139 research outputs found
The circumgalactic medium of high redshift galaxies
We study the properties of the circumgalactic medium (CGM) of high-
galaxies in the metal enrichment simulations presented in Pallottini et al.
2014. At , we find that the simulated CGM gas density profiles are
self-similar, once scaled with the virial radius of the parent dark matter
halo. We also find a simple analytical expression relating the neutral hydrogen
equivalent width () of CGM absorbers as a function of the
line of sight impact parameter (). We test our predictions against mock
spectra extracted from the simulations, and show that the model reproduces the
profile extracted from the synthetic spectra analysis.
When compared with available data, our CGM model nicely predicts the observed
in galaxies, and supports the idea that the
CGM profile does not evolve with redshift.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
The nature of the Lyman Alpha Emitter CR7: a persisting puzzle
The peculiar emission properties of the Ly emitter CR7
have been initially interpreted with the presence of either a direct collapse
black hole (DCBH) or a substantial mass of Pop III stars. Instead, updated
photometric observations by Bowler et al. (2016) seem to suggest that CR7 is a
more standard system. Here we confirm that the original DCBH hypothesis is
consistent also with the new data. Using radiation-hydrodynamic simulations, we
reproduce the new IR photometry with two models involving a Compton-thick DCBH
of mass accreting (a) metal-free
() gas with column density ,
or (b) low-metallicity gas () with
. The best fit model reproduces the
photometric data to within . Such metals can be produced by weak
star-forming activity occurring after the formation of the DCBH. The main
contribution to the Spitzer/IRAC photometric band in
both models is due to HeI/HeII emission lines, while the
contribution of [OIII] emission lines, if present, is
sub-dominant. Spectroscopic observations with JWST will be required to
ultimately clarify the nature of CR7.Comment: Accepted for publication in MNRAS Letter
Ly{\alpha} emission from galaxies in the Epoch of Reionization
The intrinsic strength of the Ly line in young, star-forming systems
makes it a special tool for studying high-redshift galaxies. However,
interpreting observations remains challenging due to the complex radiative
transfer involved. Here, we combine state-of-the-art hydrodynamical simulations
of 'Althaea', a prototypical Lyman Break Galaxy (LBG, stellar mass
at , with detailed
radiative transfer computations of dust/continuum, [CII] 158 m, and
Ly to clarify the relation between the galaxy properties and its
Ly emission. Althaea exhibits low () Ly escape
fractions and Equivalent Widths, EW Angstrom for the simulated
lines of sight, with a large scatter. The correlation between escape fraction
and inclination is weak, as a result of the rather chaotic structure of
high-redshift galaxies. Low values persist even if we artificially
remove neutral gas around star forming regions to mimick the presence of HII
regions. The high attenuation is primarily caused by dust clumps co-located
with young stellar clusters. We can turn Althaea into a Lyman Alpha Emitter
(LAE) only if we artificially remove dust from the clumps, yielding EWs up to
Angstrom. Our study suggests that the LBG-LAE duty-cycle required by
recent clustering measurements poses the challenging problem of a dynamically
changing dust attenuation. Finally, we find an anti-correlation between the
magnitude of Ly-[CII] line velocity shift and Ly luminosity.Comment: published in MNRA
Molecular clumps photoevaporation in ionized regions
We study the photoevaporation of molecular clumps exposed to a UV radiation
field including hydrogen-ionizing photons ( eV) produced by
massive stars or quasars. We follow the propagation and collision of shock
waves inside clumps and take into account self-shielding effects, determining
the evolution of clump size and density with time. The structure of the
ionization-photodissociation region (iPDR) is obtained for different initial
clump masses () and impinging fluxes ( in units of the Habing flux). The cases of molecular clumps engulfed in
the HII region of an OB star and clumps carried within quasar outflows are
treated separately. We find that the clump undergoes in both cases an initial
shock-contraction phase and a following expansion phase, which lets the
radiation penetrate in until the clump is completely evaporated. Typical
evaporation time-scales are Myr in the stellar case and 0.1 Myr
in the quasar case, where the clump mass is 0.1 and respectively. We find that clump lifetimes in quasar outflows are
compatible with their observed extension, suggesting that photoevaporation is
the main mechanism regulating the size of molecular outflows.Comment: 13 pages, 12 figures, accepted for publication in MNRA
Intensity mapping of [CII] from early galaxies
The intensity mapping of the [CII] 157.7 m fine-structure emission
line represents an ideal experiment to probe star formation activity in
galaxies, especially in those that are too faint to be individually detected.
Here, we investigate the feasibility of such an experiment for
galaxies. We construct the relation from observations
and simulations, then generate mock [CII] intensity maps by applying this
relation to halo catalogs built from large scale N-body simulations. Maps of
the extragalactic far-infrared (FIR) continuum, referred to as "foreground",
and CO rotational transition lines and [CI] fine-structure lines referred to as
"contamination", are produced as well. We find that, at 316 GHz (corresponding
to ), the mean intensities of the extragalactic FIR continuum,
[CII] signal, all CO lines from to 13 and two [CI] lines are Jy sr, Jy sr, Jy sr
and Jy sr, respectively. We discuss a method that allows us
to subtract the FIR continuum foreground by removing a spectrally smooth
component from each line of sight, and to suppress the CO/[CI] contamination by
discarding pixels that are bright in contamination emission. The [CII]
signal comes mainly from halos in the mass range ; as
this mass range is narrow, intensity mapping is an ideal experiment to
investigate these early galaxies. In principle such signal is accessible to a
ground-based telescope with a 6 m aperture, 150 K system temperature, a
pixels FIR camera in 5000 hr total integration time, however it
is difficult to perform such an experiment by using currently available
telescopes.Comment: 12 pages, 10 figures. Accepted for publication in MNRA
First CO(17-16) emission line detected in a z > 6 quasar
We report the serendipitous detection of the CO(17-16) emission line toward
the quasar SDSSJ114816.64+525150.3 (J1148) at redshift z = 6.4 obtained with
the Plateau de Bure Interferometer. The CO(17-16) line is possibly contaminated
by OH+ emission, that may account for ~ 35 - 60% of the total flux observed.
Photo-Dissociation and X-ray Dominated Regions (PDRs and XDRs) models show that
PDRs alone cannot reproduce the high luminosity of the CO(17-16) line relative
to low-J CO transitions and that XDRs are required. By adopting a composite
PDR+XDR model we derive molecular cloud and radiation field properties in the
nuclear region of J1148. Our results show that highly excited CO lines
represent a sensitive and possibly unique tool to infer the presence of X-ray
faint or obscured supermassive black hole progenitors in high-z galaxies.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Lette
Radio recombination lines from obscured quasars with the SKA
We explore the possibility of detecting hydrogen radio recombination lines
from 0 < z < 10 quasars. We compute the expected Hnalpha flux densities as a
function of absolute magnitude and redshift by considering (i) the range of
observed AGN spectral indices from UV to X-ray bands, (ii) secondary
ionizations from X-ray photons, and (iii) stimulated emission due to nonthermal
radiation. All these effects are important to determine the line fluxes. We
find that the combination of slopes: alpha_X,hard = -1.11, alpha_X,soft = -0.7,
alpha_EUV = -1.3, alpha_UV = -1.7, maximizes the expected flux, f_Hnalpha = 10
microJy for z = 7 quasars with M_AB = -27 in the n = 50 lines; allowed SED
variations produce variations by a factor of 3 around this value. Secondaries
boost the line intensity by a factor of 2 to 4, while stimulated emission in
high-z quasars with M_AB = -26 provides an extra boost to RRL flux observed at
nu = 1 GHz if recombinations arise in HII regions with T_e = 10^3-5 K, n_e =
10^3-5 cm^-3. We compute the sensitivity required for a 5sigma detection of
Hnalpha lines using the SKA, finding that the SKA-MID could detect sources with
M_AB < -27 (M_AB < -26) at z < 8 (z < 3) in less than 100 hrs of observing
time. These observations could open new paths to searches for obscured SMBH
progenitors, complementing X-ray, optical/IR and sub-mm surveys.Comment: 11 pages, 9 figures; to be published in Monthly Notices of the Royal
Astronomical Society Main Journa
Cosmic radiation backgrounds from primordial black holes
Recent measurements of the cosmic X-ray and radio backgrounds (CXB/CRB,
respectively) obtained with Chandra and ARCADE2 report signals in excess of
those expected from known sources, suggesting the presence of a yet
undiscovered population of emitters. We investigate the hypothesis that such
excesses are due to primordial black holes (PBHs) which may constitute a
substantial fraction of dark matter (DM). We present a novel semi-analytical
model which predicts X-ray and radio emission due to gas accretion onto PBHs,
assuming that they are distributed both inside DM halos and in the
intergalactic medium (IGM). Our model includes a self-consistent treatment of
heating/ionization feedback on the surrounding environment. We find that (i)
the emission from PBHs accreting in the IGM is subdominant at all times (); (ii) most of the CXB/CRB emission
comes from PBHs in DM mini-halos () at early epochs
(). While a small fraction () of DM in the form
of PBHs can account for the total observed CXB excess, the CRB one cannot be
explained by PBHs. Our results set the strongest existing constraint on in the mass range . Finally, we comment on the implications of our results on the global
21cm signal.Comment: 13 pages, 10 figures, 2 tables. Accepted for publication in MNRA
X-ray spectroscopy of the z=6.4 quasar J1148+5251
We present the 78-ks Chandra observations of the quasar SDSS
J1148+5251. The source is clearly detected in the energy range 0.3-7 keV with
42 counts (with a significance ). The X-ray spectrum is
best-fitted by a power-law with photon index absorbed by a gas
column density of .
We measure an intrinsic luminosity at 2-10 keV and 10-40 keV equal to , comparable with luminous local and
intermediate-redshift quasar properties. Moreover, the X-ray to optical
power-law slope value () of J1148 is consistent
with the one found in quasars with similar rest-frame 2500 \AA ~luminosity
(\AA). Then we use Chandra data
to test a physically motivated model that computes the intrinsic X-ray flux
emitted by a quasar starting from the properties of the powering black hole and
assuming that X-ray emission is attenuated by intervening, metal-rich () molecular clouds distributed on kpc scales in the host
galaxy. Our analysis favors a black hole mass and a molecular hydrogen mass , in good agreement with estimates obtained from previous studies. We
finally discuss strengths and limits of our analysis.Comment: 9 pages, 3 figures, 1 table, MNRAS in pres
Reionization constraints using Principal Component Analysis
Using a semi-analytical model developed by Choudhury & Ferrara (2005) we
study the observational constraints on reionization via a principal component
analysis (PCA). Assuming that reionization at z>6 is primarily driven by
stellar sources, we decompose the unknown function N_{ion}(z), representing the
number of photons in the IGM per baryon in collapsed objects, into its
principal components and constrain the latter using the photoionization rate
obtained from Ly-alpha forest Gunn-Peterson optical depth, the WMAP7 electron
scattering optical depth and the redshift distribution of Lyman-limit systems
at z \sim 3.5. The main findings of our analysis are: (i) It is sufficient to
model N_{ion}(z) over the redshift range 2<z<14 using 5 parameters to extract
the maximum information contained within the data. (ii) All quantities related
to reionization can be severely constrained for z<6 because of a large number
of data points whereas constraints at z>6 are relatively loose. (iii) The weak
constraints on N_{ion}(z) at z>6 do not allow to disentangle different feedback
models with present data. There is a clear indication that N_{ion}(z) must
increase at z>6, thus ruling out reionization by a single stellar population
with non-evolving IMF, and/or star-forming efficiency, and/or photon escape
fraction. The data allows for non-monotonic N_{ion}(z) which may contain sharp
features around z \sim 7. (iv) The PCA implies that reionization must be 99%
completed between 5.8<z<10.3 (95% confidence level) and is expected to be 50%
complete at z \approx 9.5-12. With future data sets, like those obtained by
Planck, the z>6 constraints will be significantly improved.Comment: Accepted in MNRAS. Revised to match the accepted versio
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