153 research outputs found
Missing cosmic metals revealed by X-ray absorption towards distant sources
The census of heavy elements (metals) produced by all stars through cosmic
times up to present-day is limited to ~50%; of these only half are still found
within their parent galaxy. The majority of metals is expelled from galaxies
into the circumgalactic (or even more distant, intergalactic) space by powerful
galactic winds, leaving unpleasant uncertainty on the amount, thermal
properties and distribution of these key chemical species. These dispersed
metals unavoidably absorb soft X-ray photons from distant sources. We show that
their integrated contribution can be detected in the form of increasing X-ray
absorption with distance, for all kinds of high-energy cosmic sources. Based on
extensive cosmological simulations, we assess that 10\% of all cosmic
metals reside in the intergalactic medium. Most of the X-ray absorption arises
instead from a few discrete structures along the line of sight. These extended
structures, possibly pin-pointing galaxy groups, contain million degree,
metal-enriched gas, 100-1,000 times denser than the cosmic mean. An additional
~10% of cosmic metals could reside in this phase.Comment: Accepted for publication in Astronomy & Astrophysics. 9 pages, 4
figures, 1 tabl
Mapping metals at high redshift with far-infrared lines
Cosmic metal enrichment is one of the key physical processes regulating
galaxy formation and the evolution of the intergalactic medium (IGM). However,
determining the metal content of the most distant galaxies has proven so far
almost impossible; also, absorption line experiments at become
increasingly difficult because of instrumental limitations and the paucity of
background quasars. With the advent of ALMA, far-infrared emission lines
provide a novel tool to study early metal enrichment. Among these, the [CII]
line at 157.74 m is the most luminous line emitted by the interstellar
medium of galaxies. It can also resonant scatter CMB photons inducing
characteristic intensity fluctuations () near the peak of the
CMB spectrum, thus allowing to probe the low-density IGM. We compute both [CII]
galaxy emission and metal-induced CMB fluctuations at by using
Adaptive Mesh Refinement cosmological hydrodynamical simulations and produce
mock observations to be directly compared with ALMA BAND6 data ( GHz). The [CII] line flux is correlated with as
. Such
relation is in very good agreement with recent ALMA observations (e.g. Maiolino
et al. 2015; Capak et al. 2015) of galaxies. We predict that a
() galaxy can be detected at in
(2000) hours, respectively. CMB resonant scattering can produce Jy/beam emission/absorptions features that are very challenging to be
detected with current facilities. The best strategy to detect these signals
consists in the stacking of deep ALMA observations pointing fields with known
galaxies. This would allow to simultaneously detect both
[CII] emission from galactic reionization sources and CMB fluctuations produced
by metals.Comment: 13 pages, 6 figure
The Brightest Ly Emitter: Pop III or Black Hole?
CR7 is the brightest emitter (LAE) known to date,
and spectroscopic follow-up by Sobral et al. (2015) suggests that CR7 might
host Population (Pop) III stars. We examine this interpretation using
cosmological hydrodynamical simulations. Several simulated galaxies show the
same "Pop III wave" pattern observed in CR7. However, to reproduce the extreme
CR7 /HeII1640 line luminosities () a
top-heavy IMF and a massive () PopIII burst with age
Myr are required. Assuming that the observed properties of and HeII emission are typical for Pop III, we predict that in the
COSMOS/UDS/SA22 fields, 14 out of the 30 LAEs at with should also host Pop III stars producing an
observable . As an alternate
explanation, we explore the possibility that CR7 is instead powered by
accretion onto a Direct Collapse Black Hole (DCBH). Our model predicts
, , and X-ray luminosities that are in agreement
with the observations. In any case, the observed properties of CR7 indicate
that this galaxy is most likely powered by sources formed from pristine gas. We
propose that further X-ray observations can distinguish between the two above
scenarios.Comment: 6 pages, 4 figure
ALMA suggests outflows in z ~ 5.5 galaxies
We present the first attempt to detect outflows from galaxies approaching the
Epoch of Reionization (EoR) using a sample of 9 star-forming () galaxies for which the [CII]158m line
has been previously obtained with ALMA. We first fit each line with a Gaussian
function and compute the residuals by subtracting the best fitting model from
the data. We combine the residuals of all sample galaxies and find that the
total signal is characterised by a flux excess of mJy extended over
km~s. Although we cannot exclude that part of this signal is
due to emission from faint satellite galaxies, we show that the most probable
explanation for the detected flux excess is the presence of broad wings in the
[CII] lines, signatures of starburst-driven outflows. We infer an average
outflow rate of , providing a loading
factor in agreement with observed local
starbursts. Our interpretation is consistent with outcomes from zoomed
hydro-simulations of {\it Dahlia}, a galaxy () whose feedback-regulated star formation results into an
outflow rate . The quality of the ALMA
data is not sufficient for a detailed analysis of the [CII] line profile in
individual galaxies. Nevertheless, our results suggest that starburst-driven
outflows are in place in the EoR and provide useful indications for future ALMA
campaigns. Deeper observations of the [CII] line in this sample are required to
better characterise feedback at high- and to understand the role of outflows
in shaping early galaxy formation
The impact of chemistry on the structure of high-z galaxies
To improve our understanding of high-z galaxies we study the impact of
H chemistry on their evolution, morphology and observed properties. We
compare two zoom-in high-resolution (30 pc) simulations of prototypical
galaxies at . The first, "Dahlia",
adopts an equilibrium model for H formation, while the second,
"Alth{\ae}a", features an improved non-equilibrium chemistry network. The star
formation rate (SFR) of the two galaxies is similar (within 50\%), and
increases with time reaching values close to 100 at
. They both have SFR-stellar mass relation consistent with observations,
and a specific SFR of . The main differences arise
in the gas properties. The non-equilibrium chemistry determines the
H H~transition to occur at densities ,
i.e. about 10 times larger than predicted by the equilibrium model used for
Dahlia. As a result, Alth{\ae}a features a more clumpy and fragmented
morphology, in turn making SN feedback more effective. Also, because of the
lower density and weaker feedback, Dahlia sits away from the
Schmidt-Kennicutt relation; Alth{\ae}a, instead nicely agrees with
observations. The different gas properties result in widely different
observables. Alth{\ae}a outshines Dahlia by a factor of 7 (15) in
[CII]~ (H~) line emission. Yet,
Alth{\ae}a is under-luminous with respect to the locally observed [CII]-SFR
relation. Whether this relation does not apply at high-z or the line luminosity
is reduced by CMB and metallicity effects remains as an open question
The dense molecular gas in the QSO SDSS J231038.88+185519.7 resolved by ALMA
We present ALMA observations of the CO(6-5) and [CII] emission lines and the
sub-millimeter continuum of the quasi-stellar object (QSO) SDSS
J231038.88+185519.7. Compared to previous studies, we have analyzed a synthetic
beam that is ten times smaller in angular size, we have achieved ten times
better sensitivity in the CO(6-5) line, and two and half times better
sensitivity in the [CII] line, enabling us to resolve the molecular gas
emission. We obtain a size of the dense molecular gas of kpc, and
of kpc for the 91.5 GHz dust continuum. By assuming that CO(6-5) is
thermalized, and by adopting a CO--to-- conversion factor , we infer a molecular gas mass of
. Assuming that the
observed CO velocity gradient is due to an inclined rotating disk, we derive a
dynamical mass of , which is a factor of approximately two smaller than the previously
reported estimate based on [CII]. Regarding the central black hole, we provide
a new estimate of the black hole mass based on the C~IV emission line detected
in the X-SHOOTER/VLT spectrum: . We find a molecular gas fraction of ,
where . We derive a ratio
suggesting high gas turbulence, outflows/inflows
and/or complex kinematics due to a merger event. We estimate a global Toomre
parameter , indicating likely cloud fragmentation. We compare,
at the same angular resolution, the CO(6-5) and [CII] distributions, finding
that dense molecular gas is more centrally concentrated with respect to [CII].
We find that the current BH growth rate is similar to that of its host galaxy.Comment: A&A in pres
The interstellar medium of high-redshift galaxies: Gathering clues from C III] and [C II] lines
Context. A tight relation between [C II] 158 μm line luminosity and the star formation rate (SFR) has been observed for local galaxies. At high redshift (z > 5), galaxies instead deviate downwards from the local Σ[C II] − ΣSFR relation. This deviation might be caused by different interstellar medium (ISM) properties in galaxies at early epochs.
Aims. To test this hypothesis, we combined the [C II] and SFR data with C III] 1909 Å line observations and our physical models. We additionally investigated how ISM properties, such as burstiness, κs, total gas density, n, and metallicity, Z, affect the deviation from the Σ[C II] − ΣSFR relation in these sources.
Methods. We present the VLT/X-shooter observations targeting the C III] λ1909 line emission in three galaxies at 5.5 < z < 7.0. We include archival X-shooter data of two other sources at 5.5 < z < 7.0 and the VLT/MUSE archival data of six galaxies at z ∼ 2. We extend our sample of galaxies with eleven star-forming systems at 6 < z < 7.5, with either C III] or [C II] detection reported in the literature.
Results. We detected C III] λλ1907, 1909 line emission in HZ10 and we derived the intrinsic, integrated flux of the C III] λ1909 line. We constrained the ISM properties for our sample of galaxies, κs, n, and Z, by applying our physically motivated model based on the MCMC algorithm. For the most part, high-z star-forming galaxies show subsolar metallicities. The majority of the sources have log(κs) ≳ 1, that is, they overshoot the Kennicutt–Schmidt (KS) relation by about one order of magnitude.
Conclusions. Our findings suggest that the whole KS relation might be shifted upwards at early times. Furthermore, all the high-z galaxies of our sample lie below the Σ[C II] − ΣSFR local relation. The total gas density, n, shows the strongest correlation with the deviation from the local Σ[C II] − ΣSFR relation, namely, low-density high-z systems have lower [C II] surface brightness, in agreement with theoretical models
The brightest Ly α emitter: Pop III or black hole?
CR7 is the brightest z=6.6 Ly α emitter (LAE) known to date, and spectroscopic follow-up by Sobral etal. suggests that CR7 might host Population (Pop) III stars. We examine this interpretation using cosmological hydrodynamical simulations. Several simulated galaxies show the same ‘Pop III wave' pattern observed in CR7. However, to reproduce the extreme CR7 Ly α/He ii1640 line luminosities () a top-heavy initial mass function and a massive ( ≳ 107 M⊙) Pop III burst with age ≲ 2Myr are required. Assuming that the observed properties of Ly α and He ii emission are typical for Pop III, we predict that in the COSMOS/UDS/SA22 fields, 14 out of the 30 LAEs at z=6.6 with Lα>1043.3 erg s−1 should also host Pop III stars producing an observable . As an alternate explanation, we explore the possibility that CR7 is instead powered by accretion on to a direct collapse black hole. Our model predicts Lα, , and X-ray luminosities that are in agreement with the observations. In any case, the observed properties of CR7 indicate that this galaxy is most likely powered by sources formed from pristine gas. We propose that further X-ray observations can distinguish between the two above scenario
Witnessing Galaxy Assembly at the Edge of the Reionization Epoch
We report the discovery of Serenity-18, a galaxy at z=5.939 for which we
could measure the content of molecular gas, M(H_2)~ 5 x10^9 M_sun, traced by
the CO(6-5) emission, together with the metal-poor ([Fe/H]=-3.08 +- 0.12,
[Si/H]=-2.86 +- 0.14) gas clump/filament which is possibly feeding its growth.
The galaxy has an estimated star formation rate of ~100 M_sun yr^{-1}, implying
that it is a typical main sequence galaxy at these redshifts. The metal-poor
gas is detected through a damped Lyman-alpha absorber (DLA) observed at a
spatial separation of 40 kpc and at the same redshift of Serenity-18, along the
line of sight to the quasar SDSS J2310+1855 (z_em = 6.0025). The chemical
abundances measured for the damped Lyman-alpha system are in very good
agreement with those measured for other DLAs discovered at similar redshifts,
indicating an enrichment due to massive PopII stars. The galaxy/Damped system
we discovered is a direct observational evidence of the assembly of a galaxy at
the edge of the reionization epoch.STFC
ER
The black hole and host galaxy growth in an isolated QSO observed with ALMA
The outstanding mass growth of supermassive black holes (SMBHs) at the
Reionisation Epoch and how it is related to the concurrent growth of their host
galaxies, poses challenges to theoretical models aimed at explaining how these
systems formed in short timescales (<1 Gyr). To trace the average evolutionary
paths of quasi-stellar objects (QSOs) and their host galaxies in the BH
mass-host mass () plane, we compare the star formation rate (SFR),
derived from the accurate estimate of the dust temperature and the dust mass
(), with the BH accretion rate. To this aim, we
analysed a deep, pc resolution ALMA observation of the sub-mm continuum,
[CII] and HO of the QSO J2310+1855, enabling a detailed study of
dust properties and cold gas kinematics. We performed an accurate SED analysis
obtaining a dust temperature of K and a dust mass of
. The implied AGN-corrected SFR
is , a factor of 2 smaller than previously
reported for this QSO. We derived the best estimate of the dynamical mass
within kpc, based on
a dynamical model of the system. We found that , suggesting that AGN feedback might be efficiently
acting to slow down the SMBH accretion, while the stellar mass assembly is
still vigorously taking place in the host galaxy. In addition, we were also
able to detect high-velocity emission on the red and blue sides of the [CII]
emission line, that traces a gaseous outflow, and for the first time, we mapped
a spatially-resolved water vapour disk through the HO v=0
emission line detected at GHz,
whose kinematic properties and size are broadly consistent with those of the
[CII] disk.Comment: 15 pages, 10 figures, 4 tables. Accepted in A&
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