WISE/SDSS Selected Hyper-luminous Research Articles

The WISSH quasars project

Context. Hyperluminous quasi-stellar objects (QSOs) are ideal laboratories to investigate active galactic nucleus (AGN) feedback mechanisms. Their formidable energy release causes powerful winds at all scales, and thus the maximum feedback is expected. Aims. Our aim is to derive the mean spectral energy distribution (SED) of a sample of 85 WISE-SDSS selected hyperluminous (WISSH) quasars. Since the SED provides a direct way to investigate the AGN structure, our goal is to understand if quasars at the bright end of the luminosity function have peculiar properties compared to the bulk of the QSO population. Methods. We collected all the available photometry, from X-rays to the far-infrared (FIR); each WISSH quasar is observed in at least 12 different bands. We then built a mean intrinsic SED after correcting for the dust extinction, absorption and emission lines, and intergalactic medium absorption. We also derived bolometric, IR band, and monochromatic luminosities together with bolometric corrections at λ = 5100 Å and 3 μm. We define a new relation for the 3 μm bolometric correction. Results. We find that the mean SED of hyperluminous WISSH QSOs shows some differences compared to that of less luminous sources (i.e., a lower X-ray emission and a near- and mid-IR excess which can be explained assuming a larger dust contribution. WISSH QSOs have stronger emission from both warm (T ∼ 500 − 600 K) and very hot (T ≥ 1000 K) dust, the latter being responsible for shifting the typical dip of the AGN SED from 1.3 μm to 1.1 μm. We also derived the mean SEDs of two subsamples created based on their spectral features (presence of broad absorption lines and equivalent width of CIV line). We confirm that broad absorption lines (BALs) are X-ray weak and that they have a reddened UV-optical continuum. We also find that BALs tend to have stronger emission from the hot dust component. For sources with a weaker CIV line, our main result is the confirmation of their lower X-ray emission. By populating the LIR vs. z diagram proposed by Symeonidis & Page (MNRAS, 503, 3992), we found that ∼90% of WISSH QSOs with z ≥ 3.5 have their FIR emission dominated by star-forming activity. Conclusions. This analysis suggests that hyperluminous QSOs have a peculiar SED compared to less luminous objects. It is therefore critical to use SED templates constructed exclusively from very bright quasar samples (such as this one) when dealing with particularly luminous sources, such as high-redshift QSOs.

The WISSH quasars project

Context. Sources at the brightest end of the quasi-stellar object (QSO) luminosity function, during the peak epoch in the history of star formation and black hole accretion (z ∼ 2−4, often referred to as “Cosmic noon”) are privileged sites to study the cycle of feeding & feedback processes in massive galaxies. Aims. We aim to perform the first systematic study of cold gas properties in the most luminous QSOs, by characterising their host-galaxies and environment. These targets exhibit indeed widespread evidence of outflows at nuclear and galactic scales. Methods. We analyse ALMA, NOEMA and JVLA observations of the far-infrared continuum, CO and [CII] emission lines in eight QSOs (bolometric luminosity LBol ≳ 3 × 1047 erg s−1) from the WISE-SDSS selected hyper-luminous (WISSH) QSOs sample at z ∼ 2.4−4.7. Results. We report a 100% emission line detection rate and a 80% detection rate in continuum emission, and we find CO emission to be consistent with the steepest CO ladders observed so far. Sub-millimetre data reveal presence of (one or more) bright companion galaxies around ∼80% of WISSH QSOs, at projected distances of ∼6−130 kpc. We observe a variety of sizes for the molecular gas reservoirs (∼1.7−10 kpc), mostly associated with rotating disks with disturbed kinematics. WISSH QSOs typically show lower CO luminosity and higher star formation efficiency than infrared matched, z ∼ 0−3 main-sequence galaxies, implying that, given the observed SFR ∼170−1100 M⊙ yr−1, molecular gas is converted into stars in ≲50 Myr. Most targets show extreme dynamical to black-hole mass ratios Mdyn/MBH ∼ 3−10, two orders of magnitude smaller than local relations. The molecular gas fraction in the host-galaxies of WISSH is lower by a factor of ∼10−100 than in star forming galaxies with similar M*. Conclusions. Our analysis reveals that hyper-luminous QSOs at Cosmic noon undergo an intense growth phase of both the central super-massive black hole and of the host-galaxy. These systems pinpoint the high-density sites where giant galaxies assemble, where we show that mergers play a major role in the build-up of the final host-galaxy mass. We suggest that the observed low molecular gas fraction and short depletion timescale are due to AGN feedback, whose presence is indicated by fast AGN-driven ionised outflows in all our targets.

The WISSH quasars project

Hyper-luminous quasars (Lbol ≳ 1047 erg s−1) are ideal laboratories to study the interaction and impact of the extreme radiative field and the most powerful winds in the active galactic nuclei (AGN) nuclear regions. They typically exhibit low coronal X-ray luminosity (LX) compared to the ultraviolet (UV) and mid-infrared (MIR) radiative outputs (LUV and LUV); a non-negligible fraction of them report even ∼1 dex weaker LX compared to the prediction of the well established LX–LUV and LX–LUV relations followed by the bulk of the AGN population. In our WISE/SDSS-selected Hyper-luminous (WISSH) z = 2 − 4 broad-line quasar sample, we report on the discovery of a dependence between the intrinsic 2–10 keV luminosity (L2 − 10) and the blueshifted velocity of the CIV emission line (vCIV) that is indicative of accretion disc winds. In particular, sources with the fastest winds (vCIV ≳ 3000 km s−1) possess ∼0.5–1 dex lower L2 − 10 than sources with negligible vCIV. No similar dependence is found on LUV, LUV, Lbol, the photon index, or the absorption column density. We interpret these findings in the context of accretion disc wind models. Both magnetohydrodynamic and line-driven models can qualitatively explain the reported relations as a consequence of X-ray shielding from the inner wind regions. In case of line-driven winds, the launch of fast winds is favoured by a reduced X-ray emission, and we speculate that these winds may play a role in directly limiting the coronal hard X-ray production.

The WISSH quasars project

Winds accelerated by active galactic nuclei (AGNs) are invoked in the most successful models of galaxy evolution to explain the observed physical and evolutionary properties of massive galaxies. Winds are expected to deposit energy and momentum into the interstellar medium (ISM), thus regulating both star formation and supermassive black hole (SMBH) growth. We undertook a multiband observing program aimed at obtaining a complete census of winds in a sample of WISE/SDSS selected hyper-luminous (WISSH) quasars (QSOs) at z ≈ 2–4. We analyzed the rest-frame optical (i.e. LBT/LUCI and VLT/SINFONI) and UV (i.e. SDSS) spectra of 18 randomly selected WISSH QSOs to measure the SMBH mass and study the properties of winds both in the narrow line region (NLR) and broad line region (BLR) traced by blueshifted or skewed [OIII] and CIV emission lines, respectively. These WISSH QSOs are powered by SMBH with masses ≳109 M⊙ accreting at 0.4 < λEdd < 3.1. We found the existence of two subpopulations of hyper-luminous QSOs characterized by the presence of outflows at different distances from the SMBH. One population (i.e. [OIII] sources) exhibits powerful [OIII] outflows, a rest-frame equivalent width (REW) of the CIV emission REWCIV ≈ 20–40 Å, and modest CIV velocity shift (vCIVpeak) with respect to the systemic redshift (vCIVpeak <~ 2000 km s−1). The second population (i.e. Weak [OIII] sources), representing ~70% of the analyzed WISSH QSOs, shows weak or absent [OIII] emission and an extremely large blueshifted CIV emission (vCIVpeak up to ~8000 km s−1 and REWCIV <~ 20 Å). We propose two explanations for the observed behavior of the strength of the [OIII] emission in terms of the orientation effects of the line of sight and ionization cone. The dichotomy in the presence of BLR and NLR winds could be likely due to inclination effects considering a polar geometry scenario for the BLR winds. In a few cases these winds are remarkably as powerful as those revealed in the NLR in the [OIII] QSOs (Ėkin ~ 1044−45 erg s−1). We also investigated the dependence of these CIV winds on fundamental AGN parameters such as bolometric luminosity (LBol), Eddington ratio (λEdd), and UV-to-X-ray continuum slope (αOX). We found a strong correlation with LBol and an anti-correlation with αOX whereby the higher the luminosity, the steeper the ionizing continuum described by means of αOX and the larger the blueshift of the CIV emission line. Finally, the observed dependence vCIVpeak ∝ LBol0.28 ± 0.04 is consistent with a radiatively-driven-winds scenario, where a strong UV continuum is necessary to launch the wind and a weakness of the X-rayemission is fundamental to prevent overionization of the wind itself.

The WISSH quasars project

We present an ALMA high-resolution (0.18″ × 0.21″) observation of the 840 μm continuum and [CII] λ157.74 μm line emission in the WISE-SDSS selected hyper-luminous (WISSH) quasi-stellar object (QSO) J1015+0020, at z ∼ 4.4. Our analysis reveals an exceptional overdensity of [CII]-emitting companions with a very small (<150 km s−1) velocity shift with respect to the QSO redshift. We report the discovery of the closest companion observed so far in submillimetre observations of high-z QSOs. It is only 2.2 kpc distant and merging with J1015+0020, while two other [CII] emitters are found at 8 and 17 kpc. Two strong continuum emitters are also detected at <3.5 arcsec from the QSO. They are likely associated with the same overdense structure of J1015+0020, as they exceed by a factor of 100 the number of expected sources, considering the log(N)–log(S) at 850 μm. The host galaxy of J1015+0020 shows a star formation rate (SFR) of about 100 M⊙ yr−1, while the total SFR of the QSO and its companion galaxies is a factor of ∼10 higher, indicating that substantial stellar mass assembly at early epochs may have taken place in the QSO satellites. For J1015+0020 we computed a black hole mass MBH ∼ 6 × 109 M⊙. As we resolve the [CII] emission of the QSO, we can compute a dynamical mass of Mdyn ∼ 4 × 1010 M⊙. This translates into an extreme ratio Mdyn/MBH ∼ 7, i.e. two orders of magnitude smaller than what is typically observed in local galaxies. The total stellar mass of the QSO host galaxy plus the [CII] emitters in the ALMA field of view already exceeds 1011 M⊙ at z ∼ 4.4. These sources will likely merge and develop into a giant galaxy of ∼1.3 × 1012 M⊙. Under the assumption of constant Ṁacc or λEdd equal to the observed values, we find that the growth timescale of the host galaxy of J1015+0020 is comparable or even shorter than that inferred for the SMBH.

The WISSH quasars project

We perform a survey of the X-ray properties of 41 objects from the WISE/SDSS selected Hyper-luminous (WISSH) quasars sample, composed by 86 broad-line quasars (QSOs) with bolometric luminosity $L_{Bol}\geq 2\times 10^{47}\,erg\, s^{-1}$, at z~2-4. All but 3 QSOs show unabsorbed 2-10 keV luminosities $L_{2-10}\geq10^{45} \,erg \,s^{-1}$. Thanks to their extreme radiative output across the Mid-IR-to-X-ray range, WISSH QSOs offer the opportunity to significantly extend and validate the existing relations involving $L_{2-10}$. We study $L_{2-10}$ as a function of (i) X-ray-to-Optical (X/O) flux ratio, (ii) mid-IR luminosity ($L_{MIR}$), (iii) $L_{Bol}$ as well as (iv) $\alpha_{OX}$ vs. the 2500$\mathring{A}$ luminosity. We find that WISSH QSOs show very low X/O(<0.1) compared to typical AGN values; $L_{2-10}/L_{MIR}$ ratios significantly smaller than those derived for AGN with lower luminosity; large X-ray bolometric corrections $k_{\rm Bol,X}\sim$ 100-1000; and steep $-2<\alpha_{OX}<-1.7$. These results lead to a scenario where the X-ray emission of hyper-luminous quasars is relatively weaker compared to lower-luminosity AGN. Models predict that such an X-ray weakness can be relevant for the acceleration of powerful high-ionization emission line-driven winds, commonly detected in the UV spectra of WISSH QSOs, which can in turn perturb the X-ray corona and weaken its emission. Accordingly, hyper-luminous QSOs represent the ideal laboratory to study the link between the AGN energy output and wind acceleration. Additionally, WISSH QSOs show very large BH masses ($\log[M_{\rm BH}/M_{\odot}]$>9.5). This enables a more robust modeling of the $\Gamma-M_{BH}$ relation by increasing the statistics at high masses. We derive a flatter $\Gamma$ dependence than previously found over the broad range 5 <$\log(M_{\rm BH}/M_{\odot})$ < 11.

The WISSH quasars project

Models and observations suggest that both the power and effects of AGN feedback should be maximised in hyper-luminous (LBol > 1047 erg s-1) quasars, i.e. objects at the brightest end of the AGN luminosity function. In this paper, we present the first results of a multiwavelength observing programme, focusing on a sample of WISE/SDSS selected hyper-luminous (WISSH) broad-line quasars at z ≈ 1.5–5. The WISSH quasars project has been designed to reveal the most energetic AGN-driven outflows, estimate their occurrence at the peak of quasar activity, and extend the study of correlations between outflows and nuclear properties up to poorly investigated, extreme AGN luminosities, i.e. LBol ~ 1047 − 1048 erg s-1. We present near-infrared, long-slit LBT/LUCI1 spectroscopy of five WISSH quasars at z ≈ 2.3 − 3.5, showing prominent [OIII] emission lines with broad (FWHM ~1200–2200 km s-1) and skewed profiles. The luminosities of these broad [OIII] wings are the highest measured so far, with L[OIII]broad ≳ 5 × 1044 erg s-1, and reveal the presence of powerful ionised outflows with associated mass outflow rates Ṁ ≳ 1700M⊙ yr-1 and kinetic powers Ėkin ≳ 1045 erg s-1. Although these estimates are affected by large uncertainties because of the use of [OIII] as a tracer of ionised outflows and the very basic outflow model adopted here, these results suggest that in our hyper-luminous targets the AGN is highly efficient at pushing large amounts of ionised gas outwards. Furthermore, the mechanical outflow luminosities measured for WISSH quasars correspond to higher percentages (~1–3%) of LBol than those derived for AGN with lower LBol. Our targets host very massive (MBH ≳ 2 × 109M⊙) black holes that are still accreting at a high rate (i.e. a factor of ~0.4–3 of the Eddington limit). These findings clearly demonstrate that WISSH quasars offer the opportunity to probe the extreme end of both luminosity and supermassive black holes (SMBH) mass functions and revealing powerful ionised outflows that are able to affect the evolution of their host galaxies.