Low-mass Black Holes And Their Host Galaxies

Supermassive black holes (SMBHs), with masses in the range of106-1010M☉as measured via stellar and gas kinematics, have been convincingly inferred to be present in the centers of nearby inactive massive galaxies. Moreover, there are tight relations between the BH mass (MBH) and the properties of the spheroidal component (namely, ellipticals and the classical bulges of disk galaxies). How do these SMBHs grow from "seeds" and physically connected with their host galaxies? To answer these questions, we have conducted some study on the low-mass black holes and their host galaxies.We have selected a sample of Seyfert1galaxies with low-mass BHs from Sloan Digital Sky Survey Data Release4, which is much more complete at the end of low Ed-dington ratio comparing with previous studies. We present a detailed multiwavelength study of the nuclear and host galaxy properties of UM625in the sample. The broad Ha line and luminosity indicates a low black hole mass of1.6×106M☉. A comprehensive analysis of SDSS and Hubble Space Telescope (HST) images shows that UM625is a nearly face-on SO galaxy with a pseudobulge. The extended disk is featureless, but the central～150-400pc contains a conspicuous semi-ring of bright, blue star-forming knots, whose integrated ultraviolet (UV) luminosity suggests a star formation rate of～0.3M☉yr-1. The broad-band spectral energy distribution constrains the bolometric luminosity to Lbol≈(0.5-3)×1043erg s-1and Lbol/LEdd≈0.02-0.15. Chan-dra and XMM-Newton X-ray data suggests that this system contains a heavily absorbed and intrinsically X-ray weak (αox=-1.72) nucleus. Although not strong enough to qualify as radio-loud, UM625does belong to a minority of low-mass AGNs detected in the radio. This is the fourth broad-band SED study about low-mass BHs, following NGC4395, POX52and NGC4051. The somewhat special X-ray and radio properties of UM625indicate the diversity of AGNs with low-mass BHs. Similar to other low-mass BHs, the mass of the central black hole roughly agrees with the value predicted from its bulge velocity dispersion but is significantly lower than that expected from its bulge luminosity.We have also tried to search for candidates of SMBH "seeds" from local Universe taking advantage of multi-wavelength archival data. We discovered an X-ray point source detected by XMM-Newton in the center of nearby bulgeless galaxy NGC3319. The UV image taken by HST/FOC also reveals an unresolved source. There is no bulge component in this galaxy at all, including pseudobulge, but a compact nulcear star clus-ter (NSC) appears in the center, which is very common in late-type galaxies. The spectra from ground-based telescope don’t show any obvious sign of AGN, totally dominated by stellar light. Assuming that the X-ray and UV emission come from the same source, the SED looks pretty like a low luminosity AGN yet with high Eddington rate and the MBH will be predicted as low as a few thousands of solar mass, possibly the smallest black holes ever found in galactic centers. However, all of these evidence is not di-rect and conclusive. The X-ray source could be actually an off-nuclear ULX due to the limitation of resolution; the UV emission can also be contaminated by the young stars embedded in the NSC. Further Chandra observation is necessary to pinpoint the loca-tion of the X-ray source. As a strong barred galaxy without (pseudo)bulge, it’s an ideal prototype for us to explore the co-evolution, if there exists, between SMBHs and their host galaxies at the very eary stage.Narrow-line Seyfert1galaxies (NLSls) are generally considered as young AGNs with relatively low-mass BHs yet high accretion rate, that is in the rapid-growth phase. Previous studies show that radio-loud NLSls (RL-NLSl) are very rare and their radio origin is controversial. Using newly released data from the Wide-field Infrared Sur-vey Explorer, we have, for the first time, systematically studied the infrared variability of RL-NLSls. Two objects show intraday variability, while another one has a longer measurable timescale within180days. The detection of intraday variability restricts the size of the infrared-emitting region to10-3pc, significantly smaller than the scale of the torus but consistent with the base of a jet. Together with the fact that y-ray de-tection by Fermi, our results strongly confirm the view that at least some RL-NLsls are blazars with a relativistic jet close to our line of sight. The beamed synchrotron emission from the jet contributes significantly to and probably dominates the spectra in the infrared. With updated catalog of RL-NLSls and newest WISE source catalog, we have discoved two more objects displaying infrared variability. On the other hand, there’re eight sources in total detected in γ-ray as of now. All of these observations prove that intense relativistic jets can be launched when BHs are still young, challeng-ing our knowledge of the jet production mechanism. Meanwhile, AGN feedback seems more popular than generally believed.In order to understand further the details of co-evolution of BHs, both for su-permassive and low-mass ones, and their host galaxies, we’re going to carry on more projects about this subject. It includes but not limits to Integral field spectroscopy ob-servations of gas kinematics around (low-mass) BHs; the host galaxies of RL-NLSls; the evolutionary link between type1and type2AGNs.