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Register a new userProbing the Coevolution of Supermassive Black Holes and Galaxies Using Gravitationally Lensed Quasar Hosts
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C. Y. Peng
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In the present-day universe, supermassive black hole masses (MBH) appear to be strongly correlated with their galaxys bulge luminosity, among other properties. In this study, we explore the analogous relationship between MBH, derived using the virial method, and the stellar R-band bulge luminosity (Lr) or stellar bulge mass (M*) at epochs of 1 < z < 4.5 using a sample of 31 gravitationally lensed AGNs and 20 non-lensed AGNs. At redshifts z > 1.7 (10--12 Gyrs ago), we find that the observed MBH--Lr relation is nearly the same (to within ~0.3 mag) as it is today. When the observed Lr are corrected for luminosity evolution, this means that the black holes grew in mass faster than their hosts, with the MBH/M* mass ratio being a factor of > 4(+2)(-1) times larger at z > 1.7 than it is today. By the redshift range 1<z<1.7 (8-10 Gyrs ago), the MBH/M* ratio is at most two times higher than today, but it may be consistent with no evolution. Combining the results, we conclude that the ratio MBH/M* rises with look-back time, although it may saturate at ~6 times the local value. Scenarios in which moderately luminous quasar hosts at z>1.7 were fully formed bulges that passively faded to the present epoch are ruled out.
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At low redshift, there are fundamental correlations between the mass of supermassive black holes (MBH) and the mass (Mbulge) and luminosity of the host galaxy bulge. We investigate the same relation at z>=1. Using virial mass estimates for 11 quasars at z >~2 to measure their black hole mass, we find that black holes at high z fall nearly on the same MBH versus R-band magnitude (MR) relation (to ~0.3 mag) as low-redshift active and inactive galaxies, without making any correction for luminosity evolution. Using a set of conservative assumptions about the host galaxy stellar population, we show that at z>~2 (10 Gyrs ago) the ratio of MBH/Mbulge was 3--6 times larger than today. Barring unknown systematic errors on the measurement of MBH, we also rule out scenarios in which moderately luminous quasar hosts at z>~2 were fully formed bulges that passively fade to the present epoch. On the other hand, 5 quasar hosts at z~1 are consistent with current day MBH-MR relationship after taking into account evolution, appropriate for that of E/S0 galaxies. Therefore, z~1 host galaxies appear to fit the hypothesis they are fully formed early-type galaxies. We also find that most quasar hosts with absolute magnitudes brighter than MR = -23 cannot fade below L* galaxies today, regardless of their stellar population makeup, because their black hole masses are too high and they must arrive at the local MBH-MR relationship by z=0.
We exploit the recent, wide samples of far-infrared (FIR) selected galaxies followed-up in X rays and of X-ray/optically selected active galactic nuclei (AGNs) followed-up in the FIR band, along with the classic data on AGN and stellar luminosity functions at high redshift z>1.5, to probe different stages in the coevolution of supermassive black holes (BHs) and host galaxies. The results of our analysis indicate the following scenario: (i) the star formation in the host galaxy proceeds within a heavily dust-enshrouded medium at an almost constant rate over a timescale ~0.5-1 Gyr, and then abruptly declines due to quasar feedback; over the same timescale, (ii) part of the interstellar medium loses angular momentum, reaches the circum-nuclear regions at a rate proportional to the star formation and is temporarily stored into a massive reservoir/proto-torus wherefrom it can be promptly accreted; (iii) the BH grows by accretion in a self-regulated regime with radiative power that can slightly exceed the Eddington limit L/L_Edd< 4, particularly at the highest redshifts; (iv) for massive BHs the ensuing energy feedback at its maximum exceeds the stellar one and removes the interstellar gas, thus stopping the star formation and the fueling of the reservoir; (v) afterwards, if the latter has retained enough gas, a phase of supply-limited accretion follows exponentially declining with a timescale of about 2 e-folding times. We show that the ratio of the FIR luminosity of the host galaxy to the bolometric luminosity of the AGN maps the various stages of the above sequence. Finally, we discuss how the detailed properties and the specific evolution of the reservoir can be investigated via coordinated, high-resolution observations of starforming, strongly-lensed galaxies in the (sub-)mm band with ALMA and in the X-ray band with Chandra and the next generation X-ray instruments.
Gravitational lensing assists in the detection of quasar hosts by amplifying and distorting the host light away from the unresolved quasar core images. We present the results of HST observations of 30 quasar hosts at redshifts 1 < z < 4.5. The hosts are small in size (r_e <~ 6 kpc), and span a range of morphologies consistent with early-types (though smaller in mass) to disky/late-type. The ratio of the black hole mass (MBH, from the virial technique) to the bulge mass (M_bulge, from the stellar luminosity) at 1<z<1.7 is broadly consistent with the local value; while MBH/M_bulge at z>1.7 is a factor of 3--6 higher than the local value. But, depending on the stellar content the ratio may decline at z>4 (if E/S0-like), flatten off to 6--10 times the local value (if Sbc-like), or continue to rise (if Im-like). We infer that galaxy bulge masses must have grown by a factor of 3--6 over the redshift range 3>z>1, and then changed little since z~1. This suggests that the peak epoch of galaxy formation for massive galaxies is above z~1. We also estimate the duty cycle of luminous AGNs at z>1 to be ~1%, or 10^7 yrs, with sizable scatter.
In the last decades several correlations between the mass of the central supermassive black hole (BH) and properties of the host galaxy - such as bulge luminosity and mass, central stellar velocity dispersion, Sersic index, spiral pitch angle etc. - have been found and point at a coevolution scenario of BH and host galaxy. In this article, I review some of these relations for inactive galaxies and discuss the findings for galaxies that host an active galactic nucleus/quasar. I present the results of our group that finds that active galaxies at $zlesssim 0.1$ do not follow the BH mass - bulge luminosity relation. Furthermore, I show near-infrared integral-field spectroscopic data that suggest that young stellar populations cause the bulge overluminosity and indicate that the host galaxy growth started first. Finally, I discuss implications for the BH-host coevolution.
One of the main challenges in using high redshift active galactic nuclei to study the correlations between the mass of the supermassive Black Hole (MBH) and the properties of their active host galaxies is instrumental resolution. Strong lensing magnification effectively increases instrumental resolution and thus helps to address this challenge. In this work, we study eight strongly lensed active galactic nuclei (AGN) with deep Hubble Space Telescope imaging, using the lens modelling code Lenstronomy to reconstruct the image of the source. Using the reconstructed brightness of the host galaxy, we infer the host galaxy stellar mass based on stellar population models. MBH are estimated from broad emission lines using standard methods. Our results are in good agreement with recent work based on non-lensed AGN, demonstrating the potential of using strongly lensed AGNs to extend the study of the correlations to higher redshifts. At the moment, the sample size of lensed AGN is small and thus they provide mostly a consistency check on systematic errors related to resolution for the non-lensed AGN. However, the number of known lensed AGN is expected to increase dramatically in the next few years, through dedicated searches in ground and space based wide field surveys, and they may become a key diagnostic of black hole and galaxy co-evolution.
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