Supermassive Black Holes and Galaxy Morphology

0 downloads 0 Views 217KB Size Report
Feb 1, 2008 - Keywords: black holes, galaxies, evolution, morphology. 1. Introduction. It is now widely accepted that quasars (QSOs) and Active Galactic.
Supermassive Black Holes and Galaxy Morphology F. D. Macchetto∗ Space Telescope Science Institute

arXiv:astro-ph/9910089v1 5 Oct 1999

February 1, 2008 Abstract. This review addresses one of the important topics of current astrophysical research, namely the role that supermassive black holes play in shaping the morphology of their host galaxies. There is increasing evidence for the presence of massive black holes at the centers of all galaxies and many efforts are directed at understanding the processes that lead to their formation, the duty cycle for the active phase and the question of the fueling mechanism. Related issues are the epoch of formation of the supermassive black holes, their time evolution and growth and the role they play in the early ionization of the Universe. Considerable observational and theoretical work has been carried out in this field over the last few years and I will review some of the recent key areas of progress. Keywords: black holes, galaxies, evolution, morphology

1. Introduction It is now widely accepted that quasars (QSOs) and Active Galactic Nuclei (AGN) are powered by accretion onto massive black holes. This has led to extensive theoretical and observational studies to elucidate the properties of the black holes, the characteristics of the accretion mechanisms and the mechanisms responsible for the production and transportation of the energy from the central regions to the extended radio lobes. However, over the last few years there has been an increasing realization that Massive Dark Objects (MDOs) may actually reside at the centers of all galaxies (Ho 1998; Magorrian et al. 1998; Richstone et al. 1998; Van der Marel 1998). The mean mass of these objects, of order 10−2.5 times the mass of their host galaxies, is consistent with the mass in black holes needed to produce the observed energy density in quasar light if we make reasonable assumptions about the efficiency of quasar energy production (Chokshi and Turner 1992; Blandford 1999). This raises a number of important new questions and has fundamental implications for the role of the black holes in contributing or being responsible for the ionization (or reionization) of the early universe and for their role in the processes leading to the formation of galaxies. Conversely the apparent correlation between the black hole mass and ∗

On assignment from the Astrophysics Division, Space Science Department of the European Space Agency

astro.tex; 1/02/2008; 11:31; p.1

2

F. D. Macchetto

the mass of the spheroidal component in elliptical and spirals points towards a close interaction between the galaxy size and morphology and its central black hole. Models in which elliptical galaxies form from the mergers of disk galaxies whose bulges contain black holes are consistent with the core fundamental plane, the relation between the central parameters of early-type galaxies. Furthermore it is clear that the dynamical influence of a supermassive black hole can extend far beyond the nucleus if a substantial number of stars are on orbits that carry them into the center. Work by Merritt (1998) has shown that nuclear black holes are important for understanding many of the largescale properties of galaxies, including the fact that elliptical galaxies come in two, morphologically-distinct families, the absence of bars in most disk galaxies, and the shapes of the spiral galaxy rotation curves. Since the growth of the black hole mass depends on the global morphology of the host galaxies, the link between black hole and galaxy structure implies a feedback mechanism that determines what fraction of a galaxys mass ends up in the center. 2. Dynamical Evidence for Massive Black Holes 2.1. Megamasers in NGC 4258 The best observation showing the presence of a Keplerian disk around a black hole was the VLBI observation of megamasers in the nucleus of the Seyfert 2 galaxy NGC 4258 reported by Miyoshi et al. (1995). These observations reveal individual masing knots revolving at distances ranging from ∼ 13 pc to 25 pc around the central object. These data show a near-perfect Keplerian velocity distribution, implying that almost all the mass is located well within the inner radius where the megamasers reside,and they derive a central mass of ∼ 3.6 × 107 M⊙ within the inner ∼ 13 pc. It is not possible to have a cluster of distinct, dark, massive objects (e.g., neutron stars) responsible for such gravitational potential; since the objects would escape on a relatively short time scale, and form a potential well with a different shape, which would force a departure of the megamaser emitting material from pure Keplerian motion (Maoz 1995). Thus the simplest explanation is that the central mass condensation is indeed a black hole. Given this mass and the fact that NGC 4258 is a relatively low luminosity (∼ 1042 erg s−1 ) object, the emission is sub-Eddington, with L/LE ∼ 3 × 10−4 . Such sub-Eddington sources are likely to have accretion disk structures, where the accreting gas is optically thin and radiates inefficiently, and the accretion energy that is dissipated viscously, is advected with the accretion flow (see, e.g., Ichimaru 1977;

astro.tex; 1/02/2008; 11:31; p.2

Supermassive Black Holes and Galaxy Morphology

3

Narayan & Yi 1994; Abramowicz et al. 1995). However, it is important to note that such low Eddington rate cannot be normal among quasars; if most of them radiated at such low L/LE , the black hole masses of the most luminous sources would be much larger than expected on other grounds, unless the low activity is not due to a low black hole mass, but rather to a low accretion rate (Ptak 1997). 2.2. Kinematic Studies Using Optical Emission Lines The other line of evidence for the presence of black holes in galaxies is the velocity field of the matter emitting closely to the nucleus. This approach dates back to the pioneering ground-based observations made in the late 70s, when Sargent and collaborators showed that the stellar velocity dispersion in the radio galaxy M87 increases to 350 km s−1 in the innermost 1.5′′ from the nucleus. Considerably better spatial resolution observations using the long-slit spectrograph on the Faint Object Camera of HST were carried out by Macchetto et al. (1997). They observed the ionized gas disk in the emission line of [OII]λ3727 at three different positions separated by 0.2 arcsec, with a spatial sampling of 0.03 arcsec (or ∼ 2 pc at the distance of M87). They measured the rotation curve of the inner ∼ 1′′ of the ionized gas disk to a distance as close as 0.′′ 07 (≃ 5 pc) to the dynamical center. They modeled the kinematics of the gas under the assumption of the existence of both a central black hole and an extended central mass distribution, taking into account the effects of the instrumental PSF, the intrinsic luminosity distribution of the line, and the finite size of the slit. They found that the central mass must be concentrated within a sphere whose maximum radius is ≃ 3.5 pc and showed that both the observed rotation curve and line profiles are best explained by a thin-disk in Keplerian motion. They proved that the observed motions are due to the presence of a supermassive black-hole and derived a value of MBH = (3.2 ± 0.9) × 109 M⊙ for its mass. 2.3. Virial Masses The virial masses and emission-line region sizes of Active Galactic Nuclei (AGNs) can be measured by “reverberation-mapping” techniques. Wandel, Peterson & Malkan (1999) have compiled a sample of 17 Seyfert 1 and 2 quasars with reliable reverberation and spectroscopic data and used these results to calibrate similar determinations made by photoionization models of the AGN line-emitting regions. Reverberation mapping uses the light travel-time delayed emission-line response to continuum variations to determine the size and kinematics of the emission-line region. The distance of the broad emission-line region

astro.tex; 1/02/2008; 11:31; p.3

4

F. D. Macchetto

(BLR) from the ionizing source is then combined with the velocity dispersion, derived from either the broad Hβ line width or from the variable part of the line profile to estimate the virial mass. When they compare the central masses calculated with the reverberation method to those calculated using a photoionization (Hβ line) model, they find a nearly linear correlation (Table 1). They find that the correlation between the masses is significantly better than the correlation between the corresponding BLR sizes calculated by the two methods, which further supports the conclusion that both methods measure the mass of the central black hole. They also derive the Eddington ratio, which for the objects in the sample fall in the range LV /LEdd ∼ 0.001–0.03 and Lion /LEdd ≈ 0.01–0.3.

2.4. The Black Hole Mass of a Seyfert Galaxy

In a recent study Winge et al. (1999) have analyzed both groundbased, and HST/FOC long-slit spectroscopy at subarcsecond spatial resolution of the narrow-line region (NLR) of NGC 4151. They found that the extended emission gas (R > 4′′ ) is in a normal rotation in the galactic plane, a behavior that they were able to trace even across the nuclear region, where the gas is strongly disturbed by the interaction with the radio jet and connects smoothly with the large-scale rotation defined by the neutral gas emission. The HST data, at 0.′′ 03 spatial resolution, allow for the first time truly to isolate the kinematic behavior of the individual clouds in the inner narrow-line region. They find that, underlying the perturbations introduced by the radio ejecta, the general velocity field can still be well represented by planar rotation down to a radius of ∼ 0.′′ 5 (30 pc), the distance at which the rotation curve has its turnover. The most striking result that emerges from the analysis is that the galaxy potential derived fitting the rotation curve changes from a “dark halo” at the extended narrow-line region distances to being dominated by the central mass concentration in the NLR, with an almost Keplerian falloff in the 1′′ < R < 4′′ interval. The observed velocity of the gas at 0.′′ 5 implies a mass of M ∼ 109 M⊙ within the inner 60 pc. The presence of a turnover in the rotation curve indicates that this central mass concentration is extended. The first measured velocity point (outside the region saturated by the nucleus) would imply an enclosed mass of ∼ 5×107 M⊙ within R ∼ 0.′′ 15 (10 pc), which represents an upper limit to any nuclear point mass.

astro.tex; 1/02/2008; 11:31; p.4

Table I. Reverberation BLR Sizes and Central Masses Compared with Photoionization Sizes and Masses. The last two columns give the ionizing luminosity derived from the lag and the corresponding Eddington ratio. (Wandel et al. 1999) log Rph

3C 120 3C 390.3 Akn 120 F9 1C 4329A Mrk 79 Mrk 110 Mrk 335 Mrk 509 Mrk 590 Mrk 817 NGC 3227 NGC 3783 NGC 4051 NGC 4151 NGC 5548 NGC 7469 PG 0804+762 PG 0953+414

0.92 0.89 1.07 1.13