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Register a new userChandra and MMT observations of low-mass black hole active galactic nuclei accreting at low rates in dwarf galaxies
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W. Yuan
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We report on Chandra X-ray observations of four candidate low-mass black hole (<10^6Msun) active galactic nuclei (AGNs) that have the estimated Eddington ratios among the lowest (~10^(-2)) found for this class. The aims are to validate the nature of their AGNs and to confirm the low Eddington ratios that are derived from the broad H_alpha line, and to explore this poorly studied regime in the AGN parameter space. Among them, two objects with the lowest significance of the broad lines are also observed with Multi-Mirror Telescope, and the high-quality optical spectra taken confirm them as Seyfert 1 AGNs and as having small black hole masses. X-ray emission is detected from the nuclei of two of the galaxies, which is variable on timescales of 10^3s, whereas no significant (or only marginal at best) detection is found for the remaining two. The X-ray luminosities are on the order of 10^(41) ergs/s or even lower, on the order of 10^(40) ergs/s for non-detections, which are among the lowest regimes ever probed for Seyfert galaxies. The low X-ray luminosities, compared to their black hole masses derived from H_alpha, confirm their low accretion rates assuming typical bolometric corrections. Our results hint at the existence of a possibly large population of under-luminous low-mass black holes in the local universe. An off-nucleus ultra-luminous X-ray source (ULX) in one of the dwarf galaxies is detected serendipitously, with a luminosity (6-9)x10^(39) ergs/s in 2-10 keV.
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Nearly every massive galaxy harbors a supermassive black hole (SMBH) in its nucleus. SMBH masses are millions to billions $M_{odot}$, and they correlate with properties of spheroids of their host galaxies. While the SMBH growth channels, mergers and gas accretion, are well established, their origin remains uncertain: they could have either emerged from massive seeds ($10^5-10^6 M_{odot}$) formed by direct collapse of gas clouds in the early Universe or from smaller ($100 M_{odot}$) black holes, end-products of first stars. The latter channel would leave behind numerous intermediate mass black holes (IMBHs, $10^2-10^5 M_{odot}$). Although many IMBH candidates have been identified, none is accepted as definitive, thus their very existence is still debated. Using data mining in wide-field sky surveys and applying dedicated analysis to archival and follow-up optical spectra, we identified a sample of 305 IMBH candidates having masses $3times10^4<M_{mathrm{BH}}<2times10^5 M_{odot}$, which reside in galaxy centers and are accreting gas that creates characteristic signatures of a type-I active galactic nucleus (AGN). We confirmed the AGN nature of ten sources (including five previously known objects which validate our method) by detecting the X-ray emission from their accretion discs, thus defining the first bona fide sample of IMBHs in galactic nuclei. All IMBH host galaxies possess small bulges and sit on the low-mass extension of the $M_{mathrm{BH}}-M_{mathrm{bulge}}$ scaling relation suggesting that they must have experienced very few if any major mergers over their lifetime. The very existence of nuclear IMBHs supports the stellar mass seed scenario of the massive black hole formation.
Low-mass compact galaxies (ultracompact dwarfs [UCDs] and compact ellipticals [cEs]) populate the stellar size-mass plane between globular clusters and early-type galaxies. Known to be formed either in-situ with an intrinsically low mass or resulting from the stripping of a more massive galaxy, the presence of a supermassive or an intermediate-mass black hole (BH) could help discriminate between these possible scenarios. With this aim, we have performed a multiwavelength search of active BH activity, i.e. active galactic nuclei (AGN), in a sample of 937 low-mass compact galaxies (580 UCDs and 357 cEs). This constitutes the largest study of AGN activity in these types of galaxies. Based on their X-ray luminosity, radio luminosity and morphology, and/or optical emission line diagnostic diagrams, we find a total of 11 cEs that host an AGN. We also study for the first time the location of both low-mass compact galaxies (UCDs and cEs) and dwarf galaxies hosting AGN on the BH-galaxy scaling relations, finding that low-mass compact galaxies tend to be overmassive in the BH mass-stellar mass plane but not as much in the BH mass-stellar velocity dispersion correlation. This, together with available BH mass measurements for some of the low-mass compact galaxies, supports a stripping origin for the majority of these objects that would contribute to the scatter seen at the low-mass end of the BH-galaxy scaling relations. However, the differences are too large to be explained solely by this scatter, and thus our results suggest that a flattening at such low-masses is also plausible, happening at a velocity dispersion of ~20-40 km/s.
We present a multi-wavelength study of the active galactic nucleus in the nearby ($D=14.1$ Mpc) low mass galaxy IC 750, which has circumnuclear 22 GHz water maser emission. The masers trace a nearly edge-on, warped disk $sim$0.2 pc in diameter, coincident with the compact nuclear X-ray source which lies at the base of the $sim$kpc-scale extended X-ray emission. The position-velocity structure of the maser emission indicates the central black hole (BH) has a mass less than $1.4 times 10^5~M_odot$. Keplerian rotation curves fitted to these data yield enclosed masses between $4.1 times 10^4~M_odot$ and $1.4 times 10^5~M_odot$, with a mode of $7.2 times 10^4~M_odot$. Fitting the optical spectrum, we measure a nuclear stellar velocity dispersion $sigma_* = 110.7^{+12.1}_{-13.4}$~{rm km~s}$^{-1}.$ From near-infrared photometry, we fit a bulge mass of $(7.3 pm 2.7) times 10^8~M_odot$ and a stellar mass of $1.4 times 10^{10}~M_odot$. The mass upper limit of the intermediate mass black hole in IC 750 falls roughly two orders of magnitude below the $M_{rm BH}-sigma_*$ relation and roughly one order of magnitude below the $M_{rm BH}-M_{rm Bulge}$ and $M_{rm BH}-M_*$ relations -- larger than the relations intrinsic scatters of (0.58 $pm$ 0.09) dex, 0.69 dex, and (0.65 $pm$ 0.09) dex, respectively. These offsets could be due to larger scatter at the low mass end of these relations. Alternatively, black hole growth is intrinsically inefficient in galaxies with low bulge and/or stellar masses, which causes the black holes to be under-massive relative to their hosts, as predicted by some galaxy evolution simulations.
Optical spectra and images taken with the Baade 6.5 meter Magellan telescope confirm that 2XMM J123103.2+110648, a highly variable X-ray source with an unusually soft spectrum, is indeed associated with a type 2 (narrow-line) active nucleus at a redshift of z = 0.11871. The absence of broad Halpha or Hbeta emission in an otherwise X-ray unabsorbed source suggests that it intrinsically lacks a broad-line region. If, as in other active galaxies, the ionized gas and stars in J1231+1106 are in approximate virial equilibrium, and the black hole mass versus stellar velocity dispersion relation holds, the exceptionally small velocity dispersion of 33.5 km/s for [O III] 5007 implies that the black hole mass is approximately 10^5 solar masses, among the lowest ever detected. Such a low black hole mass is consistent with the general characteristics of the host, a small, low-luminosity, low-mass disk galaxy. We estimate the Eddington ratio of the black hole to be > 0.5, in good agreement with expectations based on the X-ray properties of the source.
Supermassive black holes (SMBHs) have been found to be ubiquitous in the nuclei of early-type galaxies and of bulges of spirals. There are evidences of a tight correlation between the SMBH masses, the velocity dispersions of stars in the spheroidal components galaxies and other galaxy properties. Also the evolution of the luminosity density due to nuclear activity is similar to that due to star formation. All that suggests an evolutionary connection between Active Galactic Nuclei (AGNs) and their host galaxies. After a review of these evidences this lecture discusses how AGNs can affect the host galaxies. Other feedback processes advocated to account for the differences between the halo and the stellar mass functions are also briefly introduced.
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