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Register a new userThe XMM-Newton view of AGN with intermediate mass black holes
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We have observed with XMM-Newton four radiatively efficient active type 1 galaxies with black hole masses < 10^6 Msun, selected optically from the SDSS. We show here that their soft X-ray spectrum exhibits a soft excess with the same characteristics as that observed ubiquitously in radio-quiet Seyfert 1 galaxies and type 1 quasars, both in terms of temperatures and strength. However, even when the soft X-ray excess is modelled with a pure thermal disc, its luminosity turns out to be much lower than that expected from accretion theory for the given temperature, casting further doubts on a thermal interpretation for soft excesses. While alternative scenarios for the nature of the soft excess (namely smeared ionized absorption and disc reflection) cannot be distinguished on a pure statistical basis, we point out that the absorption model produces a strong correlation between absorbing column density and ionization state, which may be difficult to interpret and is most likely spurious. As for reflection, it does only invoke standard ingredients of any accretion model for radiatively efficient sources such as a hard X-rays source and a relatively cold (though partially ionized) accretion disc, and therefore seems the natural choice to explain the soft excess in most (if not all) cases. The reflection model is also consistent with the additional presence of a thermal disc component with the theoretically expected temperature (although, again, with smaller-than-expected total luminosity). The observed active galaxies are among the most variable in X-rays and their excess variance is among the largest. This is in line with their relatively small black hole mass and with expectations from simple power spectra models. (abridged)
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We describe ongoing searches for intermediate-mass black holes with M_BH ~ 100-10^5 M_sun. We review a range of search mechanisms, both dynamical and those that rely on accretion signatures. We find that dynamical and accretion signatures alike point to a high fraction of 10^9-10^10 M_sun galaxies hosting black holes with M_BH<10^5 M_sun. In contrast, there are no solid detections of black holes in globular clusters. There are few observational constraints on black holes in any environment with M_BH ~ 100-10^4 M_sun. Considering low-mass galaxies with dynamical black hole masses and constraining limits, we find that the M_BH-sigma_* relation continues unbroken to M_BH~10^5 M_sun, albeit with large scatter. We believe the scatter is at least partially driven by a broad range in black hole mass, since the occupation fraction appears to be relatively high in these galaxies. We fold the observed scaling relations with our empirical limits on occupation fraction and the galaxy mass function to put observational bounds on the black hole mass function in galaxy nuclei. We are pessimistic that local demographic observations of galaxy nuclei alone could constrain seeding mechanisms, although either high-redshift luminosity functions or robust measurements of off-nuclear black holes could begin to discriminate models.
Observational evidence has been mounting for the existence of intermediate mass black holes (IMBHs, 10^2-10^5 Msun), but observing them at all, much less constraining their masses, is very challenging. In one theorized formation channel, IMBHs are the seeds for supermassive black holes in the early universe. As a result, IMBHs are predicted to exist in the local universe in dwarf galaxies, as well as wandering in more massive galaxy halos. However, these environments are not conducive to the accretion events or dynamical signatures that allow us to detect IMBHs. The Laser Interferometer Space Antenna (LISA) will demystify IMBHs by detecting the mergers of these objects out to extremely high redshifts, while measuring their masses with extremely high precision. These observations of merging IMBHs will allow us to constrain the formation mechanism and subsequent evolution of massive black holes, from the dark ages to the present day, and reveal the role that IMBHs play in hierarchical galaxy evolution.
We report on the nuclear X-ray properties of the radio galaxy NGC 6251 observed with XMM-Newton. NGC 6251 is a well-known radio galaxy with intermediate FRI/II radio properties. It is optically classified as a Seyfert 2 and hosts a supermassive black hole with mass~6e8 solar masses. The 0.4-10 keV EPIC pn continuum is best fitted by two thermal components (kT~0.5 and 1.4 keV, respectively), plus a power law with photon index ~1.9 absorbed by a column density NH~5e20 cm-2. We confirm the previous ASCA detection of a strong iron line. The line, resolved in the EPIC pn spectrum, is adequately fitted with a broad (sigma~0.6 keV) Gaussian at rest-frame energy 6.4 keV with EW 220 eV. We also detect, for the first time, short-term, low-amplitude variability of the nuclear flux on a timescale of a few ks. The spectral properties argue in favor of the presence of a standard accretion disk, ruling out the base of the jet as the sole origin of the X-rays. The moderate X-ray luminosity and lack of strong intrinsic absorption suggest that NGC 6251 is a ``pure type 2 AGN which lacks a broad-line region.
For a sample of nine Galactic globular clusters we measured the inner kinematic profiles with integral-field spectroscopy that we combined with existing outer kinematic measurements and HST luminosity profiles. With this information we are able to detect the crucial rise in the velocity-dispersion profile which indicates the presence of a central black hole. In addition, N-body simulations compared to our data will give us a deeper insight in the properties of clusters with black holes and stronger selection criteria for further studies. For the first time, we obtain a homogeneous sample of globular cluster integral- field spectroscopy which allows a direct comparison between clusters with and without an intermediate-mass black hole.
Intermediate-mass black holes (IMBHs), with masses in the range $100-10^{6}$ M$_{odot}$, are the link between stellar-mass BHs and supermassive BHs (SMBHs). They are thought to be the seeds from which SMBHs grow, which would explain the existence of quasars with BH masses of up to 10$^{10}$ M$_{odot}$ when the Universe was only 0.8 Gyr old. The detection and study of IMBHs has thus strong implications for understanding how SMBHs form and grow, which is ultimately linked to galaxy formation and growth, as well as for studies of the universality of BH accretion or the epoch of reionisation. Proving the existence of seed BHs in the early Universe is not yet feasible with the current instrumentation; however, those seeds that did not grow into SMBHs can be found as IMBHs in the nearby Universe. In this review I summarize the different scenarios proposed for the formation of IMBHs and gather all the observational evidence for the few hundreds of nearby IMBH candidates found in dwarf galaxies, globular clusters, and ultraluminous X-ray sources, as well as the possible discovery of a few seed BHs at high redshift. I discuss some of their properties, such as X-ray weakness and location in the BH mass scaling relations, and the possibility to discover IMBHs through high velocity clouds, tidal disruption events, gravitational waves, or accretion disks in active galactic nuclei. I finalize with the prospects for the detection of IMBHs with up-coming observatories.
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