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Register a new userThe host galaxies of z=7 quasars: predictions from the BlueTides simulation
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We examine the properties of the host galaxies of $z=7$ quasars using the large volume, cosmological hydrodynamical simulation BlueTides. We find that the 10 most massive black holes and the 191 quasars in the simulation (with $M_{textrm{UV,AGN}}<M_{textrm{UV,host}}$) are hosted by massive galaxies with stellar masses $log(M_ast/M_odot)=10.8pm0.2$, and $10.2pm0.4$, which have large star formation rates, of $513substack{+1225 -351}M_odot/rm{yr}$ and $191substack{+288 -120}M_odot/rm{yr}$, respectively. The hosts of the most massive black holes and quasars in BlueTides are generally bulge-dominated, with bulge-to-total mass ratio $B/Tsimeq0.85pm0.1$, however their morphologies are not biased relative to the overall $z=7$ galaxy sample. We find that the hosts of the most massive black holes and quasars are significantly more compact, with half-mass radii $R_{0.5}=0.41substack{+0.18 -0.14}$ kpc and $0.40substack{+0.11 -0.09}$ kpc respectively; galaxies with similar masses and luminosities have a wider range of sizes with a larger median value, $R_{0.5}=0.71substack{+0.28 -0.25}$ kpc. We make mock James Webb Space Telescope (JWST) images of these quasars and their host galaxies. We find that distinguishing the host from the quasar emission will be possible but still challenging with JWST, due to the small sizes of quasar hosts. We find that quasar samples are biased tracers of the intrinsic black hole--stellar mass relation, following a relation that is 0.2 dex higher than that of the full galaxy sample. Finally, we find that the most massive black holes and quasars are more likely to be found in denser environments than the typical $M_{textrm{BH}}>10^{6.5}M_odot$ black hole, indicating that minor mergers play at least some role in growing black holes in the early Universe.
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The bright emission from high-redshift quasars completely conceals their host galaxies in the rest-frame ultraviolet/optical, with detection of the hosts in these wavelengths eluding even the Hubble Space Telescope (HST) using detailed point spread function (PSF) modelling techniques. In this study we produce mock images of a sample of z=7 quasars extracted from the BlueTides simulation, and apply Markov Chain Monte Carlo-based PSF modelling to determine the detectability of their host galaxies with the James Webb Space Telescope (JWST). While no statistically significant detections are made with HST, we predict that at the same wavelengths and exposure times JWST NIRCam imaging will detect ~50% of quasar host galaxies. We investigate various observational strategies, and find that NIRCam wide-band imaging in the long-wavelength filters results in the highest fraction of successful quasar host detections, detecting >80% of the hosts of bright quasars in exposure times of 5 ks. Exposure times of ~5 ks are required to detect the majority of host galaxies in the NIRCam wide-band filters, however even 10 ks exposures with MIRI result in <30% successful host detections. We find no significant trends between galaxy properties and their detectability. The PSF modelling can accurately recover the host magnitudes, radii, and spatial distribution of the larger-scale emission, when accounting for the central core being contaminated by residual quasar flux. Care should be made when interpreting the host properties measured using PSF modelling.
Supermassive blackholes with masses of a billion solar masses or more are known to exist up to $z=7$. However, the present-day environments of the descendants of first quasars is not well understood and it is not known if they live in massive galaxy clusters or more isolated galaxies at $z=0$. We use a dark matter-only realization (BTMassTracer) of the BlueTides cosmological hydrodynamic simulation to study the halo properties of the descendants of the most massive black holes at $z=8$. We find that the descendants of the quasars with most massive black holes are not amongst the most massive halos. They reside in halos of with group-like ($sim 10^{14}M_{odot}$) masses, while the most massive halos in the simulations are rich clusters with masses $sim 10^{15} M_{odot}$. The distribution of halo masses at low redshift is similar to that of the descendants of least massive black holes, for a similar range of halo masses at $z=8$, which indicates that they are likely to exist in similar environments. By tracing back to the $z = 8$ progenitors of the most massive (cluster sized) halos at $z=0$; we find that their most likely black hole mass is less than $10^7 M_{odot}$; they are clearly not amongst the most massive black holes. We also provide estimates for the likelihood of finding a high redshift quasar hosting a black hole with masses above $10^{7} M_{odot}$ for a given halo mass at $z=0$. For halos above $10^{15} M_{odot}$, there is only $20 %$ probability that their $z=8$ progenitors hosted a black hole with mass above $10^{7} M_{odot}$.
We employ the very large cosmological hydrodynamical simulation BLUETIDES to investigate the predicted properties of the galaxy population during the epoch of reionisation ($z>8$). BLUETIDES has a resolution and volume ($(400/happrox 577)^{3},{rm cMpc^3}$) providing a population of galaxies which is well matched to depth and area of current observational surveys targeting the high-redshift Universe. At $z=8$ BLUETIDES includes almost 160,000 galaxies with stellar masses $>10^{8},{rm M_{odot}}$. The population of galaxies predicted by BLUETIDES closely matches observational constraints on both the galaxy stellar mass function and far-UV ($150,{rm nm}$) luminosity function. Galaxies in BLUETIDES are characterised by rapidly increasing star formation histories. Specific star formation rates decrease with redshift though remain largely insensitive to stellar mass. As a result of the enhanced surface density of metals more massive galaxies are predicted to have higher dust attenuation resulting in a significant steepening of the observed far-UV luminosity function at high luminosities. The contribution of active SMBHs to the UV luminosities of galaxies with stellar masses $10^{9-10},{rm M_{odot}}$ is around $3%$ on average. Approximately $25%$ of galaxies with $M_{*}approx 10^{10},{rm M_{odot}}$ are predicted to have active SMBH which contribute $>10%$ of the total UV luminosity.
The most distant known quasar recently discovered by Ba~nados et al. (2018) is at $z=7.5$ (690 Myr after the Big Bang), at the dawn of galaxy formation. We explore the host galaxy of the brightest quasar in the large volume cosmological hydrodynamic simulation BlueTides, which in Phase II has reached these redshifts. The brightest quasar in BlueTides has a luminosity of a $sim$ few $10^{13} L_{odot}$ and a black hole mass of $6.4 times 10^{8} M_{odot}$ at $z sim 7.5$, comparable to the observed quasar (the only one in this large volume). The quasar resides in a rare halo of mass $M_{H} sim 10^{12} M_{odot}$ and has a host galaxy of stellar mass of $4 times 10^{10}M_{odot}$ with an ongoing (intrinsic) star formation rate of $sim 80 M_{odot} yr^{-1}$. The corresponding intrinsic UV magnitude of the galaxy is $-23.1$, which is roughly $2.7$ magnitudes fainter than the quasars magnitude of $-25.9$. We find that the galaxy is highly metal enriched with a mean metallicity equal to the solar value. We derive quasar and galaxy spectral energy distribution (SED) in the mid and near infrared JWST bands. We predict a significant amount of dust attenuation in the rest-frame UV corresponding to $A_{1500} sim 1.7$ giving an UV based SFR of $sim 14 M_{odot} yr^{-1}$. We present mock JWST images of the galaxy with and without central point source, in different MIRI and NIRCam filters. The host galaxy is detectable in NIRCam filters, but it is extremely compact ($R_{E}=0.35$ kpc). It will require JWSTs exquisite sensitivity and resolution to separate the galaxy from the central point source. Finally within the FOV of the quasar in BlueTides there are two more sources that would be detectable by JWST.
The discovery of luminous quasars at redshifts up to 7.5 demonstrates the existence of several billion M_sun supermassive black holes (SMBHs) less than a billion years after the Big Bang. They are accompanied by intense star formation in their host galaxies, pinpointing sites of massive galaxy assembly in the early universe, while their absorption spectra reveal an increasing neutral intergalactic medium (IGM) at the epoch of reionization. Extrapolating from the rapid evolution of the quasar density at z=5-7, we expect that there is only one luminous quasar powered by a billion M_sun SMBH in the entire observable universe at z~9. In the next decade, new wide-field, deep near-infrared (NIR) sky surveys will push the redshift frontier to the first luminous quasars at z~9-10; the combination with new deep X-ray surveys will probe fainter quasar populations that trace earlier phases of SMBH growth. The identification of these record-breaking quasars, and the measurements of their BH masses and accretion properties require sensitive spectroscopic observations with next generation of ground-based and space telescopes at NIR wavelengths. High-resolution integral-field spectroscopy at NIR, and observations at millimeter and radio wavelengths, will together provide a panchromatic view of the quasar host galaxies and their galactic environment at cosmic dawn, connecting SMBH growth with the rise of the earliest massive galaxies. Systematic surveys and multiwavelength follow-up observations of the earliest luminous quasars will strongly constrain the seeding and growth of the first SMBHs in the universe, and provide the best lines of sight to study the history of reionization.
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