309 research outputs found
Starburst Intensity Limit of Galaxies at z~5-6
The peak star formation intensity in starburst galaxies does not vary
significantly from the local universe to redshift z~6. We arrive at this
conclusion through new surface brightness measurements of 47 starburst galaxies
at z~5-6, doubling the redshift range for such observations. These galaxies are
spectroscopically confirmed in the Hubble Ultra Deep Field (HUDF) through the
GRism ACS program for Extragalactic Science (GRAPES) project. The starburst
intensity limit for galaxies at z~5-6 agree with those at z~3-4 and z~0 to
within a factor of a few, after correcting for cosmological surface brightness
dimming and for dust. The most natural interpretation of this constancy over
cosmic time is that the same physical mechanisms limit starburst intensity at
all redshifts up to z~6 (be they galactic winds, gravitational instability, or
something else). We do see two trends with redshift: First, the UV spectral
slope of galaxies at z~5-6 is bluer than that of z~3 galaxies, suggesting an
increase in dust content over time. Second, the galaxy sizes from z~3 to z~6
scale approximately as the Hubble parameter 1/H(z). Thus, galaxies at z~6 are
high redshift starbursts, much like their local analogs except for slightly
bluer colors, smaller physical sizes, and correspondingly lower overall
luminosities. If we now assume a constant maximum star formation intensity, the
differences in observed surface brightness between z~0 and z~6 are consistent
with standard expanding cosmology and strongly inconsistent with tired light
model.Comment: Accepted for publication in ApJ (23 pages, 5 figures). Minor changes
to tex
The VIMOS Ultra Deep Survey: Lyα emission and stellar populations of star-forming galaxies at 2 < z < 2.5
The aim of this paper is to investigate spectral and photometric properties of 854 faint (i_(AB) ≲ 25 mag) star-forming galaxies (SFGs) at 2  0 Å), and Lyα emitters (LAEs; EW ≥ 20 Å). The fraction of LAEs at these redshifts is ~10%, which is consistent with previous observations. We compared best-fitSED-estimated stellar parameters of the SFG_N, SFG_L and LAE samples. For the luminosities probed here (~ L^∗), we find that galaxies with and without Lyα in emission have small but significant differences in their SED-based properties. We find that LAEs have less dust, and lower star-formation rates (SFR) compared to non-LAEs. We also find that LAEs are less massive compared to non-LAEs, though the difference is smaller and less significant compared to the SFR and Es(B−V). When we divide the LAEs according to their Spitzer/IRAC 3.6 μm fluxes, we find that the fraction of IRAC-detected (m_(3.6) ≲ 25 mag) LAEs is much higher than the fraction of IRAC-detected narrow band (NB)-selected LAEs at z ≃ 2–3. This could imply that UV-selected LAEs host a more evolved stellar population, which represents a later stage of galaxy evolution, compared to NB-selected LAEs
The VIMOS Ultra Deep Survey: Lyα emission and stellar populations of star-forming galaxies at 2 < z < 2.5
The aim of this paper is to investigate spectral and photometric properties of 854 faint (i_(AB) ≲ 25 mag) star-forming galaxies (SFGs) at 2  0 Å), and Lyα emitters (LAEs; EW ≥ 20 Å). The fraction of LAEs at these redshifts is ~10%, which is consistent with previous observations. We compared best-fitSED-estimated stellar parameters of the SFG_N, SFG_L and LAE samples. For the luminosities probed here (~ L^∗), we find that galaxies with and without Lyα in emission have small but significant differences in their SED-based properties. We find that LAEs have less dust, and lower star-formation rates (SFR) compared to non-LAEs. We also find that LAEs are less massive compared to non-LAEs, though the difference is smaller and less significant compared to the SFR and Es(B−V). When we divide the LAEs according to their Spitzer/IRAC 3.6 μm fluxes, we find that the fraction of IRAC-detected (m_(3.6) ≲ 25 mag) LAEs is much higher than the fraction of IRAC-detected narrow band (NB)-selected LAEs at z ≃ 2–3. This could imply that UV-selected LAEs host a more evolved stellar population, which represents a later stage of galaxy evolution, compared to NB-selected LAEs
Tracing Galaxy Assembly: Tadpole Galaxies in the Hubble Ultra Deep Field
In the Hubble Ultra Deep Field (HUDF) an abundance of galaxies is seen with a
knot at one end plus an extended tail, resembling a tadpole. These "tadpole
galaxies" appear dynamically unrelaxed--presumably in an early merging
state--where tidal interactions likely created the distorted knot-plus-tail
morphology. Here we systematically select tadpole galaxies from the HUDF and
study their properties as a function of their photometric redshifts. In a
companion HUDF variability study, Cohen et al. (2005) revealed a total of 45
variable objects believed to be Active Galactic Nuclei (AGN). Here we show that
this faint AGN sample has no overlap with the tadpole galaxy sample, as
predicted by theoretical work. The tadpole morphology--combined with the lack
of overlap with the variable objects--supports the idea that these galaxies are
in the process of an early-stage merger event, i.e., at a stage that likely
precedes the "turn-on" of any AGN component and the onset of any point-source
variability.Comment: 7 pages, 4 figures. Accepted for publication by Astrophysical Journa
Emission-Line Galaxies from the HST PEARS Grism Survey I: The South Fields
We present results of a search for emission-line galaxies in the Southern
Fields of the Hubble Space Telescope PEARS (Probing Evolution And Reionization
Spectroscopically) grism survey. The PEARS South Fields consist of five ACS
pointings (including the Hubble Ultra Deep Field) with the G800L grism for a
total of 120 orbits, revealing thousands of faint object spectra in the
GOODS-South region of the sky. Emission-line galaxies (ELGs) are one subset of
objects that are prevalent among the grism spectra. Using a 2-dimensional
detection and extraction procedure, we find 320 emission lines orginating from
226 galaxy "knots'' within 192 individual galaxies. Line identification results
in 118 new grism-spectroscopic redshifts for galaxies in the GOODS-South Field.
We measure emission line fluxes using standard Gaussian fitting techniques. At
the resolution of the grism data, the H-beta and [OIII] doublet are blended.
However, by fitting two Gaussian components to the H-beta and [OIII] features,
we find that many of the PEARS ELGs have high [OIII]/H-beta ratios compared to
other galaxy samples of comparable luminosities. The star-formation rates
(SFRs) of the ELGs are presented, as well as a sample of distinct giant
star-forming regions at z~0.1-0.5 across individual galaxies. We find that the
radial distances of these HII regions in general reside near the galaxies'
optical continuum half-light radii, similar to those of giant HII regions in
local galaxies.Comment: 15 pages, 13 figures; Accepted for publication in A
Dust extinction from Balmer decrements of star-forming galaxies at 0.75<z<1.5 with HST/WFC3 spectroscopy from the WISP survey
Spectroscopic observations of Halpha and Hbeta emission lines of 128
star-forming galaxies in the redshift range 0.75<z<1.5 are presented. These
data were taken with slitless spectroscopy using the G102 and G141 grisms of
the Wide-Field-Camera 3 (WFC3) on board the Hubble Space Telescope as part of
the WFC3 Infrared Spectroscopic Parallel (WISP) survey. Interstellar dust
extinction is measured from stacked spectra that cover the Balmer decrement
(Halpha/Hbeta). We present dust extinction as a function of Halpha luminosity
(down to 3 x 10^{41} erg/s), galaxy stellar mass (reaching 4 x 10^{8} Msun),
and rest-frame Halpha equivalent width. The faintest galaxies are two times
fainter in Halpha luminosity than galaxies previously studied at z~1.5. An
evolution is observed where galaxies of the same Halpha luminosity have lower
extinction at higher redshifts, whereas no evolution is found within our error
bars with stellar mass. The lower Halpha luminosity galaxies in our sample are
found to be consistent with no dust extinction. We find an anti-correlation of
the [OIII]5007/Halpha flux ratio as a function of luminosity where galaxies
with L_{Halpha}<5 x 10^{41} erg/s are brighter in [OIII]5007 than Halpha. This
trend is evident even after extinction correction, suggesting that the
increased [OIII]5007/Halpha ratio in low luminosity galaxies is likely due to
lower metallicity and/or higher ionization parameters.Comment: 11 pages, 9 figures, 2 tables; version addressing the referee
comment
Constraining the Distribution of L- & T-Dwarfs in the Galaxy
We estimate the thin disk scale height of the Galactic population of L- &
T-dwarfs based on star counts from 15 deep parallel fields from the Hubble
Space Telescope. From these observations, we have identified 28 candidate L- &
T- dwarfs based on their (i'-z') color and morphology. By comparing these star
counts to a simple Galactic model, we estimate the scale height to be 350+-50
pc that is consistent with the increase in vertical scale with decreasing
stellar mass and is independent of reddening, color-magnitude limits, and other
Galactic parameters. With this refined measure, we predict that less than 10^9
M_{sol} of the Milky Way can be in the form L- & T- dwarfs, and confirm that
high-latitude, z~6 galaxy surveys which use the i'-band dropout technique are
97-100% free of L- & T- dwarf interlopers.Comment: 4 pages, 4 figures, accepted to ApJ
A Self-consistent Model for Brown Dwarf Populations
We present a self-consistent model of the Milky Way to reproduce the observed distributions (spectral type, absolute J-band magnitude, effective temperature) and total velocity dispersion of brown dwarfs. For our model, we adopt parametric forms for the star formation history and initial-mass function, published evolutionary models, and theoretical age–velocity relations. Using standard Markov Chain Monte Carlo methods, we derive a power-law index of the initial-mass function of α = −0.71 ± 0.11, which is an improvement over previous studies. We consider a gamma-function form for the star formation history, though we find that this complex model is only slightly favored over a declining exponential. We find that a velocity variance that linearly increases with age and has an initial value of km s−1 best reproduces the total velocity dispersions. Given the similarities to main-sequence stars, this suggests brown dwarfs likely form via similar processes, but we recognize that the sizable uncertainties on σ0 preclude firm conclusions. To further refine these conclusions, we suggest that wide-field infrared imaging or low-resolution spectroscopic surveys, such as with the Nancy Grace Roman Space Telescope or Euclid, could provide large samples of brown dwarfs with robust spectral types that could probe the thickness of the thin disk. In this way, the number counts and population demographics could probe the same physical processes as with the kinematic measurements, however may provide larger samples and be subject to different selection biases
Stellar Populations of Lyman Break Galaxies at z=1-3 in the HST/WFC3 Early Release Science Observations
We analyze the spectral energy distributions (SEDs) of Lyman break galaxies
(LBGs) at z=1-3 selected using the Hubble Space Telescope (HST) Wide Field
Camera 3 (WFC3) UVIS channel filters. These HST/WFC3 observations cover about
50 sq. arcmin in the GOODS-South field as a part of the WFC3 Early Release
Science program. These LBGs at z=1-3 are selected using dropout selection
criteria similar to high redshift LBGs. The deep multi-band photometry in this
field is used to identify best-fit SED models, from which we infer the
following results: (1) the photometric redshift estimate of these dropout
selected LBGs is accurate to within few percent; (2) the UV spectral slope
(beta) is redder than at high redshift (z>3), where LBGs are less dusty; (3) on
average, LBGs at z=1-3 are massive, dustier and more highly star-forming,
compared to LBGs at higher redshifts with similar luminosities
(0.1L*<~L<~2.5L*), though their median values are similar within 1-sigma
uncertainties. This could imply that identical dropout selection technique, at
all redshifts, find physically similar galaxies; and (4) stellar masses of
these LBGs are directly proportional to their UV luminosities with a
logarithmic slope of ~0.46, and star-formation rates are proportional to their
stellar masses with a logarithmic slope of ~0.90. These relations hold true ---
within luminosities probed in this study --- for LBGs from z~1.5 to 5. The
star-forming galaxies selected using other color-based techniques show similar
correlations at z~2, but to avoid any selection biases, and for direct
comparison with LBGs at z>3, a true Lyman break selection at z~2 is essential.
The future HST UV surveys, both wider and deeper, covering a large luminosity
range are important to better understand LBG properties, and their evolution.Comment: Accepted for publication in ApJ (29 pages, 9 figures
Spectroscopic Discovery of the Supernova 2003dh Associated with GRB 030329
We present early observations of the afterglow of the Gamma-Ray Burst (GRB)
030329 and the spectroscopic discovery of its associated supernova SN 2003dh.
We obtained spectra of the afterglow of GRB 030329 each night from March 30.12
(0.6 days after the burst) to April 8.13 (UT) (9.6 days after the burst). The
spectra cover a wavelength range of 350 nm to 850 nm. The early spectra consist
of a power-law continuum (F_nu ~ nu^{-0.9}) with narrow emission lines
originating from HII regions in the host galaxy, indicating a low redshift of
z=0.1687. However, our spectra taken after 2003 Apr. 5 show broad peaks in flux
characteristic of a supernova. Correcting for the afterglow emission, we find
the spectrum of the supernova is remarkably similar to the type Ic `hypernova'
SN 1998bw. While the presence of supernovae have been inferred from the light
curves and colors of GRB afterglows in the past, this is the first direct,
spectroscopic confirmation that a subset of classical gamma-ray bursts
originate from supernovae.Comment: published by ApJ Letters; additional material avilable at
http://cfa-www.harvard.edu/cfa/oir/Research/GRB
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