We improve the dynamical black hole (BH) mass estimates in three nearby low-mass early-type galaxies: NGC 205, NGC 5102, and NGC 5206. We use new Hubble Space Telescope (HST)/STIS spectroscopy to fit the star formation histories of the nuclei in these galaxies, and use these measurements to create local color–mass-to-light ratio (M/L) relations. We then create new mass models from HST imaging and combined with adaptive optics kinematics, we use Jeans dynamical models to constrain their BH masses. The masses of the central BHs in NGC 5102 and NGC 5206 are both below one million solar masses and are consistent with our previous estimates, M _⊙ and M _⊙ (3σ errors), respectively. However, for NGC 205, the improved models suggest the presence of a BH for the first time, with a best-fit mass of M _⊙ (3σ errors). This is the least massive central BH mass in a galaxy detected using any method. We discuss the possible systematic errors of this measurement in detail. Using this BH mass, the existing upper limits of both X-ray, and radio emissions in the nucleus of NGC 205 suggest an accretion rate ≲10⁻⁵ of the Eddington rate. We also discuss the color–M/L _eff relations in our nuclei and find that the slopes of these vary significantly between nuclei. Nuclei with significant young stellar populations have steeper color–M/L _eff relations than some previously published galaxy color–M/L _eff relations.

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 the 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 and NGC 5206 are estimated for the first time and both found to be <10⁶ 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 ( M _⊙; km s⁻¹) early-type galaxies host BHs. Such a high occupation fraction suggests that 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 to 73 × 10⁶ M _⊙ and compare these masses to scaling relations for NSCs based primarily on photometric mass estimates. Color gradients suggest that 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 best fit with multiple morphological components (NGC 5102 and NGC 5206). The NSC kinematics show they are rotating, especially in M32 and NGC 5102 ( ).

We explore the nucleus of the nearby 10⁹ M ⊙ early-type galaxy, NGC 404, using Hubble Space Telescope (HST)/STIS spectroscopy and WFC3 imaging. We first present evidence for nuclear variability in UV, optical, and infrared filters over a time period of 15 years. This variability adds to the already substantial evidence for an accreting black hole at the center of NGC 404. We then redetermine the dynamical black hole mass in NGC 404 including modeling of the nuclear stellar populations. We combine HST/STIS spectroscopy with WFC3 images to create a local color–M/L relation derived from stellar population modeling of the STIS data. We then use this to create a mass model for the nuclear region. We use Jeans modeling to fit this mass model to adaptive optics stellar kinematic observations from Gemini/NIFS. From our stellar dynamical modeling, we find a 3σ upper limit on the black hole mass of 1.5 × 10 5 M ⊙ . Given the accretion evidence for a black hole, this upper limit makes NGC 404 the lowest mass central black hole with dynamical mass constraints. We find that the kinematics of H₂ emission line gas show evidence for non-gravitational motions preventing the use of gas dynamical modeling to constrain the black hole mass. Our stellar population modeling also reveals that the central, counter-rotating region of the nuclear cluster is dominated by ∼1 Gyr old populations.

We examine the internal properties of the most massive ultracompact dwarf galaxy (UCD), M59-UCD3, by combining adaptive-optics-assisted near-IR integral field spectroscopy from Gemini/NIFS and Hubble Space Telescope (HST) imaging. We use the multiband HST imaging to create a mass model that suggests and accounts for the presence of multiple stellar populations and structural components. We combine these mass models with kinematics measurements from Gemini/NIFS to find a best-fit stellar mass-to-light ratio (M/L) and black hole (BH) mass using Jeans anisotropic models (JAMs), axisymmetric Schwarzschild models, and triaxial Schwarzschild models. The best-fit parameters in the JAM and axisymmetric Schwarzschild models have BHs between 2.5 and 5.9 million solar masses. The triaxial Schwarzschild models point toward a similar BH mass but show a minimum χ ² at a BH mass of ∼0. Models with a BH in all three techniques provide better fits to the central V _rms profiles, and thus we estimate the BH mass to be 4.2 − 1.7 + 2.1 × 10 6 M _⊙ (estimated 1σ uncertainties). We also present deep radio imaging of M59-UCD3 and two other UCDs in Virgo with dynamical BH mass measurements, and we compare these to X-ray measurements to check for consistency with the fundamental plane of BH accretion. We detect faint radio emission in M59cO but find only upper limits for M60-UCD1 and M59-UCD3 despite X-ray detections in both these sources. The BH mass and nuclear light profile of M59-UCD3 suggest that it is the tidally stripped remnant of a ∼10⁹–10¹⁰ M _⊙ galaxy.

We estimate the mass of the intermediate-mass black hole at the heart of the dwarf elliptical galaxy NGC 404 using Atacama Large Millimetre/submillimetre Array (ALMA) observations of the molecular interstellar medium at an unprecedented linear resolution of ≈0.5 pc, in combination with existing stellar kinematic information. These ALMA observations reveal a central disc/torus of molecular gas clearly rotating around the black hole. This disc is surrounded by a morphologically and kinematically complex flocculent distribution of molecular clouds, that we resolve in detail. Continuum emission is detected from the central parts of NGC 404, likely arising from the Rayleigh–Jeans tail of emission from dust around the nucleus, and potentially from dusty massive star-forming clumps at discrete locations in the disc. Several dynamical measurements of the black hole mass in this system have been made in the past, but they do not agree. We show here that both the observed molecular gas and stellar kinematics independently require a ≈5 × 10⁵ M_⊙ black hole once we include the contribution of the molecular gas to the potential. Our best estimate comes from the high-resolution molecular gas kinematics, suggesting the black hole mass of this system is 5.5|$^{+4.1}_{-3.8}\times 10^5$| M_⊙ (at the 99 per cent confidence level), in good agreement with our revised stellar kinematic measurement and broadly consistent with extrapolations from the black hole mass–velocity dispersion and black hole mass–bulge mass relations. This highlights the need to accurately determine the mass and distribution of each dynamically important component around intermediate-mass black holes when attempting to estimate their masses.

Multiwavelength observations toward protostars reveal complex properties of dust polarization, which are challenging to interpret. Here we study the physical processes inducing the alignment of the grain axis of the maximum inertia moment with the angular momentum ( J ; i.e., internal alignment) and of J with the magnetic field (i.e., external alignment) of very large grains (VLGs; of radius a > 10 μm) using the alignment framework based on radiative torques (RATs) and mechanical torques (METs). We derive analytical formulae for critical sizes of grain alignment, assuming grains aligned at low-J and high-J attractors by RATs (METs). For protostellar cores, we find that super-Barnett relaxation induces efficient internal alignment for VLGs with large iron inclusions, but inelastic relaxation is efficient for VLGs regardless of composition aligned at high-J attractors by RATs (METs). For external alignment, VLGs with iron inclusions aligned at high-J attractors have magnetic alignment by RATs (B-RAT) or METs (B-MET), enabling dust polarization as a reliable tracer of magnetic fields in dense regions. Still, grains at low-J attractors or without iron inclusions have alignment with J along the radiation direction (k-RAT) or gas flow (v-MET). For protostellar disks, we find that super-Barnett relaxation is efficient for grains with large iron inclusions in the outer disk thanks to spin-up by METs, but inelastic relaxation is inefficient. VLGs aligned at low-J attractors can have k-RAT (v-MET) alignment, but grains aligned at high-J attractors likely exhibit B-RAT (B-MET) alignment. We also find that grain alignment by METs is more important than that by RATs in protostellar disks.