Are nuclear star clusters the precursors of massive black holes?

We present new upper limits for black hole masses in extremely late type spiral galaxies. We confirm that this class of galaxies has black holes with masses less than 10^6 Msolar, if any. We also derive new upper limits for nuclear star cluster (NC) masses in massive galaxies with previously determined black hole masses. We use the newly derived upper limits and a literature compilation to study the low mass end of the global-to-nucleus relations. We find the following (1) The M_BH-sigma relation cannot flatten at low masses, but may steepen. (2) The M_BH-M_bulge relation may well flatten in contrast. (3) The M_BH-Sersic n relation is able to account for the large scatter in black hole masses in low-mass disk galaxies. Outliers in the M_BH-Sersic n relation seem to be dwarf elliptical galaxies. When plotting M_BH versus M_NC we find three different regimes: (a) nuclear cluster dominated nuclei, (b) a transition region, and (c) black hole-dominated nuclei. This is consistent with the picture, in which black holes form inside nuclear clusters with a very low-mass fraction. They subsequently grow much faster than the nuclear cluster, destroying it when the ratio M_BH/M_NC grows above 100. Nuclear star clusters may thus be the precursors of massive black holes in galaxy nuclei.Comment: This version has a corrected value for Sersic n for NGC205, which got mixed up in the original version. None of the conclusions chang

The nucleus of Centaurus A

At less than 4 Mpc away, Centaurus A (NGC 5128) is the nearest massive elliptical galaxy, the nearest radio galaxy, and the nearest recent merger. It is the ideal laboratory to study the connection between merging, massive black holes, the source of radio jets, and merger-induced star formation in detail. Using Naos-Conica and SINFONI at the ESO Very Large Telescope (VLT), we obtained adaptive optics (AO) assisted data at unprecedented spatial resolution. We demonstrate that thorough kinematical modelling of AO data is feasible and leads to an accurate measurement of the black hole mass. We find that depending on their ionisation level, different gas species display different flux distributions and velocity structures. In this respect, integral-field-unit data are crucial to identify non-gravitational gas motions. The H2 gas kinematics inside the central r<1.5 arcsec of the active galactic nucleus are successfully described by a (warped) gas disk, rotating in the joint gravitational potential of the stars and a black hole of Mbh~7x10^7 Msolar. With our revised Mbh estimate, that is a factor ~3 lower than previous measurements, Cen A is no longer a dramatic outlier in the Mbh-sigma relation. Near-infrared images in JHK reveal marginally resolved stellar clusters, comparable in their properties to young starburst clusters found close to the Galactic Centre

First Gaia dynamical model of the Milky Way disc with six phase space coordinates: a test for galaxy dynamics

We construct the first comprehensive dynamical model for the high-quality subset of stellar kinematics of the Milky Way disc, with full 6D phase-space coordinates, provided by the Gaia Data Release 2. We adopt an axisymmetric approximation and use an updated Jeans Anisotropic Modelling (JAM) method, which allows for a generic shape and radial orientation of the velocity ellipsoid, as indicated by the Gaia data, to fit the mean velocities and all three components of the intrinsic velocity dispersion tensor. The Milky Way is the first galaxy for which all intrinsic phase space coordinates are available, and the kinematics are superior to the best integral-field kinematics of external galaxies. This situation removes the long-standing dynamical degeneracies and makes this the first dynamical model highly over-constrained by the kinematics. For these reasons, our ability to fit the data provides a fundamental test for both galaxy dynamics and the mass distribution in the Milky Way disc. We tightly constrain the volume average total density logarithmic slope, in the radial range 3.6--12 kpc, to be $\alpha_{\rm tot}=-2.149\pm 0.055$ and find that the dark halo slope must be significantly steeper than $\alpha_{\rm DM}=-1$ (NFW). The dark halo shape is close to spherical and its density is $\rho_{\rm DM}(R_\odot)=0.0115\pm0.0020$ M$_\odot$ pc$^{-3}$ ($0.437\pm0.076$ GeV cm$^{-3}$), in agreement with previous estimates. The circular velocity at the solar position $v_{\rm circ}(R_{\odot}) = 236.5\pm 3.1$ km s$^{-1}$ (including systematics) and its gently declining radial trends are also consistent with recent determinations.Comment: 10 pages, 4 figures, 1 table. Accepted for publication in MNRA

Nuclear star clusters as probes of dark matter halos: the case of the Sagittarius Dwarf Spheroidal Galaxy

The Sagittarius dwarf spheroidal (Sgr dSph) galaxy is currently being accreted and disrupted by the tidal field of the Milky Way. Recent observations have shown that the central region of the dwarf hosts at least three different stellar populations, ranging from old and metal-poor over intermediate metal-rich to young metal-rich. While the intermediate-age metal-rich population has been identified as part of the galaxy, the oldest and youngest populations belong to M54, the nuclear star cluster (NSC) of the Sgr dSph galaxy. The old metal-poor component of M54 has been interpreted as at least one decayed GC which was initially orbiting its host galaxy. The youngest population formed in situ from gas accreted into M54 after its arrival at the centre of the host. In this work, we use the observed properties of M54 to explore the shape of the inner density profile of the Sgr dSph galaxy. To do so, we simulate the decay of M54 towards the centre of the dark matter (DM) halo of its host. We model the DM density profile using different central slopes, and we compare the results of the simulations to the most recent observations of the structural properties of M54. From this comparison, we conclude that a GC that decays in a DM halo with a density profile $\propto r^{-\gamma}$ and $\gamma \leq 1$ shows a rotational signal and flattening comparable to those observed for M54. Steeper profiles produce, instead, highly rotating and more flattened NSCs which do not match the properties of M54.Comment: 11 pages, 4 figures, 1 table. Accepted for publication in MNRA

GALACTICNUCLEUS: A high-angular-resolution JHKsJHK_s imaging survey of the Galactic centre. IV. Extinction maps and de-reddened photometry

The extreme extinction ($A_V\sim30$\,mag) and its variation on arc-second scales towards the Galactic centre hamper the study of its stars. Their analysis is restricted to the near infrared (NIR) regime, where the extinction curve can be approximated by a broken power law. Therefore, correcting for extinction is fundamental to analyse the structure and stellar population of the central regions of our Galaxy. We aim to, (1) discuss different strategies to de-redden the photometry and check the usefulness of extinction; (2) build extinction maps for the NIR bands $JHK_s$ and make them publicly available; (3) create a de-reddened catalogue of the GALACTICNUCLEUS (GNS) survey, identifying foreground stars; and (4) perform a preliminary analysis of the de-reddened $K_s$ luminosity functions (KLFs). We used photometry from the GNS survey to create extinction maps for the whole catalogue. We took red clump (RC) and red giant stars of similar brightnesses as a reference to build the maps and de-reddened the GNS photometry. We discussed the limitations of the process and analysed non-linear effects of the de-reddening. We obtained high resolution ($\sim3''$) extinction maps with low uncertainties ($\lesssim5$\,\%) and computed average extinctions for each of the regions covered by the GNS. We checked that our maps effectively correct the differential extinction reducing the spread of the RC features by a factor of $\sim2$. We assessed the validity of the broken power law approach computing two equivalent extinction maps $A_H$ using either $JH$ and $HK_s$ photometry for the same reference stars and obtained compatible average extinctions within the uncertainties. Finally, we analysed de-reddened KLFs for different lines of sight and found that the regions belonging to the NSD contain a homogeneous stellar population that is significantly different from that in the innermost bulge regions.Comment: Updated to the final version accepted for publication in Astronomy & Astrophysics. 17 pages, 11 figure