172 research outputs found
Evolution in the Escape Fraction of Ionizing Photons and the Decline in Strong Lya Emission from z>6 Galaxies
The rapid decline in the number of strong Lyman Alpha (Lya) emitting galaxies
at z > 6 provides evidence for neutral hydrogen in the IGM, but is difficult to
explain with plausible models for reionization. We demonstrate that the
observed reduction in Lya flux from galaxies at z > 6 can be explained by
evolution in the escape fraction of ionizing photons, f_esc. We find that the
median observed drop in the fraction of galaxies showing strong Lya emission,
as well as the observed evolution of the Lya luminosity function both follow
from a small increase in f_esc of Delta f_esc ~ 0.1 from f_esc ~ 0.6 at z ~ 6.
This high escape fraction may be at odds with current constraints on the
ionising photon escape fraction, which favor smaller values of f_esc < 20%.
However, models that invoke a redshift evolution of f_ esc that is consistent
with these constraints can suppress the z~7 Lya flux to the observed level, if
they also include a small evolution in global neutral fraction of Delta x_HI ~
0.2. Thus, an evolving escape fraction of ionising photons can be a plausible
part of the explanation for evolution in the Lya emission of high redshift
galaxies. More generally, our analysis also shows that the drop in the Lya
fraction is quantitatively consistent with the observed evolution in the Lya
luminosity functions of Lya Emitters.Comment: 9 pages, 3 figures. Accepted for publication in MNRAS. Added
references & brief discussion on line shape evolutio
Can the intergalactic medium cause a rapid drop in Lyman alpha emission at z>6?
The large cross-section of the Lyman alpha (Lya) line makes it a sensitive
probe of the ionization state of the intergalactic medium (IGM). Here we
present the most complete study to date of the IGM Lya opacity, and its
application to the redshift evolution of the 'Lya fraction', i.e. the fraction
of color-selected galaxies with a detectable Lya emission line. We use a tiered
approach, which combines large-scale semi-numeric simulations of reionization
with moderate-scale hydrodynamic simulations of the ionized IGM. This allows us
to simultaneously account for evolution in both: (i) the opacity from an
incomplete (patchy) reionization, parameterized by the filling factor of
ionized regions, Q_HII; and (ii) the opacity from self-shielded systems in the
ionized IGM, parameterized by the average photo-ionization rate inside HII
regions, \Gamma. In contrast to recent empirical models, attenuation from
patchy reionization has a unimodal distribution along different sightlines,
while attenuation from self-shielded systems is more bimodal. We quantify the
average IGM transmission in our (Q_HII, \Gamma) parameter space, which can
easily be used to interpret new data sets. Using current observations, we
predict that the Lya fraction cannot drop by more than a factor of ~2 with IGM
attenuation alone, even for HII filling factors as low as Q_HII>0.1. Larger
changes in the Lya fraction could result from a co-evolution with galaxy
properties. Marginalizing over \Gamma, we find that current observations
constrain Q_HII < 0.6 at z=7 [68% confidence level (C.L.)]. However, all of our
parameter space is consistent with observations at 95% C.L., highlighting the
need for larger observational samples at z >= 6.Comment: 12 pages, 10 figures, MNRAS submitte
The clustering properties of high-redshift passive galaxies
We investigate the clustering properties of 3<z<5 candidate passive galaxies
from the Merlin et al. (2019) sample residing in the GOODS-North (35 sources)
and GOODS-South (33 sources) fields. Within the large uncertainties due to the
paucity of sources we do not detect clustering signal in GOODS-North, while
this is present in GOODS-South, highlighting the importance of the effects of
cosmic variance. The estimated correlation length in GOODS-South is
r_0=12^+4_-5 Mpc, while the estimated minimum mass for a halo capable to host
one of such high-redshift quenched galaxies is log10(M_min/M_sun) =13.0\pm 0.3,
once also the constraints from their space density are taken into account. Both
values are compatible with the results from GOODS-North. Putting the above
findings in a cosmological context, these suggest no evolution of the dark
matter content of the hosts of passive galaxies during the past 12.5 Gyr, i.e.
during more than 90% of the age of the Universe. We discuss possible scenarios
for the observed trend.Comment: 6 pages, 3 figures, letter to appear in MNRA
The Spiderweb galaxy: a forming massive cluster galaxy at z~2
We present a deep image of the radio galaxy MRC 1138-262 taken with the
Hubble Space Telescope (HST) at a redshift of z = 2.2. The galaxy is known to
have properties of a cD galaxy progenitor and be surrounded by a 3 Mpc-sized
structure, identified with a protocluster. The morphology shown on the new deep
HST/ACS image is reminiscent of a spider's web. More than 10 individual clumpy
features are observed, apparently star-forming satellite galaxies in the
process of merging with the progenitor of a dominant cluster galaxy 11 Gyr ago.
There is an extended emission component, implying that star formation was
occurring over a 50 times 40 kpc region at a rate of more than 100 M_sun/yr. A
striking feature of the newly named ``Spiderweb galaxy'' is the presence of
several faint linear galaxies within the merging structure. The dense
environments and fast galaxy motions at the centres of protoclusters may
stimulate the formation of these structures, which dominate the faint resolved
galaxy populations in the Hubble Ultra Deep Field. The new image provides a
unique testbed for simulations of forming dominant cluster galaxies.Comment: 5 pages, 2 figures (reduced to grayscale); ApJ Letter
Beacons into the Cosmic Dark Ages: Boosted transmission of Ly from UV bright galaxies at
Recent detections of Lyman alpha (Ly) emission from galaxies
were somewhat unexpected given a dearth of previous non-detections in this era
when the intergalactic medium (IGM) is still highly neutral. But these
detections were from UV bright galaxies, which preferentially live in
overdensities which reionize early, and have significantly Doppler-shifted
Ly line profiles emerging from their interstellar media (ISM), making
them less affected by the global IGM state. Using a combination of reionization
simulations and empirical ISM models we show, as a result of these two effects,
UV bright galaxies in overdensities have higher transmission through
the IGM than typical field galaxies, and this boosted transmission is
enhanced as the neutral fraction increases. The boosted transmission is not
sufficient to explain the observed high Ly fraction of galaxies (Stark et al. 2017), suggesting Ly emitted by
these galaxies must be stronger than expected due to enhanced production and/or
selection effects. Despite the bias of UV bright galaxies to reside in
overdensities we show Ly observations of such galaxies can accurately
measure the global neutral hydrogen fraction, particularly when Ly from
UV faint galaxies is extinguished, making them ideal candidates for
spectroscopic follow-up into the cosmic Dark Ages.Comment: 6 pages, 5 figures. Accepted for publication in ApJ
VANDELS: a deep VIMOS survey of the Candels UDS and CDFS fields
VANDELS is a spectroscopic VIMOS public survey of the Candels UDS and CDFS fields aimed at exploring the high-z Universe. VANDELS, contrary to previous spectroscopic surveys, is designed to move beyond redshift acquisition, obtaining spectra with high enough signal-to-noise to derive galaxies' physical parameters e.g. metallicities and velocity offsets, from the very early phases when the Universe was still partly neutral (z∼7) to the epoch when the peak of the star-formation occurred (z∼2). To this end, integration times of 20hrsint<80 hrs have been planned. VANDELS will have an important legacy value to the astronomy community. All raw data will be immediately public on the ESO archive, while the reduced 1D and 2D spectra will be made available ∼9 months after observations. VANDELS pushes today's instrumentations opening the road towards our understanding of high-z galaxy physics. The findings will drive future observations, e.g. providing z=3-5 targets for detailed IFU observations using E-ELT
A needle in a haystack? Catching Pop\ III stars in the Epoch of Reionization: I. Pop\ III star forming environments
Despite extensive search efforts, direct observations of the first (Pop III)
stars have not yet succeeded. Theoretical studies have suggested that late Pop
III star formation (SF) is still possible in pristine clouds of high-mass
galaxies, coexisting with Pop II stars, down to the Epoch of Reionization
(EoR). Here we reassess this finding by exploring Pop III SF in eight simulations performed with the hydrodynamical code
dustyGadget. We find that Pop III SF () is still occurring down to ,
i.e. well within the reach of deep JWST surveys. At these epochs, of galaxies with are
found to host Pop III stars, although with a Pop III/Pop II mass fraction
. Regardless of their mass, Pop\ III hosting galaxies are
mainly found on the main sequence, at high SFRs, probably induced by accretion
of pristine gas. This scenario is also supported by their increasing SF
histories and their preferential location in high-density regions of the cosmic
web. Pop\ III stars are found both in the outskirts of metal-enriched regions
and in isolated, pristine clouds. In the latter case, their signal may be less
contaminated by Pop IIs, although its detectability will strongly depend on the
specific line-of-sight to the source, due to the complex morphology of the host
galaxy and its highly inhomogeneous dust distribution.Comment: Submitted to MNRAS, comments welcom
Inferences on the Timeline of Reionization at z~8 From the KMOS Lens-Amplified Spectroscopic Survey
Detections and non-detections of Lyman alpha (Ly) emission from
galaxies ( Gyr after the Big Bang) can be used to measure the timeline of
cosmic reionization. Of key interest to measuring reionization's mid-stages,
but also increasing observational challenge, are observations at z > 7, where
Ly redshifts to near infra-red wavelengths. Here we present a search
for z > 7.2 Ly emission in 53 intrinsically faint Lyman Break Galaxy
candidates, gravitationally lensed by massive galaxy clusters, in the KMOS
Lens-Amplified Spectroscopic Survey (KLASS). With integration times of ~7-10
hours, we detect no Ly emission with S/N>5 in our sample. We determine
our observations to be 80% complete for 5 spatially and spectrally
unresolved emission lines with integrated line flux erg
s cm. We define a photometrically selected sub-sample of 29
targets at , with a median 5 Ly EW limit of 58A.
We perform a Bayesian inference of the average intergalactic medium (IGM)
neutral hydrogen fraction using their spectra. Our inference accounts for the
wavelength sensitivity and incomplete redshift coverage of our observations,
and the photometric redshift probability distribution of each target. These
observations, combined with samples from the literature, enable us to place a
lower limit on the average IGM neutral hydrogen fraction of at z ~ 8, providing further evidence of rapid reionization
at z~6-8. We show that this is consistent with reionization history models
extending the galaxy luminosity function to , with
low ionizing photon escape fractions, .Comment: Accepted for publication in MNRA
HST Grism Observations of a Gravitationally Lensed Redshift 10 Galaxy
We present deep spectroscopic observations of a Lyman-break galaxy candidate
(hereafter MACS1149-JD) at with the Space
Telescope () WFC3/IR grisms. The grism observations were taken at
4 distinct position angles, totaling 34 orbits with the G141 grism, although
only 19 of the orbits are relatively uncontaminated along the trace of
MACS1149-JD. We fit a 3-parameter (, F160W mag, and Ly equivalent
width) Lyman-break galaxy template to the three least contaminated grism
position angles using an MCMC approach. The grism data alone are best fit with
a redshift of ( confidence), in
good agreement with our photometric estimate of
( confidence). Our analysis
rules out Lyman-alpha emission from MACS1149-JD above a equivalent
width of 21 \AA{}, consistent with a highly neutral IGM. We explore a scenario
where the red /IRAC color of the galaxy
previously pointed out in the literature is due to strong rest-frame optical
emission lines from a very young stellar population rather than a 4000 \AA{}
break. We find that while this can provide an explanation for the observed IRAC
color, it requires a lower redshift (), which is less preferred
by the imaging data. The grism data are consistent with both
scenarios, indicating that the red IRAC color can still be explained by a 4000
\AA{} break, characteristic of a relatively evolved stellar population. In this
interpretation, the photometry indicate that a Myr stellar
population is already present in this galaxy only after
the Big Bang.Comment: Accepted to ApJ. This is the accepted versio
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