Dense and Warm Molecular Gas and Warm Dust in Nearby Galaxies

We performed 12CO(1-0), 13CO(1-0), and HCN(1-0) single-dish observations (beam size ~14"-18") toward nearby starburst and non-starburst galaxies using the Nobeyama 45 m telescope. The 13CO(1-0) and HCN(1-0) emissions were detected from all the seven starburst galaxies, with the intensities of both lines being similar (i.e., the ratios are around unity). On the other hand, for case of the non-starburst galaxies, the 13CO(1-0) emission was detected from all three galaxies, while the HCN(1-0) emission was weakly or not detected in past observations. This result indicates that the HCN/13CO intensity ratios are significantly larger (~1.15+-0.32) in the starburst galaxy samples than the non-starburst galaxy samples (<0.31+-0.14). The large-velocity-gradient model suggests that the molecular gas in the starburst galaxies have warmer and denser conditions than that in the non-starburst galaxies, and the photon-dominated-region model suggests that the denser molecular gas is irradiated by stronger interstellar radiation field in the starburst galaxies than that in the non-starburst galaxies. In addition, HCN/13CO in our sample galaxies exhibit strong correlations with the IRAS 25 micron flux ratios. It is a well established fact that there exists a strong correlation between dense molecular gas and star formation activities, but our results suggest that molecular gas temperature is also an important parameter.Comment: 14 pages, 6 figures. Accepted for publication in PAS

NMA Survey of CO and HCN Emission from Nearby Active Galaxies

High resolution (a few arcseconds) observations of CO(1-0) and HCN(1-0) emission from nearby Seyfert galaxies have been conducted with the Nobeyama Millimeter Array. Based on the observed CO distributions and kinematics,we suggest that a small scale (a few 100 pc - a few kpc) distortion of the underlying potential seems to be necessary for Seyfert activity, although it is not a sufficient condition. We also find that the Toomre's Q values in the centers of Seyfert galaxies tend to be larger than unity, indicating the circumnuclear molecular gas disks around Seyfert nuclei would be gravitationally stable. The HCN/CO integrated intensity ratios (R_HCN/CO) range over an order of magnitude, from 0.086 to 0.6. The Seyfert galaxies with high R_HCN/CO may have an extended (r ~ 100 pc scale) envelope of obscuring material. The presence of kpc scale jet/ outflow might be also related to the extremely high R_HCN/CO.Comment: To appear in the Proceedings of the 3rd Cologne-Zermatt Symposium, ``The Physics and Chemistry of the Interstellar Medium'

An Extragalactic 12CO J=3-2 survey with the Heinrich-Hertz-Telescope

We present results of a ^{12}CO J = 3-2 survey of 125 nearby galaxies obtained with the 10-m Heinrich-Hertz-Telescope, with the aim to characterize the properties of warm and dense molecular gas in a large variety of environments. With an angular resolution of 22'', ^{12}CO 3-2 emission was detected in 114 targets. Based on 61 galaxies observed with equal beam sizes the ^{12}CO 3-2/1-0 integrated line intensity ratio R_{31} is found to vary from 0.2 to 1.9, with an average value of 0.81. No correlations are found for R_{31} to Hubble type and far infrared luminosity. Possible indications for a correlation with inclination angle and the 60mum/100mum color temperature of the dust are not significant. Higher R_{31} ratios than in ``normal'' galaxies, hinting at enhanced molecular excitation, may be found in galaxies hosting active galactic nuclei. Even higher average values are determined for galaxies with bars or starbursts, the latter being identified by the ratio of infrared luminosity versus isophotal area, log[(L_{FIR}/L_{SUN})/(D_{25}/kpc)^2)] > 7.25. (U)LIRGs are found to have the highest averaged R_{31} value. This may be a consequence of particularly vigorous star formation activity, triggered by galaxy interaction and merger events. The nuclear CO luminosities are slightly sublinearly correlated with the global FIR luminosity in both the ^{12}CO J = 3-2 and the 1-0 lines. The slope of the log-log plots rises with compactness of the respective galaxy subsample, indicating a higher average density and a larger fraction of thermalized gas in distant luminous galaxies. While linear or sublinear correlations for the ^{12}CO J = 3-2 line can be explained, if the bulk of the observed J = 3-2 emission originates from molecular gas with densities below the critical one, the case of the ^{12}CO J = 1-0 line with its small critical density remains a puzzle.Comment: 26 pages, 9 figures, 4 tables, Accepted for publication in The Astrophysical Journal (Part 1

12CO, 13CO and C18O observations along the major axes of nearby bright infrared galaxies

We present simultaneous observations of CO,13CO and C18O J=1-0 emission in 11 nearby (cz<1000 km/s) bright infrared galaxies. Both 12CO and 13CO are detected in the centers of all galaxies, except for 13CO in NGC 3031. We have also detected C18O, CS J=2-1, and HCO+ J=1-0 emission in the nuclear regions of M82 and M51. These are the first systematical extragalactic detections of 12CO and its isotopes from the PMO 14m telescope. We have conducted half-beam spacing mapping of M82 over an area of 4'*2.5' and major axis mapping of NGC 3627, NGC 3628, NGC 4631, and M51. The radial distributions of 12CO and 13CO in NGC 3627, NGC 3628, and M51 can be well fitted by an exponential profile. The 12CO/13CO intensity ratio,R,decreases monotonically with galactocentric radius in all mapped sources. The average R in the center and disk of the galaxies are 9.9+/-3.0 and 5.6+/-1.9 respectively, much lower than the peculiar R(~24) found in the center of M82. The intensity ratios of 13CO/C18O, 13CO/HCO+ and 13CO/CS (either ours or literature data) show little variations with galactocentric radius, in sharp contrast with the greatly varied R. This supports the notion that the observed gradient in R could be the results of the variations of the physical conditions across the disks. The H_2 column density derived from C18O shows that the Galactic standard conversion factor (X-factor) overestimates the amount of the molecular gas in M82 by a factor of ~2.5. These observations suggest that the X-factor in active star-forming regions (i.e., nuclear regions) should be lower than that in normal star-forming disks, and the gradient in R can be used to trace the variations of the X-factor.Comment: 27 pages, 7 figures, accepted by RA

Formation of a Massive Black Hole at the Center of the Superbubble in M82

We performed 12CO(1-0), 13CO(1-0), and HCN(1-0) interferometric observations of the central region (about 450 pc in radius) of M82 with the Nobeyama Millimeter Array, and have successfully imaged a molecular superbubble and spurs. The center of the superbubble is clearly shifted from the nucleus by 140 pc. This position is close to that of the massive black hole (BH) of >460 Mo and the 2.2 micron secondary peak (a luminous supergiant dominated cluster), which strongly suggests that these objects may be related to the formation of the superbubble. Consideration of star formation in the cluster based on the infrared data indicates that (1) energy release from supernovae can account for the kinetic energy of the superbubble, (2) the total mass of stellar-mass BHs available for building-up the massive BH may be much higher than 460 Mo, and (3) it is possible to form the middle-mass BH of 100-1000 Mo within the timescale of the superbubble. We suggest that the massive BH was produced and is growing in the intense starburst region.Comment: 9 pages, 3 figures, to appear in ApJ Lette

Molecular gas in nearby low-luminosity QSO host galaxies

This paper addresses the global molecular gas properties of a representative sample of galaxies hosting low-luminosity quasistellar objects. An abundant supply of gas is necessary to fuel both the active galactic nucleus and any circum-nuclear starburst activity of QSOs. We selected a sample of nearby low-luminosity QSO host galaxies that is free of infrared excess biases. All objects are drawn from the Hamburg-ESO survey for bright UV-excess QSOs, have DEC>-30 degrees and redshifts that do not exceed z=0.06. The IRAM 30m telescope was used to measure the CO(1-0) and CO(2-1) transition in parallel. 27 out of 39 galaxies in the sample have been detected. The molecular gas masses of the detected sources range from 0.4E9 M_sun to 9.7E9 M_sun. We can confirm that the majority of galaxies hosting low-luminosity QSOs are rich in molecular gas. The properties of galaxies hosting brighter type I AGN and circumnuclear starformation regions differ from the properties of galaxies with fainter central regions. The overall supply of molecular gas and the spread of the line width distribution is larger. When comparing the far-infrared with the CO luminosities, the distribution can be separated into two different power-laws: one describing the lower activity Seyfert I population and the second describing the luminous QSO population. The separation in the L_FIR/L'_CO behavior may be explainable with differing degrees of compactness of the emission regions. We provide a simple model to describe the two power-laws. The sample studied in this paper is located in a transition region between the two populations

Molecular Gas in NUclei of GAlaxies (NUGA) I.The counter-rotating LINER NGC4826

We present new high-resolution observations of the nucleus of the counter-rotating LINER NGC4826, made in the J=1-0 and J=2-1 lines of 12CO with the IRAM Plateau de Bure mm-interferometer(PdBI).The CO maps, which achieve 0.8''(16pc) resolution in the 2-1 line, fully resolve an inner molecular gas disk which is truncated at an outer radius of 700pc. The total molecular gas mass is distributed in a lopsided nuclear disk of 40pc radius and two one-arm spirals, which develop at different radii in the disk. The distribution and kinematics of molecular gas in the inner 1kpc of NGC4826 show the prevalence of different types of m=1 perturbations in the gas. Although dominated by rotation, the gas kinematics are perturbed by streaming motions related to the m=1 instabilities. The non-circular motions associated with the inner m=1 perturbations agree qualitatively with the pattern expected for a trailing wave developed outside corotation ('fast' wave). In contrast, the streaming motions in the outer m=1 spiral are better explained by a 'slow' wave. A paradoxical consequence is that the inner m=1 perturbations would not favour AGN feeding. An independent confirmation that the AGN is not being generously fueled at present is found in the low values of the gravitational torques exerted by the stellar potential for R<530pc. The distribution of star formation in the disk of NGC4826 is also strongly asymmetrical. Massive star formation is still vigorous, fed by the significant molecular gas reservoir at R<700pc. There is supporting evidence for a recent large mass inflow episode in NGC4826. These observations have been made in the context of the NUclei of GAlaxies (NUGA) project, aimed at the study of the different mechanisms for gas fueling of AGN.Comment: A&A, 2003, Paper accepted (04/06/03). For a full-resolution version of this paper see http://www.oan.es/preprint

CO emission in optically obscured (type-2) quasars at redshifts z=0.1-0.4

We present a search for CO emission in a sample of ten type-2 quasar host galaxies with redshifts of z=0.1-0.4. We detect CO(J=1-0) line emission with >=5sigma in the velocity integrated intensity maps of five sources. A sixth source shows a tentative detection at the ~4.5sigma level of its CO(J=1-0) line emission. The CO emission of all six sources is spatially coincident with the position at optical, infrared or radio wavelengths. The spectroscopic redshifts derived from the CO(J=1-0) line are very close to the photometric ones for all five detections except for the tentative detection for which we find a much larger discrepancy. We derive gas masses of ~(2-16)x10^9Msun for the CO emission in the six detected sources, while we constrain the gas masses to upper limits of Mgas<=8x10^9Msun for the four non-detections. These values are of the order or slightly lower than those derived for type-1 quasars. The line profiles of the CO(J=1-0) emission are rather narrow (<=300km/s) and single peaked, unveiling no typical signatures for current or recent merger activity, and are comparable to that of type-1 quasars. However, at least one of the observed sources shows a tidal-tail like emission in the optical that is indicative for an on-going or past merging event. We also address the problem of detecting spurious ~5sigma emission peaks within the field of view.Comment: accepted for publication in ApJ; 10 pages, 6 figures, 4 tables; format is emulateap

The impact of bars on the mid-infrared dust emission of spiral galaxies: global and circumnuclear properties

We study the mid-infrared properties of a sample of 69 nearby spiral galaxies, selected to avoid Seyfert activity contributing a significant fraction of the central energetics, or strong tidal interaction, and to have normal infrared luminosities. These observations were obtained with ISOCAM, which provides an angular resolution of the order of 10 arcsec (half-power diameter of the point spread function) and low-resolution spectro-imaging information. Between 5 and 18 microns, we mainly observe two dust phases, aromatic infrared bands and very small grains, both out of thermal equilibrium. On this sample, we show that the global F15/F7 colors of galaxies are very uniform, the only increase being found in early-type strongly barred galaxies, consistent with previous IRAS studies. The F15/F7 excesses are unambiguously due to galactic central regions where bar-induced starbursts occur. However, the existence of strongly barred early-type galaxies with normal circumnuclear colors indicates that the relationship between a distortion of the gravitational potential and a central starburst is not straightforward. As the physical processes at work in central regions are in principle identical in barred and unbarred galaxies, and since this is where the mid-infrared activity is mainly located, we investigate the mid-infrared circumnuclear properties of all the galaxies in our sample. We show how surface brightnesses and colors are related to both the available molecular gas content and the mean age of stellar populations contributing to dust heating. Therefore, the star formation history in galactic central regions can be constrained by their position in a color-surface brightness mid-infrared diagram.Comment: 22 pages, 25 figures, accepted for publication in A&A ; small errors corrected and references update

Mid-infrared spectroscopy of infrared-luminous galaxies at z~0.5-3

We present results on low-resolution mid-infrared (MIR) spectra of 70 infrared-luminous galaxies obtained with the Infrared Spectrograph (IRS) onboard Spitzer. We selected sources from the European Large Area Infrared Survey (ELAIS) with S15 > 0.8 mJy and photometric or spectroscopic z > 1. About half of the sample are QSOs in the optical, while the remaining sources are galaxies, comprising both obscured AGN and starbursts. We classify the spectra using well-known infrared diagnostics, as well as a new one that we propose, into three types of source: those dominated by an unobscured AGN (QSOs), obscured AGN, and starburst-dominated sources. Starbursts concentrate at z ~ 0.6-1.0 favored by the shift of the 7.7-micron PAH band into the selection 15 micron band, while AGN spread over the 0.5 < z < 3.1 range. Star formation rates (SFR) are estimated for individual sources from the luminosity of the PAH features. An estimate of the average PAH luminosity in QSOs and obscured AGN is obtained from the composite spectrum of all sources with reliable redshifts. The estimated mean SFR in the QSOs is 50-100 Mo yr^-1, but the implied FIR luminosity is 3-10 times lower than that obtained from stacking analysis of the FIR photometry, suggesting destruction of the PAH carriers by energetic photons from the AGN. The SFR estimated in obscured AGN is 2-3 times higher than in QSOs of similar MIR luminosity. This discrepancy might not be due to luminosity effects or selection bias alone, but could instead indicate a connection between obscuration and star formation. However, the observed correlation between silicate absorption and the slope of the near- to mid-infrared spectrum is compatible with the obscuration of the AGN emission in these sources being produced in a dust torus.Comment: 32 pages, 24 figures, 15 tables, accepted for publication in MNRA