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Growth of massive black holes in the early universe
Growth of massive black holes in the early universe
QSONG (Quasar Spectroscopic Observations with Near-IR Grism) was one of the AKARI space telescope's major mission program (2006-2010), pointing 155 quasars in the distant universe (3.3<z<6.4). Taking rest-frame optical spectra (or listening to the songs from the quasars in frequency space) for 154 sources in the Grism mode (R~120) and 11 in the Prism mode (R~20), we obtained 43 black hole mass estimates using the well calibrated H-alpha line, in the order of 10 billion solar masses (Jun et al. 2015a). These values indicate the most massive black hole growth has taken place up to z=5 or beyond, and hints early/vigorous black hole growth compared to the mass assembly of the most massive galaxies. As a follow-up study with NASA's IRTF for 26 quasars at 0.7<z<2.5 with higher resolution spectra (Jun et al. 2017), we discovered that black hole mass estimates exceeding 10 billion solar masses often display double-peaked profiles for Balmer lines, and broad/blueshifted profiles for CIV lines, so that the current best estimate for the most massive limit is ~10 billion solar mass.
Since then, JWST (James Webb Space Telescope) with an unprecedented sensitivity led to discovering the properties of the high redshift quasars. I have participated in tracking the cosmic reionization, measuring outflows, and locating the environments. Now, I look forward to using SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer) low-resolution spectra for the brighter quasar population to do the same thing.
AGN evolution
AGN evolution
AGN sitting in the center of most galaxies, are often obscured by gas and dust in the form of a central pc-scale dusty structure, and a kpc-scale host galaxy. Following the discovery of quasars almost without infrared emission (Jiang et al. 2010), we additionally identified 233 dust-poor quasars out of 41,000 SDSS quasars matched with near- and mid-IR surveys. Being ~3 sigma outliers in the distribution of IR-to-optical flux ratios, dust-poor quasars were found more frequent with higher accretion rates and at higher redshift, at given luminosity. This indicates that dust-poor quasars are rapidly growing in the early universe with little dust surrounding, or are simply viewed face-on as to explain their narrower observed line widths. We will further tell between geometric and evolutionary scenarios for these objects, using rest-frame optical spectroscopy of z~3.5 sources with Gemini/GNIRS (2020-2021).
Contrary to dust-poor quasars, Hot DOGs (Hot Dust-Obscured Galaxies) were identified using the same IR survey data (Eisenhardt et al. 2012, Wu et al. 2012), showing the largest amounts of extinction with E(B-V) several to a few tens of magnitudes. The rest-frame UV-optical spectra for 12 Hot DOGs at z~2 reveals that Hot DOGs are narrow-lined AGN with large amounts of outflowing gas as shown by their broad/blueshifted profiles in various spatial scales and chemical species (Jun et al. 2020).
This hints that luminous quasars provide strong feedback to their surrounding, with enough kinetic energy delivered to their host galaxies to quench star formation. We showed that the relative contribution of outflows should be dominating over direct expansion of obscuring material by radiation pressure, when the AGN accretion reaches quasar luminosity (Jun et al. 2021a).
AGN variability
AGN variability
Thanks to wide-field, moderate-cadence nature of IR space observatories (WISE, and SPHEREx to come), we were able to reproduce the optically periodic nature of PG 1302-102 in the mid-IR (Jun et al. 2015b), with a time-lag best explained by a light-travel-time to the dusty structure surrounding the central accretion disk. In addition to measuring periodic light curves, we are currently measuring highly-variable mid-IR light curves as well as performing spectro-photometric followup to understand their nature.
AGN hosts and environment
AGN hosts and environment
Using the Spitzer space telescope's sensitivity from space, we constructed the fundamental plane - the planar relation between the size, mean surface brightness, and velocity dispersion of 56 local early-type galaxies (Jun & Im 2008), which host supermassive black holes. This relation gives the slope of the dynamical mass-to-light ratio versus luminosity, which is found closer to a constant in the mid-IR compared to the optical/near-IR, suggesting that the mid-IR luminosity is a better tracer of galaxy mass.
To understand the galaxy environment of AGN, we are working on a pilot field of a z=1.47 extremely massive quasar, PG 1630+377, using UKIRT broad- and narrow-band filter observations. The redshifted H-alpha line from star-forming galaxies will show an excess of narrow-band fluxes over the broad, being H-alpha emitters (HAEs). The number density map of luminous HAEs shows a structure with overdensity delta=6.6, ~2.1Mpc from the massive quasar, which is the strongest overdensity in the ~500 arcmin^2 survey area. Further analysis of the entire survey is needed, but this case demonstrates the likelihood of massive quasars being signposts for distant proto-clusters (Jun et al. 2021b).
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