We present new HST I-band images of a sample of 77 nearby, late-type spiral galaxies with low inclination. The main purpose of this catalog is to study the frequency and properties of nuclear star clusters. In 59 galaxies of our sample, we 1 Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with proposal # 8599.

We study the nuclear star clusters (NCs) in spiral galaxies of various Hubble types using spectra obtained with the STIS on board the Hubble Space Telescope (HST ). We observed the nuclear clusters in 40 galaxies, selected from two previous HST WFPC2 imaging surveys. At a spatial resolution of $0B2 the spectra provide a better separation of cluster light from underlying galaxy light than is possible with ground-based spectra. Approximately half of the spectra have a sufficiently high signal-to-noise ratio for detailed stellar population analysis. For the other half we only measure the continuum slope, as quantified by the B À V color. To infer the star formation history, metallicity, and dust extinction, we fit weighted superpositions of single-age stellar population templates to the high signal-to-noise ratio spectra. We use the results to determine the luminosity-weighted age, mass-to-light ratio, and masses of the clusters. Approximately half of the sample clusters contain a population younger than 1 Gyr. The luminosity-weighted ages range from 10 Myr to 10 Gyr. The stellar populations of NCs are generally best fit as a mixture of populations of different ages. This indicates that NCs did not form in a single event, but that instead they had additional star formation long after the oldest stars formed. On average, the sample clusters in late-type spirals have a younger luminosityweighted mean age than those in early-type spirals (h log i L ¼ 8:37 AE 0:25 vs. 9:23 AE 0:21). The average mass-weighted ages are older by $0.7 dex, indicating that there often is an underlying older population that does not contribute much light but does contain most of the mass. The average cluster masses are smaller in late-type spirals than in early-type spirals (log M ¼ 6:25 AE 0:21 vs. 7:63 AE 0:24) and exceed the masses typical of globular clusters. The cluster mass correlates loosely with total galaxy luminosity. It correlates more strongly with both the Hubble type of the host galaxy and the luminosity of its bulge. The latter correlation has the same slope as the well-known correlation between supermassive black hole mass and bulge luminosity. The properties of both nuclear clusters and black holes in the centers of spiral galaxies are therefore intimately connected to the properties of the host galaxy, and in particular its bulge component. Plausible formation scenarios have to account for this. We discuss various possible selection biases in our results, but conclude that none of them can explain the differences seen between clusters in early-and late-type spirals. The inability to infer spectroscopically the populations of faint clusters does introduce a bias toward younger ages, but not necessarily toward higher masses.

In the last decade star clusters have been found in the centers of spiral galaxies across all Hubble types. We here present a spectroscopic study of the exceptionally bright (10 6 -10 8 L ⊙ ) but compact (r e ∼ 5 pc) nuclear star clusters in very late type spirals with UVES at the VLT. We find the velocity dispersions of the nine clusters in our sample to range from 13 to 34 km s −1 . Using photometric data from the HST/WFPC2 and spherically symmetric dynamical models we determine masses between 8 × 10 5 and 6 × 10 7 M ⊙ . The mass to light ratios range from 0.2 to 1.5 in the I band This indicates a young mean age for most clusters, in agreement with previous studies. Given their high masses and small sizes we find that nuclear clusters are among the objects with the highest mean surface density known (up to 10 5 M ⊙ pc −2 ). From their dynamical properties we infer that, rather than small bulges, the closest structural kin of nuclear clusters appear to be massive compact star clusters. This includes such different objects as globular clusters, "super star clusters", ultra compact dwarf galaxies and the nuclei of dwarf elliptical galaxies. It is a challenge to explain why, despite the wildly different current environments, all different types of massive star clusters share very similar and structural properties. A possible explanation links UCDs and massive globular clusters to nuclear star clusters through stripping of nucleated dwarf galaxies in a merger event. The extreme properties of this type of clusters would then be a consequence of their location in the centers of their respective host galaxies.

As part of an ongoing effort to study the stellar nuclei of very late-type, bulgeless spirals, we present results from a high-resolution spectroscopic survey of nine such nuclear star clusters, undertaken with VLT/UVES. We fit the spectra with population synthesis models and measure Lick-type indices to determine mean luminosity-weighted ages, which range from 4.1 × 10 7 to 1.1 × 10 10 years and are insensitive to assumed metallicity or internal extinction. The average metallicity of nuclear clusters in late-type spirals is slightly sub-solar ( Z = 0.015) but shows significant scatter. Most of the clusters have moderate extinctions of 0.1 to 0.3 mags in the I-band. The fits also show that the nuclear cluster spectra are best described by a mix of several generations of stars. This is supported by the fact that only models with composite stellar populations yield mass-tolight ratios that match those obtained from dynamical measurements. For our nine sample clusters, the last star formation episode was on average 34 Myr ago, while all clusters experienced some star formation in the last 100 Myr. We thus conclude that the nuclear clusters undergo repeated episodes of star formation. The robustness of our results with respect to possible contamination from the underlying galaxy disk is demonstrated by comparison to a similar analysis using smaller-aperture spectra obtained with HST/STIS. Combining these results with those from Walcher et al. (2005), we have thus shown that the stellar nuclei of these bulge-less galaxies are massive and dense star clusters that form stars -2recurrently until the present day. This set of properties is unique among the various classes of star clusters. It is almost inevitable to associate these unique properties with the location of the cluster in its host galaxy. It remains a challenging question to elucidate exactly how very late-type spirals manage to create nuclei with such extreme characteristics.

We review the current knowledge about nuclear star clusters (NSCs), the spectacularly dense and massive assemblies of stars found at the centers of most galaxies. Recent observational and theoretical works suggest that many NSC properties, including their masses, densities, and stellar populations, vary with the properties of their host galaxies. Understanding the formation, growth, and ultimate fate of NSCs, therefore, is crucial for a complete picture of galaxy evolution. Throughout the review, we attempt to combine and distill the available evidence into a coherent picture of NSC evolution. Combined, this evidence points to a clear transition mass in galaxies of $$\sim 10^9\,M_\odot$$ ∼ 10 9 M ⊙ where the characteristics of nuclear star clusters change. We argue that at lower masses, NSCs are formed primarily from globular clusters that inspiral into the center of the galaxy, while at higher masses, star formation within the nucleus forms the bulk of the NSC. We also discuss the co-existence of NSCs and central black holes, and how their growth may be linked. The extreme densities of NSCs and their interaction with massive black holes lead to a wide range of unique phenomena including tidal disruption and gravitational-wave events. Finally, we review the evidence that many NSCs end up in the halos of massive galaxies stripped of the stars that surrounded them, thus providing valuable tracers of the galaxies’ accretion histories.

We present a catalogue of photometric and structural properties of 228 nuclear star clusters (NSCs) in nearby late-type disk galaxies. These new measurements are derived from a homogeneous analysis of all suitable WFPC2 images in the HST archive. The luminosity and size of each NSC is derived from an iterative PSF-fitting technique, which adapts the fitting area to the effective radius (r eff ) of the NSC, and uses a WFPC2-specific PSF model tailored to the position of each NSC on the detector.The luminosities of NSCs are 10 8 L V, , and their integrated optical colours suggest a wide spread in age. We confirm that most NSCs have sizes similar to Globular Clusters (GCs), but find that the largest and brightest NSCs occupy the regime between Ultra Compact Dwarf (UCD) and the nuclei of early-type galaxies in the size-luminosity plane. The overlap in size, mass, and colour between the different incarnations of compact stellar systems provides a support for the notion that at least some UCDs and the most massive Galactic GCs, may be remnant nuclei of disrupted disk galaxies.We find tentative evidence for the NSCs' r eff to be smaller when measured in bluer filters, and discuss possible implications of this result. We also highlight a few examples of complex nuclear morphologies, including double nuclei, extended stellar structures, and nuclear F 606W excess from either recent (circum-)nuclear star formation and/or a weak AGN. Such examples may serve as case studies for ongoing NSC evolution via the two main suggested mechanisms, namely cluster merging and in situ star formation.

Galactic nuclei typically host either a Nuclear Star Cluster (NSC, prevalent in galaxies with masses 10 10 M ) or a Massive Black Hole (MBH, common in galaxies with masses 10 12 M ). In the intermediate mass range, some nuclei host both a NSC and a MBH. In this paper, we explore scaling relations between NSC mass (M NSC ) and host galaxy total stellar mass (M ,gal ) using a large sample of NSCs in late-and earlytype galaxies, including a number of NSCs harboring a MBH. Such scaling relations reflect the underlying physical mechanisms driving the formation and (co)evolution of these central massive objects. We find ∼ 1.5σ significant differences between NSCs in late-and early-type galaxies in the slopes and offsets of the relations r eff,NSC -M NSC , r eff,NSC -M ,gal and M NSC -M ,gal , in the sense that i) NSCs in late-types are more compact at fixed M NSC and M ,gal ; and ii) the M NSC -M ,gal relation is shallower for NSCs in late-types than in early-types, similar to the M BH -M ,bulge relation. We discuss these results in the context of the (possibly ongoing) evolution of NSCs, depending on host galaxy type. For NSCs with a MBH, we illustrate the possible influence of a MBH on its host NSC, by considering the ratio between the radius of the MBH sphere of influence and r eff,NSC . NSCs harbouring a sufficiently massive black hole are likely to exhibit surface brightness profile deviating from a typical King profile.

We present a detailed study of the nuclear star clusters (NSCs) and massive black holes (BHs) of four of the nearest low-mass early-type galaxies: M32, NGC 205, NGC 5102, and NGC 5206. We measure dynamical masses of both the BHs and NSCs in these galaxies using Gemini/NIFS or VLT/SINFONI stellar kinematics, Hubble Space Telescope (HST ) imaging, and Jeans Anisotropic Models. We detect massive BHs in M32, NGC 5102, and NGC 5206, while in NGC 205, we find only an upper limit. These BH mass estimates are consistent with previous measurements in M32 and NGC 205, while those in NGC 5102 & NGC 5206 are estimated for the first time, and both found to be <10 6 M . This adds to just a handful of galaxies with dynamically measured sub-million M central BHs. Combining these BH detections with our recent work on NGC 404's BH, we find that 80% (4/5) of nearby, low-mass (10 9 − 10 10 M ; σ ∼ 20 − 70 km s −1 ) early-type galaxies host BHs. Such a high occupation fraction suggests the BH seeds formed in the early epoch of cosmic assembly likely resulted in abundant seeds, favoring a low-mass seed mechanism of the remnants, most likely from the first generation of massive stars. We find dynamical masses of the NSCs ranging from 2 − 73 × 10 6 M and compare these masses to scaling relations for NSCs based primarily on photometric mass estimates. Color gradients suggest younger stellar populations lie at the centers of the NSCs in three of the four galaxies (NGC 205, NGC 5102, and NGC 5206), while the morphology of two are complex and are best-fit with multiple morphological components (NGC 5102 and NGC 5206). The NSC kinematics show they are rotating, especially in M32 and NGC 5102 (V /σ ∼ 0.7).