ABSTRACT
High-$z$ AGNs hosted in gas-rich galaxies are expected to grow through significantly obscured accretion phases. This may limit or bias their observability. In this work, we use bluetides, a ...large volume cosmological simulation of galaxy formation to examine quasar obscuration for the highest redshift ($z$ ≥ 7) supermassive black holes residing in the centre of galaxies. We find that for the bright quasars, most of the high-column density gas ($\rm {\gt} 90 {\rm {per\ cent}}$) resides in the innermost regions of the host galaxy (typically within <10 ckpc), while the gas in the outskirts is a minor contributor to the NH. The brightest quasars can have large angular variations in galactic obscuration, over 2 orders of magnitude (ranging from column density $N_\mathrm{H} \sim 10^{21.5 \!-\! 24}\, \rm {cm}^{-2}$), where the lines of sight with the lowest obscuration are those formed via strong gas outflows driven by AGN feedback. The obscured fraction P(NH > 1023 cm−2) typically ranges from 0.6 to 1.0 for increasing LX (with $L_\mathrm{ X} \gt 10^{43} \, \rm {erg\, s}^{-1}$), with no clear trend of redshift evolution. Due to the angular variation in NH, all relations between NH and LX, MBH, and galaxy host properties (global M*, $M_{\rm H_2}$, and star formation rate) show appreciable scatter. The dust optical depth in the UV band τUV has tight positive correlation with NH. Our dust-extincted UV luminosity function (UVLF) is about 1.5 dex lower than the intrinsic UVLF, implying that more than 99 per cent of the $z$ ∼ 7 AGNs are heavily dust extincted and therefore would be missed by the UV-band observation.
We report on the spectroscopic confirmation of a large-scale structure around the luminous
z
= 6.31 quasi-stellar object (QSO) SDSS J1030+0524, powered by a one billion solar mass black hole. The ...structure is populated by at least six members, namely, four Lyman-break galaxies (LBGs), and two Lyman alpha emitters (LAEs). The four LBGs were identified among a sample of 21
i
-band dropouts with
z
AB
< 25.5 selected up to projected separations of 5 physical Mpc (15 arcmin) from the QSO. Their redshifts were determined through multi-object spectroscopic observations at 8−10 m class telescopes lasting up to eight hours. The two LAEs were identified in a 6 h VLT/MUSE observation centered on the QSO. The redshifts of the six galaxies cover the range between 6.129−6.355. Assuming that the peculiar velocities are negligible, this range corresponds to radial separations of ±5 physical Mpc from the QSO, that is comparable to the projected scale of the observed LBG distribution on the sky. We conservatively estimate that this structure is significant at a level > 3.5
σ
and that the level of the galaxy overdensity is at least 1.5−2 within the large volume sampled (∼780 physical Mpc
3
). The spectral properties of the six member galaxies (Ly
α
strength and UV luminosity) are similar to those of field galaxies at similar redshifts. This is the first spectroscopic identification of a galaxy overdensity around a supermassive black hole in the first billion years of the Universe. Our finding lends support to the idea that the most distant and massive black holes form and grow within massive (>10
12
M
⊙
) dark matter halos in large-scale structures and that the absence of earlier detections of such systems is likely due to observational limitations.
Using Chandra observations in the 2.15 deg2 COSMOS-legacy field, we present one of the most accurate measurements of the Cosmic X-ray Background (CXB) spectrum to date in the 0.3-7 keV energy band. ...The CXB has three distinct components: contributions from two Galactic collisional thermal plasmas at kT ∼ 0.27 and 0.07 keV and an extragalactic power law with a photon spectral index Γ = 1.45 0.02. The 1 keV normalization of the extragalactic component is 10.91 0.16 keV cm−2 s−1 sr−1 keV−1. Removing all X-ray-detected sources, the remaining unresolved CXB is best fit by a power law with normalization 4.18 0.26 keV cm−2 s−1 sr−1 keV−1 and photon spectral index Γ = 1.57 0.10. Removing faint galaxies down to leaves a hard spectrum with and a 1 keV normalization of ∼1.37 keV cm−2 s−1 sr−1 keV−1. This means that ∼91% of the observed CXB is resolved into detected X-ray sources and undetected galaxies. Unresolved sources that contribute ∼8%-9% of the total CXB show marginal evidence of being harder and possibly more obscured than resolved sources. Another ∼1% of the CXB can be attributed to still undetected star-forming galaxies and absorbed active galactic nuclei. According to these limits, we investigate a scenario where early black holes totally account for non-source CXB fraction and constrain some of their properties. In order to not exceed the remaining CXB and the accreted mass density, such a population of black holes must grow in Compton-thick envelopes with 1.6 × 1025 cm−2 and form in extremely low-metallicity environments .
A new, faint population of X-ray transients Bauer, Franz E.; Treister, Ezequiel; Schawinski, Kevin ...
Monthly notices of the Royal Astronomical Society,
06/2017, Letnik:
467, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Abstract
We report on the detection of a remarkable new fast high-energy transient found in the Chandra Deep Field-South, robustly associated with a faint (mR = 27.5 mag, zph ∼ 2.2) host in the ...CANDELS survey. The X-ray event is comprised of 115$^{+12}_{-11}$ net 0.3–7.0 keV counts, with a light curve characterized by an ≈100 s rise time, a peak 0.3–10 keV flux of ≈5 × 10−12 erg s−1 cm−2 and a power-law decay time slope of −1.53 ± 0.27. The average spectral slope is $\Gamma = 1.43^{+0.23}_{-0.13}$, with no clear spectral variations. The X-ray and multiwavelength properties effectively rule out the vast majority of previously observed high-energy transients. A few theoretical possibilities remain: an ‘orphan’ X-ray afterglow from an off-axis short-duration gamma-ray burst (GRB) with weak optical emission, a low-luminosity GRB at high redshift with no prompt emission below ∼20 keV rest frame, or a highly beamed tidal disruption event (TDE) involving an intermediate-mass black hole and a white dwarf with little variability. However, none of the above scenarios can completely explain all observed properties. Although large uncertainties exist, the implied rate of such events is comparable to those of orphan and low-luminosity GRBs as well as rare TDEs, implying the discovery of an untapped regime for a known transient class, or a new type of variable phenomena whose nature remains to be determined.
Abstract
By characterizing the contribution of stray light to large data sets from the CXB Measurement X-ray observatory collected over 2012–2017, we report a measurement of the cosmic X-ray ...background (CXB) in the 3–20 keV energy range. These data represent ∼20% sky coverage while avoiding Galactic ridge X-ray emission and are less weighted by deep survey fields than previous measurements with CXB Measurement. Images in narrow energy bands are stacked in detector space and spatially fit with a model representing the stray light and uniform pattern expected from the CXB and the instrumental background, respectively. We establish baseline flux values from Earth-occulted data and validate the fitting method on stray-light observations of the Crab, which further serve to calibrate the resulting spectra. We present independent spectra of the CXB with the focal plane module FPMA and FPMB detector arrays, which are in excellent agreement with the canonical characterization by HEAO 1 and are 10% lower than most subsequent measurements:
F
3
−
20
keV
FPMA
=
2.63
×
10
−
11
erg
s
−
1
cm
−
2
deg
−
2
and
F
3
–
20
keV
FPMB
=
2.58
×
10
−
11
erg
s
−
1
cm
−
2
deg
−
2
. We discuss these results in light of previous measurements of the CXB and consider the impact of systematic uncertainties on our spectra.
We present the first detection of an X-ray flare from an ultracool dwarf of spectral class L. The event was identified in the EXTraS database of
XMM-Newton
variable sources, and its optical ...counterpart, J0331−27, was found through a cross-match with the Dark Energy Survey Year 3 release. Next to an earlier four-photon detection of Kelu-1, J0331−27 is only the second L dwarf detected in X-rays, and much more distant than other ultracool dwarfs with X-ray detections (photometric distance of 240 pc). From an optical spectrum with the VIMOS instrument at the VLT, we determine the spectral type of J0331−27 to be L1. The X-ray flare has an energy of
E
X, F
∼ 2 × 10
33
erg, placing it in the regime of superflares. No quiescent emission is detected, and from 2.5 Ms of
XMM-Newton
data we derive an upper limit of
L
X, qui
< 10
27
erg s
−1
. The flare peak luminosity (
L
X, peak
= 6.3 × 10
29
erg s
−1
), flare duration (
τ
decay
≈ 2400 s), and plasma temperature (≈16 MK) are similar to values observed in X-ray flares of M dwarfs. This shows that strong magnetic reconnection events and the ensuing plasma heating are still present even in objects with photospheres as cool as ∼2100 K. However, the absence of any other flares above the detection threshold of
E
X, F
∼ 2.5 × 10
32
erg in a total of ∼2.5 Ms of X-ray data yields a flare energy number distribution inconsistent with the canonical power law d
N
/d
E
∼
E
−2
, suggesting that magnetic energy release in J0331−27 – and possibly in all L dwarfs – takes place predominantly in the form of giant flares.
Abstract
Spatially resolved observations of active galactic nuclei (AGN) host galaxies undergoing feedback processes are one of the most relevant avenues through which galactic evolution can be ...studied, given the long-lasting effects AGN feedback has on gas reservoirs, star formation, and AGN environments at all scales. Within this context, we report results from Very Large Telescope/MUSE integral field optical spectroscopy of TN J1049-1258, one of the most powerful radio sources known, at a redshift of 3.7. We detected extended (∼18 kpc) Ly
α
emission, spatially aligned with the radio axis, redshifted by 2250 ± 60 km s
−1
with respect to the host galaxy systemic velocity, and cospatial with UV continuum emission. This Ly
α
emission could arise from a companion galaxy, although there are arguments against this interpretation. Alternatively, it might correspond to an outflow of ionized gas stemming from the radio galaxy. The outflow would be the highest redshift spatially resolved ionized outflow to date. The enormous amount of energy injected, however, appears to be unable to quench the host galaxy’s prodigious star formation, occurring at a rate of ∼4500
M
⊙
yr
−1
, estimated using its far-infrared luminosity. Within the field, we also found two companion galaxies at projected distances of ∼25 and ∼60 kpc from the host, which suggests the host galaxy is harbored within a protocluster.
In this work we performed a spectral energy distribution (SED) analysis in the optical/infrared band of the host galaxy of a proto-brightest bluster galaxy (BCG, NVSS J103023 + 052426) in a ...proto-cluster at z = 1.7. We found that it features a vigorous star formation rate (SFR) of ∼570 M⊙/yr and a stellar mass of M*∼3.7×1011M⊙; the high corresponding specific SFR = 1.5±0.5Gyr−1 classifies this object as a starburst galaxy that will deplete its molecular gas reservoir in ∼3.5×108 yr. Thus, this system represents a rare example of a proto-BCG caught during the short phase of its major stellar mass assembly. Moreover, we investigated the nature of the host galaxy emission at 3.3 mm. We found that it originates from the cold dust in the interstellar medium, even though a minor non-thermal AGN contribution cannot be completely ruled out. Finally, we studied the polarized emission of the lobes at 1.4 GHz. We unveiled a patchy structure where the polarization fraction increases in the regions in which the total intensity shows a bending morphology; in addition, the magnetic field orientation follows the direction of the bendings. We interpret these features as possible indications of an interaction with the intracluster medium. This strengthens the hypothesis of positive AGN feedback, as inferred in previous studies of this object on the basis of X-ray/mm/radio analysis. In this scenario, the proto-BCG heats the surrounding medium and possibly enhances the SFR in nearby galaxies.
RADIO LOUD AGNs ARE MERGERS Chiaberge, Marco; Gilli, Roberto; M. Lotz, Jennifer ...
Astrophysical journal/The Astrophysical journal,
06/2015, Letnik:
806, Številka:
2
Journal Article
Recenzirano
Odprti dostop
ABSTRACT We measure the merger fraction of Type 2 radio-loud and radio-quiet active galactic nuclei (AGNs) at using new samples. The objects have Hubble Space Telescope (HST) images taken with Wide ...Field Camera 3 (WFC3) in the IR channel. These samples are compared to the 3CR sample of radio galaxies at and to a sample of non-active galaxies. We also consider lower redshift radio galaxies with HST observations and previous generation instruments (NICMOS and WFPC2). The full sample spans an unprecedented range in both redshift and AGN luminosity. We perform statistical tests to determine whether the different samples are differently associated with mergers. We find that all (92% ) radio-loud galaxies at are associated with recent or ongoing merger events. Among the radio-loud population there is no evidence for any dependence of the merger fraction on either redshift or AGN power. For the matched radio-quiet samples, only 38% are merging systems. The merger fraction for the sample of non-active galaxies at is indistinguishable from radio-quiet objects. This is strong evidence that mergers are the triggering mechanism for the radio-loud AGN phenomenon and the launching of relativistic jets from supermassive black holes (SMBHs). We speculate that major black hole (BH)-BH mergers play a major role in spinning up the central SMBHs in these objects.
Population synthesis models of actively accreting supermassive black holes (or active galactic nuclei (AGN)) predict a large fraction that must grow behind dense, obscuring screens of gas and dust. ...Deep X-ray surveys are thought to have provided the most complete and unbiased samples of AGN, but there is strong observational evidence that a portion of the population of obscured AGN is being missed. In this paper, we use a sample of AGN derived from the deepest X-ray survey to date, the Chandra 7 Ms GOODS-South Survey, to investigate the nature of low-flux X-ray sources. We make full use of the extensive multiwavelength coverage of the GOODS-South field and cross-match our objects with wavelengths from the radio to the IR. We find that the low X-ray flux AGN in our sample have X-ray luminosities that indicate low-luminosity AGN classification, while their radio, IR, and optical counterparts indicate moderate to powerful AGN classification. We find that the predicted column densities are, on average, an order of magnitude higher than the calculated column densities via X-ray detections for X-ray-faint sources. We interpret our results as evidence of obscured AGN disguised as low-luminosity AGN via their X-ray luminosities. When we compare the estimation of the obscured AGN space density with and without these objects, we find a difference of 40% in the lowest X-ray luminosity regime probed by our sample.
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