, written-response surveys to qualitatively evaluate this curriculum\u27s impact on participants. Qualitative data from 15 iterations were analyzed via directed thematic analysis, coding language, and concepts to identify key themes. Results: Three core themes emerged from analysis of participants\u27 comments. First, participants valued the curriculum\u27s focus on the application of the structural competency framework in real-world clinical, community, and policy contexts. Second, participants with clinical experience (residents, fellows, and faculty) reported that the curriculum helped them reframe how they thought about patients. Third, participants reported feeling reconnected to their original motivations for entering the health professions. Discussion: This structural competency curriculum fills a gap in health professional education by equipping learners to understand and respond to the role that social, economic, and political structural factors play in patient and community health
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