ORIGIN of RADIO EMISSION from NEARBY LOW-LUMINOSITY ACTIVE GALACTIC NUCLEI Qingwen Wu1, 2 and Xinwu Cao1 Receivved 2004 July 16; Accepted 2004 Novvember 3
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The Astrophysical Journal, 621:130–138, 2005 March 1 # 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A. ORIGIN OF RADIO EMISSION FROM NEARBY LOW-LUMINOSITY ACTIVE GALACTIC NUCLEI Qingwen Wu1, 2 and Xinwu Cao1 Receivved 2004 July 16; accepted 2004 Novvember 3 ABSTRACT We use the observational data in radio, optical, and X-ray wave bands for a sample of active galactic nuclei (AGNs) with measured black hole masses to explore the origin of radio emission from nearby low-luminosity active galactic nuclei (LLAGNs). The maximal luminosity of an advection-dominated accretion flow (ADAF) can be calculated for a given black hole mass, as there is a critical accretion rate above which the ADAF is no longer present. We find that the radio luminosities are higher than the maximal luminosities expected from the ADAF model for most sources in this sample. This implies that the radio emission is predominantly from the jets in these sources. The radio emission from a small fraction of the sources (15/60; referred to as radio-weak sources) in this sample can be explained by the ADAF model. However, comparing the observed multiband emission data with the spectra calculated for the ADAF or adiabatic inflow-outflow solution (ADIOS) cases, we find that neither ADAF nor ADIOS models can reproduce the observed multiband emission simultaneously, with reasonable magnetic field strengths, for these radio-weak sources. A variety of other possibilities are discussed, and we suggest that the radio emission is probably dominated by jet emission even in these radio-weak LLAGNs. Subject headinggs: accretion, accretion disks — black hole physics — galaxies: active — galaxies: jets — galaxies: nuclei 1. INTRODUCTION radii, in order to explain the observed spectra. Several inves- tigations were carried out to fit the spectra of LLAGNs, such as There is increasing evidence for supermassive black holes in M81 and NGC 4579, using the ADAF+SD (standard disk) mod- both distant luminous active galactic nuclei (AGNs) and nearby galaxies. Black hole accretion is thought to power AGNs, els (Quataert et al. 1999). This model can successfully explain most important features of LLAGNs, but it underpredicts the and the UV/optical bump observed in luminous quasars is detected low-frequency radio flux by more than 1 order of mag- naturally interpreted as blackbody emission from standard thin disks (e.g., Sun & Malkan 1989). A small fraction of qua- nitude (Falcke et al. 2004; Falcke & Markoff 2000; Yuan et al. 2002). They proposed that outflows or jets might be in these sars (10%) are radio-loud, defined by a radio loudness R sources and that the presence of such outflows or jets is sup- L (6 cm)=L (4400 8) > 10 (Kellermann et al. 1989, 1994; ported by theoretical calculations on ADAFs, as they have posi- Visnovsky et al. 1992; Stocke et al. 1992; Hooper et al. 1995). tive Bernoulli constants (Narayan & Yi 1994, 1995a). Recently, High-resolution radio observations show that the radio emis- the modified ADAF model, the powerful windy ADAF (adia- sion of these luminous radio-loud quasars is usually related to batic inflow-outflow solutions [ADIOSs]; Blandford & Begelman their relativistic jets. It is found that some nearby active galax- 1999), and convection-dominated flows (CDAFs; Narayan et al. ies share similar characteristics with those luminous quasars, 2000; Stone et al. 1999) were proposed to explain the observa- but with relatively weaker broad-line emission and radio cores tional features of LLAGNs. Some LLAGNs have been observed (Junor & Biretta 1995; Ho et al. 1997b; Bietenholz et al. 2000; Falcke et al. 2000; Nagar et al. 2002). These low-luminosity ac- and detected with the Very Long Baseline Array (VLBA) at different frequencies (Ulvestad & Ho 2001; Nagar et al. 2001; tive galactic nuclei (LLAGNs) are very common in the local Anderson et al. 2004), which implies that the jet should dominate universe. According to the Palomar survey (Ho et al. 1997a), the radio emission from the nuclei in some sources. The VLBI more than 40% of all nearby galaxies are brighter than BT ¼ 12:5 mag and emit AGN-like optical spectra. observations show that the radio core sizes are very small in most Most of these LLAGNs are highly sub-Eddington systems, of these sources. In this paper we use a sample of LLAGNs with À2 measured black hole masses to explore the origin of their radio i.e., Lbol=LEdd < 10 (Ho 2003). Accreting at less than a few percent of the Eddington rate, the black hole may accrete via emission. an advection-dominated accretion flow (ADAF; Narayan & Yi 2. THE SAMPLE 1994, 1995a, 1995b; Abramowicz et al. 1995; Chen et al. 1995). The ADAF model can successfully explain most observational The LLAGN sample used for the present investigation is features of Sagittarius A* at the center of our Galaxy (Narayan selected from Ho (2002). The sources in this sample, which et al. 1995, 1998; Manmoto et al. 1997; Mahadevan 1998) and have the most up-to-date estimated black hole masses, have other LLAGNs, such as NGC 4258 (Lasota et al. 1996; Gammi high angular resolution optical and radio measurements. This et al. 1999). sample covers a wide range of nuclear activity, from nearby Lasota et al. (1996) suggested that the ADAFs in most inactive nuclei to classical Seyfert 1 nuclei and luminous qua- LLAGNs should be truncated to standard thin disks at certain sars. In our present investigation we focus on the LLAGNs, and the Palomar-Green quasars are not included, as they are well- known luminous quasars. The optical emission from the host 1 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, 200030, China; [email protected], [email protected]. galaxies dominates over that from the nuclei for most LLAGNs. 2 Graduate School of the Chinese Academy of Sciences, Beijing, 100039, In some extreme cases, the nucleus accounts for merely 0.01% China. of the integrated light (Ho & Peng 2001). The emission from 130 ORIGIN OF RADIO EMISSION FROM LOW-LUMINOSITY AGNs 131 the nuclei reflects the nature of the central engines in these For an ADIOS flow, the accretion rate is a function of radius r LLAGNs. The core radio emission has been measured by the instead of being a constant independent of r, as in a pure ADAF Very Large Array (VLA) at 6 cm, with a beam size P500.The model (Blandford & Begelman 1999; Quataert & Narayan 1999). p nuclear spectral power P6, nuc at 6 cm, computed from the ob- A dimensionless accretion rate, m˙ ¼ m˙ out(r=rmax) , is assumed served flux density, represents the emission associated with for the ADIOS flow, where m˙ out is the mass accretion rate at the ‘‘active’’ (nonstellar) nucleus. Since a number of objects rmax and p is a parameter describing the strength of the winds have type 2 nuclei, which may be hidden from direct view (Blandford & Begelman 1999). For strong winds (correspond- or at least appreciably extincted, the known relation between ing to a large p), the spectrum emitted from the flow is modified, H luminosity and B-band absolute magnitude for type 1 AGNs as its structure is significantly altered by the winds. Both syn- 0 is used to estimate the MB; nuc of these LLAGNs (Ho & Peng chrotron and bremsstrahlung emission decrease with increasing 2001). Ho (2002) introduced a modified radio loudness R0 p, as a large p leads to lower density and lower electron temper- 0 0 8 L;c(6 cm)=L;c(4400 ) to measure the radio activities in these ature in the inner region of the flow. An ADIOS is described by a 0 0 8 LLAGNs, where L;c(6 cm) and L;c(4400 )arethecorelu- similar set of equations to that for an ADAF, with an r-dependent minosities as measured by the VLA and Hubble Space Tele- accretion rate m˙ (r) assumed. The spectral calculations for the scope (HST ), respectively. It is found that all these LLAGNs ADIOS flows are presented in Chang et al. (2002) and are briefly are radio-loud (R0 > 10) by this definition, instead of radio- summarized here (several errors in their paper have been fixed). quiet in the conventional definition. The total heating rate Q+ of the ADIOS flow is given by X-ray observations can also provide a direct probe of the cen- tral engines, if the X-ray emission is not obscured. We searched 1 À 1 Qþ ¼ 9:39 ; 1038 c mm˙ rÀp r pÀ1 À r pÀ1 ; the literature and compiled the X-ray emission data measured f 3 out max p À 1 max min at different energy bands, which were converted to Lk(2 keV). ð4Þ For the sources without measured photon indices, we adopt the conventional assumption that photon index À ¼ 2. As a result, where p < 1, and r is the outer radius of the ADIOS flow, we have a sample of 62 LLAGNs, of which 57 have measured max which extends to rmin . optical nuclei, 60 have measured radio nuclei, and 57 have mea- The total ion-electron heating rate for the electrons in the sured X-ray emission. All the data in this sample are listed in flow is Table 1, and more details can be found in Ho (2002). 1 ie ; 38 À2 À2 2 À2p 3. SPECTRA OF ADAF/ADIOS Q ¼ 1:2 10 g(e) c1 c3 mm˙ out rmax 2p À 1 In this work, we use the dimensionless variables ; 2pÀ1 2pÀ1 r max À r min ; ð5Þ M R M˙ m ¼ bh ; r ¼ ; m˙ ¼ ; ð1Þ M RSchw M˙ Edd where p 6¼ 1=2, c1 and c3 are the constants as defined in Narayan & Yi (1995b), and g(e) is given by equation (11) in where the Schwarzschild radius is defined as M97.