[en] Over the last decade, HST imaging studies have revealed that the centers of most galaxies are occupied by massive, yet compact, stellar clusters. These 'nuclear star clusters' (NSCs) are found in a wide range of Hubble types, suggesting that their formation is intricately linked to galaxy evolution. In this review, I briefly summarize what has been learned about NSCs, mention some ideas for their formation, and touch on more speculative links between NSCs, super-massive black holes, and globular clusters.

[en] We study the neutral hydrogen properties of a sample of 20 bulgeless disk galaxies (Sd-Sdm Hubble types), an interesting class that can be used to constrain galaxy formation and evolution, especially the role of mergers versus internal processes. Our sample is composed of nearby (within 32 Mpc), moderately inclined galaxies that bracket the circular velocity of 120 km s^-1, which has been found to be associated with a transition in dust scale heights in edge-on, late-type disks. Here, we present H I channel maps, line profiles, and integrated intensity maps. We also derive kinematic parameters, including the circular velocity, from rotation curve analyses and calculate the integrated H I flux and H I mass for each galaxy in the sample. Three of the 20 galaxies in our sample have kinematically distinct outer components with major axes that differ by 30⁰-90⁰ from the main disk. These distinct outer components may be due to a recent interaction, which would be somewhat surprising because the disks do not contain bulges. We will use the data products and derived properties in subsequent investigations into star formation and secular evolution in bulgeless disks with circular velocities above and below 120 km s^-1.

[en] We study the relation between the surface density of gas and star formation rate in 20 moderately inclined, bulgeless disk galaxies (Sd-Sdm Hubble types) using CO(1-0) data from the IRAM 30 m telescope, H I emission line data from the VLA/EVLA, Hα data from the MDM Observatory, and polycyclic aromatic hydrocarbon emission data derived from Spitzer IRAC observations. We specifically investigate the efficiency of star formation as a function of circular velocity (v_circ). Previous work found that the vertical dust structure and disk stability of edge-on, bulgeless disk galaxies transition from diffuse dust lanes with large scale heights and gravitationally stable disks at v_circ < 120 km s^–1 (M_* ∼< 10¹⁰ M_☉) to narrow dust lanes with small scale heights and gravitationally unstable disks at v_circ > 120 km s^–1. We find no transition in star formation efficiency (Σ_SFR/Σ_Hi+H2) at v_circ = 120 km s^–1 or at any other circular velocity probed by our sample (v_circ = 46-190 km s^–1). Contrary to previous work, we find no transition in disk stability at any circular velocity in our sample. Assuming our sample has the same dust structure transition as the edge-on sample, our results demonstrate that scale height differences in the cold interstellar medium of bulgeless disk galaxies do not significantly affect the molecular fraction or star formation efficiency. This may indicate that star formation is primarily affected by physical processes that act on smaller scales than the dust scale height, which lends support to local star formation models.

[en] We report Chandra observations of the Scd galaxy NGC 2139, which is known to host a recently formed (10^7.6 yr) nuclear star cluster. The star cluster is undetected in X-rays, with an upper bound on 0.5-7 keV luminosity of L_X < 7.1 × 10³⁷ erg s^–1. This bound implies a bolometric accretion luminosity <0.3% of the Eddington luminosity for a black hole with the mass (∼3400 M_☉ ) expected from extrapolation of the M – σ relation. The lack of X-ray emission indicates that a black hole, if present, is not undergoing significant accretion at the current time. While the central cluster is undetected, the data reveal a substantial population of bright X-ray point sources elsewhere in this galaxy, with eight qualifying as ultraluminous X-ray sources with L_X > 10³⁹ erg s^–1. We use archival Hubble Space Telescope images to identify candidate optical counterparts for seven Chandra sources, which in most cases have optical luminosities and spatial profiles consistent with star clusters. Compared with other galaxies, the number of luminous X-ray sources in NGC 2139 is larger by a factor of ∼4-10 than expected based on its present star formation rate and stellar mass. This finding can be understood if NGC 2139 has concluded a burst of star formation in the recent past, and suggests that this galaxy could be important for testing the use of X-ray source populations as a chronometer of star formation history.

[en] The detection and characterization of supermassive black holes (SMBHs) in local low mass galaxies is crucial to our understanding of the origins of SMBHs. This statement assumes that low mass galaxies have had a relatively quiet cosmic history, so that their black holes have not undergone significant growth and therefore can be treated as relics of the original SMBH seeds. While recent studies have found optical signatures of active galactic nuclei (AGNs) in a growing population of dwarf galaxies, these studies are biased against low metallicity and relatively merger-free galaxies, thus missing precisely the demographic in which to search for the relics of SMBH seeds. Here, we report the detection of the [Si vi]1.963 μm coronal line (CL), a robust indicator of an AGN in the galaxy SDSS J160135.95+311353.7, a nearby (z = 0.031) low metallicity galaxy with a stellar mass approximately an order of magnitude lower than the LMC (M _* ≈ 10^8.56 M _⊙) and no optical evidence for an AGN. The AGN bolometric luminosity implied by the CL detection is ≈10⁴² erg s⁻¹, precisely what is predicted from its near-infrared continuum emission based on well-studied AGNs. Our results are consistent with a black hole of mass ≈10⁵ M _⊙, in line with expectations based on its stellar mass. This is the first time a near-infrared CL has been detected in a low mass, low metallicity galaxy with no optical evidence for AGN activity, providing confirmation of the utility of infrared CLs in finding AGNs in low mass galaxies when optical diagnostics fail. These observations highlight a powerful avenue of investigation to hunt for low mass black holes in the James Webb Space Telescope era.

[en] Wu & Peek predict SDSS-quality spectra based on Pan-STARRS broadband grizy images using machine learning (ML). In this article, we test their prediction for a unique object, UGC 2885 (“Rubin’s galaxy”), the largest and most massive, isolated disk galaxy in the local universe (D < 100 Mpc). After obtaining the ML predicted spectrum, we compare it to all existing spectroscopic information that is comparable to an SDSS spectrum of the central region: two archival spectra, one extracted from the VIRUS-P observations of this galaxy, and a new, targeted MMT/Binospec observation. Agreement is qualitatively good, though the ML prediction prefers line ratios slightly more toward those of an active galactic nucleus (AGN), compared to archival and VIRUS-P observed values. The MMT/Binospec nuclear spectrum unequivocally shows strong emission lines except Hβ, the ratios of which are consistent with AGN activity. The ML approach to galaxy spectra may be a viable way to identify AGN supplementing NIR colors. How such a massive disk galaxy (M* = 10¹¹ M _⊙), which uncharacteristically shows no sign of interaction or mergers, manages to fuel its central AGN remains to be investigated.