Potential Black Hole Seeding of the Spiral Galaxy NGC 4424 via an Infalling Star Cluster

Galaxies have long been known to merge. Smaller galaxies rain down on bigger galaxies, stoking their growth (Lindblad, 1927; Spitzer & Baade, 1951; Baade & Minkowski, 1954; Vorontsov-Vel’Yaminov, 1959; Ibata et al., 2019b). This evolution has become a defining tenet of the cold dark matter model. The ongoing discovery of (infalling) satellites continues to test and support the hierarchical growth scenario for galaxies. The burgeoning detection of drawn-out, low surface brightness, debris streams of former galaxies in the outskirts of larger galaxies (Putman et al., 1998; Belokurov et al., 2006; Martin et al., 2014; Drlica-Wagner et al., 2015; Koposov et al., 2015; Shipp et al., 2018; Martinez-Delgado et al., 2021) are starting to reveal the extent of the hierarchical model, at least in regard to building stellar halos. The closest known tidal stream to the center of our galaxy is 10-20 kpc distant from the galaxy center (Yuan et al., 2020; Malhan et al., 2021).

The dense and compact nuclear star clusters—which are common features at the centers of low ($\approx$10⁷–10${}^{9}\,M_{\odot}$) and intermediate stellar mass ($\approx$$10^{9}$–$10^{10.5}\,M_{\odot}$) galaxies(e.g. Sandage et al., 1985; Böker et al., 2002; Graham & Guzmán, 2003; Balcells et al., 2003, 2007; Scott & Graham, 2013; Sánchez-Janssen et al., 2019)—are more strongly gravitationally bound than their host galaxies and are thus more able to resist disruption during the cannibalistic process (e.g., Bekki et al., 2001, 2003). Indeed, the dual and multiple nuclei of massive early-type galaxies is a testament to the robustness of these captured star clusters during the galaxy assimilation process (e.g., Jenner, 1974; Petrosian et al., 1978; Koroviakovskii et al., 1981; Tonry, 1985; Blakeslee & Tonry, 1992; Bonfini & Graham, 2016). Furthermore, the abundance of kinematically distinct cores and nuclear stellar disks in early-type galaxies (e.g. Franx et al., 1989; Forbes et al., 1995; Graham et al., 1998; Hau et al., 1999) reveals a more complete journey to the center of these galaxies, in stark contrast with an eternity spent in the halo.

Such galaxy growth, revealed by off-center nuclei, can be harder to spot in late-type galaxies due to confusion with knots of star formation. This ambiguity makes the later stages of the merger/digestion process more challenging to establish in late-type galaxies. Furthermore, without a massive stellar bulge, dynamical friction will be less in the inner regions, reducing the extent to which minor mergers contribute to the nuclei and bulges. The abundance of low bulge-to-total stellar mass ratios in late-type galaxies identified by Graham & Worley (2008) and Weinzirl et al. (2009) was later used by Kormendy et al. (2010) to challenge the hierarchical growth of most late-type galaxies. It was speculated that the bulges were too small to be built by mergers and were thus considered the result of internal secular processes. The detection of an infalling nuclear star cluster near the center of a late-type spiral galaxy would reveal that the Roche-lobe destruction of galactic neighbors does not just produce streams that build diffuse halos, but that tidal-shredding also contributes to the galaxy proper, including small bulges. Moreover, such digestion could potentially seed and feed spiral galaxies with massive black holes.

Among the Virgo cluster’s spiral galaxies, the ‘Sa? pec’ (Sandage & Tammann, 1981) galaxy NGC 4424 (VCC 979), with a disk-inclination of 70$\deg$ from face-on, is of particular interest. While there is no detectable X-ray point source at the center of NGC 4424, there is a nearby ($\sim$5$\arcsec$) ‘off-center’ source, NGC 4424 X-3 ($L_{0.5-8\,{\rm keV}}\approx 6\times 10^{38}$ erg s^-1). First publicly reported by Boselli et al. (2018), they suggested it is either an X-ray binary (XRB) or the nucleus of a merged galaxy. Two brighter X-ray point sources plus a fainter fourth point source reside in the outskirts of this galaxy.

Although Feng & Soria (2011), Bachetti et al. (2014) and Kaaret et al. (2017) reveal why super-Eddington accretion (Alexander & Natarajan, 2014; Soria et al., 2014) is thought to explain many of the off-center ultraluminous X-ray sources (ULX: $L_{\rm X}\approx 3\times 10^{39}$ to $10^{41}$ erg s^-1), some might be intermediate-mass black holes (IMBHs) from accreted galaxies, particularly the hyperluminous X-ray sources (HLXs: $L_{\rm X}>10^{41}$ erg s^-1, Gao et al., 2003) such as ESO 243-49 (Farrell et al., 2009; Soria et al., 2010) and others (Sutton et al., 2012; Barrows et al., 2019; Earnshaw et al., 2019). Here, we report the association of NGC 4424 X-3 with a red, elongated star cluster.

In Section 2, we provide some background information and references to NGC 4424 before presenting optical and near-infrared images along with an X-ray map. We perform a re-analysis of the Hubble Space Telescope (HST) image, the Chandra X-ray Observatory (CXO) and XMM-Newton data, and provide an extended discussion. Section 3 offers a prediction for the mass of the central black hole in both NGC 4424 and the suspected infaller. Finally, a more detailed discussion, centered around off-centered sources, is provided in Section 5.

Previously, in Graham et al. (2019), we used the mean redshift-independent distance modulus for NGC 4424 of 30.6$\pm$1.0 from the NASA/IPAC Extragalactic Database (NED)¹¹1http://nedwww.ipac.caltech.edu. Here, we adopt the Cepheid-based distance modulus of 31.080$\pm$0.292 (luminosity distance $D=16.44^{+2.37}_{-2.07}$ Mpc) from Riess et al. (2016), giving a scale of 79 pc arcsec${-1}$. This encompasses and agrees with an array of recent results, including Hatt et al. (2018), who report a ‘tip of the red giant branch’ distance modulus of $31.00\pm 0.03_{\rm stat}\pm 0.06_{\rm sys}$ ($15.8\pm 0.2_{\rm stat}\pm 0.4_{\rm sys}$ Mpc), and with Munari et al. (2013), who derived a SN Ia distance modulus of 30.95, and with Cortés et al. (2008), who reported a value of 30.91 based on the Tully & Fisher (1977) relation.

NGC 4424 might harbour two massive black holes, possibly with one yet to settle to the center of this merged system (Merritt & Milosavljević, 2005; Barausse, 2012; Tremmel et al., 2018), thereby explaining NGC 4424 X-3 in the elongated, off-center star cluster. In this scenario, the second (speculated) massive black hole is quiescent, residing at the center of NGC 4424.

In what follows, we first report on the predicted mass for such a galaxy-centric black hole. We then provide a prediction for the mass of the black hole in the star cluster Nikhuli.

Lacking an outward-pointing, comet-like appearance, we suspect that this active¹⁵¹⁵15We use the term ‘active’ to denote a star cluster with an active black hole; in this case, X-ray active. star cluster is probably not an ejected nucleus from a gravitational wave recoil event (Blecha & Loeb, 2008; Holley-Bockelmann et al., 2008; Komossa & Merritt, 2008; Askar et al., 2021; Hogg et al., 2021; Ward et al., 2021). Although probably evident to many readers, we note that Nikhuli has a lopsided shape, unlike that of a (background) galaxy. The significant elongation of Nikhuli in the direction of the NGC 4424 galaxy center favors an infall scenario. Conceivably, three additional knots to the north may delineate the orbital path of the infalling galaxy. Tidal stretching (Roche, 1850) of an accreted nuclear star cluster, and possibly the paltry remains of the accreted galaxy still surrounding the nuclear star cluster, is evident.

There is an abundance of nucleated dwarf and nucleated spiral galaxies (Böker et al., 2002; Graham & Guzmán, 2003), and their nuclear star cluster can contain a massive black hole (e.g. Graham & Spitler, 2009). The accretion of these galaxies, coupled with dynamical friction, can drive their nuclear star clusters toward the center of the accreting galaxy. Massive structures, such as dense star clusters, can sink due to the braking force and thus orbital decay, arising from the dynamical friction with the surrounding galaxy (Chandrasekhar & von Neumann, 1943; Baranov & Batrakov, 1974; Tremaine et al., 1975; Inoue, 2011; Arca-Sedda & Capuzzo-Dolcetta, 2014a, b; Chen et al., 2021; Morton et al., 2021).

Differing from the naked¹⁶¹⁶16The term ‘naked’ is used to denote the absence of a surrounding star cluster. black holes in the merger simulations of Bellovary et al. (2021) and Ma et al. (2021), which tend not to sink to the center of the final galaxy but remain off-center, the shrouded black hole in Nikhuli may have more success (Pfister et al., 2019; Ma et al., 2021). Bekki (2010) have shown that star clusters with masses greater than $2\times 10^{5}\,M_{\odot}$, i.e., less than 10 times the mass of Nikhuli, experience significant orbital decay within 1 Gyr. Other simulations have found that black holes can form close pairs in dwarf galaxies, which likely merge to build the foundation of the central supermassive black hole and produce gravitational wave signals detectable using the planned Laser Interferometer Space Antenna (e.g., Bellovary et al., 2019). In addition, Oh & Lin (2000) report that globular cluster capture may explain some of the nucleated dwarf galaxies in the Virgo cluster, with the clusters reaching the core within a Hubble time.

Following Binney & Tremaine (1987, their eq. 7.18), a crude estimate for the time, $t$, it would take Nikhuli (with NGC 4424 X-3) to sink to the center of NGC 4424 can be made when assuming the inner region of NGC 4424 is dominated by stars with a roughly homogeneous, isothermal distribution. Their dynamical friction timescale is given by

While Carollo (1999) adopted $\log\Lambda=10$ for Coulomb’s logarithm, we use $\ln\Lambda=10$, which is 2.3 times smaller. Substituting in $M=3.44\times 10^{6}$ M_⊙ for Nikhuli, along with $r=400$ pc and $v_{c}=\sqrt{2}\sigma$, where $\sigma=57$ km s^-1, one obtains a dynamical friction time of $\sim$100 Myr. If one assumes that Nikhuli is at a non-projected distance of 1 kpc from the galaxy center, then one obtains a timescale of $\sim$0.64 Gyr. Such timescales mesh well with those seen in Lotz et al. (2001, see their Figure 2), who used a value of $\ln\Lambda\approx 7.3$. If we adopt the value $\log\Lambda=2$ found by Velazquez & White (1999) for bulgeless, exponential disks in dark matter halos with NFW (Navarro et al., 1996) profiles, then the above timescales increase to $\sim$0.5 and $\sim$3.2 Gyr. However, as Milosavljević (2004) note, disk perturbations, which can include spiral arms and bars, can result in both positive and negative torques on an infalling satellite. The bar-like feature in NGC 4424 may facilitate the inspiral of the Nikhuli/black hole system (Bortolas et al., 2021). Whether or not Nikhuli makes it all the way to the center of NGC 4424, and how long that may take, will also depend on the time spent in the disk plane and requires modeling beyond the scope of this paper.

In low-mass spheroids with low Sérsic indices, and thus a shallow gradient to their central gravitational potential (e.g., Terzić & Graham, 2005; Terzić & Sprague, 2007), star clusters can wander about their galaxy’s center, typically displaced by $\sim$100 parsecs (Binggeli et al., 2000). Massive black holes can also be offset (Boldrini et al., 2020; Ricarte et al., 2021). A similar situation occurs in the partially depleted cores of massive spheroids, where infalling perturbers can stall outside of the cores with shallow density profiles (Goerdt et al., 2006; Read et al., 2006; Inoue, 2009; Petts et al., 2015; Bonfini & Graham, 2016; Banik & van den Bosch, 2021). However, Figure 3 does not suggest that Nikhuli has stalled, and as such we can not conclude that Nikhuli is destined to become an eternally wandering black hole. We can, however, derive an additional estimate of the dynamical friction time scale for a shallow potential.

Assuming that Nikhuli is now orbiting within the pseudobulge of NGC 4424, and assuming this pseudobulge dominates the inner gravitational potential, we can use the Sérsic function which was found to describe this structure (Figure 4) to estimate the infall time for Nikhuli. Building on Arca-Sedda & Capuzzo-Dolcetta (2014a, their equation 21), Arca-Sedda et al. (2015, their equation 8) provide an (orbital eccentricity)-dependent variant from which the following expression was obtained. The equation depends on the inner profile slope and yields the time an infalling star cluster of mass $M_{\rm sc}$ at radius $r$ will take to get to the center of a bulge/halo with mass $M_{\rm b}$, scale radius $r_{s}$, and negative logarithmic slope of the inner density profile $\gamma_{0}$.

From our fitted Sérsic function, we derived a mass for the pseudobulge¹⁷¹⁷17Using the pseudobulge mass in the $M_{\rm bh}$–$M_{\rm bulge}$ relation for late-type galaxies (Davis et al., 2019; Sahu et al., 2019) gives a predicted black hole mass of $\log(M_{\rm bh}/{\rm M}_{\odot})=3.8$. However, the unrelaxed nature of this pseudobulge makes this a dubious prediction. such that $\log(M_{\rm b}/{\rm M}_{\odot})=8.48\pm 0.34$, assuming $M/L_{F160W}=0.5$. This is notably greater than the mass of Nikhuli with $M_{\rm sc}=(3.44\pm 0.93)\times 10^{6}\,{\rm M}_{\odot}$, which we take to be at a deprojected radius $r=5\arcsec$ (395 pc) from the center of NGC 4424. Graham et al. (2006, their equation 23) provide an equation for the negative logarithmic slope, $\gamma_{0}$, of the model from Prugniel & Simien (1997), which uses the Sérsic model parameters to provide an expression which approximates the deprojected Sérsic (1963) $R^{1/n}$ model. For small radii, $\gamma_{0}$ depends solely on $n$. We found the pseudobulge in NGC 4424 is well-described with $n=0.47\pm 0.08$, for which we obtain $\gamma_{0}\approx 0$. Arca-Sedda et al. (2015, their equation 5) provide an expression to derive the appropriate scale radius $r_{s}$ from the projected ‘effective half-mass radius’ of the bulge/halo — taken here to be the ‘effective half-light radius’ of our equivalent-axis (aka geometric-mean axis) model for the pseudobulge. With $R_{\rm e,eq}=3\farcs 87\pm 0\farcs 39$ from our Sérsic model fit to the pseudobulge, we have that $r_{s}\approx 0.35R_{\rm e,eq}=1\farcs 35$, or $\approx$106 pc. Finally, for an eccentricity $e=0$ and $\gamma_{0}=0$, the $g(e,\gamma_{0})$ term above, from Arca-Sedda et al. (2015, their equation 10), equals 5.83. Inserting the above values into equation 5, one obtains a dynamical friction time of 220 Myr.

Alternatively, we can approximate the total (bulge $+$ bar $+$ disk) light over $\sim$1 to $\sim$8 arcseconds in Figure 4 with an $n=1$ model having a half-light radius of 8$\farcs$4. The stellar mass of this model, within this half-light radius, is approximately $0.8\times 10^{9}$ M_⊙, with an extrapolation of the profile to infinity doubling this mass to $1.6\times 10^{9}$ M_⊙. Under the approximation of a spherical system, this model corresponds to $\gamma_{0}=0.44$, $r_{s}=3\farcs 5$ (277 pc), and $g(e,\gamma_{0})\approx 5.2$, and it yields a similar infall time of 206 Myr. Increasing the assumed distance of Nikhuli from the galaxy center by a factor of 3 lengthens the infall time to $\sim$1.4 Gyr, while assuming an eccentricity of 0.5 will reduce the times by a factor of 0.65, giving $t=0.9$ Myr in this instance. The star cluster(s) seen within the inner arcsecond may be a testament to these dynamical times.

The phenomenon of an infalling galaxy undergoing assimilation has been offered to explain the offset IMBH in ESO 243-49 (Farrell et al., 2009; Bellovary et al., 2010; Mapelli et al., 2012; Webb et al., 2017; Bellovary et al., 2021; Ricarte et al., 2021) and the ULX sources likely associated with a massive black hole in galaxies such as IC 4320 (Sutton et al., 2012), NGC 5252 (Kim et al., 2015, 2020), NGC 2276-3c (Mezcua et al., 2015) and others. One such interesting example can be seen in the Virgo cluster galaxy NGC 4651 (Martínez-Delgado et al., 2010; Foster et al., 2014; Morales et al., 2018), where the elongation process associated with infall has produced a tidal stream (Newberg, 2016) which is immediately evident in optical images. An earlier stage of such shredding, of an infalling dwarf galaxy, has been reported in a galaxy at a redshift $z=0.145$ by Forbes et al. (2003). See also UGC 10214 (Vorontsov-Vel’Yaminov, 1959; Miskolczi et al., 2011), NGC 7714 (Soria & Motch, 2004), and M32 giving itself to M31 (Arp, 1964; Graham, 2002; Gordon et al., 2006). Various stages of the interaction process are evident in the catalog of Vorontsov-Velyaminov (1977), and the scenario in which a galaxy is threshed and reduced to its dense core has been developed by (Bekki & Freeman, 2003).

In and around our Galaxy, we know that the Sagittarius dwarf galaxy is undergoing disruption (Ibata et al., 1994; Koposov et al., 2015), the Cetus and Indus stellar streams are the remnants of accreted dwarf galaxies Chang et al. (2020); Hansen et al. (2021), and the Gaia-Enceladus-Sausage was acquired long ago (Belokurov et al., 2018; Helmi et al., 2018). Furthermore, Ibata et al. (2019a) revealed the impressive tidal stream associated with $\omega$ Centauri, possibly the pared-core of an accreted dwarf galaxy but currently lacking confirmation of a central black hole (Zocchi et al., 2019; Baumgardt et al., 2019). Additional globular cluster streams, evidence of accretion and possibly remnant galaxy nuclei in the Milky Way are known (e.g. Bonaca et al., 2021; Woody & Schlaufman, 2021; Pfeffer et al., 2021; Thomas et al., 2020; Ferguson et al., 2021; Piatti et al., 2021). Yuan et al. (2020) and Malhan et al. (2021) discovered and investigated the low-mass stream LMS-1, revealing the remnants of a low-mass galaxy now just 10 to 20 kpc from our Galaxy’s center, making it the closest known stream to the Galaxy’s center thus far. They conclude that the “globular cluster” NGC 5024 or NGC 5053 is likely the former nuclear star cluster of this low-mass galaxy, akin to NGC 5824 possibly being the remnant nuclear star cluster of the dwarf progenitor to the Cetus stream (Chang et al., 2020). The slightly more massive (than globular cluster) UCD galaxies (e.g. Ahn et al., 2017) have also been caught in the act of stripping. They are considered to be the former nuclei of galaxies (e.g., VUCD3: Liu et al., 2015) and are known to contain massive black holes (Graham, 2020, and references therein).

Globular clusters are known to contain XRBs (e.g. Hut et al., 1992; Becker et al., 2003; Maccarone et al., 2003), and it has been speculated that their capture may contribute to the LMXB population in galaxies (Lehmer et al., 2020). With a stellar mass of $\sim$3$\times 10^{6}\,M_{\odot}$, Nikhuli is more massive than most globular clusters. Furthermore, the width of the Nikhuli stream is some 35 pc across. As such, we are probably not seeing a captured globular cluster—which have half light radii typically less than 4 pc (Jordán et al., 2005)—but more likely the nucleus of a stripped galaxy. Conceivably, the color or metallicity of Nikhuli might shed light on the full stellar mass of the progenitor via the color-magnitude or mass-metallicity relation for dwarf galaxies. However, given that the remnant nuclear star cluster light will now dominate the flux and color, this would yield questionable results.

Although Nikhuli is unlikely to be a captured globular cluster, if nuclear star clusters have a similar LMXB formation efficiency as globular clusters, then Nikhuli might contain a LMXB. It would therefore be desirable to observe Doppler-broadened, optical emission lines coming from NGC 4424 X-3 to verify the existence of the potential massive black hole. Such confirmation of a massive black hole mass might be possible if (with sufficiently high spatial resolution in order to reduce the stellar noise) broad optical emission lines are observed from NGC 4424 X-3. An alternative approach to estimate the black hole mass could come from the ‘fundamental plane of black hole activity’. As given by Plotkin et al. (2012), it predicts a radio luminosity $\nu\,L_{\nu}$(5 GHz) of $10^{33.9}$ erg s^-1 for $M_{\rm bh}=0.7\times 10^{5}\,M_{\odot}$ and $L_{0.5-10\,{\rm keV}}=0.7\times 10^{39}$ erg s^-1. At a distance of 16.4 Mpc, this corresponds to 5 $\mu$Jy, tantalizingly within the reach of the Karl G. Jansky Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA), which have probed down to root mean square (rms) noise levels of $\sim$1–2 $\mu$Jy beam^-1 (Strader et al., 2012; Tremou et al., 2018).

An incoming black hole of equal mass to that already (presumed to be) at the center of NGC 4424 might imply a major merger between two spiral galaxies. However, such a scenario is unlikely to result in a post-merger galaxy looking like a spiral galaxy. On the other hand, early-type galaxies have smaller stellar masses for a given black hole mass than late-type galaxies (Sahu et al., 2019). As such, a minor merger can bring in an equal mass black hole. This suggestion, which we think is presented here for the first time, may also explain why the extreme ULX and HLX off-center sources found by Sutton et al. (2012) in eight nearby ($<$100 Mpc) galaxies are not in galaxies displaying signs of a recent major merger.

A minor merger in NGC 4424 also meshes with the notion of an unequal (stellar mass) merger reported by Boselli et al. (2018). From Figure 11 in Sahu et al. (2019), one can see that an early-type galaxy with a black mass of $10^{4.8}\,M_{\odot}$ (see section 3.2) will have a stellar mass of $\sim 0.6\times 10^{9}\,M_{\odot}$, which is just 6% of the stellar mass of NGC 4424 (see section 2.2). Suppose such an early-type galaxy is what fell into NGC 4424. In that case, (ignoring gaseous processes), the coalescence of NGC 4424’s initial and incoming black hole will double the central black hole mass, with the stellar mass increasing by only $\sim$6%. We speculate that such minor mergers may help maintain the steepness of the non-linear, near-cubic $M_{\rm bh}$–$M_{\rm*,gal}$ relation for spiral galaxies (Savorgnan et al., 2016; Davis et al., 2018). Furthermore, compact spheroids and the bulges of early-type galaxies formed early in the Universe (Graham et al., 2015, and references therein) and were likely the formation sites of the first massive black holes. Some of these galaxies (and their black holes) may have subsequently been sewn (and sown) into late-type spiral galaxies.

Given the similarity of the predicted (central and infalling) black hole masses in NGC 4424, and the absence of an X-ray point source at the (very) center of NGC 4424, it might, but need not, be the case that NGC 4424 currently has no central black hole. The delivery of NGC 4424 X-3 via a low-mass early-type galaxy might demonstrate how some apparently bulgeless late-type galaxies, like, for example, M33 (Merritt et al., 2001), could eventually be both seeded with massive black holes and commence building their bulge. The median bulge-to-total flux ratios in Sc and Sd galaxies are only 8% (Graham & Worley, 2008). The bulge growth from minor mergers might be slow at first, and initially go undetected while the bulge-to-total stellar mass ratio is small or ill-defined, leading to ‘bulgeless galaxies’ with massive black holes, such as NGC 4395, NGC 2748 and NGC 6926 (Davis et al., 2019). Indeed, Cortés et al. (2006) has suggested that NGC 4424 will become a lenticular galaxy with a small bulge over the next 3 Gyr. The merger-induced bulge growth arises not just from the delivery of new stellar material sewn into the galaxy, but also from the redistribution of gas toward the galaxy center, which subsequently undergoes star formation.

In a downsizing scenario, such minor mergers would represent the late-time seeding of massive black holes into late-type spiral galaxies. Seeding by accretion may, of course, occur over a range of redshifts. This is different to high-$z$ quasars getting a kick start in life from a ‘black hole seed’ that is an IMBH (e.g., Düchting, 2004; Koushiappas et al., 2004), unless the origin of massive black holes in dwarf early-type galaxies is the same as high-$z$ quasars. While the old nuclear star clusters of five low-mass dwarf early-type galaxies (Paudel et al., 2011) may support this, those clusters could represent globular clusters captured through dynamical friction. Furthermore, it is not yet clear that today’s dwarf early-type galaxies, a source of minor mergers for spiral galaxies, ubiquitously contain a massive black hole. Although Graham et al. (2021) report that the detection of an X-ray point-source at the center of low-mass late-type galaxies is $\sim$40%, while it is just $\sim$10% in low-mass early-type galaxies, they suggest that this may reflect the availability of cold gas for igniting the candidate AGN in these galaxies, rather than representing a measure of the massive black hole occupation fraction. Further work is required to establish the true massive black hole occupation fraction in low-mass galaxies.

In addition to a ‘classical’ (i.e., merger built) bulge, an incoming perturber might cause a thin in-plane bar to experience an instability leading to the creation of an X/(peanut shell)-shaped ‘pseudobulge’ structure (Combes et al., 1990; Athanassoula, 2005; Ciambur & Graham, 2016). Such a feature may be apparent in NGC 4424 (Cortés et al., 2006, see their Figure 2), where, interestingly, there are also signs that this structure forms the inner half of a figure-of-eight pattern, such as that more clearly seen in NGC 4429 (Frei et al., 1996; Baillard et al., 2011; Cappellari et al., 2011, their Figure 6). In passing, we speculate that NGC 4429 may, therefore, represent a more evolved state of NGC 4424. NGC 4608 (Cappellari et al., 2011) may offer a more face-on representation of NGC 4429. If so, then the figure-of-eight pattern may arise from an X-shaped pseudobulge coupled with the ansae/ring at the end of the original bar.

At $6\times 10^{10}\,M_{\odot}$ (Licquia & Newman, 2015), the Milky Way has a stellar mass some six times bigger than NGC 4424, and a clear X/P pseudobulge (Ciambur et al., 2017, and references therein). This pseudobulge, which peaks at roughly half the length of the bar, likely formed from perturbations to the bar due to by minor fly-bys (Kumar et al., 2021) or minor mergers which also contribute to the classical bulge (Nataf, 2017; Zoccali et al., 2018). If six minor merger events have brought in six $10^{5}\,M_{\odot}$ black holes, one might (naively) expect a central black hole with a minimum mass of $0.6\times 10^{6}\,M_{\odot}$, with gas fuelling and stellar capture perhaps increasing this to the observed value of $4\times 10^{6}\,M_{\odot}$ (Boehle et al., 2016). The zoom-in hydrodynamical simulation of (Bonoli et al., 2016) supports such growth by infall and black hole merging. It would also explain the tentative suggestions of IMBHs infalling/wandering in the Milky Way (Takekawa et al., 2020, and references therein).

We have discovered, in HST imaging, a red, elongated star cluster located $\sim$400 pc ($\sim$5$\arcsec$), in projection, from the center of the post-merger galaxy NGC 4424. This may be the remnants of the infalling galaxy responsible for the known, past minor-merger event that disturbed NGC 4424. The star cluster, referred to as Nikhuli, is stretched in the direction of NGC 4424’s center, and could be the nuclear star cluster of the captured galaxy.

Based on the $3\times 10^{6}\,M_{\odot}$ stellar mass of Nikhuli, we used the $M_{\rm bh}$–$M_{\rm nc}$ scaling relation to predict that it may harbour an intermediate-mass black hole of $7\times 10^{4}\,M_{\odot}$, although this estimate has a considerable 1.6 dex of uncertainty. Interestingly, Nikhuli corresponds spatially with a CXO X-ray point-source having $L_{\rm X}\equiv L_{0.5-10\,{\rm keV}}=0.7\times 10^{39}$ erg s^-1 for a photon index $\Gamma=1.7$. While this source may be a super-Eddington stellar-mass black hole, we argue that it is more likely to be a sub-Eddington massive black hole.

Based on the stellar mass and velocity dispersion of NGC 4424, it is expected to possess an intermediate-mass black hole with $\log M_{\rm bh}/{\rm M}_{\odot}=4.9\pm 0.6$. While the X-ray point-source in Nikhuli is highly unlikely to be a high-mass X-ray binary, we estimate that there is a 7% chance that it may be a low-mass X-ray binary if the formation efficiency of LMXBs in nuclear star clusters is 1000 times greater than that of a galaxy’s field stars. On the other hand, given that nuclear star clusters and massive black holes are known to regularly coexist, Nikhuli may be the delivery vehicle for seeding a, or at least helping to grow the, massive black hole in NGC 4424. At the same time, Nikhuli’s now disrupted host galaxy may have been sewn into the fabric of NGC 4424, contributing to the build up of the stellar halo or a classical bulge depending on how far the stars penetrate.

We plan to search for more such hidden structures, like Nikhuli, in 74 other Virgo cluster spiral galaxies imaged by CXO (Soria et al., 2021). To date, a lot of the tidal stream discoveries have occurred in the halos of galaxies. Our galaxy capture-and-subtract process will hopefully enable one to better probe the inner regions. In particular, we intend to cross-match any new discoveries with the off-center ULXs identified by (Soria et al., 2021). This may provide a statistical argument for how important minor merging could be as either a black hole seeding or growth mechanism for late-type spiral galaxies.