780 research outputs found
Filamentary Large-scale Structure Traced by Six Lyα Blobs at z = 2.3
Extended nebulae of Lyα emission ("Lyα blobs") are known to be associated with overdense regions at high redshift. Here we present six large Lyα blobs in a previously known protocluster with galaxy overdensity δ ~ 7 at z = 2.3; this is the richest field of giant Lyα blobs detected to date. The blobs have linear sizes of ≳ 100 kpc and Lyα luminosities of ~10^(43) erg s^(–1). The positions of the blobs define two linear filaments with an extent of at least 12 comoving Mpc; these filaments intersect at the center of one of the blobs. Measurement of the position angles of the blobs indicates that five of the six are aligned with these filaments to within ~10°, suggesting a connection between the physical processes powering extended Lyα emission and those driving structure on larger scales
X-ray properties of UV-selected star forming galaxies at z~1 in the Hubble Deep Field North
We present an analysis of the X-ray emission from a large sample of
ultraviolet (UV) selected, star forming galaxies with 0.74<z<1.32 in the Hubble
Deep Field North (HDF-N) region. By excluding all sources with significant
detected X-ray emission in the 2 Ms Chandra observation we are able to examine
the properties of galaxies for which the emission in both UV and X-ray is
expected to be predominantly due to star formation. Stacking the X-ray flux
from 216 galaxies in the soft and hard bands produces significant detections.
The derived mean 2-10 keV rest-frame luminosity is 2.97+/-0.26x10^(40) erg/s,
corresponding to an X-ray derived star formation rate (SFR) of 6.0+/-0.6
Msolar/yr. Comparing the X-ray value with the mean UV derived SFR, uncorrected
for attenuation, we find that the average UV attenuation correction factor is
\~3. By binning the galaxy sample according to UV magnitude and colour,
correlations between UV and X-ray emission are also examined. We find a strong
positive correlation between X-ray emission and rest-frame UV emission. A
correlation between the ratio of X-ray-to-UV emission and UV colour is also
seen, such that L(X)/L(UV) increases for redder galaxies. Given that X-ray
emission offers a view of star formation regions that is relatively unaffected
by extinction, results such as these can be used to evaluate the effects of
dust on the UV emission from high-z galaxies. For instance we derive a
relationship for estimating UV attenuation corrections as a function of colour
excess. The observed relation is inconsistent with the Calzetti et al. (2000)
reddening law which over predicts the range in UV attenuation corrections by a
factor of ~100 for the UV selected z~1 galaxies in this sample (abridged).Comment: 10 pages, 7 figures, accepted for publication in MNRA
Diffuse Lyman Alpha Emitting Halos: A Generic Property of High Redshift Star Forming Galaxies
Using a sample of 92 UV continuum-selected, spectroscopically identified
galaxies with = 2.65, all of which have been imaged in the Ly-a line with
extremely deep narrow-band imaging, we examine galaxy Ly-a emission profiles to
very faint surface brightness limits. The galaxies are representative of
spectroscopic samples of LBGs at similar redshifts in terms of apparent
magnitude, UV luminosity, inferred extinction, and star formation rate, and
were selected without regard to Ly-a emission properties. We use extremely deep
stacks of UV continuum and Ly-a emission line images to show that all
sub-samples exhibit diffuse Ly-a emission to radii of at least 10" (80 physical
kpc), including galaxies whose spectra exhibit Ly-a in net absorption. The
intensity scaling, but not the surface brightness distribution, is strongly
correlated with the emission observed in the central ~1". The characteristic
scale length for Ly-a line emission exceeds that of the UV continuum light for
the same galaxies by factors of at least 5-10, regardless of the spectral
morphology of Ly-a. Including the extended Ly-a halos increases the total Ly-a
flux [and rest equivalent width W_0(Lya)] by an average factor of 5. We argue
that most, if not all, of the observed Ly-a emission in the diffuse halos
originates in the galaxy H II regions and is scattered in our direction by H I
gas in the galaxy's circum-galactic medium (CGM). We show that whether or not a
galaxy is classified as a giant "Lyman-a Blob" (LAB) depends sensitively on the
Ly-a surface brightness threshold reached by an observation. Accounting for
diffuse Ly-a halos, all LBGs would be LABs if surveys were routinely sensitive
to 10 times lower surface brightness thresholds; also, essentially all LBGs
would qualify as LAEs (W_0(Lya) > 20 A).Comment: Updated to match final version to appear in ApJ; 20 pages, 14 figure
Galaxy Clustering at z~3
Galaxies at very high redshift (z~3 or greater) are now accessible to
wholesale observation, making possible for the first time a robust statistical
assessment of their spatial distribution at lookback times approaching ~90% of
the age of the Universe. This paper summarizes recent progress in understanding
the nature of these early galaxies, concentrating in particular on the
clustering properties of photometrically selected ``Lyman break'' galaxies.
Direct comparison of the data to predictions and physical insights provided by
galaxy and structure formation models is particularly straightforward at these
early epochs, and results in critical tests of the ``biased'', hierarchical
galaxy formation paradigm.Comment: Presented at Royal Society Discussion Meeting, March 1998, "Large
Scale Structure in the Universe", 14 pages LaTeX, 7 ps figures, uses
rspublic.sty (included
The Structure and Kinematics of the Circum-Galactic Medium from Far-UV Spectra of z~2-3 Galaxies
We present new results on the kinematics and spatial distribution of
metal-enriched gas within 125 kpc (physical) of Lyman Break galaxies at
redshifts z~2-3. In particular, we demonstrate how rest-UV galaxy spectra can
be used to obtain key spatial and spectral information more efficiently than
possible with QSO sightlines. After recalibrating the measurement of galaxy
systemic redshifts from their UV spectra, we investigate the kinematics of
galaxy-scale outflows via the strong interstellar (IS) absorption and Lya
emission lines (when present), as well as their dependence on other physical
properties of the galaxies. We construct a sample of 512 close (1-15 arcsec)
angular pairs of z~2-3 LBGs in which the spectra background galaxies probe the
circumgalactic gas surrrounding those in the foreground. The close pairs,
together with spectra of the foreground galaxies themselves, sample
galactocentric impact parameters b=0-125 kpc (physical) at =2.2. The
ensemble provides a spatial map of cool gas as a function of galactocentric
distance for a well-characterized population of galaxies. We propose a simple
model that simultaneously matches the kinematics, depth, and profile shape of
IS absorption and Lya emission lines, as well as the observed variation of
absorption line strength (of HI, CII, CIV, SiII, SiIV) versus galactocentric
impact parameter. We discuss the results of the observations in the context of
"cold accretion", in which cool gas accretes via filamentary streams directly
onto the central regions of galaxies. At present, we find little observational
support for cool infalling material, whereas evidence supporting the
large-scale effects of outflows is strong. Reconciling theory and observation
on the subject of gas flows into and out of forming galaxies seems necessary.Comment: To appear in ApJ; minor revisions to match journal version; added 1
figure, several references, and a subsection discussing the inferred rate of
gas outflow into the IG
The Direct Detection of Lyman Continuum Emission from Star-forming Galaxies at z~3
We present the results of rest-frame UV spectroscopic observations of a sample of 14 z ~ 3 star-forming galaxies in the SSA 22a field. These spectra are characterized by unprecedented depth in the Lyman continuum region. For the first time, we have detected escaping ionizing radiation from individual galaxies at high redshift, with 2 of the 14 objects showing significant emission below the Lyman limit. We also measured the ratio of emergent flux density at 1500 Å to that in the Lyman continuum region, for the individual detections (C49 and D3) and the sample average. If a correction for the average IGM opacity is applied to the spectra of the objects C49 and D3, we find f_(1500)/f_(900,corr,C49) = 4.5 and f_(1500)/f_(900,corr,D3) = 2.9. The average emergent flux density ratio in our sample is = 22, implying an escape fraction ~4.5 times lower than inferred from the composite spectrum from Steidel and coworkers. If this new estimate is representative of LBGs, their contribution to the metagalactic ionizing radiation field is J_ν(900) ~ 2.6 × 10^(-22) ergs s^(-1) cm^(-2) Hz^(-1) sr^(-1), comparable to the contribution of optically selected quasars at the same redshift. The sum of the contributions from galaxies and quasars is consistent with recent estimates of the level of the ionizing background at z ~ 3, inferred from the H I Lyα forest optical depth. There is significant variance among the emergent far-UV spectra in our sample, yet the factors controlling the detection or nondetection of Lyman continuum emission from galaxies are not well determined. Because we do not yet understand the source of this variance, significantly larger samples will be required to obtain robust constraints on the galaxy contribution to the ionizing background at z ~ 3 and beyond
The Kinematics of Morphologically Selected z~2 Galaxies in the GOODS-N Field
We present near-IR spectra of H-alpha emission from 13 galaxies at z~2 in the
GOODS-N field. The galaxies were selected primarily because they appear to have
elongated morphologies, and slits were aligned with the major axes (as
determined from the rest-frame UV emission) of 11 of the 13. If the galaxies
are elongated because they are highly inclined, alignment of the slit and major
axis should maximize the observed velocity and reveal velocity shear, if
present. In spite of this alignment, we see spatially resolved velocity shear
in only two galaxies. We show that the seeing makes a large difference in the
observed velocity spread of a tilted emission line, and use this information to
place limits on the velocity spread of the ionized gas of the galaxies in the
sample: we find that all 13 have v_{0.5} < 110 km/s, where v_{0.5} is the
velocity shear (half of the velocity range of a tilted emission line) that
would be observed under our best seeing conditions of ~0.5". When combined with
previous work, our data also indicate that aligning the slit along the major
axis does not increase the probability of observing a tilted emission line. We
then focus on the one-dimensional velocity dispersion \sigma, which is much
less affected by the seeing, and see that the elongated subsample exhibits a
significantly lower velocity dispersion than galaxies selected at random from
our total H-alpha sample, not higher as one might have expected. We also see
some evidence that the elongated galaxies are less reddened than those randomly
selected using only UV colors. Both of these results are counter to what would
be expected if the elongated galaxies were highly inclined disks. It is at
least as likely that the galaxies' elongated morphologies are due to merging
subunits.Comment: 9 pages, 5 figures. Accepted for publication in Ap
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