138 research outputs found
Prospects for Observing the low-density Cosmic Web in Lyman-alpha Emission
Mapping the intergalactic medium (IGM) in Lyman- emission would yield
unprecedented tomographic information on the large-scale distribution of
baryons and potentially provide new constraints on the UV background and
various feedback processes relevant to galaxy formation. Here, we use a
cosmological hydrodynamical simulation to examine the Lyman- emission
of the IGM due to collisional excitations and recombinations in the presence of
a UV background. We focus on gas in large-scale-structure filaments in which
Lyman- radiative transfer effects are expected to be moderate. At low
density the emission is primarily due to fluorescent re-emission of the
ionising UV background due to recombinations, while collisional excitations
dominate at higher densities. We discuss prospects of current and future
observational facilities to detect this emission and find that the emission of
filaments of the cosmic web will typically be dominated by the halos and
galaxies embedded in them, rather than by the lower density filament gas
outside halos. Detecting filament gas directly would require a very long
exposure with a MUSE-like instrument on the ELT. Our most robust predictions
that act as lower limits indicate this would be slightly less challenging at
lower redshifts (). We also find that there is a large amount of
variance between fields in our mock observations. High-redshift protoclusters
appear to be the most promising environment to observe the filamentary IGM in
Lyman- emission.Comment: 20 pages, 13 figures. Accepted for publication in Astronomy &
Astrophysics. Accepted version contains several revisions following
suggestions made in the review proces
The Sizes of Candidate Galaxies: confirmation of the bright CANDELS sample and relation with luminosity and mass
Recently, a small sample of six candidates was discovered in
CANDELS that are more luminous than any of the previous
galaxies identified over the HUDF/XDF and CLASH fields. We measure
the sizes of these candidates to map out the size evolution of galaxies from
the earliest observable times. Their sizes are also used to provide a valuable
constraint on whether these unusual galaxy candidates are at high redshift.
Using galfit to derive sizes from the CANDELS F160W images of these candidates,
we find a mean size of 0.130.02" (or 0.50.1 kpc at ). This
handsomely matches the 0.6 kpc size expected extrapolating lower redshift
measurements to , while being much smaller than the 0.59" mean size
for lower-redshift interlopers to photometric selections lacking
the blue IRAC color criterion. This suggests that source size may be an
effective constraint on contaminants from selections lacking IRAC
data. Assuming on the basis of the strong photometric evidence that the Oesch
et al. 2014 sample is entirely at , we can use this sample to extend
current constraints on the size-luminosity, size-mass relation, and size
evolution of galaxies to . We find that the candidate
galaxies have broadly similar sizes and luminosities as -8 counterparts
with star-formation-rate surface densities in the range of . The stellar mass-size
relation is uncertain, but shallower than those inferred for lower-redshift
galaxies. In combination with previous size measurements at z=4-7, we find a
size evolution of with for galaxies,
consistent with the evolution previously derived from galaxies.Comment: 9 figures, 5 tables, accepted by Ap
The Star Formation Rate Function for Redshift z~4-7 Galaxies: Evidence for a Uniform Build-Up of Star-Forming Galaxies During the First 3 Gyr of Cosmic Time
We combine recent estimates of dust extinction at z~4-7 with UV luminosity
function (LF) determinations to derive star formation rate (SFR) functions at
z~4, 5, 6 and 7. SFR functions provide a more physical description of galaxy
build-up at high redshift and allow for direct comparisons to SFRs at lower
redshifts determined by a variety of techniques. Our SFR functions are derived
from well-established z~4-7 UV LFs, UV-continuum slope trends with redshift and
luminosity, and IRX-beta relations. They are well-described by Schechter
relations. We extend the comparison baseline for SFR functions to z~2 by
considering recent determinations of the H{\alpha} and mid-IR luminosity
functions. The low-end slopes of the SFR functions are flatter than for the UV
LFs, \Delta\alpha\sim+0.13, and show no clear evolution with cosmic time
(z~0-7). In addition, we find that the characteristic value SFR* from the
Schechter fit to SFR function exhibits consistent, and substantial, linear
growth as a function of redshift from ~5 M_sun/yr at z~8, 650 Myr after the Big
Bang, to ~100 M_sun/yr at z~2, ~2.5 Gyr later. Recent results at z~10, close to
the onset of galaxy formation, are consistent with this trend. The uniformity
of this evolution is even greater than seen in the UV LF over the redshift
range z~2-8, providing validation for our dust corrections. These results
provide strong evidence that galaxies build up uniformly over the first 3 Gyr
of cosmic time.Comment: Added an appendix, 1 figure and 3 tables: 9 pages, 5 figures, 4
tables, ApJ, in pres
Lyman-alpha Emission from a Luminous z=8.68 Galaxy: Implications for Galaxies as Tracers of Cosmic Reionization
We report the discovery of Lyman-alpha emission (Ly) in the bright
galaxy EGSY-2008532660 (hereafter EGSY8p7) using the MOSFIRE spectrograph at
the Keck Observatory. First reported by Roberts-Borsani et al. (2015), it was
selected for spectroscopic observations because of its photometric redshift
(), apparent brightness (H)
and red Spitzer/IRAC [3.6]-[4.5] color indicative of contamination by strong
oxygen emission in the [4.5] band. With a total integration of 4.3 hours,
our data reveal an emission line at 11776 {\AA} which we argue is
likely Ly at a redshift , in good
agreement with the photometric estimate. The line was detected independently on
two nights using different slit orientations and its detection significance is
. An overlapping skyline contributes significantly to the
uncertainty on the total line flux although the significance of the detected
line is robust to a variety of skyline-masking procedures. By direct addition
and a Gaussian fit, we estimate a 95\% confidence range of
1.0--2.5 erg s cm, corresponding to a rest-frame
equivalent width of 17--42 {\AA}. EGSY8p7 is the most distant galaxy confirmed
spectroscopically to date, and the third luminous source in the EGS field
beyond with detectable Ly emission viewed at a
time when the intergalactic medium is believed to be fairly neutral. Although
the reionization process was probably patchy, we discuss whether luminous
sources with prominent IRAC color excesses may harbor harder ionizing spectra
than the dominant fainter population thereby creating earlier ionized bubbles.
Further spectroscopic follow-up of such bright sources promises important
insight into the early formation of galaxies.Comment: V3: ApJL accepted; 7 pages, 4 figures, 1 tabl
Inferred Hα flux as a star formation rate indicator at z ∼ 4–5: implications for dust properties, burstiness, and the z = 4–8 star formation rate functions
We derive Hα fluxes for a large spectroscopic and photometric-redshift-selected sample of sources over GOODSNorth and South in the redshift range z = 3.8–5.0 with deep Hubble Space Telescope (HST), Spitzer/IRAC, and ground-based observations. The Hα flux is inferred based on the offset between the IRAC 3.6 μm flux and that predicted from the best-fit spectral energy distribution (SED). We demonstrate that the Hα flux correlates well with dust-corrected UV star formation rate (SFR) and therefore can serve as an independent SFR indicator. However, we also find a systematic offset in the SFR SFR H UV a +b ratios for z ∼ 4–5 galaxies relative to local relations (assuming the same dust corrections for nebular regions and stellar light). We show that we can resolve the modest tension in the inferred SFRs by assuming bluer intrinsic UV slopes (increasing the dust correction), a rising star formation history, or assuming a low-metallicity stellar population with a hard ionizing spectrum (increasing the LHa SFR ratio). Using Hα as an SFR indicator, we find a normalization of the star formation main sequence in good agreement with recent SED-based determinations and also derive the SFR functions at z ~ 4 8– . In addition, we assess for the first time the burstiness of star formation in z ~ 4 galaxies on <100 Myr timescales by comparing UV and Hα-based sSFRs; their one-to-one relationship argues against significantly bursty star formation histories
High-precision Photometric Redshifts from Spitzer/IRAC: Extreme [3.6]-[4.5] Colors Identify Galaxies in the Redshift Range z~6.6-6.9
One of the most challenging aspects of studying galaxies in the z>~7 universe
is the infrequent confirmation of their redshifts through spectroscopy, a
phenomenon thought to occur from the increasing opacity of the intergalactic
medium to Lya photons at z>6.5. The resulting redshift uncertainties inhibit
the efficient search for [C II] in z~7 galaxies with sub-mm instruments such as
ALMA, given their limited scan speed for faint lines. One means by which to
improve the precision of the inferred redshifts is to exploit the potential
impact of strong nebular emission lines on the colors of z~4-8 galaxies as
observed by Spitzer/IRAC. At z~6.8, galaxies exhibit IRAC colors as blue as
[3.6]-[4.5] ~-1, likely due to the contribution of [O III]+Hb to the 3.6 mum
flux combined with the absence of line contamination in the 4.5 mum band. In
this paper we explore the use of extremely blue [3.6]-[4.5] colors to identify
galaxies in the narrow redshift window z~6.6-6.9. When combined with an
I-dropout criterion, we demonstrate that we can plausibly select a relatively
clean sample of z~6.8 galaxies. Through a systematic application of this
selection technique to our catalogs from all five CANDELS fields, we identify
20 probable z~6.6-6.9 galaxies. We estimate that our criteria select the ~50%
strongest line emitters at z~6.8 and from the IRAC colors we estimate a typical
[O III]+Hb rest-frame equivalent width of 1085A for this sample. The small
redshift uncertainties on our sample make it particularly well suited for
follow-up studies with facilities such as ALMA.Comment: In submission to the Astrophysical Journal, updated in response to
the referee report, 13 pages, 11 figures, 1 tabl
The H{\alpha} Luminosity Function of Galaxies at z {\sim} 4.5
We present the H{\alpha} luminosity function (LF) derived from a large sample
of Lyman break galaxies at z {\sim} 4.5 over the GOODS-South and North fields.
This study makes use of the new, full-depth Spitzer/IRAC [3.6] and [4.5]
imaging from the GOODS Re-ionization Era wide-Area Treasury from the Spitzer
program. The H{\alpha} flux is derived from the offset between the continuum
flux estimated from the best-fit spectral energy distribution, and the observed
photometry in IRAC [3.6]. From these measurements, we build the H{\alpha} LF
and study its evolution providing the best constraints of this property at high
redshift, where spectroscopy of H{\alpha} is not yet available. Schechter
parameterizations of the H{\alpha} LF show a decreasing evolution of
{\Phi^\star} with redshift, increasing evolution in L{^\star}, and no
significant evolution in the faint-end slope at high z. We find that star
formation rates (SFRs) derived from H{\alpha} are higher than those derived
from the rest-frame UV for low SFR galaxies but the opposite happens for the
highest SFRs. This can be explained by lower mass galaxies (also lower SFR)
having, on average, rising star formation histories (SFHs), while at the
highest masses the SFHs may be declining. The SFR function is steeper, and
because of the excess SFR(H{\alpha}) compared to SFR(UV) at low SFRs, the SFR
density estimated from H{\alpha} is higher than the previous estimates based on
UV luminosities.Comment: Astrophysical Journal, Accepted, 17 pages, 16 figure
Spatially resolved Kennicutt-Schmidt relation at z ≈ 7 and its connection with the interstellar medium properties
We exploit moderately resolved [O III], [C II] and dust continuum ALMA observations to derive the gas density (n), the gas-phase metallicity (Z) and the deviation from the Kennicutt-Schmidt (KS) relation (κs) on ≈sub−kpc
scales in the interstellar medium (ISM) of five bright Lyman Break Galaxies at the Epoch of Reionization (z ≈ 7). To do so, we use GLAM, a state-of-art, physically motivated Bayesian model that links the [C II]and [O III] surface brightness (Σ[CII], Σ[OIII]) and the SFR surface density (ΣSFR) to n, κs, and Z. All five sources are characterized by a central starbursting region, where the Σgas vs ΣSFR align ≈10 × above the KS relation (κs ≈ 10). This translates into gas depletion times in the range tdep ≈ 80 − 250 Myr. The inner starbursting centers are characterized by higher gas density (log (n/cm−3) ≈ 2.5 − 3.0) and higher metallicity (log (Z/Z⊙) ≈ −0.5) than the galaxy outskirts. We derive marginally negative radial metallicity gradients (∇log Z ≈ −0.03 ± 0.07 dex/kpc), and a dust temperature (Td ≈ 32 − 38 K) that anticorrelates with the gas depletion time
Radio and Far-IR Emission Associated with a Massive Star-forming Galaxy Candidate at z6.8: A Radio-Loud AGN in the Reionization Era?
We report the identification of radio (0.144-3 GHz), mid-IR, far-IR, and
sub-mm (24-850m) emission at the position of one of 41 UV-bright
(M) Lyman-break galaxy
candidates in the 1.5 deg COSMOS field. This source, COS-87259, exhibits a
sharp flux discontinuity (factor 3) between two narrow/intermediate bands at
9450 and 9700 Angstroms and is undetected in all nine bands blueward of 9600
Angstroms, as expected from a Lyman-alpha break at . The full
multi-wavelength (X-ray through radio) data of COS-87529 can be
self-consistently explained by a very massive (M
M) and extremely red (rest-UV slope )
galaxy with hyperluminous infrared emission (L
L) powered by both an intense burst of highly-obscured star formation
(SFR1800 M yr) and an obscured () radio-loud (L W
Hz) AGN. The radio emission is compact (1.040.12 arcsec) and
exhibits an ultra-steep spectrum between 1.32-3 GHz
() that flattens at lower frequencies
( between 0.144-1.32 GHz), consistent with known
radio galaxies. We also demonstrate that COS-87259 may reside in a
significant (11) galaxy overdensity at , as common for
systems hosting radio-loud AGN. Nonetheless, a spectroscopic redshift will
ultimately be required to establish the true nature of COS-87259 as we cannot
yet completely rule out low-redshift solutions. If confirmed to lie at
, the properties of COS-87259 would be consistent with a picture
wherein AGN and highly-obscured star formation activity are fairly common among
very massive (M M) reionization-era galaxies.Comment: 14 pages, 6 figures. Accepted in MNRAS with minor revisions. This
accepted version considers very recent data from LOFAR and MeerKAT which
improve our analyses on the radio slope and luminosit
ALMA Confirmation of an Obscured Hyperluminous Radio-Loud AGN at Associated with a Dusty Starburst in the 1.5 deg COSMOS Field
We present band 6 ALMA observations of a heavily-obscured radio-loud
( W Hz) AGN candidate at
found in the 1.5 deg COSMOS field. The ALMA
data reveal detections of exceptionally strong [CII]158m
() and underlying dust continuum emission from this
object (COS-87259), where the [CII] line luminosity, line width, and 158m
continuum luminosity are comparable to that seen from sub-mm galaxies
and quasar hosts. The 158m continuum detection suggests a total infrared
luminosity of with corresponding very large obscured
star formation rate (1300 /yr) and dust mass (
). The strong break seen between the VIRCam and IRAC photometry
perhaps suggests that COS-87259 is an extremely massive reionization era galaxy
with . Moreover, the MIPS, PACS, and
SPIRE detections imply that this object harbors an AGN that is heavily obscured
() with a bolometric luminosity of
approximately . Such a very high AGN luminosity
suggests this object is powered by an 1.6 10
black hole if accreting near the Eddington limit, and is effectively a
highly-obscured version of an extremely UV-luminous ()
quasar. Notably, these quasars are an exceedingly rare
population (0.001 deg) while COS-87259 was identified over a
relatively small field. Future very wide-area surveys with, e.g., Roman and
Euclid have the potential to identify many more extremely red yet UV-bright
objects similar to COS-87259, providing richer insight into the
occurrence of intense obscured star formation and supermassive black hole
growth among this population.Comment: 12 pages, 7 figures, 1 table. Updated to accepted version (MNRAS
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