They certainly have not been found in every case that has been looked. Kormendy has undertaken a systematic survey of a modest sample of galaxies (E-Sb), and his detection rate has been about 20% (KR). But, of course, many factors conspire against the detection of MDOs, and this estimate should be regarded as firm lower limit. If one takes seriously the M- Mbul relation described above, it is possible that every bulge contains a SMBH with an appropriately scaled size. This view is supported by the statistical analysis of Magorrian et al. (1998). In fact, the detection of an MDO in NGC 4945 (Section 4.2) and the presence of a bona fide AGN in NGC 4395 indicate that perhaps even some galaxies without bulges may have SMBHs.
Figure 9. (a) Detection rate and (b) number distribution of AGNs as a function of Hubble type in the spectroscopic survey of Ho et al. (1995, 1997). "Type 1" AGNs (those with broad H) are shown separately from the total population (types 1 and 2).
Additional support for this picture comes from the growing evidence that nonstellar nuclear activity is very common in galaxies, much more so than conventionally believed based on the statistics of bright AGNs and quasars. A recent spectroscopic survey of a large, statistically complete sample of nearby galaxies finds that over 40% of all bright (BT 12.5 mag) galaxies have nuclei that can be classified as "active," and the percentage is even higher among early-type systems (E-Sbc), approaching 50%-75% (Ho, Filippenko, & Sargent 1997). Most of the nearby AGNs have much lower luminosities than traditionally studied active galaxies, and a greater heterogeneity in spectral types is found (low-ionization nuclei, or LINERs, are common, for example), but the evidence is overwhelming that many of these nuclei are truly accretion-powered sources (see Filippenko 1996; Ho et al. 1997). Moreover, intrinsically weak, compact radio cores are known to be present in a significant fraction of elliptical and S0 galaxies (Sadler, Jenkins, & Kotanyi 1989; Wrobel & Heeschen 1991), almost all of which spectroscopically qualify as AGNs (Ho 1998).
Within the conventional AGN paradigm, the observed widespread nuclear activity implies that SMBHs are a generic component of many, perhaps most, present-day bulge-dominated galaxies, consistent with the picture emerging from the kinematic studies. This is a remarkable statement. It implies that SMBHs should not be regarded as "freaks of nature" that exist in only a handful of galaxies; rather, they must be accepted and understood as a normal component of galactic structure, one that arises naturally in the course of galaxy formation and evolution.