Warm gas in central regions of nearby galaxies. Extended mapping of CO(3-2) emission

We have mapped the CO(3-2) line emission from several nearby galaxies, using the Heinrich-Hertz-Telescope. Our investigation includes twelve galaxies of various types and in different stages of star forming activity. The main results of this CO(3-2) survey are: 1. In none of the observed objects the emission is confined to the nucleus, as claimed in some earlier publications. CO(3-2) emission can be detected for some objects to the same extent as the CO(2-1) and the CO(1-0) lines. 2. The emission is more concentrated to the vicinity of star forming structures than the lower CO transitions. This is shown by decreasing line ratios from the very centres towards larger radii. 3. The CO(3-2) luminosity is stronger in objects that contain a nuclear starburst or morphological peculiarities. 4. The line ratios seem to be independent of Hubble type, color or luminosity. Most galaxies with enhanced central star formation show line ratios of ~1.3 in the very centre and \~1.0 at a radius of about 1kpc. Objects with a ring-like molecular gas distribution or normal galaxies show lower ratios. 5. Starburst galaxies show CO(3-2) emission also in their disks. The line intensities there are higher than that of normal galaxies

A sensitive search for CO J=1-0 emission in 4C 41.17: high-excitation molecular gas at z=3.8

We report sensitive imaging observations of the CO J=1-0 line emission in the powerful high-redshift radio galaxy 4C 41.17 (z=3.8) with the NRAO Very Large Array (VLA), conducted in order to detect the large concomitant H_2 gas reservoir recently unveiled in this system by De Breuck et al (2005) via the emission of the high excitation J=4-3 line. Our observations fail to detect the J=1-0 line but yield sensitive lower limits on the R_43=(4-3)/(1-0) brightness temperature ratio of R_43 ~ 0.55 - >1.0 for the bulk of the H_2 gas mass. Such high ratios are typical of the high-excitation molecular gas phase ``fueling'' the star formation in local starbursts, but quite unlike these objects, much of the molecular gas in 4C 41.17 seems to be in such a state, and thus participating in the observed starburst episode. The widely observed and unique association of highly excited molecular gas with star forming sites allows CO line emission with large (high-J)/(low-J) intensity ratios to serve as an excellent ``marker'' of the spatial distribution of star formation in distant dust-obscured starbursts, unaffected by extinction.Comment: 7 Pages including 8 PostScript figures. Accepted for publication in Astronomy & Astrophysic

Dust properties in M31.I.Basic properties and a discussion on age-dependent dust heating

Context. Spitzer Space Telescope observations and dust emission models are used to discuss the distribution of dust and its characteristics in M31. Together with GALEX FUV, NUV, and SDSS images we studied the age dependence of the dust heating process. Methods.Spitzer IRAC/MIPS maps of M31 were matched together and compared to dust emission models allowing to constrain the dust mass, the intensity of the mean radiation field, the abundance of Polycyclic Aromatic Hydrocarbons (PAH) particles. The total infrared emission (TIR) was analyzed in function of UV and Optical colors and compared to predictions of models which consider the age-dependent dust heating. Results. We demonstrate that cold-dust component emission dominates the infrared spectral energy distribution of M31. The mean intensity of the radiation field heating the dust is low (typically U<2, where U=1 is the value in the solar surrounding). Due to the lack of submillimetric measurements the dust mass (M_{dust}) is only weakly constrained by the infrared spectrum. We show that across the spiral-ring structure of M31 a fraction >3% of the total dust mass is in PAHs. UV and optical colors are correlated to (TIR/FUV) ratios in \sim 670 pc-sized regions overall the disk of M31, although deviating from the IRX-beta relationship for starburst galaxies. We derived that in 83% of the regions analyzed across the 10kpc ring more than 50% of the energy absorbed by the dust is rediated at \lambda > 4000 \AA and that dust in M31 appears mainly heated by populations a few Gyr old even across the star-forming ring. The attenuation is varying radially peaking near 10kpc and decreasing faster in the inner regions of M31 than in the outer regions. We finally derived the attenuation map of M31 at 6"/px resolution (\sim 100 pc/px along the plane of M31).[abridged]Comment: 21 pages, 18 figures, accepted for publication in A&A. Only low resolution images included, full resolution images will be avaiable in the journal electronic version. Fig.14 and Fig.17 will be avaiable via CD

The Herschel Exploitation of Local Galaxy Andromeda (HELGA). I: Global far-infrared and sub-mm morphology

We have obtained Herschel images at five wavelengths from 100 to 500 micron of a ~5.5x2.5 degree area centred on the local galaxy M31 (Andromeda), our nearest neighbour spiral galaxy, as part of the Herschel guaranteed time project "HELGA". The main goals of HELGA are to study the characteristics of the extended dust emission, focusing on larger scales than studied in previous observations of Andromeda at an increased spatial resolution, and the obscured star formation. In this paper we present data reduction and Herschel maps, and provide a description of the far-infrared morphology, comparing it with features seen at other wavelengths. We use high--resolution maps of the atomic hydrogen, fully covering our fields, to identify dust emission features that can be associated to M31 with confidence, distinguishing them from emission coming from the foreground Galactic cirrus. Thanks to the very large extension of our maps we detect, for the first time at far-infrared wavelengths, three arc-like structures extending out to ~21, ~26 and ~31 kpc respectively, in the south-western part of M31. The presence of these features, hosting ~2.2e6 Msol of dust, is safely confirmed by their detection in HI maps. Overall, we estimate a total dust mass of ~5.8e7 Msol, about 78% of which is contained in the two main ring-like structures at 10 and 15 kpc, at an average temperature of 16.5 K. We find that the gas-to-dust ratio declines exponentially as a function of the galacto-centric distance, in agreement with the known metallicity gradient, with values ranging from 66 in the nucleus to ~275 in the outermost region. [Abridged]Comment: 15 Pages, 9 Figures. Accepted for publication in Astronomy and Astrophysics. A high resolution version of the paper can be found at http://wazn.ugent.be/jfritz/HelgaI_final.pd

Dynamically Influenced Molecular Clouds in the Nucleus of NGC 6946: Variations in the CO Isotopic Line Ratios

We present high resolution (~5'') maps of the J = 1 - 0 transitions of ^{13}CO and C^{18}O towards the nucleus of NGC 6946, made with the Owens Valley Millimeter Array. The images are compared with existing ^{12}CO(1-0) maps to investigate localized changes in gas properties across the nucleus. As compared to ^{12}CO, both ^{13}CO and C^{18}O are more confined to the central ring of molecular gas associated with the nuclear star formation; that is, ^{12}CO is stronger relative to ^{13}CO and C^{18}O away from the nucleus and along the spiral arms. The ^{12}CO(1-0)/^{13}CO(1-0) line ratio reaches very high values of >40. We attribute the relative ^{13}CO weakness to a rapid change in the interstellar medium from dense star forming cores in a central ring to diffuse, low density molecular gas in and behind the molecular arms. This change is abrupt, occurring in less than a beamsize (90 pc), about the size of a giant molecular cloud. Column densities determined from ^{13}CO(1-0), C^{18}O(1-0), and 1.4 mm dust continuum all indicate that the standard Galactic conversion factor, X_{CO}, overestimates the amount of molecular gas in NGC 6946 by factors of ~3-5 towards the central ring and potentially even more so in the diffuse gas away from the central starburst. We suggest that the nuclear bar acts to create coherent regions of molecular clouds with distinct and different physical conditions. The ^{12}CO(1-0)/^{13}CO(1-0) line ratio in galactic nuclei can be a signpost of a dynamically evolving ISM.Comment: 38 pages, 9 figures. Accepted to the Astronomical Journa

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

Photon dominated regions in the spiral arms of M83 and M51

We present CI 3P1-3P0 spectra at four spiral arm positions and the nuclei of the nearby galaxies M83 and M51 obtained at the JCMT. This data is complemented with maps of CO 1-0, 2-1, and 3-2, and ISO/LWS far-infrared data of CII (158 micron), OI (63 micron), and NII (122 micron) allowing for the investigation of a complete set of all major gas cooling lines. From the intensity of the NII line, we estimate that between 15% and 30% of the observed CII emission originate from the dense ionized phase of the ISM. The analysis indicates that emission from the diffuse ionized medium is negligible. In combination with the FIR dust continuum, we find gas heating efficiencies below ~0.21% in the nuclei, and between 0.25 and 0.36% at the outer positions. Comparison with models of photon-dominated regions (PDRs) of Kaufman et al. (1999) with the standard ratios OI(63)/CII_PDR and (OI(63)+CII_PDR) vs. TIR, the total infrared intensity, yields two solutions. The physically most plausible solution exhibits slightly lower densities and higher FUV fields than found when using a full set of line ratios, CII_PDR/CI(1-0), CI(1-0)/CO(3-2), CO(3-2)/CO(1-0), CII/CO(3-2), and, OI(63)/CII_PDR. The best fits to the latter ratios yield densities of 10^4 cm^-3 and FUV fields of ~G_0=20-30 times the average interstellar field without much variation. At the outer positions, the observed total infrared intensities are in perfect agreement with the derived best fitting FUV intensities. The ratio of the two intensities lies at 4-5 at the nuclei, indicating the presence of other mechanisms heating the dust

Radial distribution of dust, stars, gas, and star-formation rate in DustPedia face-on galaxies

Aims. The purpose of this work is the characterization of the radial distribution of dust, stars, gas, and star-formation rate (SFR) in a sub-sample of 18 face-on spiral galaxies extracted from the DustPedia sample. Methods. This study is performed by exploiting the multi-wavelength DustPedia database, from ultraviolet (UV) to sub-millimeter bands, in addition to molecular (12CO) and atomic (Hi) gas maps and metallicity abundance information available in the literature. We fitted the surface-brightness profiles of the tracers of dust and stars, the mass surface-density profiles of dust, stars, molecular gas, and total gas, and the SFR surface-density profiles with an exponential curve and derived their scale-lengths. We also developed a method to solve for the CO-to-H2 conversion factor (αCO) per galaxy by using dust- and gas-mass profiles. Results. Although each galaxy has its own peculiar behavior, we identified a common trend of the exponential scale-lengths versus wavelength. On average, the scale-lengths normalized to the B-band 25 mag/arcsec2 radius decrease from UV to 70 μm, from 0.4 to 0.2, and then increase back up to ~0.3 at 500 microns. The main result is that, on average, the dust-mass surface-density scale-length is about 1.8 times the stellar one derived from IRAC data and the 3.6 μm surface brightness, and close to that in the UV. We found a mild dependence of the scale-lengths on the Hubble stage T: the scale-lengths of the Herschel bands and the 3.6 μm scale-length tend to increase from earlier to later types, the scale-length at 70 μm tends to be smaller than that at longer sub-mm wavelength with ratios between longer sub-mm wavelengths and 70 μm that decrease with increasing T. The scale-length ratio of SFR and stars shows a weak increasing trend towards later types. Our αCO determinations are in the range (0.3−9) M⊙ pc-2 (K km s-1)-1, almost invariant by using a fixed dust-to-gas ratio mass (DGR) or a DGR depending on metallicity gradient

Dynamics of Disks and Warps

This chapter reviews theoretical work on the stellar dynamics of galaxy disks. All the known collective global instabilities are identified, and their mechanisms described in terms of local wave mechanics. A detailed discussion of warps and other bending waves is also given. The structure of bars in galaxies, and their effect on galaxy evolution, is now reasonably well understood, but there is still no convincing explanation for their origin and frequency. Spiral patterns have long presented a special challenge, and ideas and recent developments are reviewed. Other topics include scattering of disk stars and the survival of thin disks.Comment: Chapter accepted to appear in Planets, Stars and Stellar Systems, vol 5, ed G. Gilmore. 32 pages, 17 figures. Includes minor corrections made in proofs. Uses emulateapj.st

The nature and origin of substructure in the outskirts of M31-II. Detailed star formation histories

While wide-field surveys of M31 have revealed much substructure at large radii, understanding the nature and origin of this material is not straightforward from morphology alone. Using deep HST/ACS data, we have derived further constraints in the form of quantitative star formation histories (SFHs) for 14 inner halo fields which sample diverse substructures. In agreement with our previous analysis of colour-magnitude diagram morphologies, we find the resultant behaviours can be broadly separated into two categories. The SFHs of 'disc-like' fields indicate that most of their mass has formed since z~1, with one quarter of the mass formed in the last 5 Gyr. We find 'stream-like' fields to be on average 1.5 Gyr older, with <10 percent of their stellar mass formed within the last 5 Gyr. These fields are also characterised by an age--metallicity relation showing rapid chemical enrichment to solar metallicity by z=1, suggestive of an early-type progenitor. We confirm a significant burst of star formation 2 Gyr ago, discovered in our previous work, in all the fields studied here. The presence of these young stars in our most remote fields suggests that they have not formed in situ but have been kicked-out from through disc heating in the recent past.Comment: 14 pages, 10 figures (+12 figures in appendix). MNRAS, in pres