arXiv subjects

E. Treister

Publications and source records attributed to E. Treister.

At least 37 records · Page 2Linked to original sources

arXiv2020

A Hard X-ray Test of HCN Enhancements as a Tracer of Embedded Black Hole Growth

Enhanced emission from the dense gas tracer HCN (relative to HCO$^+ $) has been proposed as a signature of active galactic nuclei (AGN). In a previous single-dish millimeter line survey we identified galaxies with HCN/HCO$ ^+ $ (1-0) intensity ratios consistent with those of many AGN but whose mid-infrared spectral diagnostics are consistent with little to no ( $\lesssim15\% $) contribution of an AGN to the bolometric luminosity. To search for putative heavily obscured AGN, we present and analyze \nustar hard X-ray (3-79 keV) observations of four such galaxies from the Great Observatories All-sky LIRG Survey. We find no X-ray evidence for AGN in three of the systems and place strong upper limits on the energetic contribution of any heavily obscured ($N_{\rm H}>10^{24}$ cm$^{-2}$) AGN to their bolometric luminosity. The X-ray flux upper limits are presently an order of magnitude below what XDR-driven chemistry model predict are necessary to drive HCN enhancements. In a fourth system we find a hard X-ray excess consistent with the presence of an AGN, but contributing only $\sim3\%$ of the bolometric luminosity. It is also unclear if the AGN is spatially associated with the HCN enhancement. We further explore the relationship between HCN/HCO$^+$ (for several $\mathrm{J}_\mathrm{upper}$ levels) and $L_\mathrm{AGN}/L_\mathrm{IR}$ for a larger sample of systems in the literature. We find no evidence for correlations between the line ratios and the AGN fraction derived from X-rays, indicating that HCN/HCO$^+$ intensity ratios are not driven by the energetic dominance of AGN, nor are they reliable indicators of whether SMBH accretion is ongoing.

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arXiv2020

The Molecular Gas in the NGC 6240 Merging Galaxy System at the Highest Spatial Resolution

We present the highest resolution --- 15 pc (0.03'') --- ALMA $^{12}$CO(2-1) line emission and 1.3mm continuum maps, tracers of the molecular gas and dust, respectively, in the nearby merging galaxy system NGC 6240, that hosts two supermassive black holes growing simultaneously. These observations provide an excellent spatial match to existing Hubble optical and near-infrared observations of this system. A significant molecular gas mass, $\sim$9$\times$10$^9$M$_\odot$, is located in between the two nuclei, forming a clumpy stream kinematically dominated by turbulence, rather than a smooth rotating disk as previously assumed from lower resolution data. Evidence for rotation is seen in the gas surrounding the southern nucleus, but not in the northern one. Dynamical shells can be seen, likely associated with nuclear supernovae remnants. We further detect the presence of significant high velocity outflows, some of them reaching velocities $>$500 km/s, affecting a significant fraction, $\sim$11\% of the molecular gas in the nuclear region. Inside the spheres of influence of the northern and southern supermassive black holes we find molecular masses of 7.4$\times$10$^8$M$_\odot$ and 3.3$\times$10$^9$M$_\odot$, respectively. We are thus directly imaging the reservoir of gas that can accrete onto each supermassive black hole. These new ALMA maps highlight the critical need for high resolution observations of molecular gas in order to understand the feeding of supermassive black holes and its connection to galaxy evolution in the context of a major galaxy merger.

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arXiv2019

The ALMA Frontier Fields survey V: ALMA Stacking of Lyman-Break Galaxies in Abell 2744, Abell 370, Abell S1063, MACSJ0416.1-2403 and MACSJ1149.5+2223

The Hubble Frontier Fields offer an exceptionally deep window into the high-redshift universe, covering a substantially larger area than the Hubble Ultra-Deep field at low magnification and probing 1--2 mags deeper in exceptional high-magnification regions. We aim to leverage Atacama Large Millimetre Array (ALMA) band 6 ($\approx$263\,GHz) mosaics in the central portions of five Frontier Fields to characterize the infrared (IR) properties of $1582$ ultraviolet (UV)-selected Lyman-Break Galaxies (LBGs) at redshifts of $z {\sim}$2--8. We investigated individual and stacked fluxes and IR excess (IRX) values of the LBG sample as functions of stellar mass ($\mathrm{M}_{\bigstar}$), redshift, UV luminosity and slope $β$, and lensing magnification. Two (2) LBG candidates were individually detected above a significance of $4.1{-}σ$, while stacked samples of the remaining LBG candidates yielded no significant detections. We investigated our detections and upper limits in the context of the IRX-$\mathrm{M}_{\bigstar}$ and IRX-$β$ relations, probing at least one dex lower in stellar mass than past studies have done. Our upper limits exclude substantial portions of parameter space and they are sufficiently deep in a handful of cases to create mild tension with the typically assumed attenuation and consensus relations. We observe a clear and smooth trend between $\mathrm{M}_{\bigstar}$ and $β$, which extends to low masses and blue (low) $β$ values, consistent with expectations from previous works.

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arXiv2019

On the Gas Content, Star Formation Efficiency, and Environmental Quenching of Massive Galaxies in Proto-Clusters at z~2.0-2.5

We present ALMA Band 6 (nu=233GHz, lambda=1.3mm) continuum observations towards 68 'normal' star-forming galaxies within two Coma-like progenitor structures at z=2.10 and 2.47, from which ISM masses are derived, providing the largest census of molecular gas mass in overdense environments at these redshifts. Our sample comprises galaxies with a stellar mass range of 1x10^9M_sun - 4x10^11M_sun with a mean M_*~6x10^10M_sun. Combining these measurements with multiwavelength observations and SED modeling, we characterize the gas mass fraction and the star formation efficiency, and infer the impact of the environment on galaxies' evolution. Most of our detected galaxies (~70%) have star formation efficiencies and gas fractions similar to those found for coeval field galaxies and in agreement with the field scaling relations. However, we do find that the proto-clusters contain an increased fraction of massive, gas-poor galaxies, with low gas fractions (f_gas<6-10%) and red rest-frame ultraviolet/optical colors typical of post-starburst and passive galaxies. The relatively high abundance of passive galaxies suggests an accelerated evolution of massive galaxies in proto-cluster environments. The large fraction of quenched galaxies in these overdense structures also implies that environmental quenching takes place during the early phases of cluster assembly, even before virialization. From our data, we derive a quenching efficiency of E_q~0.45 and an upper limit on the quenching timescale of T_q<1Gyr.

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arXiv2018

A Serendipitous Hard X-ray Detection of the Blazar LBQS 1319+0039

We report a serendipitous hard X-ray (3-24 keV) detection of the blazar LBQS 1319+0039. The X-ray spectrum is consistent with powerlaw emission with a photon index of $Γ= 1.78$. The observed emission and published redshift imply an observed 2-10 keV luminosity of $3.4\times10^{45}$ erg s$^{-1}$. This is consistent with extrapolations from a previous soft X-ray detection, thus there is no evidence for strong variability.

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arXiv2018

BAT AGN Spectroscopic Survey -- XII. The relation between coronal properties of Active Galactic Nuclei and the Eddington ratio

The bulk of the X-ray emission in Active Galactic Nuclei (AGN) is produced very close to the accreting supermassive black hole (SMBH), in a corona of hot electrons which up scatters optical and ultraviolet photons from the accretion flow. The cutoff energy ($E_{\rm C}$) of the primary X-ray continuum emission carries important information on the physical characteristics of the X-ray emitting plasma, but little is currently known about its potential relation with the properties of accreting SMBHs. Using the largest broad-band (0.3-150 keV) X-ray spectroscopic study available to date, we investigate how the corona is related to the AGN luminosity, black hole mass and Eddington ratio ($λ_{\rm Edd}$). Assuming a slab corona the median values of the temperature and optical depth of the Comptonizing plasma are $kT_{\rm e}=105 \pm 18$ keV and $τ=0.25\pm0.06$, respectively. When we properly account for the large number of $E_{\rm C}$ lower limits, we find a statistically significant dependence of the cutoff energy on the Eddington ratio. In particular, objects with $ λ_{\rm Edd}>0.1$ have a significantly lower median cutoff energy ($E_{\rm C}=160\pm41$ keV) than those with $λ_{\rm Edd}\leq 0.1$ ($E_{\rm C}=370\pm51$ keV). This is consistent with the idea that radiatively compact coronae are also cooler, because they tend to avoid the region in the temperature-compactness parameter space where runaway pair production would dominate. We show that this behaviour could also straightforwardly explain the suggested positive correlation between the photon index ($Γ$) and the Eddington ratio, being able to reproduce the observed slope of the $Γ-λ_{\rm Edd}$ trend.

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arXiv2018

Two Separate Outflows in the Dual Supermassive Black Hole System NGC 6240

Theoretical models and numerical simulations have established a framework of galaxy evolution in which galaxies merge and create dual supermassive black holes (with separations of one to ten kiloparsecs), which eventually sink into the centre of the merger remnant, emit gravitational waves and coalesce. The merger also triggers star formation and supermassive black hole growth, and gas outflows regulate the stellar content. Although this theoretical picture is supported by recent observations of starburst-driven and supermassive black hole-driven outflows, it remains unclear how these outflows interact with the interstellar medium. Furthermore, the relative contributions of star formation and black hole activity to galactic feedback remain unknown. Here we report observations of dual outflows in the central region of the prototypical merger NGC 6240. We find a black-hole-driven outflow of [O III] to the northeast and a starburst-driven outflow of Hα to the northwest. The orientations and positions of the outflows allow us to isolate them spatially and study their properties independently. We estimate mass outflow rates of 10 and 75 solar masses per year for the Hα bubble and the [O III] cone, respectively. Their combined mass outflow is comparable to the star formation rate, suggesting that negative feedback on star formation is occurring.

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arXiv2018

Optical, near-IR and sub-mm IFU Observations of the nearby dual AGN Mrk 463

We present optical and near-IR Integral Field Unit (IFU) and ALMA band 6 observations of the nearby dual Active Galactic Nuclei (AGN) Mrk 463. At a distance of 210 Mpc, and a nuclear separation of $\sim$4 kpc, Mrk 463 is an excellent laboratory to study the gas dynamics, star formation processes and supermassive black hole (SMBH) accretion in a late-stage gas-rich major galaxy merger. The IFU observations reveal a complex morphology, including tidal tails, star-forming clumps, and emission line regions. The optical data, which map the full extent of the merger, show evidence for a biconical outflow and material outflowing at $>$600 km s$^{-1}$, both associated with the Mrk 463E nucleus, together with large scale gradients likely related to the ongoing galaxy merger. We further find an emission line region $\sim$11 kpc south of Mrk 463E that is consistent with being photoionized by an AGN. Compared to the current AGN luminosity, the energy budget of the cloud implies a luminosity drop in Mrk 463E by a factor 3-20 over the last 40,000 years. The ALMA observations of $^{12}$CO(2-1) and adjacent 1mm continuum reveal the presence of $\sim$10$^{9}$M$_\odot$ in molecular gas in the system. The molecular gas shows velocity gradients of $\sim$800 km/s and $\sim$400 km/s around the Mrk 463E and 463W nuclei, respectively. We conclude that in this system the infall of $\sim$100s $M_\odot$/yr of molecular gas is in rough balance with the removal of ionized gas by a biconical outflow being fueled by a relatively small, $<$0.01% of accretion onto each SMBH.

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arXiv2018

The NuSTAR Extragalactic Surveys: X-ray spectroscopic analysis of the bright hard-band selected sample

We discuss the spectral analysis of a sample of 63 Active Galactic Nuclei (AGN) detected above a limiting flux of $S(8-24$ keV$)=7\times10^{-14}$ erg/s/cm$^2$ in the multi-tiered NuSTAR Extragalactic Survey program. The sources span a redshift range z=0-2.1 (median $\langle$z$\rangle=$0.58). The spectral analysis is performed over the broad 0.5-24 keV energy range, combining NuSTAR with Chandra and/or XMM-Newton data and employing empirical and physically motivated models. This constitutes the largest sample of AGN selected at $ >10$ keV to be homogeneously spectrally analyzed at these flux levels. We study the distribution of spectral parameters such as photon index, column density ($N_{\rm H}$), reflection parameter ($R$) and 10-40 keV luminosity ($L_{X}$). Heavily obscured ($\log [N_{\rm H}/cm^{-2}]\ge23$) and Compton Thick (CT; $\log [N_{\rm H}/cm^{-2}]\ge24$) AGN constitute $\sim$25% (15-17 sources) and $\sim$2-3% ( 1-2 sources) of the sample, respectively. The observed $N_{\rm H}$ distribution fairly agrees with predictions of Cosmic X-ray Background population synthesis models (CXBPSM). We estimate the intrinsic fraction of AGN as a function of $N_{H}$, accounting for the bias against obscured AGN in a flux-selected sample. The fraction of CT AGN relative to $\log [N_{\rm H}/cm^{-2}]=20-24$ AGN is poorly constrainted, formally in the range 2-56% (90% upper limit of 66%). We derived a fraction ($f_{abs}$) of obscured AGN ($\log [N_{\rm H}/cm^{-2}]=22-24$) as a function of $L_{X}$ in agreement with CXBPSM and previous z < 1 X-ray determinations. Furthermore $f_{abs}$ at z=0.1-0.5 and $\log (L_{X}/erg$ $s^{-1})\approx43.6-44.3$ agrees with observational measurements/trends obtained over larger redshift intervals. We report a significant anti-correlation of $R$ with $L_{X}$ (confirmed by our companion paper on stacked spectra) with considerable scatter around the median $R$ values.

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arXiv2018

The NuSTAR Extragalactic Surveys: source catalog and the Compton-thick fraction in the UDS field

We present the results and the source catalog of the NuSTAR survey in the UKIDSS Ultra Deep Survey (UDS) field, bridging the gap in depth and area between NuSTAR's ECDFS and COSMOS surveys. The survey covers a $\sim 0.6$ deg$^2$ area of the field for a total observing time of $\sim$ 1.75 Ms, to a half-area depth of $\sim$ 155 ks corrected for vignetting at $3-24$ keV, and reaching sensitivity limits at half-area in the full ($3-24$ keV), soft ($3-8$ keV) and hard ($8-24$ keV) bands of $2.2 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$, $1.0 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$, and $2.7 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$, respectively. A total of 67 sources are detected in at least one of the three bands, 56 of which have a robust optical redshift with a median of $\langle z\rangle \sim 1.1$. Through a broadband ($0.5-24$ keV) spectral analysis of the whole sample combined with the NuSTAR hardness ratios, we compute the observed Compton-thick (CT; $N_{\rm H} > 10^{24}$ cm$^{-2}$) fraction. Taking into account the uncertainties on each $N_{\rm H}$ measurement, the final number of CT sources is $6.8\pm1.2$. This corresponds to an observed CT fraction of $(11.5\pm2.0)\%$, providing a robust lower limit to the intrinsic fraction of CT AGN and placing constraints on cosmic X-ray background synthesis models.

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arXiv2017

LLAMA: Normal star formation efficiencies of molecular gas in the centres of luminous Seyfert galaxies

Using new APEX and JCMT spectroscopy of the CO 2-1 line, we undertake a controlled study of cold molecular gas in moderately luminous Active Galactic Nuclei (AGN) and inactive galaxies from the Luminous Local AGN with Matched Analogs (LLAMA) survey. We use spatially resolved infrared photometry of the LLAMA galaxies from 2MASS, WISE, IRAS & Herschel, corrected for nuclear emission using multi-component spectral energy distribution (SED) fits, to examine the dust-reprocessed star-formation rates (SFRs), molecular gas fractions and star formation efficiencies (SFEs) over their central 1 - 3 kpc. We find that the gas fractions and central SFEs of both active and inactive galaxies are similar when controlling for host stellar mass and morphology (Hubble type). The equivalent central molecular gas depletion times are consistent with the discs of normal spiral galaxies in the local Universe. Despite energetic arguments that the AGN in LLAMA should be capable of disrupting the observable cold molecular gas in their central environments, our results indicate that nuclear radiation only couples weakly with this phase. We find a mild preference for obscured AGN to contain higher amounts of central molecular gas, which suggests a connection between AGN obscuration and the gaseous environment of the nucleus. Systems with depressed SFEs are not found among the LLAMA AGN. We speculate that the processes that sustain the collapse of molecular gas into dense pre-stellar cores may also be a prerequisite for the inflow of material on to AGN accretion disks.

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arXiv2017

Investigating the evolution of the dual AGN system ESO~509-IG066

We analyze the evolution of the dual AGN in ESO 509-IG066, a galaxy pair located at z=0.034 whose nuclei are separated by 11 kpc. Previous observations with XMM-Newton on this dual AGN found evidence for two moderately obscured ($N_H\sim10^{22}$ cm$^{-2}$) X-ray luminous ($L_X\sim10^{43}$ erg/s) nuclear sources. We present an analysis of subsequent Chandra, NuSTAR and Swift/XRT observations that show one source has dropped in flux by a factor of 10 between 2004 and 2011, which could be explained by either an increase in the absorbing column or an intrinsic fading of the central engine possibly due to a decrease in mass accretion. Both of these scenarios are predicted by galaxy merger simulations. The source which has dropped in flux is not detected by NuSTAR, which argues against absorption, unless it is extreme. However, new Keck/LRIS optical spectroscopy reveals a previously unreported broad H-alpha line which is highly unlikely to be visible under the extreme absorption scenario. We therefore conclude that the black hole in this nucleus has undergone a dramatic drop in accretion rate. From AO-assisted near-infrared integral-field spectroscopy of the other nucleus, we find evidence that the galaxy merger is having a direct effect on the kinematics of the gas close to the nucleus of the galaxy, providing a direct observational link between the galaxy merger and the mass accretion rate on to the black hole.

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arXiv2017

The NuSTAR Extragalactic Survey: Average broad-band X-ray spectral properties of the NuSTAR detected AGN

We present a study of the average X-ray spectral properties of the sources detected by the NuSTAR extragalactic survey, comprising observations of the E-CDFS, EGS and COSMOS fields. The sample includes 182 NuSTAR sources (64 detected at 8-24 keV), with 3-24 keV fluxes ranging between $f_{\rm 3-24 keV}\approx10^{-14}$ and $6\times10^{-13}$ erg/cm$^2$/s ($f_{\rm 8-24 keV}\approx3\times10^{-14}-3\times10^{-13}$ erg/cm$^2$/s) and redshifts of $z=0.04-3.21$. We produce composite spectra from the Chandra+NuSTAR data ($E\approx2-40$ keV, rest frame) for all the sources with redshift identifications (95%) and investigate the intrinsic, average spectra of the sources, divided into broad-line (BL) and narrow-line (NL) AGN, and also in different bins of X-ray column density and luminosity. The average power-law photon index for the whole sample is $Γ=1.65_{-0.03}^{+0.03}$, flatter than $Γ\approx1.8$ typically found for AGN. While the spectral slope of BL and X-ray unabsorbed AGN is consistent with typical values ($Γ=1.79_{-0.01}^{+0.01}$), a significant flattening is seen in NL AGN and heavily-absorbed sources ($Γ=1.60_{-0.05}^{+0.08}$ and $Γ=1.38_{-0.12}^{+0.12}$, respectively), likely due to the effect of absorption and to the contribution from Compton reflection to the high-energy flux (E>10 keV). We find that the typical reflection fraction in our spectra is $R\approx0.5$ (for $Γ=1.8$), with a tentative indication of an increase of the reflection strength with column density. While there is no significant evidence for a dependence of the photon index with X-ray luminosity in our sample, we find that $R$ decreases with luminosity, with relatively high levels of reflection ($R\approx1.2$) for $L_{\rm 10-40 keV}<10^{44}$ erg/s and $R\approx0.3$ for $L_{\rm 10-40 keV}>10^{44}$ erg/s AGN, assuming $Γ=1.8$.

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arXiv2017

The NuSTAR Serendipitous Survey: Hunting for The Most Extreme Obscured AGN at >10 keV

We identify sources with extremely hard X-ray spectra (i.e., with photon indices of Gamma<0.6 in the 13 sq. deg. NuSTAR serendipitous survey, to search for the most highly obscured AGNs detected at >10 keV. Eight extreme NuSTAR sources are identified, and we use the NuSTAR data in combination with lower energy X-ray observations (from Chandra, Swift XRT, and XMM-Newton) to characterize the broad-band (0.5-24 keV) X-ray spectra. We find that all of the extreme sources are highly obscured AGNs, including three robust Compton-thick (CT; N_H > 1.5e24 cm^-2) AGNs at low redshift (z<0.1), and a likely-CT AGN at higher redshift (z=0.16). Most of the extreme sources would not have been identified as highly obscured based on the low energy (<10 keV) X-ray coverage alone. The multiwavelength properties (e.g., optical spectra and X-ray/MIR luminosity ratios) provide further support for the eight sources being significantly obscured. Correcting for absorption, the intrinsic rest-frame 10-40 keV luminosities of the extreme sources cover a broad range, from ~ 5 x 10^42 to 10^45 erg s^-1. The estimated number counts of CT AGNs in the NuSTAR serendipitous survey are in broad agreement with model expectations based on previous X-ray surveys, except for the lowest redshifts (z<0.07) where we measure a high CT fraction of f_CT^obs = 30 (+16 -12) %. For the small sample of CT AGNs, we find a high fraction of galaxy major mergers (50 +/- 33%) compared to control samples of "normal" AGNs.

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arXiv2017

The ALMA Frontier Fields Survey - II. Multiwavelength Photometric analysis of 1.1mm continuum sources in Abell 2744, MACSJ0416.1-2403 and MACSJ1149.5+2223

[abridged] The Hubble and Spitzer Space Telescope surveys of the Frontier Fields (FF) provide extremely deep images around six massive, strong-lensing clusters of galaxies. The ALMA FF survey aims to cover the same fields at 1.1mm, with maps reaching (unlensed) sensitivities of $<$70$μ$Jy, in order to explore the properties of background dusty star-forming galaxies. We report on the multi-wavelength photometric analysis of all 12 significantly detected ($>$5$σ$) sources in the first three FF clusters observed by ALMA, based on data from Hubble and Spitzer, the VLT and Herschel. We measure the total photometry in all available bands and determine the photometric redshifts and the physical properties of the counterparts via SED-fitting. In particular, we carefully estimate the FIR photometry using 1.1mm priors to limit the misidentification of blended FIR counterparts, which strongly affect some flux estimates in previous FIR catalogs. We identify robust near-infrared (NIR) counterparts for all 11 sources with K$_s$ detection, the majority of which are quite red, with eight having $F814W-K_s\gtrsim 4$ and five having $F160W-[4.5]\gtrsim3$. From the FIR point of view, all our objects have $z_{phot}$$\sim$1--3, whereas based on the optical SED one object prefers a high-$z$ solution ($z\geq\ $7). Five objects among our sample have spectroscopic redshifts from the GLASS survey for which we can reproduce their SEDs with existing templates. This verification confirms the validity of our photometric redshift methodology. The mean redshift of our sample is $z_{phot}$=1.99$\pm$0.27. All 1.1mm selected objects are massive (10.0$<\log[M_{\star}(M_{\odot})]<$ 11.5), with high star formation rates ($<\log[SFR(M_{\odot}/yr)]> \approx$1.6) and high dust contents (8.1 $<\log[M_{dust} (M_{\odot})]<$8.8), consistent with previous ALMA surveys.

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arXiv2017

Inferring Compton-thick AGN candidates at z>2 with Chandra using the >8 keV restframe spectral curvature

To fully understand cosmic black hole growth we need to constrain the population of heavily obscured active galactic nuclei (AGN) at the peak of cosmic black hole growth ($z\sim$1-3). Sources with obscuring column densities higher than $\mathrm{10^{24}}$ atoms $\mathrm{cm^{-2}}$, called Compton-thick (CT) AGN, can be identified by excess X-ray emission at $\sim$20-30 keV, called the "Compton hump". We apply the recently developed Spectral Curvature (SC) method to high-redshift AGN (2 5\times 10^{43}}$ erg $\mathrm{s^{-1}}$. In the Cosmological evolution survey (COSMOS), we find an observed CT fraction of $\mathrm{15^{+4}_{-3}\%}$ (40/272) or $\mathrm{32\pm11 \%}$ when corrected for the survey sensitivity. When comparing to low redshift AGN with similar X-ray luminosities, our results imply the CT AGN fraction is consistent with having no redshift evolution. Finally, we provide SC equations that can be used to find high-redshift CT AGN (z>1) for current (XMM-Newton) and future (eROSITA and ATHENA) X-ray missions.

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arXiv2017

Growing supermassive black holes in the late stages of galaxy mergers are heavily obscured

Mergers of galaxies are thought to cause significant gas inflows to the inner parsecs, which can activate rapid accretion onto supermassive black holes (SMBHs), giving rise to Active Galactic Nuclei (AGN). During a significant fraction of this process, SMBHs are predicted to be enshrouded by gas and dust. Studying 52 galactic nuclei in infrared-selected local Luminous and Ultra-luminous infrared galaxies in different merger stages in the hard X-ray band, where radiation is less affected by absorption, we find that the amount of material around SMBHs increases during the last phases of the merger. We find that the fraction of Compton-thick (CT, $N_{\rm\,H}\geq 10^{24}\rm\,cm^{-2}$) AGN in late merger galaxies is higher ($f_{\rm\,CT}=65^{+12}_{-13}\%$) than in local hard X-ray selected AGN ($f_{\rm\,CT}=27\pm 4\%$), and that obscuration reaches its maximum when the nuclei of the two merging galaxies are at a projected distance of $D_{12}\simeq0.4-10.8$ kiloparsecs ($f_{\rm\,CT}=77_{-17}^{+13}\%$). We also find that all AGN of our sample in late merger galaxies have $N_{\rm\,H}> 10^{23}\rm\,cm^{-2}$, which implies that the obscuring material covers $95^{+4}_{-8}\%$ of the X-ray source. These observations show that the material is most effectively funnelled from the galactic scale to the inner tens of parsecs during the late stages of galaxy mergers, and that the close environment of SMBHs in advanced mergers is richer in gas and dust with respect to that of SMBHs in isolated galaxies, and cannot be explained by the classical AGN unification model in which the torus is responsible for the obscuration.

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arXiv2017

A New, Faint Population of X-ray Transients

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 ($m_{\rm R}=27.5$ mag, $z_{\rm ph}$$\sim$2.2) host in the CANDELS survey. The X-ray event is comprised of 115$^{+12}_{-11}$ net 0.3-7.0 keV counts, with a light curve characterised by a $\approx$100 s rise time, a peak 0.3-10 keV flux of $\approx$5$\times$10$^{-12}$ erg s$^{-1}$ cm$^{-2}$, and a power-law decay time slope of $-1.53\pm0.27$. The average spectral slope is $Γ=1.43^{+0.23}_{-0.13}$, with no clear spectral variations. The \hbox{X-ray} and multi-wavelength 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 $\sim$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.

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