About 12 billion years ago, the Universe was first experiencing light again after the dark ages, and galaxies filled the environment with stars, metals and dust. How efficient was this process? How ...fast did these primordial galaxies form stars and dust? We can answer these questions by tracing the Star Formation Rate Density (SFRD) back to its widely unknown high redshift tail, traditionally observed in the Near-InfraRed (NIR), Optical and UV bands. Thus, the objects with a high amount of dust were missing. We aim to fill this knowledge gap by studying Radio Selected NIR-dark (\textit{RS-NIRdark}) sources, i.e. sources not having a counterpart at UV-to-NIR wavelengths. We widen the sample by Talia et al. (2021) from 197 to 272 objects in the COSMic evolution Survey (COSMOS) field, including also photometrically contaminated sources, previously excluded. Another important step forward consists in the visual inspection of each source in the bands from u* to MIPS-24\(\mu\)m. According to their "environment" in the different bands, we are able to highlight different cases of study and calibrate an appropriate photometric procedure for the objects affected by confusion issues. We estimate that the contribution of RS-NIRdark to the Cosmic SFRD at 3\(<\)z\(<\)5 is \(\sim\)10--25\(\%\) of that based on UV-selected galaxies.
How and when did galaxies form and assemble their stars and stellar mass? The answer to these questions, so crucial to astrophysics and cosmology, requires the full reconstruction of the so called ...cosmic star formation rate density (SFRD), i.e. the evolution of the average star formation rate per unit volume of the universe. While the SFRD has been reliably traced back to 10-11 billion years ago, its evolution is still poorly constrained at earlier cosmic epochs, and its estimate is mainly based on galaxies luminous in the ultraviolet and with low obscuration by dust. This limited knowledge is largely due to the lack of an unbiased census of all types of star-forming galaxies in the early universe. We present a new approach to find dust-obscured star-forming galaxies based on their emission at radio wavelengths coupled with the lack of optical counterparts. Here, we present a sample of 197 galaxies selected with this method. These systems were missed by previous surveys at optical and near-infrared wavelengths, and 22 of them are at very high redshift (i.e. z > 4.5). The contribution of these elusive systems to the SFRD is substantial and can be as high as 40% of the previously known SFRD based on UV-luminous galaxies. The mere existence of such heavily obscured galaxies in the first two billion years after the Big Bang opens new avenues to investigate the early phases of galaxy formation and evolution, and to understand the links between these systems and the massive galaxies which ceased their star formation at later cosmic times.
A&A 663, A159 (2022) Explaining the existence of $\gtrsim10^8\,\mathrm{M_\odot}$ SMBHs at $z>6$ is
a persistent challenge to modern astrophysics. Multi-wavelength observations of
$z\gtrsim6$ QSOs ...reveal that, on average, their accretion physics is similar to
that of their counterparts at lower redshift. However, QSOs showing properties
that deviate from the general behavior can provide useful insights into the
physical processes responsible for the rapid growth of SMBHs in the early
universe. We present X-ray (XMM-Newton, 100 ks) follow-up observations of a
$z\approx6$ QSO, J1641+3755, which was found to be remarkably X-ray bright in a
2018 Chandra dataset. J1641+3755 is not detected in the 2021 XMM-Newton
observation, implying that its X-ray flux decreased by a factor $\gtrsim7$ on a
notably short timescale (i.e., $\approx115$ rest-frame days), making it the
$z>4$ QSO with the largest variability amplitude. We also obtained rest-frame
UV spectroscopic and photometric data with textit{LBT}, and compared them with
archival datasets. Surprisingly, we found that J1641+3755 became brighter in
the rest-frame UV band from 2003 to 2016, while no strong variation occurred
from 2016 to 2021. Multiple narrow absorption features are detected in its
rest-frame UV spectrum, and several of them can be associated with an
intervening system at $z=5.67$. The variability properties of J1641+3755 can be
due to intrinsic variations of the accretion rate, a small-scale obscuration
event, gravitational lensing due to an intervening object, or an unrelated
X-ray transient in a foreground galaxy in 2018. Accounting for all of the $z>6$
QSOs with multiple X-ray observations separated by $>10$ rest-frame days, we
found an enhancement of strongly (i.e., by a factor $>3$) X-ray variable
objects compared to QSOs at later cosmic times. This finding may be related to
the physics of fast accretion in high-redshift QSOs.
We present the radio properties of 66 spectroscopically-confirmed normal
star-forming galaxies (SFGs) at $4.4<z<5.9$ in the COSMOS field that were C
II detected in the Atacama Large Millimeter Array ...(ALMA) Large Program to
INvestigate C II at Early times (ALPINE). We separate these galaxies
("CII-detected-all") into lower redshift ("CII-detected-lz", $\langle
z\rangle=4.5$) and higher redshift ("CII-detected-hz", $\langle z\rangle=5.6$)
sub-samples and stack multi-wavelength imaging for each sub-sample from X-ray
to radio bands. A radio signal is detected in the stacked 3 GHz image of
CII-detected-all and -lz samples at $\gtrsim3\sigma$. We find that the
infrared-radio correlation of our sample, quantified by $q_{\mathrm{TIR}}$, is
lower than the local relation for normal SFGs at $\sim$3$\sigma$ significance
level, and is instead broadly consistent with that of bright sub-mm galaxies at
$2<z<5$. Neither of these samples show evidence of dominant AGN activity in
their stacked Spectral Energy Distributions (SEDs), rest-frame UV spectra, or
X-ray images. Although we cannot rule out the possible effect of the assumed
spectral index and the applied infrared SED templates as at least partially
causing these differences, the lower obscured fraction of star formation than
at lower redshift can alleviate the tension between our stacked
$q_{\mathrm{TIR}}$s and that of local normal SFGs. It is possible that the dust
buildup, which primarily governs the IR emission in addition to older stellar
populations, has not had enough time to occur fully in these galaxies, whereas
the radio emission can respond on a more rapid timescale. Therefore, we might
expect a lower $q_{\mathrm{TIR}}$ to be a general property of high-redshift
SFGs.
The discovery and spectroscopic confirmation of Hyperion, a proto-supercluster at z~2.47, provides an unprecedented opportunity to study distant galaxies in the context of their large-scale ...environment. We carry out deep narrow-band imaging of a ~1*1 deg^2 region around Hyperion and select 157 Lya emitters (LAEs). The inferred LAE overdensity is delta_g~40 within an effective volume of 30*20*15 cMpc^3, consistent with the fact that Hyperion is composed of multiple protoclusters and will evolve into a super-cluster with a total mass of M_tot ~1.4*10^15 M_sun at z=0. The distribution of LAEs closely mirrors that of known spectroscopic members, tracing the protocluster cores and extended filamentary arms connected to them, suggesting that they trace the same large-scale structure. By cross-correlating the LAE positions with HI tomography data, we find weak evidence that LAEs may be less abundant in the highest HI regions, perhaps because Lya is suppressed in such regions. The Hyperion region hosts a large population of active galactic nuclei (AGN), ~12 times more abundant than that in the field. The prevalence of AGN in protocluster regions hints at the possibility that they may be triggered by physical processes that occur more frequently in dense environments, such as galaxy mergers. Our study demonstrates LAEs as reliable markers of the largest cosmic structures. When combined with ongoing and upcoming imaging and spectroscopic surveys, wide-field narrow-band imaging has the potential to advance our knowledge in the formation and evolution of cosmic structures and of their galaxy inhabitants.
A&A 630, A118 (2019) X-ray emission from QSOs has been used to assess SMBH accretion properties up
to $z$~6. However, at $z>6$ only ~15 QSOs are covered by sensitive X-ray
observations, preventing a ...statistically significant investigation of the X-ray
properties of QSOs in the first Gyr of the Universe. We present new Chandra
observations of 10 $z>6$ QSOs, selected to have virial black-hole mass
estimates from Mg II line spectroscopy. Adding archival X-ray data for an
additional 15 $z>6$ QSOs, we investigate the X-ray properties of the QSO
population in the first Gyr of the Universe, focusing in particular on the
$L_{UV}-L_{X}$ relation, which is traced by the $\alpha_{ox}$ parameter, and
the shape of their X-ray spectra. We performed photometric analyses to derive
estimates of the X-ray luminosities, and thus the $\alpha_{ox}$ values and
bolometric corrections ($K_{bol}=L_{bol}/L_{X}$). We compared the resulting
$\alpha_{ox}$ and $K_{bol}$ distributions with the results found for QSO
samples at lower redshift. Finally, we performed a basic X-ray spectral
analysis of the brightest $z>6$ QSOs to derive their individual photon indices,
and joint spectral analysis of the whole sample to estimate the average photon
index. We confirm a lack of significant evolution of $\alpha_{ox}$ with
redshift, extending the results from previous works up to $z>6$, and the trend
of an increasing bolometric correction with increasing luminosity found for
QSOs at lower redshifts. The average power-law photon index of our sample
($\Gamma=2.20_{-0.34}^{+0.39}$ and $\Gamma=2.13_{-0.13}^{+0.13}$ for sources
with $<30$ and $>30$ net counts, respectively) is slightly steeper than, but
still consistent with, typical QSOs at $z=1-6$. All these results point toward
a lack of substantial evolution of the inner accretion-disk/hot-corona
structure in QSOs from low redshift to $z>6$. Our data hint at generally high
Eddington ratios at $z>6$.
A&A 649, A133 (2021) The discovery of hundreds of QSOs in the first Gyr of the Universe powered by
already grown SMBHs challenges our knowledge of SMBH formation. In particular,
investigations of ...$z>6$ QSOs presenting notable properties can provide unique
information on the physics of fast SMBH growth in the early universe. We
present the results of follow-up observations of the $z=6.515$ radio-quiet QSO
PSO167-13, which is interacting with a close companion galaxy. The PSO167-13
system has been recently proposed to host the first heavily obscured X-ray
source at high redshift. We observed PSO167-13 with Chandra/ACIS-S (177 ks),
and obtained new spectroscopic observations (7.2 h) with Magellan/FIRE. No
significant X-ray emission is detected from the PSO167-13 system, suggesting
that the obscured X-ray source previously tentatively detected was either due
to a strong background fluctuation or is highly variable. The upper limit (90%
confidence level) on the X-ray emission of PSO167-13
($L_{2-10\,\mathrm{keV}}<8.3\times10^{43}\,\mathrm{erg s^{-1}}$) is the lowest
available for a $z>6$ QSO. The ratio between the X-ray and UV luminosity of
$\alpha_{ox}<-1.95$ makes PSO167-13 a strong outlier from the
$\alpha_{ox}-L_{UV}$ and $L_X-L_{\mathrm{bol}}$ relations. In particular, its
X-ray emission is $>6$ times weaker than the expectation based on its UV
luminosity. The new Magellan/FIRE spectrum of PSO167-13 is strongly affected by
the unfavorable sky conditions, but the tentatively detected C IV and Mg II
emission lines appear strongly blueshifted. The most plausible explanations for
the X-ray weakness of PSO167-13 are intrinsic weakness or small-scale
absorption by Compton-thick material. The possible strong blueshift of its
emission lines hints at the presence of nuclear winds, which could be related
to its X-ray weakness.
A&A 628, L6 (2019) While theoretical arguments predict that most of the early growth of
supermassive black holes (SMBHs) happened during heavily obscured phases of
accretion, current methods used for ...selecting $z>6$ quasars (QSOs) are strongly
biased against obscured QSOs, thus considerably limiting our understanding of
accreting SMBHs during the first Gyr of the Universe from an observational
point of view. We report the $Chandra$ discovery of the first heavily obscured
QSO candidate in the early universe, hosted by a close ($\approx5$ kpc) galaxy
pair at $z=6.515$. One of the members is an optically classified type 1 QSO,
PSO167-13. The companion galaxy was first detected as a C II emitter by ALMA.
An X-ray source is significantly ($P=0.9996$) detected by $Chandra$ in the 2-5
keV band, with $<1.14$ net counts in the 0.5-2 keV band, although the current
positional uncertainty does not allow a conclusive association with either
PSO167-13 or its companion galaxy. From X-ray photometry and hardness-ratio
arguments, we estimated an obscuring column density of
$N_H>2\times10^{24}\,\mathrm{cm^{-2}}$ and
$N_H>6\times10^{23}\,\mathrm{cm^{-2}}$ at $68\%$ and $90\%$ confidence levels,
respectively. Thus, regardless of which of the two galaxies is associated with
the X-ray emission, this source is the first heavily obscured QSO candidate at
$z>6$.
A&A 646, A76 (2021) We present the first CII 158 $\mu$m luminosity function (LF) at $z\sim 5$
from a sample of serendipitous lines detected in the ALMA Large Program to
INvestigate CII at Early times ...(ALPINE). A search performed over the 118
ALPINE pointings revealed several serendipitous lines. Based on their fidelity,
we selected 14 lines for the final catalog. According to the redshift of their
counterparts, we identified 8 out of 14 detections as CII lines at $z\sim 5$,
and two as CO transitions at lower redshifts. The remaining 4 lines have an
elusive identification in the available catalogs and we considered them as
CII candidates. We used the 8 confirmed CII and the 4 CII candidates to
build one of the first CII LFs at $z\sim 5$. We found that 11 out of these 12
sources have a redshift very similar to that of the ALPINE target in the same
pointing, suggesting the presence of overdensities around the targets.
Therefore, we split the sample in two (a "clustered" and "field" sub-sample)
according to their redshift separation and built two separate LFs. Our
estimates suggest that there could be an evolution of the CII LF between $z
\sim 5$ and $z \sim 0$. By converting the CII luminosity to star formation
rate we evaluated the cosmic star formation rate density (SFRD) at $z\sim 5$.
The clustered sample results in a SFRD $\sim 10$ times higher than previous
measurements from UV-selected galaxies. On the other hand, from the field
sample (likely representing the average galaxy population) we derived a SFRD
$\sim 1.6$ higher compared to current estimates from UV surveys but compatible
within the errors. Because of the large uncertainties, observations of larger
samples are necessary to better constrain the SFRD at $z\sim 5$. This study
represents one of the first efforts aimed at characterizing the demography of
CII emitters at $z\sim 5$ using a mm-selection of galaxies.
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