Expected intermediate-mass black holes in the Virgo cluster - II. Late-type galaxies

The Chandra X-ray Observatory's Cycle 18 Large Program titled `Spiral galaxies of the Virgo Cluster' will image 52 galaxies with the ACIS-S detector. Combined with archival data for an additional 22 galaxies, this will represent the complete sample of 74 spiral galaxies in the Virgo cluster with star formation rates ≳0.3 M_⊙ yr^-1. Many of these galaxies are expected to have an active nucleus, signalling the presence of a central black hole. In preparation for this survey, we predict the central black hole masses using the latest black hole scaling relations based on the spiral arm pitch angle ϕ, velocity dispersion σ, and total stellar mass M_*,galaxy. With a focus on intermediate-mass black holes (IMBHs; 10² < M_bh/M_⊙ < 10⁵), we highlight NGC 4713 and NGC 4178, both with M_bh ≈ 10³-10⁴ (an estimate that is further supported in NGC 4178 by its nuclear star cluster mass). From Chandra archival data, we find that both galaxies have a point-like nuclear X-ray source, with unabsorbed 0.3-10 keV luminosities of a few times 10³⁸ erg s^-1. In NGC 4178, the nuclear source has a soft, probably thermal, spectrum consistent with a stellar-mass black hole in the high/soft state, while no strong constraints can be derived for the nuclear emission of NGC 4713. In total, 33 of the 74 galaxies are predicted to have M_bh < (10⁵-10⁶) M_⊙, and several are consistently predicted to have masses of 10⁴-10⁵ M_⊙, such as IC 3392, NGC 4294, and NGC 4413. We speculate that a sizeable population of IMBHs may reside in late-type spiral galaxies with low stellar mass (M_{\ast }≲ 10^{10} M_{⊙}).