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Register a new userThe nature of X-ray absorbed starburst QSOs and the QSO evolutionary scheme
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M.J.Page
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In contradiction to the simple AGN unification schemes, there exists a significant population of broad line, z~2 QSOs which have heavily absorbed X-ray spectra. These objects have luminosities and redshifts characteristic of the sources that produce the bulk of the QSO luminosity in the universe. Our follow up observations in the submillimetre show that these QSOs are embedded in ultraluminous starburst galaxies, unlike most unabsorbed QSOs at the same redshifts and luminosities. The radically different star formation properties between the absorbed and unabsorbed QSOs implies that the X-ray absorption is unrelated to the torus invoked in AGN unification schemes. The most puzzling question about these objects is the nature of the X-ray absorber. We present our study of the X-ray absorbers based on deep (50-100ks) XMM-Newton spectroscopy. The hypothesis of a normal QSO continuum, coupled with a neutral absorber is strongly rejected. We consider the alternative hypotheses for the absorber, originating either in the QSO or in the surrounding starburst. Finally we discuss the implications for QSO/host galaxy formation, in terms of an evolutionary sequence of star formation and black hole growth. We propose that both processes occur simultaneously in the gas-and-dust-rich heavily obscured centres of young galaxies, and that absorbed QSOs form a transitional stage, between the main obscured growth phase, and the luminous QSO.
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There exists a significant population of broad line, z~2 QSOs which have heavily absorbed X-ray spectra. Follow up observations in the submillimetre show that these QSOs are embedded in ultraluminous starburst galaxies, unlike most unabsorbed QSOs at the same redshifts and luminosities. Here we present X-ray spectra from XMM-Newton for a sample of 5 such X-ray absorbed QSOs that have been detected at submillimetre wavelengths. We also present spectra in the restframe ultraviolet from ground based telescopes. All 5 QSOs are found to exhibit strong C IV absorption lines in their ultraviolet spectra with equivalent width > 5 Angstroms. The X-ray spectra are inconsistent with the hypothesis that these objects show normal QSO continua absorbed by low-ionization gas. Instead, the spectra can be modelled successfully with ionized absorbers, or with cold absorbers if they posess unusually flat X-ray continuum shapes and unusual optical to X-ray spectral energy distributions. We show that the ionized absorber model provides the simplest, most self-consistent explanation for their observed properties. We estimate that the fraction of radiated power that is converted into kinetic luminosity of the outflowing winds is typically ~4 per cent, in agreement with recent estimates for the kinetic feedback from QSOs required to produce the M - sigma relation, and consistent with the hypothesis that the X-ray absorbed QSOs represent the transition phase between obscured accretion and the luminous QSO phase in the evolution of massive galaxies.
It is suggested that many of the ultraluminous compact x-ray sources now being found in the main bodies of galaxies, particularly those that are active, like M82, NGC 3628 and others, are local QSOs, or BL Lac objects, with high intrinsic redshifts in the process of being ejected from those galaxies. Evidence in support of this hypothesis is summarized.
We have assembled a sample of 5 X-ray-absorbed and submm-luminous type 1 QSOs at $z sim 2$ which are simultaneously growing their central black holes through accretion and forming stars copiously. We present here the analysis of their rest-frame UV to submm Spectral Energy Distributions (SEDs), including new Herschel data. Both AGN (direct and reprocessed) and Star Formation (SF) emission are needed to model their SEDs. From the SEDs and their UV-optical spectra we have estimated the masses of their black holes $M_{BH}sim 10^{9}-10^{10},M_{odot}$, their intrinsic AGN bolometric luminosities $L_{BOL}sim(0.8 - 20)times 10^{13} L_{odot}$, Eddington ratios $L_{BOL}/L_{Edd}sim 0.1 - 1.1$ and bolometric corrections $L_{BOL}/L_{X,2-10}sim 30 - 500$. These values are common among optically and X-ray-selected type 1 QSOs (except for RX~J1249), except for the bolometric corrections, which are higher. These objects show very high far-infrared luminosities $L_{FIR}sim$ (2 - 8)$times10^{12},M_{odot}$ and Star Formation Rates SFR$sim 1000 M_{odot}/$y. From their $L_{FIR}$ and the shape of their FIR-submm emission we have estimated star-forming dust masses of $M_{DUST}sim 10^9,M_odot$. We have found evidence of a tentative correlation between the gas column densities of the ionized absorbers detected in X-ray (N$_{H_{ion}}$) and $SFR$. Our computed black hole masses are amongst the most massive known.
We cross-correlate QSOs from the 2dF Survey with galaxy groups. The galaxy samples are limited to B < 20.5. We use an objective algorithm to detect galaxy groups. A 3sigma anti-correlation is observed between QSOs and galaxy groups. This paucity of faint QSOs around groups is neither a selection effect nor due to restrictions on the placement of 2dF fibres. By observing the colours of QSOs on the scales of the anti-correlation, we limit dust in galaxy groups, finding a maximum reddening of E(b_j-r) < 0.012 at the 95% level. The small amount of dust thus inferred is insufficient to cause the anti-correlation, supporting the suggestion by Croom & Shanks that the signal is due to gravitational lensing. The possibility remains that tailored dust models, such as grey dust, heavy patches of dust or a combination of dust and lensing, could explain the anti-correlation. Assuming the signal is caused by lensing rather than dust, we measure the average velocity dispersion of a Singular Isothermal Sphere that would cause the anti-correlation as around 1150 km/s. Simulations reject 600 km/s at the 5% significance level. We also model foreground lenses as NFW haloes and measure the typical mass within 1.5 Mpc/h of the halo centre as M_{1.5} = (1.2 +/- 0.9) x 10^{15} solarmasses/h. Regardless of whether we utilise a SIS or NFW dark matter profile, our model favours more mass in groups than accounted for in a universe with density parameter Omega_m = 0.3. Detailed simulations and galaxy group redshifts will significantly reduce the current systematic uncertainties in these $Omega_m$ estimates. Reducing the remaining uncertainty will require larger QSO and galaxy group surveys (abridged).
In this paper we report on the broadband X-ray properties of a complete sample of 33 absorbed Seyfert galaxies hard X-ray selected with integral. The high quality broadband spectra obtained with both xmm, and integral-IBIS data are well reproduced with an absorbed primary emission with a high energy cutoff and its scattered fraction below 2-3 keV, plus the Compton reflection features. A high energy cut-off is found in 30% of the sample, with an average value below 150 keV. The diagnostic plot NH vs Fobs(2-10 keV)/F(20-100 keV) allowed the isolation of the Compton thick objects, and may represent a useful tool for future hard X-ray observations of newly discovered AGN. We are unable to associate the reflection components with the absorbing gas as a torus, a more complex scenario being necessary. In the Compton thin sources, a fraction (but not all) of the Fe K line needs to be produced in a gas possibly associated with the optical Broad Line Region, responsible also for the absorption. We still need a Compton thick medium (not intercepting the line of sight) likely associated to a torus, which contributes to the Fe line intensity and produces the observed reflection continuum above 10 keV. The so-called Iwasawa-Taniguchi effect can not be confirmed with our data. Finally, the comparison with a sample of unobscured AGN shows that, type 1 and type 2 (once corrected for absorption) Seyfert are characterized by the same nuclear/accretion properties (luminosity, bolometric luminosity, Eddington ratio), supporting the unified view.
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