SHAO Astrophysics Colloquium

Title: Black hole accretion in the present-day and early universe

Speaker: Kohei Inayoshi  (KIAA/Peking Univ)

Time: 10:00 am, January 18 (Friday)

Location: Lecture Hall, 3rd floor

Abstract: 

Supermassive black holes (SMBHs) are almost ubiquitously harbored at the centers of massive nearby galaxies. The existence of SMBHs is consistent with the number and energetics of bright quasars, which are associated with efficient gas accretion onto SMBHs. Among them, the SMBH population in the early universe (z>6) provides us an important constraint on their formation process and requires rapid growth of their seeds. In the present-day universe, however only a few percent of SMBHs are observed as luminous active galactic nuclei (AGN). A majority of them are nearly quiescent and known as low luminosity AGN. We study the two different accretion domains performing 1D/2D radiation-hydrodynamical simulations. For the higher-rate case, we find the global accretion solutions from outside the Bondi radius at super-Eddington accretion rates (>500 Mdot_Edd), unimpeded by radiation feedback. We apply this result to SMBHs embedded in protogalaxies and discuss their subsequent growth. For the lower-rate case (< Mdot_Edd), the BH accretion is either suppressed due to convective motion (L<10^-7 L_Edd) or led through a cold, geometrically-thin disk (L~10^-3 L_Edd). The transitional behavior of accreting BHs in galactic nuclei naturally explain (1) the reason for the offset between the observed luminosities and theoretical predictions for nearby quiescent SMBHs, and (2) the conditions to fuel gas into the nuclear SMBH.

Special Astrophysics Colloquium

Title: Demystifying the Diverse IR SEDs of Type-1 AGNs from z~0 to z~6

Speaker: Jianwei Lyu (University of Arizona)

Location:  the middle conference room , 3rd floor

Time: 10:00 am, January 16th (Wednesday)

Abstract:

To gain insights into the nature of AGN dust environment and explore its possible evolution, we have conducted a systematic research to reveal the connections behind the variations of AGN SEDs through a comparative study of local AGNs with high-z objects, covering broad ranges of luminosity (LAGN~108 – 1014 L⊙) and redshift (z~0–6) (Lyu, Rieke & Alberts 2016, Lyu, Rieke & Shi 2017; Lyu & Rieke 2017; Lyu & Rieke 2018). We have shown a “minimalist” two-free-parameter semi-empirical model is good enough to reconcile the IR SED shapes of all major populations of type-1 AGNs, including local Seyfert-1 nuclei, normal blue quasars, extremely red quasars at z ？2–3 (Ross et al. 2015), AGNs with mid-IR excess emission at z ？0.7–2 (Xu et al. 2015), hot dust-obscured galaxies at z ？1.5–4 (Eisenhardt et al. 2012), and dust-free quasars at z ？ 6 (Jiang et al. 2010). In fact, the commonly seen UV-optical extinction, the unexpected strong mid-IR polar dust emission, and the poorly-understood near- to mid-IR SED variations in type-1 AGNs can be physically connected in a similar fashion. It is highly likely that most type-1 AGNs, regardless of their luminosities and redshifts, share similar torus properties but differ mainly due to the extended (sub-kpc to kpc scales) polar dust component. By grouping similar SED behaviors together and summarizing observations in the literature, we have proposed that the AGN environment could be grouped into three general categories, in which feedbacks from the AGN and/or the host galaxy would influence the behavior of the polar dust component. The possibly frequent appearance of the AGN polar dust emission greatly complicates the interpretation of AGN IR properties and undermines the validity of estimating the torus dust covering factor from SEDs. The templates developed from this work can be generally applied in e.g., SED decompositions of galaxies with possible AGN contribution, IR AGN selections and the search for AGN candidates with strong polar dust emission.