Molecular gas in NUclei of GAlaxies (NUGA) VII. NGC4569, a large scale bar funnelling gas into the nuclear region

This work is part of the NUGA survey of CO emission in nearby active galaxies. We present observations of NGC4569, a member of the Virgo Cluster. We analyse the molecular gas distribution and kinematics in the central region and we investigate a possible link to the strong starburst present at the nucleus. 70% of the 1.1x10^9 Msolar of molecular gas detected in the inner 20" is found to be concentrated within the inner 800 pc and is distributed along the large scale stellar bar seen in near-infrared observations. A hole in the CO distribution coincides with the nucleus where most of the Halpha emission and blue light are emitted. The kinematics are modelled in three different ways, ranging from the purely geometrical to the most physical. This approach allows us to constrain progressively the physical properties of the galaxy and eventually to emerge with a reasonable fit to an analytical model of orbits in a barred potential. Fitting an axisymmetric model shows that the non-circular motions must be comparable in amplitude to the circular motions (120 km/s). Fitting a model based on elliptical orbits allows us to identify with confidence the single inner Lindblad resonance (ILR) of the large scale bar. Finally, a model based on analytical solutions for the gas particle orbits in a weakly barred potential constrained by the ILR radius reproduces the observations well. The mass inflow rate is then estimated and discussed based on the best fit model solution. The gravitational torques implied by this model are able to efficiently funnel the gas inside the ILR down to 300 pc, although another mechanism must take over to fuel the nuclear starburst inside 100 pc.Comment: accepted for publication in A&

Molecular Gas in Spiral Galaxies

In this review, I highlight a number of recent surveys of molecular gas in nearby spiral galaxies. Through such surveys, more complete observations of the distribution and kinematics of molecular gas have become available for galaxies with a wider range of properties (e.g., brightness, Hubble type, strength of spiral or bar structure). These studies show the promise of both interferometers and single-dish telescopes in advancing our general understanding of molecular gas in spiral galaxies. In particular, I highlight the contributions of the recent BIMA Survey of Nearby Galaxies (SONG).Comment: 8 pages, 1 figure. To appear in the proceedings of the 4th Cologne-Bonn-Zermatt-Symposium, "The Dense Interstellar Medium in Galaxies", which was held in Zermatt, Switzerland in September 200

Galaxies with unusually high abundances of molecular hydrogen

A sample of 66 galaxies from the catalog of Bettoni et al. (CISM) with anomalously high molecular-to-atomic hydrogen mass ratios (M_{mol}/M_{HI}>2) is considered. The sample galaxies do not differ systematically from other galaxies in the catalog with the same morphological types, in terms of their photometric parameters, rotational velocities, dust contents, or the total mass of gas in comparison with galaxies of similar linear sizes and disk angular momentum. This suggests that the overabundance of $H_2$ is due to transition of HI to H_2. Galaxies with bars and active nuclei are found more frequently among galaxies which have M_{mol} estimates in CISM. In a small fraction of galaxies, high M_{mol}/M_{HI} ratios are caused by the overestimation of M_{mol} due to a low conversion factor for the translation of CO-line intensities into the number of H_2 molecules along the line of sight. It is argued that the "molecularization" of the bulk of the gas mass could be due 1) to the concentration of gas in the inner regions of the galactic disks, resulting to a high gas pressure and 2) to relatively low star-formation rate per unit mass of molecular gas which indeed takes place in galaxies with high M_{mol}/M_{HI} ratios.Comment: 11 pages,7 figures, published in Astronomy Report

Measuring Gas Accretion and Angular Momentum near Simulated Supermassive Black Holes

Using cosmological simulations with a dynamic range in excess of 10 million, we study the transport of gas mass and angular momentum through the circumnuclear region of a disk galaxy containing a supermassive black hole (SMBH). The simulations follow fueling over relatively quiescent phases of the galaxy's evolution (no mergers) and without feedback from active galactic nuclei (AGNs), as part of the first stage of using state-of-the-art, high-resolution cosmological simulations to model galaxy and black hole co-evolution. We present results from simulations at different redshifts (z=6, 4, and 3) and three different black hole masses (30 million, 90 million, and 300 million solar masses; at z=4), as well as a simulation including a prescription that approximates optically thick cooling in the densest regions. The interior gas mass throughout the circumnuclear disk shows transient and chaotic behavior as a function of time. The Fourier transform of the interior gas mass follows a power law with slope -1 throughout the region, indicating that, in the absence of the effects of galaxy mergers and AGN feedback, mass fluctuations are stochastic with no preferred timescale for accretion over the duration of each simulation (~ 1-2 Myr). The angular momentum of the gas disk changes direction relative to the disk on kiloparsec scales over timescales less than 1 Myr, reflecting the chaotic and transient gas dynamics of the circumnuclear region. Infalling clumps of gas, which are driven inward as a result of the dynamical state of the circumnuclear disk, may play an important role in determining the spin evolution of an SMBH, as has been suggested in stochastic accretion scenarios.Comment: 13 pages, 10 figures; accepted to ApJ; corrected minor typos and reference error

The Star Formation Rate in disk galaxies: thresholds and dependence on gas amount

We reassess the applicability of the Toomre criterion in galactic disks and we study the local star formation law in 16 disk galaxies for which abundance gradients are published. The data we use consists of stellar light profiles, atomic and molecular gas (deduced from CO with a metallicity-dependent conversion factor), star formation rates (from H-alpha emissivities), metallicities, dispersion velocities and rotation curves. We show that the Toomre criterion applies successfully to the case of the Milky Way disk, but it has limited success with the data of our sample; depending on whether the stellar component is included or not in the stability analysis, we find average values for the threshold ratio of the gas surface density to the critical surface density in the range 0.5 to 0.7. We also test various star formation laws proposed in the literature, i.e. either the simple Schmidt law or modifications of it, that take into account dynamical factors. We find only small differences among them as far as the overall fit to our data is concerned; in particular, we find that all three SF laws (with parameters derived from the fits to our data) match particularly well observations in the Milky Way disk. In all cases we find that the exponent n of our best fit SFR has slightly higher values than in other recent works and we suggest several reasons that may cause that discrepancy.Comment: 18 pages, 12 figures, accepted in MNRA

The Westerbork HI Survey of spiral and irregular galaxies III: HI observations of early-type disk galaxies

We present HI observations of 68 early-type disk galaxies from the WHISP survey. They have morphological types between S0 and Sab and absolute B-band magnitudes between -14 and -22. These galaxies form the massive, high surface-brightness extreme of the disk galaxy population, few of which have been imaged in HI before. The HI properties of the galaxies in our sample span a large range; the average values of M_HI/L_B and D_HI/D_25 are comparable to the ones found in later-type spirals, but the dispersions around the mean are larger. No significant differences are found between the S0/S0a and the Sa/Sab galaxies. Our early-type disk galaxies follow the same HI mass-diameter relation as later-type spiral galaxies, but their effective HI surface densities are slightly lower than those found in later-type systems. In some galaxies, distinct rings of HI emission coincide with regions of enhanced star formation, even though the average gas densities are far below the threshold of star formation derived by Kennicutt (1989). Apparently, additional mechanisms, as yet unknown, regulate star formation at low surface densities. Many of the galaxies in our sample have lopsided gas morphologies; in most cases this can be linked to recent or ongoing interactions or merger events. Asymmetries are rare in quiescent galaxies. Kinematic lopsidedness is rare, both in interacting and isolated systems. In the appendix, we present an atlas of the HI observations: for all galaxies we show HI surface density maps, global profiles, velocity fields and radial surface density profiles.Comment: 24 pages, 11 figures. Accepted for publication in A&A. A version with the full atlas can be downloaded from http://www.astro.rug.nl/~edo/WHISPIII.ps.gz (gzipped postscript, 9.3Mb

Detectors for the James Webb Space Telescope Near-Infrared Spectrograph I: Readout Mode, Noise Model, and Calibration Considerations

We describe how the James Webb Space Telescope (JWST) Near-Infrared Spectrograph's (NIRSpec's) detectors will be read out, and present a model of how noise scales with the number of multiple non-destructive reads sampling-up-the-ramp. We believe that this noise model, which is validated using real and simulated test data, is applicable to most astronomical near-infrared instruments. We describe some non-ideal behaviors that have been observed in engineering grade NIRSpec detectors, and demonstrate that they are unlikely to affect NIRSpec sensitivity, operations, or calibration. These include a HAWAII-2RG reset anomaly and random telegraph noise (RTN). Using real test data, we show that the reset anomaly is: (1) very nearly noiseless and (2) can be easily calibrated out. Likewise, we show that large-amplitude RTN affects only a small and fixed population of pixels. It can therefore be tracked using standard pixel operability maps.Comment: 55 pages, 10 figure

The Pressure of an Equilibrium Interstellar Medium in Galactic Disks

Based on an axisymmetric galactic disk model, we estimate the equilibrium gas pressure P/k in the disk plane as a function of the galactocentric distance R for several galaxies (MW, M33, M51, M81, M100, M101, M106, and the SMC). For this purpose, we solve a self-consistent system of equations by taking into account the gas self-gravity and the presence of a dark pseudo-isothermal halo. We assume that the turbulent velocity dispersions of the atomic and molecular gases are fixed and that the velocity dispersion of the old stellar disk corresponds to its marginal stability (except for the Galaxy and the SMC). We also consider a model with a constant disk thickness. Of the listed galaxies, the SMC and M51 have the highest pressure at a given relative radius R/R_25, while M81 has the lowest pressure. The pressure dependence of the relative molecular gas fraction confirms the existence of a positive correlation between these quantities, but it is not so distinct as that obtained previously when the pressure was estimated very roughly. This dependence breaks down for the inner regions of M81 and M106, probably because the gas pressure has been underestimated in the bulge region. We discuss the possible effects of factors other than the pressure affecting the relative content of molecular gas in the galaxies under consideration.Comment: 10 pages, 5 figure