  We have conducted a high-resolution spectroscopic study using Spitzer of 18
bulgeless (Sd/Sdm) galaxies that show no definitive signatures of nuclear
activity in their optical spectra. This is the first systematic mid-IR search
for weak or hidden AGNs in a statistically significant sample of bulgeless disk
galaxies. Based on the detection of the high-ionization [NeV] line, we report
the discovery of an AGN in one out of the 18 galaxies in the sample. This
galaxy, NGC 4178, is a nearby edge-on Sd galaxy, which likely hosts a prominent
nuclear star cluster (NSC). The bolometric luminosity of the AGN inferred from
the [NeV] luminosity is ~ 8e41 ergs/s. This is almost two orders of magnitude
greater than the luminosity of the AGN in NGC 4395, the best studied AGN in a
bulgeless disk galaxy. Assuming that the AGN in NGC 4178 is radiating below the
Eddington limit, the lower mass limit for the black hole is ~ 6e3M_sun. The
fact that none of the other galaxies in the sample shows any evidence for an
AGN demonstrates that while the AGN detection rate based on mid-IR diagnostics
is high (30-40%) in optically quiescent galaxies with pseudobulges, it drops
drastically in Sd/Sdm galaxies. Our observations therefore confirm that AGNs in
completely bulgeless disk galaxies are not hidden in the optical but truly are
rare. Of the three Sd galaxies with AGNs known so far, all have prominent NSCs,
suggesting that in the absence of a well-defined bulge, the galaxy must possess
a NSC in order to host an AGN. While the presence of a NSC appears to be a
requirement for hosting an AGN in bulgeless galaxies, neither the properties of
the NSC nor those of the host galaxy appear exceptional in late-type AGN hosts.
The recipe for forming and growing a central black hole in a bulgeless galaxy
therefore remains unknown.
