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Radio Sources in Low-Luminosity Active Galactic Nuclei

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Radio Sources in Low-Luminosity Active Galactic Nuclei A&A 392, 53–82 (2002) Astronomy DOI: 10.1051/0004-6361:20020874 & c ESO 2002 Astrophysics Radio sources in low-luminosity active galactic nuclei III. “AGNs” in a distance-limited sample of “LLAGNs” N. M. Nagar1, H. Falcke2,A.S.Wilson3, and J. S. Ulvestad4 1 Arcetri Observatory, Largo E. Fermi 5, Florence 50125, Italy 2 Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, 53121 Bonn, Germany e-mail: [email protected] 3 Department of Astronomy, University of Maryland, College Park, MD 20742, USA Adjunct Astronomer, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA e-mail: [email protected] 4 National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801, USA e-mail: [email protected] Received 23 January 2002 / Accepted 6 June 2002 Abstract. This paper presents the results of a high resolution radio imaging survey of all known (96) low-luminosity active galactic nuclei (LLAGNs) at D ≤ 19 Mpc. We first report new 2 cm (150 mas resolution using the VLA) and 6 cm (2 mas resolution using the VLBA) radio observations of the previously unobserved nuclei in our samples and then present results on the complete survey. We find that almost half of all LINERs and low-luminosity Seyferts have flat-spectrum radio cores when observed at 150 mas resolution. Higher (2 mas) resolution observations of a flux-limited subsample have provided a 100% (16 of 16) detection rate of pc-scale radio cores, with implied brightness temperatures ∼>108 K. The five LLAGNs with the highest core radio fluxes also have pc-scale “jets”. Compact radio cores are almost exclusively found in massive ellipticals and in type 1 nuclei (i.e. nuclei with broad Hα emission). Only a few “transition” nuclei have compact radio cores; those detected in the radio have optical emission-line diagnostic ratios close to those of LINERs/Seyferts. This indicates that some transition nuclei are truly composite Seyfert/LINER+H II region nuclei, with the radio core power depending on the strength of the former component. The core radio power is correlated with the nuclear optical “broad” Hα luminosity, the nuclear optical “narrow” emission-line luminosity and width, and with the galaxy luminosity. In these correlations LLAGNs fall close to the low-luminosity extrapolations of more powerful AGNs. The scalings suggest that many of the radio-non-detected LLAGNs are simply lower power versions of the radio-detected LLAGNs. The ratio of core radio power to nuclear optical emission- line luminosity increases with increasing bulge luminosity for all LLAGNs. Also, there is evidence that the luminosity of the disk component of the galaxy is correlated with the nuclear emission-line luminosity (but not the core radio power). About half of all LLAGNs with multiple epoch data show significant inter-year radio variability. Investigation of a sample of ∼150 nearby bright galaxies, most of them LLAGNs, shows that the nuclear (≤150 mas size) radio power is strongly correlated with both the black hole mass and the galaxy bulge luminosity; linear regression fits to all ∼150 galaxies give: log P2cm = −2 −3 1.31(0.16) log MMDO + 8.77 and log P2cm = 1.89(0.21) log LB(bulge) − 0.17. Low accretion rates (≤10 −10 of the Eddington rate) are implied in both advection- and jet-type models. In brief, all evidence points towards the presence of accreting massive black holes in a large fraction, perhaps all, of LLAGNs, with the nuclear radio emission originating in either the accretion inflow onto the massive black hole or from jets launched by this black hole–accretion disk system. Key words. accretion, accretion disks – galaxies: active – galaxies: jets – galaxies: nuclei – radio continuum: galaxies – surveys 1. Introduction definition, Ho et al. 1997a, hereafter H97a) can be modeled in terms of photoionization by hot, young stars (Terlevich & The debate on the power source of low-luminosity active galac- Melnick 1985; Filippenko & Terlevich 1992; Shields 1992), tic nuclei (LLAGNs, i.e. low-luminosity Seyferts, LINERs, by collisional ionization in shocks (Koski & Osterbrock 1976; and “transition” nuclei [nuclei with spectra intermediate be- Fosbury et al. 1978; Heckman 1980; Dopita & Sutherland tween those of LINERs and H II regions]) is a continuing one. 40 −1 1995) or by aging starbursts (Alonso-Herrero et al. 1999). Their low emission-line luminosities (LHα ≤ 10 erg s by On the other hand, evidence has been accumulating that at Send offprint requests to: N. M. Nagar, least some fraction of LLAGNs share characteristics in com- e-mail: [email protected] mon with their more powerful counterparts – radio galaxies Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20020874 54 N. M. Nagar et al.: AGNs in a distance-limited sample of LLAGNs. III. and powerful Seyfert galaxies. These similarities include the disk of the galaxy, even at Chandra’s sub arcsecond resolution. presence of compact nuclear radio cores (Heckman 1980), wa- Further, all these indicators may be affected by viewing geom- ter vapor megamasers (Braatz et al. 1997), nuclear point-like etry, obscuration, and the signal-to-noise of the observations, UV sources (Maoz et al. 1995; Barth et al. 1998), broad Hα the last problem being exacerbated by the low optical, UV, and lines (hereafter H97c Ho et al. 1997c), and broader Hα lines in X-ray luminosities of LLAGNs and the need to subtract the polarized emission than in total emission (Barth et al. 1999). If starlight. The radio regime, however, offers several advantages. LLAGNs are truly scaled down AGNs then the challenge is to Gigahertz (cm wave) radiation does not suffer the obscuration explain their much lower accretion luminosities. This requires that affects the UV to infra-red. Also, at tens of gigahertz the either very low accretion rates (∼10−8 of the Eddington accre- problems of free-free absorption can be avoided in most cases. tion rate, LEdd)orradiativeefficiencies (the ratio of radiated Finally, high resolution, high sensitivity radio maps can be rou- energy to accreted mass) much lower than the typical value of tinely made with an investment of less than an hour per source ∼10% (e.g. Chap. 7.8 of Frank et al. 1995) assumed for power- at the Very Large Array1 (VLA; Thompson et al. 1980) and ful AGNs. the Very Long Baseline Array (VLBA; Napier et al. 1994). At One well-known property of some powerful AGNs is a their resolutions of ∼100 milli-arcsec (mas) and ∼1 mas, re- compact (sub-parsec), flat-spectrum (usually defined as α ≥ spectively, it is easy to pick out the AGN, since any other radio α −0.5, S ν ∝ ν ) nuclear radio source, usually interpreted emission from the galaxy is usually resolved out. as the synchrotron self-absorbed base of the jet which fuels Closely related to these theoretical and observational is- larger-scale radio emission. Astrophysical jets are known to be sues are the increasing number of accurate mass determina- produced in systems undergoing accretion onto a compact ob- tions for “massive dark objects” (MDOs) in nearby galactic ject (e.g. Pringle 1993; Blandford 1993) so such compact ra- nuclei. Indeed, dynamic signatures of nuclear black holes are dio sources in galactic nuclei may reasonably be considered a being found in almost every nearby galaxy studied. Mass es- signature of an AGN. Much theoretical work (e.g. Begelman timates from high-resolution stellar-, maser-, or gas-dynamics et al. 1984; Lovelace & Romanova 1996; Falcke & Biermann are probably accurate to a factor of two (Richstone et al. 1998; 1999) has been devoted to this disk-jet relationship in the case Gebhardt et al. 2000). Slightly less reliable (factor ∼2–5) es- of galactic nuclei and it has been suggested that scaled-down timates of MMDO for large samples are possible using the versions of AGN jets can produce flat-spectrum radio cores in tight correlation between central stellar velocity dispersion of LLAGNs (Falcke & Biermann 1999). Compact nuclear radio the galaxy bulge (σc)andMMDO (Merritt & Ferrarese 2001; emission with a flat to inverted spectrum is also expected from Gebhardt et al. 2000). In this paper we assume that the few 5−9 the accretion inflow in advection-dominated (ADAF; Narayan ×10 M MDOs traced by dynamic methods are black holes, et al. 1998) or convection-dominated (CDAF; Narayan et al. and interchangeably use the terms MDO and black hole. With 2000) accretion flows, possible forms of accretion onto a black the new MDO results the degeneracy between black hole mass hole at low accretion rates (Rees et al. 1982). Flat-spectrum and accretion rate when accounting for radiated luminosity radio sources can also result through thermal emission from is removed. Given AGN spectral energy densities and black ionized gas in normal H II regions or through free-free absorp- hole masses in large samples of nearby AGNs, one can at- tion of non-thermal radio emission, a process which proba- tempt to isolate factors which determine accretion rates and bly occurs in compact nuclear starbursts (Condon et al. 1991). luminosities. The brightness temperature, Tb, in such starbursts is limited to A weak correlation between the nuclear centimeter radio < log [Tb (K)] ∼ 5 (Condon et al. 1991). Thus it is necessary luminosity and the mass of the nuclear black hole has been to show that Tb exceeds this limit before accretion onto a black found by several authors, e.g. Franceschini et al. (1998), Yi & hole can be claimed as the power source. Boughn (1999) and Di Matteo et al. (2001). This result has From the observational perspective, Heckman (1980) been used as evidence for the existence of an ADAF-like in- showed that LINER nuclei tend to be associated with a com- flow since, in such a flow, the radio luminosity is predicted pact radio source, and compact, flat-spectrum radio cores are to scale with the black hole mass and the accretion rate (e.g.
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