Radiatively driven mass accretion on to galactic nuclei by circumnuclear starbursts

Masayuki Umemura 2 Jun Fukue 1 Shin Mineshige 0 0 Department of Astronomy, Kyoto University , Sakyo-ku, Kyoto 606 , Japan 1 Astronomical Institute, Osaka Kyoiku University , Asahigaoka, Kashiwara, Osaka 582 , Japan 2 Center for Computational Physics, University of Tsukuba , Tsukuba, Ibaraki 305 , Japan A B S T R A C T We examine the physical processes of radiatively driven mass accretion on to galactic nuclei, owing to intensive radiation from circumnuclear starbursts. The radiation from a starburst not only causes the inner gas disc to contract via radition flux force, but also extracts angular momentum owing to relativistic radiation drag, thereby inducing an avalanche of the surface layer of the disc. To analyse such a mechanism, the radiation -hydrodynamical equations are solved, including the effects of the radiation drag force as well as the radiation flux force. As a result, it is found that the mass accretion rate owing to the radiative avalanche is given by M r hL=c2r=R2DR=R1 et at radius r, where the efficiency h ranges from 0.2 up to 1, L and R are respectively the bolometric luminosity and the radius of the starburst ring, DR is the extent of the emission regions, and t is the face-on optical depth of the disc. In an optically thick regime, the rate depends upon neither the optical depth nor the surface mass density distribution of the disc. The present radiatively driven mass accretion may provide a physical mechanism which enables mass accretion from 100-pc scales down to ,parsec scales, and it may eventually be linked to advection-dominated viscous accretion on to a massive black hole. The radiation-hydrodynamical and self-gravitational instabilities of the disc are briefly discussed. In particular, the radiative acceleration possibly builds up a dusty wall, which 'shades' the nucleus in edge-on views. This provides another version of the model for the formation of an obscuring torus. I N T R O D U C T I O N In the standard picture of active galactic nuclei (AGN), a viscous accretion disc of size & a parsec surrounding a putative supermassive black hole is thought to be the central engine for the enormous energy output. The mass supply on to such a tiny region is an issue of growing interest. Accretion driven by a non-axisymmetric wave or a bar is a possible mechanism (Shore & White 1982; Shlosman, Frank & Begelman 1989), but it has been found so far that bar-driven accretion is effective only in the galactic nuclear regions beyond ,100 pc (Wada & Habe 1995). Hence another mechanism that enables mass accretion from 100-pc down to parsec scales is required. Recently, Umemura, Fukue & Mineshige (1997) have proposed a possible physical mechanism on the relevant scales: that is, mass accretion driven by radiation from circumnuclear starbursts. *E-mail: (MU); (JF); minesige@kusastro;kyoto-u.ac.jp (SM) In recent years, there has been accumulated observational evidence for starbursts in the circumnuclear regions of AGN. Among this evidence, it is remarkable that molecular gas of mass at least 1011 M( and dust of mass ,108 M( are found towards a QSO (BR 1202 0725) at redshift z 4:69 (Ohta et al. 1996; Omont et al. 1996). This, taking into account the age of the Universe of ,1 Gyr at z 4:69 and the inferred metallicity, suggests that the QSO inhabits a massive host galaxy undergoing starbursts. Also, recent Hubble Space Telescope (HST ) images of nearby luminous QSOs have shown that luminous QSO phenomena occur preferentially in luminous host galaxies, often ellipticals (McLeod & Rieke 1995; Bahcall et al. 1997; Hooper, Impey & Foltz 1997). In addition, AGN events appear to be frequently accompanied by starbursts (Soifer et al. 1986; Scoville et al. 1986; Heckman 1991; Rieke 1992; Scoville 1992; Filippenko 1992). The IRAS survey has revealed that, at bolometric luminosities greater than 1012 L(, the space density of starburst galaxies is just comparable to that of QSOs in the same luminosity range (Soifer et al. 1986). Millimetre observations of infrared galaxies indicate that all are exceedingly rich in molecular gas with up to 2 1010 M( in their central regions (Scoville et al. 1986). The correlations between X-ray and CO luminosities, as well as between CO and far-infrared luminosities, in Seyfert galaxies also suggest that more powerful AGN inhabit more active star-forming galaxies (Yamada 1994). Such increasing evidence suggests a possible physical connection between AGN events and starburst activity (Norman & Scoville 1988; RowanRobinson & Crawford 1989; Terlevich 1992; Scoville 1992; Terlevich 1992; Perry 1992; Taniguchi 1992; Rowan-Robinson 1995). As for the structure of starburst regions, recent high-resolution observations of Seyfert nuclei, including those from the HST, have revealed circumnuclear starburst regions with radial extents from a few tens of parsecs up to kiloparsecs, which often exhibit ring-like features (Wilson et al. 1991; Forbes et al. 1994; Mauder et al. 1994; Buta, Purcell & Crocker 1995; Barth et al. 1995; Maoz et al. 1996; Storchi-Bergman, Wilson & Baldwin 1996; Leitherer et al. 1996). The rings frequently contain a number of extremely compact star clusters of extent < 5 pc, which are composed of hundreds of O-type stars. Recently, an unexpectedly large (,100 pc) disc of gas and dust has been discovered by the HST around the bright nucleus in the active galaxy NGC 4261 (Jaffe et al. 1993; Ferrarese, Ford & Jaffe 1996). This disc is surrounded by a fat torus composed of early-type stars. Umemura et al. (1997, hereafter Paper I) have considered the radiative effects on nuclear discs owing to circumnuclear starbursts, and proposed the possibility of a radiative avalanche: that is, mass accretion driven by the relativistic radiation drag. In Paper I, the effects of radiation drag were analysed solely by solving the azimuthal equation of motion. In this paper, we extensively explore the radiative avalanche, including radial motion driven by the radiation flux force. Also, the radiationhydrodynamical instability of the nuclear region is analysed, including vertical motion as well, so that the radiative build-up of a dusty obscuring wall is predicted. Some related topics, such as supernova effects, the generation of seed magnetic fields, and a possible link to an advection-dominated viscous flow, are discussed. In Section 2, the basic equations which govern the radiationhydrodynamical disc evolution are provided. In Section 3, the time-scales for a radiative avalanche, the disc evolution, and the resultant mass accretion rate are presented. In Section 4, the radiationhydrodynamical/gravitational instabilities are analysed. In Section 5, we discuss the effects of stellar evolution, the generation of magnetic fields, and a link between a radiative avalanche and advection-dominated accretion on to a black hole. Section 6 is devoted to our conclusions. B A S I C E Q U AT I O N S Radiation fields from starbursts Here, we suppo (...truncated)