Spectroastrometry of rotating gas disks for the detection of supermassive black holes in galactic nuclei. III. CRIRES observations of the Circinus galaxy

We present new CRIRES spectroscopic observations of BrGamma in the nuclear region of the Circinus galaxy, obtained with the aim of measuring the black hole (BH) mass with the spectroastrometric technique. The Circinus galaxy is an ideal benchmark for the spectroastrometric technique given its proximity and secure BH measurement obtained with the observation of its nuclear H2O maser disk. The kinematical data have been analyzed both with the classical method based on the analysis of the rotation curves and with the new method developed by us and based on spectroastrometry. The classical method indicates that the gas disk rotates in the gravitational potential of an extended stellar mass distribution and a spatially unresolved mass of (1.7 +- 0.2) 10^7 Msun, concentrated within r < 7 pc. The new method is capable of probing gas rotation at scales which are a factor ~3.5 smaller than those probed by the rotation curve analysis. The dynamical mass spatially unresolved with the spectroastrometric method is a factor ~2 smaller, 7.9 (+1.4 -1.1) 10^6 Msun indicating that spectroastrometry has been able to spatially resolve the nuclear mass distribution down to 2 pc scales. This unresolved mass is still a factor ~4.5 larger than the BH mass measurement obtained with the H2O maser emission indicating that it has not been possible to resolve the sphere of influence of the BH. Based on literature data, this spatially unresolved dynamical mass distribution is likely dominated by molecular gas and it has been tentatively identified with the circum-nuclear torus which prevents a direct view of the central BH in Circinus. This mass distribution, with a size of ~2pc, is similar in shape to that of the star cluster of the Milky Way suggesting that a molecular torus, forming stars at a high rate, might be the earlier evolutionary stage of the nuclear star clusters which are common in late type spirals.Comment: A&A in press. We wish to honor the memory of our great friend and colleague David Axon. He will be greatly missed by all of us. arXiv admin note: text overlap with arXiv:1110.093

Spectroastrometry of rotating gas disks for the detection of supermassive black holes in galactic nuclei. I. Method and simulations

This is the first in a series of papers in which we study the application of spectroastrometry in the context of gas kinematical studies aimed at measuring the mass of supermassive black holes. The spectroastrometrical method consists in measuring the photocenter of light emission in different wavelength or velocity channels. In particular we explore the potential of spectroastrometry of gas emission lines in galaxy nuclei to constrain the kinematics of rotating gas disks and to measure the mass of putative supermassive black holes. By means of detailed simulations and test cases, we show that the fundamental advantage of spectroastrometry is that it can provide information on the gravitational potential of a galaxy on scales significantly smaller (~ 1/10) than the limit imposed by the spatial resolution of the observations. We then describe a simple method to infer detailed kinematical informations from spectroastrometry in longslit spectra and to measure the mass of nuclear mass concentrations. Such method can be applied straightforwardly to integral field spectra, which do not have the complexities due to a partial spatial covering of the source in the case of longslit spectra.Comment: Accepted for publication in A&

LSD: Lyman-break galaxies Stellar populations and Dynamics. I: Mass, metallicity and gas at z~3.1

We present the first results of a project, LSD, aimed at obtaining spatially-resolved, near-infrared spectroscopy of a complete sample of Lyman-Break Galaxies at z~3. Deep observations with adaptive optics resulted in the detection of the main optical lines, such as [OII], Hbeta and [OIII], which are used to study sizes, SFRs, morphologies, gas-phase metallicities, gas fractions and effective yields. Optical, near-IR and Spitzer/IRAC photometry is used to measure stellar mass. We obtain that morphologies are usually complex, with the presence of several peaks of emissions and companions that are not detected in broad-band images. Typical metallicities are 10-50% solar, with a strong evolution of the mass-metallicity relation from lower redshifts. Stellar masses, gas fraction, and evolutionary stages vary significantly among the galaxies, with less massive galaxies showing larger fractions of gas. In contrast with observations in the local universe, effective yields decrease with stellar mass and reach solar values at the low-mass end of the sample. This effect can be reproduced by gas infall with rates of the order of the SFRs. Outflows are present but are not needed to explain the mass-metallicity relation. We conclude that a large fraction of these galaxies are actively creating stars after major episodes of gas infall or merging.Comment: MNRAS, in pres

Spectroastrometry of rotating gas disks for the detection of supermassive black holes in galactic nuclei. II. Application to the galaxy Centaurus A (NGC 5128)

We measure the black hole mass in the nearby active galaxy Centaurus A (NGC 5128) using a new method based on spectroastrometry of a rotating gas disk. The spectroastrometric approach consists in measuring the photocenter position of emission lines for different velocity channels. In a previous paper we focused on the basic methodology and the advantages of the spectroastrometric approach with a detailed set of simulations demonstrating the possibilities for black hole mass measurements going below the conventional spatial resolution. In this paper we apply the spectroastrometric method to multiple longslit and integral field near infrared spectroscopic observations of Centaurus A. We find that the application of the spectroastrometric method provides results perfectly consistent with the more complex classical method based on rotation curves: the measured BH mass is nearly independent of the observational setup and spatial resolution and the spectroastrometric method allows the gas dynamics to be probed down to spatial scales of ~0.02", i.e. 1/10 of the spatial resolution and ~1/50 of BH sphere of influence radius. The best estimate for the BH mass based on kinematics of the ionized gas is then log(MBH (sin i)^2/M\odot)=7.5 \pm 0.1 which corresponds to MBH = 9.6(+2.5-1.8) \times 10^7 M\odot for an assumed disk inclination of i = 35deg. The complementarity of this method with the classic rotation curve method will allow us to put constraints on the disk inclination which cannot be otherwise derived from spectroastrometry. With the application to Centaurus A, we have shown that spectroastrometry opens up the possibility of probing spatial scales smaller than the spatial resolution, extending the measured MBH range to new domains which are currently not accessible: smaller BHs in the local universe and similar BHs in more distant galaxies

Gas accretion as the origin of chemical abundance gradients in distant galaxies

It has recently been suggested that galaxies in the early Universe can grow through the accretion of cold gas, and that this may have been the main driver of star formation and stellar mass growth. Because the cold gas is essentially primordial, it has a very low abundance of elements heavier than helium (metallicity). As it is funneled to the centre of a galaxy, it will lead the central gas having an overall lower metallicity than gas further from the centre, because the gas further out has been enriched by supernovae and stellar winds, and not diluted by the primordial gas. Here we report chemical abundances across three rotationally-supported star-forming galaxies at z~3, only 2 Gyr after the Big Bang. We find an 'inverse' gradient, with the central, star forming regions having a lower metallicity than less active ones, opposite to what is seen in local galaxies. We conclude that the central gas has been diluted by the accretion of primordial gas, as predicted by 'cold flow' models.Comment: To Appear in Nature Oct 14, 2010; Supplementary Information included her

How well can we Measure the Intrinsic Velocity Dispersion of Distant Disk Galaxies?

The kinematics of distant galaxies, from z=0.1 to z>2, play a key role in our understanding of galaxy evolution from early times to the present. One of the important parameters is the intrinsic, or local, velocity dispersion of a galaxy, which allows one to quantify the degree of non-circular motions such as pressure support. However, this is difficult to measure because the observed dispersion includes the effects of (often severe) beam smearing on the velocity gradient. Here we investigate four methods of measuring the dispersion that have been used in the literature, to assess their effectiveness at recovering the intrinsic dispersion. We discuss the biasses inherent in each method, and apply them to model disk galaxies in order to determine which methods yield meaningful quantities, and under what conditions. All the mean weighted dispersion estimators are affected by (residual) beam smearing. In contrast, the dispersion recovered by fitting a spatially and spectrally convolved disk model to the data is unbiassed by the beam smearing it is trying to compensate. Because of this, and because the bias it does exhibit depends only on the signal-to-noise, it can be considered reliable. However, at very low signal-to-noise, all methods should be used with caution.Comment: accepted by Ap

Physical properties of dense molecular gas in centres of Seyfert galaxies

We present new ~1" resolution data of the dense molecular gas in the central 50-100 pc of four nearby Seyfert galaxies. PdBI observations of HCN and, in 2 of the 4 sources, simultaneously HCO+ allow us to carefully constrain the dynamical state of the dense gas surrounding the AGN. Analysis of the kinematics shows large line widths of 100-200 km/s FWHM that can only partially arise from beam smearing of the velocity gradient. The observed morphological and kinematic parameters (dimensions, major axis position angle, red and blue channel separation, and integrated line width) are well reproduced by a thick disk, where the emitting dense gas has a large intrinsic dispersion (20-40 km/s), implying that it exists at significant scale heights (25-30% of the disk radius). To put the observed kinematics in the context of the starburst and AGN evolution, we estimate the Toomre Q parameter. We find this is always greater than the critical value, i.e. Q is above the limit such that the gas is stable against rapid star formation. This is supported by the lack of direct evidence, in these 4 Seyfert galaxies, for on-going star formation close around the AGN. Instead, any current star formation tends to be located in a circumnuclear ring. We conclude that the physical conditions are indeed not suited to star formation within the central ~100 pc.Comment: 15 pages, 13 figures, 7 tables. Accepted for pubblication in MNRA

Tides in colliding galaxies

Long tails and streams of stars are the most noticeable upshots of galaxy collisions. Their origin as gravitational, tidal, disturbances has however been recognized only less than fifty years ago and more than ten years after their first observations. This Review describes how the idea of galactic tides emerged, in particular thanks to the advances in numerical simulations, from the first ones that included tens of particles to the most sophisticated ones with tens of millions of them and state-of-the-art hydrodynamical prescriptions. Theoretical aspects pertaining to the formation of tidal tails are then presented. The third part of the review turns to observations and underlines the need for collecting deep multi-wavelength data to tackle the variety of physical processes exhibited by collisional debris. Tidal tails are not just stellar structures, but turn out to contain all the components usually found in galactic disks, in particular atomic / molecular gas and dust. They host star-forming complexes and are able to form star-clusters or even second-generation dwarf galaxies. The final part of the review discusses what tidal tails can tell us (or not) about the structure and content of present-day galaxies, including their dark components, and explains how tidal tails may be used to probe the past evolution of galaxies and their mass assembly history. On-going deep wide-field surveys disclose many new low-surface brightness structures in the nearby Universe, offering great opportunities for attempting galactic archeology with tidal tails.Comment: 46 pages, 13 figures, Review to be published in "Tidal effects in Astronomy and Astrophysics", Lecture Notes in Physics. Comments are most welcom