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Register a new userLow-Level Nuclear Activity in Nearby Spiral Galaxies
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Himel Ghosh
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We are conducting a search for supermassive black holes (SMBHs) with masses below 10^7 M_sun by looking for signs of extremely low-level nuclear activity in nearby galaxies that are not known to be AGNs. Our survey has the following characteristics: (a) X-ray selection using the Chandra X-ray Observatory, since x-rays are a ubiquitous feature of AGNs; (b) Emphasis on late-type spiral and dwarf galaxies, as the galaxies most likely to have low-mass SMBHs; (c) Use of multiwavelength data to verify the source is an AGN; and (d) Use of the highest angular resolution available for observations in x-rays and other bands, to separate nuclear from off-nuclear sources and to minimize contamination by host galaxy light. Here we show the feasibility of this technique to find AGNs by applying it to six nearby, face-on spiral galaxies (NGC 3169, NGC 3184, NGC 4102, NGC 4647, NGC 4713, NGC 5457) for which data already exist in the Chandra archive. All six show nuclear x-ray sources. The data as they exist at present are ambiguous regarding the nature of the nuclear x-ray sources in NGC 4713 and NGC 4647. We conclude, in accord with previous studies, that NGC 3169 and NGC 4102 are almost certainly AGNs. Most interestingly, a strong argument can be made that NGC 3184 and NGC 5457, both of type Scd, host AGNs.
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We study the effects of bulge elongation on the star formation activity in the centers of spiral galaxies using the data from the Sloan Digital Sky Survey Data Release 7. We construct a volume-limited sample of face-on spiral galaxies with $M_r < -$19.5 mag at 0.02 $leq z <$ 0.055 by excluding barred galaxies, where the aperture of the SDSS spectroscopic fibre covers the bulges of the galaxies. We adopt the ellipticity of bulges measured by Simard et al. (2011) who performed two-dimensional bulge+disc decompositions using the SDSS images of galaxies, and identify nuclear starbursts using the fibre specific star formation rates derived from the SDSS spectra. We find a statistically significant correlation between bulge elongation and nuclear starbursts in the sense that the fraction of nuclear starbursts increases with bulge elongation. This correlation is more prominent for fainter and redder galaxies, which exhibit higher ratios of elongated bulges. We find no significant environmental dependence of the correlation between bulge elongation and nuclear starbursts. These results suggest that non-axisymmetric bulges can efficiently feed the gas into the centre of galaxies to trigger nuclear starburst activity.
We have studied the X-ray nuclear activity of 187 nearby (distance < 15 Mpc) galaxies observed with Chandra/ACIS. We found that 86 of them have a point-like X-ray core, consistent with an accreting black hole (BH). We argue that the majority of them are nuclear BHs, rather than X-ray binaries. The fraction of galaxies with an X-ray detected nuclear BH is higher (~60 per cent) for ellipticals and early-type spirals (E to Sb), and lower (~30 per cent) for late-type spirals (Sc to Sm). There is no preferential association of X-ray cores with a large-scale bar; in fact, strongly barred galaxies appear to have slightly lower detection fraction and luminosity for their nuclear X-ray sources, compared with non-barred or weakly barred galaxies of similar Hubble types. The cumulative luminosity distribution of the nuclear sources in the 0.3-8 keV band is a power-law with slope ~-0.5, from ~2 x 10^{38} erg/s to ~10^{42} erg/s. The Eddington ratio is lower for ellipticals (L_{X}/L_{Edd} ~ 10^{-8}) and higher for late-type spirals (up to L_{X}/L_{Edd} ~ 10^{-4}), but in all cases, the accretion rate is low enough to be in the radiatively-inefficient regime. The intrinsic NH is generally low, especially for the less luminous sources: there appear to be no Type-2 nuclear BHs at luminosities <~ 10^{39} erg/s. The lack of a dusty torus or of other sources of intrinsic absorption (e.g., an optically-thick disk wind) may be directly related to the lack of a standard accretion disk around those faint nuclear BHs. The fraction of obscured sources increases with the nuclear BH luminosity: 2/3 of the sources with L_{X} > 10^{40} erg/s have a fitted NH > 10^{22} cm^{-2}. This is contrary to the declining trend of the obscured fraction with increasing luminosities, observed in more luminous AGN and quasars.
Dust enshrouded activity can ideally be studied by mid-infrared (MIR) observations. In order to explore the AGN versus star forming origin of the nuclear MIR emission of galaxies, observations of high spatial resolution are required. Here we report on 11.3mic. observations with VISIR at the VLT, reaching 0.35 spatial resolution (FWHM). During the scientific verification of VISIR we have observed a sample of 36 nearby galaxies having a variety of optically classified nuclear activity: 17 black hole driven active galactic nuclei (AGN), 10 starbursts (SBs) and 9 quiet spirals. 16/17 AGN are detected and unresolved, 5/10 SBs are detected and resolved with structured emission up to a few arcsec, while for 5/10 SB and all 9 quiet nuclei low upper limits are provided. The morphology of the resolved SB nuclei follows that seen at radio frequencies. The compactness of AGN and the extent of the SB nuclei is consistent with predictions from radiative transfer models and with MIR spectra of lower spatial resolution. We explore the nuclear MIR surface brightness as a quantitative measure. While AGN and SB cannot be distinguished with MIR data from 4m class telescopes, our data provide evidence that, up to a distance of 100 Mpc, AGN and SB can well be separated by means of MIR surface brightness when using 8m class telescopes.
In this short write-up, I will concentrate on a few topics of interest. In the 1970s I found very extended HI disks in galaxies such as NGC 5055 and NGC 2841, out to 2 - 2.5 times the Holmberg radius. Since these galaxies are warped, a tilted ring model allows rotation curves to be derived, and evidence for dark matter to be found. The evaluation of the amount of dark matter is hampered by a disk-halo degeneracy, which can possibly be broken by observations of velocity dispersions in both the MgI region and the CaII region.
We have performed an X-ray study of the nearby barred spiral galaxy NGC1672, primarily to ascertain the effect of the bar on its nuclear activity. We use both Chandra and XMM-Newton observations to investigate its X-ray properties, together with supporting high-resolution optical imaging data from the Hubble Space Telescope (HST), infrared imaging from the Spitzer Space Telescope, and ATCA ground-based radio data. We detect 28 X-ray sources within the D25 area of the galaxy, many of which correlate spatially with star-formation in the bar and spiral arms, while two are identified as background galaxies in the HST images. Nine of the X-ray sources are ULXs, with the three brightest (LX > 5E39 erg/s) located at the ends of the bar. With the spatial resolution of Chandra, we are able to show for the first time that NGC1672 possesses a hard (Gamma~1.5) nuclear X-ray source with a 2-10 keV luminosity of 4E38 erg/s. This is surrounded by an X-ray bright circumnuclear star-forming ring, comprised of point sources and hot gas, which dominates the 2-10 keV emission in the central region of the galaxy. The spatially resolved multiwavelength photometry indicates that the nuclear source is a low-luminosity AGN (LLAGN), but with star formation activity close to the central black hole. A high-resolution multiwavelength survey is required to fully assess the impact of both large-scale bars and smaller-scale phenomena such as nuclear bars, rings and nuclear spirals on the fueling of LLAGN.
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