80 research outputs found
A distortion of very--high--redshift galaxy number counts by gravitational lensing
The observed number counts of high-redshift galaxy candidates have been used
to build up a statistical description of star-forming activity at redshift z >~
7, when galaxies reionized the Universe. Standard models predict that a high
incidence of gravitational lensing will probably distort measurements of flux
and number of these earliest galaxies. The raw probability of this happening
has been estimated to be ~ 0.5 percent, but can be larger owing to
observational biases. Here we report that gravitational lensing is likely to
dominate the observed properties of galaxies with redshifts of z >~ 12, when
the instrumental limiting magnitude is expected to be brighter than the
characteristic magnitude of the galaxy sample. The number counts could be
modified by an order of magnitude, with most galaxies being part of multiply
imaged systems, located less than 1 arcsec from brighter foreground galaxies at
z ~ 2. This lens-induced association of high-redshift and foreground galaxies
has perhaps already been observed among a sample of galaxy candidates
identified at z ~ 10.6. Future surveys will need to be designed to account for
a significant gravitational lensing bias in high-redshift galaxy samples.Comment: Nature, Jan. 13, 2011 issue (in press
H-alpha +[NII] Observations of the HII Regions in M81
In a first of a series of studies of the H-alpha + [NII] emission from nearby
spiral galaxies, we present measurements of H-alpha + [NII] emission from HII
regions in M81. Our method uses large-field-CCD images and long-slit spectra,
and is part of the ongoing Beijing-Arizona-Taipei-Connecticut Sky Survey. The
CCD images are taken with the NAOC 0.6/0.9m f/3 Schmidt telescope at the
Xinglong Observing Station, using a multicolor filter set. Spectra of 10 of the
brightest HII regions are obtained using the NAOC 2.16m telescope with a Tek
1024 X 1024 CCD. The continua of the spectra are calibrated by flux-calibrated
images taken from the Schmidt observations. We determine the continuum
component of our H-alpha + [NII] image via interpolation from the more
accurately-measured backgrounds (M81 starlight) obtained from the two
neighboring (in wavelength) BATC filter images. We use the calibrated fluxes of
H-alpha + [NII] emission from the spectra to normalize this interpolated,
continuum-subtracted H-alpha + [NII] image. We estimate the zero point
uncertainty of the measured H-alpha + [NII] emission flux to be 8%. A
catalogue of H-alpha + [NII] fluxes for 456 HII regions is provided, with those
fluxes are on a more consistent linear scale than previously available. The
logarithmically-binned H-alpha + [NII] luminosity function of HII regions is
found to have slope = -0.70, consistent with previous results (which
allowed ). From the overall H-alpha + [NII] luminosity
of the HII regions, the star formation rate of M81 is found to be , modulo uncertainty with extinction corrections.Comment: 18 pages, 7 figures, accepted for publication in the Astronomical
Journa
Resolving the extragalactic hard X-ray background
The origin of the hard (2-10 keV) X-ray background has remained mysterious
for over 35 years. Most of the soft (0.5-2 keV) X-ray background has been
resolved into discrete sources, which are primarily quasars; however, these
sources do not have the flat spectral shape required to match the X-ray
background spectrum. Here we report the results of an X-ray survey 30 times
more sensitive than previous studies in the hard band and four times more
sensitive in the soft band. The sources detected in our survey account for at
least 75 per cent of the hard X-ray background. The mean X-ray spectrum of
these sources is in good agreement with that of the background. The X-ray
emission from the majority of the detected sources is unambiguously associated
with either the nuclei of otherwise normal bright galaxies or optically faint
sources, which could either be active nuclei of dust enshrouded galaxies or the
first quasars at very high redshifts.Comment: Nature article in pres
The Galaxy Structure-Redshift Relationship
There exists a gradual, but persistent, evolutionary effect in the galaxy
population such that galaxy structure and morphology change with redshift. This
galaxy structure-redshift relationship is such that an increasingly large
fraction of all bright and massive galaxies at redshifts 2 < z < 3 are
morphologically peculiar at wavelengths from rest-frame ultraviolet to
rest-frame optical. There are however examples of morphologically selected
spirals and ellipticals at all redshifts up to z ~ 3. At lower redshift, the
bright galaxy population smoothly transforms into normal ellipticals and
spirals. The rate of this transformation strongly depends on redshift, with the
swiftest evolution occurring between 1 < z < 2. This review characterizes the
galaxy structure-redshift relationship, discusses its various physical causes,
and how these are revealing the mechanisms responsible for galaxy formation.Comment: 20 pages, 8 figures. Invited Review to appear in "Penetrating Bars
Through Masks of Cosmic Dust: The Hubble Tuning Fork Strikes A New Note", ed.
D. Block et a
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SUPERNOVA REMNANTS AND THE INTERSTELLAR MEDIUM OF M83: IMAGING AND PHOTOMETRY WITH THE WIDE FIELD CAMERA 3 ON THE HUBBLE SPACE TELESCOPE
We present Wide Field Camera 3 images taken with the Hubble Space Telescope within a single field in the southern grand design star-forming galaxy M83. Based on their size, morphology, and photometry in continuum-subtracted Hα, [S II], Hβ, [O III], and [O II] filters, we have identified 60 supernova remnant (SNR) candidates, as well as a handful of young ejecta-dominated candidates. A catalog of these remnants, their sizes and, where possible, their Hα fluxes are given. Radiative ages and pre-shock densities are derived from those SNRs that have good photometry. The ages lie in the range 2.62 \u3c log (τrad/yr) \u3c 5.0, and the pre-shock densities at the blast wave range over 0.56 \u3c n 0/cm-3 \u3c 1680. Two populations of SNRs have been discovered. These divide into a nuclear and spiral arm group and an inter-arm population. We infer an arm to inter-arm density contrast of 4. The surface flux in diffuse X-rays is correlated with the inferred pre-shock density, indicating that the warm interstellar medium (ISM) is pressurized by the hot X-ray plasma. We also find that the ISM in the nuclear region of M83 is characterized by a very high porosity and pressure, and infer an SNR rate of 1 per 70-150 yr for the nuclear (R \u3c 300 pc) region. On the basis of the number of SNRs detected and their radiative ages, we infer that the lower mass of Type II SNe in M83 is M min = 16+7 –5 M . Finally, we give evidence for the likely detection of the remnant of the historical supernova, SN1968L
FIGS-Faint Infrared Grism Survey: Description and Data Reduction
The Faint Infrared Grism Survey (FIGS) is a deep Hubble Space Telescope (HST) WFC3/IR (Wide Field Camera 3 Infrared) slitless spectroscopic survey of four deep fields. Two fields are located in the Great Observatories Origins Deep Survey-North (GOODS-N) area and two fields are located in the Great Observatories Origins Deep Survey-South (GOODS-S) area. One of the southern fields selected is the Hubble Ultra Deep Field. Each of these four fields were observed using the WFC3/G102 grism (0.8 μm–1.15 μm continuous coverage) with a total exposure time of 40 orbits (≈100 kilo-seconds) per field. This reaches a continuum depth of AB magnitudes and probes emission lines to . This paper details the four FIGS fields and the overall observational strategy of the project. A detailed description of the Simulation Based Extraction (SBE) method used to extract and combine over 10,000 spectra of over 2000 distinct sources brighter than mag is provided. High fidelity simulations of the observations is shown to significantly improve the background subtraction process, the spectral contamination estimates, and the final flux calibration. This allows for the combination of multiple spectra to produce a final high quality, deep, 1D spectra for each object in the survey
Discovery of a z = 7.452 High Equivalent Width Lyman-α Emitter from the Hubble Space Telescope Faint Infrared Grism Survey
We present the results of an unbiased search for Lyα emission from continuum-selected 5.6 4σ) emission lines using two different automated detection methods, free of any visual inspection biases. Applying these methods on photometrically selected high-redshift candidates between 5.6 7 (140.3 ± 19.0 Å)
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An Ultra Deep Field survey with WFIRST
Studying the formation and evolution of galaxies at the earliest cosmic
times, and their role in reionization, requires the deepest imaging possible.
Ultra-deep surveys like the HUDF and HFF have pushed to mag \mAB30,
revealing galaxies at the faint end of the LF to 911 and
constraining their role in reionization. However, a key limitation of these
fields is their size, only a few arcminutes (less than a Mpc at these
redshifts), too small to probe large-scale environments or clustering
properties of these galaxies, crucial for advancing our understanding of
reionization. Achieving HUDF-quality depth over areas 100 times larger
becomes possible with a mission like the Wide Field Infrared Survey Telescope
(WFIRST), a 2.4-m telescope with similar optical properties to HST, with a
field of view of 1000 arcmin, 100 the area of the
HST/ACS HUDF.
This whitepaper motivates an Ultra-Deep Field survey with WFIRST, covering
100300 the area of the HUDF, or up to 1 deg, to
\mAB30, potentially revealing thousands of galaxies and AGN at the
faint end of the LF, at or beyond \,\,910 in the epoch of
reionization, and tracing their LSS environments, dramatically increasing the
discovery potential at these redshifts.
(Note: This paper is a somewhat expanded version of one that was submitted as
input to the Astro2020 Decadal Survey, with this version including an Appendix
(which exceeded the Astro2020 page limits), describing how the science drivers
for a WFIRST Ultra Deep Field might map into a notional observing program,
including the filters used and exposure times needed to achieve these depths.
Secular Evolution and the Growth of Pseudobulges in Disk Galaxies
Galaxy evolution is in transition from an early universe dominated by
hierarchical clustering to a future dominated by secular processes. These
result from interactions involving collective phenomena such as bars, oval
disks, spiral structure, and triaxial dark halos. This paper summarizes a
review by Kormendy & Kennicutt (2004) using, in part, illustrations of
different galaxies. In simulations, bars rearrange disk gas into outer rings,
inner rings, and galactic centers, where high gas densities feed starbursts.
Consistent with this picture, many barred and oval galaxies have dense central
concentrations of gas and star formation rates that can build bulge-like
stellar densities on timescales of a few billion years. We conclude that
secular evolution builds dense central components in disk galaxies that look
like classical, merger-built bulges but that were made slowly out of disk gas.
We call these pseudobulges. Many pseudobulges can be recognized because they
have characteristics of disks: (1) flatter shapes than those of classical
bulges, (2) correspondingly large ratios of ordered to random velocities, (3)
small velocity dispersions, (4) spiral structure or nuclear bars, (5) nearly
exponential brightness profiles, and (6) starbursts. These structures occur
preferentially in barred and oval galaxies in which secular evolution should be
most rapid. Thus a variety of observational and theoretical results contribute
to a new paradigm of secular evolution that complements hierarchical
clustering.Comment: 19 pages, 9 Postscript figures; requires kapproc.cls and procps.sty;
to appear in "Penetrating Bars Through Masks of Cosmic Dust: The Hubble
Tuning Fork Strikes a New Note", ed. Block, Freeman, Puerari, Groess, and
Block, Dordrecht: Kluwer, in press; for a version with full resolution
figures, see http://chandra.as.utexas.edu/~kormendy/ar3ss.htm
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