S. Garcia-Burillo

A Multi-Transition HCN and HCO+ Study of 12 Nearby Active Galaxies: Active Galactic Nucleus versus Starburst Environments

Recent studies have indicated that the HCN-to-CO(J = 1–0) and HCO+-to-HCN(J = 1–0) ratios are significantly different between galaxies with AGN (active galactic nucleus) and SB (starburst) signatures. In order to study the molecular gas properties in active galaxies and search for differences between AGN and SB environments, we observed the HCN(J = 1–0), (J = 2–1), (J = 3–2), HCO+(J = 1–0), and HCO+(J = 3–2) emission with the IRAM 30 m in the center of 12 nearby active galaxies which either exhibit nuclear SB and/or AGN signatures. Consistent with previous results, we find a significant difference of the HCN(J = 2–1)-to-HCN(J = 1–0), HCN(J = 3–2)-to-HCN(J = 1–0), HCO+(J = 3–2)-to-HCO+(J = 1–0), and HCO+-to-HCN intensity ratios between the sources dominated by an AGN and those with an additional or pure central SB: the HCN, HCO+, and HCO+-to-HCN intensity ratios tend to be higher in the galaxies of our sample with a central SB as opposed to the pure AGN cases, which show rather low intensity ratios. Based on an LVG analysis of these data, i.e., assuming purely collisional excitation, the (average) molecular gas densities in the SB-dominated sources of our sample seem to be systematically higher than in the AGN sources. The LVG analysis seems to further support systematically higher HCN and/or lower HCO+ abundances as well as similar or higher gas temperatures in AGNs compared to the SB sources of our sample. In addition, we find that the HCN-to-CO ratios decrease with increasing rotational number J for the AGNs while they stay mostly constant for the SB sources.

Molecular gas in NUclei of GAlaxies (NUGA). IV. Gravitational torques and AGN feeding

We discuss the efficiency of stellar gravity torques as a mechanism to account for the feeding of the central engines of four low luminosity Active Galactic Nuclei (AGN): NGC 4321 (HII nucleus/LINER), NGC 4826 (HII nucleus/LINER), NGC 4579 (LINER 1.9/Seyfert 1.9) and NGC 6951 (Seyfert 2). These galaxies have been observed as part of the NUclei of GAlaxies–(NUGA) CO project, aimed at the study of AGN fueling mechanisms. Our calculations allow us to derive the characteristic time-scales for gas flows and discuss whether torques from the stellar potentials are efficient enough to drain the gas angular momentum in the inner 1 kpc of these galaxies. The stellar potentials are derived using high-resolution near infrared (NIR) images and the averaged effective torques on the gas are estimated using the high-resolution (∼0.5 �� –2 �� ) CO maps of the galaxies. Results indicate paradoxically that feeding should be thwarted close to the AGNs: in the four cases analyzed, gravity torques are mostly positive inside r ∼ 200 pc, resulting in no inflow on these scales. As a possible solution for the paradox, we speculate that the agent responsible for driving inflow to still smaller radii is transient and thus presently absent in the stellar potential. Alternatively, the gravity torque barrier associated with the Inner Lindblad Resonance of the bars in these galaxies could be overcome by other mechanisms that become competitive in due time against gravity torques. In particular, we estimate on a case-by-case basis the efficiency of viscosity versus gravity torques to drive AGN fueling. We find that viscosity can counteract moderate-to-low gravity torques on the gas if it acts on a nuclear ring of high gas surface density contrast and ∼a few 100 pc size. We propose an evolutionary scenario in which gravity torques and viscosity act in concert to produce recurrent episodes of activity during the typical lifetime of any galaxy. In this scenario the recurrence of activity in galaxies is indirectly related to that of the bar instabilities although the active phases are not necessarily coincident with the maximum strength of a single bar episode. The general implications of these results for the current understanding of fueling of low-luminosity AGN are discussed.

Molecular gas chemistry in AGN - I. The IRAM 30 m survey of NGC 1068

There is observational evidence that nuclear winds and X-rays can heavily influence the physical conditions and chemical abundances of molecular gas in the circumnuclear disks (CND) of Active Galactic Nuclei (AGN). In this paper we probe the chemical status of molecular gas in the CND of NGC 1068, a prototypical Seyfert 2 galaxy. Precedent claims that the chemistry of molecular gas in the nucleus of NGC 1068 is abnormal by galactic standards were based on the high HCN/CO luminosity ratio measured in the CND. Results from new observations obtained in this survey have served to derive abundances of molecular species such as SiO, CN, HCO + , HOC + ,H 13 CO + and HCO. These estimates are complemented by a re-evaluation of molecular abundances for HCN, CS and CO, based on previously published single-dish and interferometer observations of NGC 1068. We report on the first detection of SiO emission in the CND of NGC 1068. The estimated large abundance of SiO in the CND, X(SiO) ∼ (5−10) × 10 −9 , cannot be attributed to shocks related to star formation, as there is little evidence of a recent starburst in the nucleus of NGC 1068. Alternatively, we propose that silicon chemistry is driven by intense X-ray processing of molecular gas. We also report on the first extragalactic detection of the reactive ion HOC + .M ost remarkably, the estimated HCO + /HOC + abundance ratio in the nucleus of NGC 1068, ∼30-80, is the smallest ever measured in molecular gas. The abundances derived for all molecules that have been the subject of this survey are compared with the predictions of models invoking either oxygen-depletion or X-ray chemistry in molecular gas. Our conclusions favour an overall scenario where the CND of NGC 1068 has become a giant X-ray Dominated Region (XDR).

Evidence of enhanced star formation efficiency in luminous and ultraluminous infrared galaxies

We present new observations made with the IRAM 30 m telescope of the J = 1−0 and 3–2 lines of HCN and HCO + used to probe the dense molecular gas content in a sample of 17 local luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). These observations have allowed us to derive an updated version of the power law describing the correlation between the FIR luminosity (LFIR )a nd the HCN(1−0) luminosity (L �−0) ) of local and high-redshift galaxies. We present the first clear observational evidence that the star formation efficiency of the dense gas (SFEdense), measured as the LFIR/L �−0) ratio, is significantly higher in LIRGs and ULIRGs than in normal galaxies, a result that has also been found recently in high-redshift galaxies. This may imply a statistically significant turn upward in the Kennicutt-Schmidt law derived for the dense gas at LFIR ≥ 10 11 L� . We used a one-phase large velocity gradient (LVG) radiative transfer code to fit the three independent line ratios derived from our observations. The results of this analysis indicate that the [HCN]/[HCO + ] abundance ratios could be up to one order of magnitude higher than normal in a significant number

Is HCN a True Tracer of Dense Molecular Gas in Luminous and Ultraluminous Infrared Galaxies

We present the results of the first HCO+ survey probing the dense molecular gas content of a sample of 16 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). Previous work, based on HCN (1-0) observations, had shown that LIRGs and ULIRGs possess a significantly higher fraction of dense molecular gas compared to normal galaxies. While the picture issued from HCO+ partly confirms this result, we have discovered an intriguing correlation between the HCN (1-0)/HCO+ (1-0) luminosity ratio and the IR luminosity of the galaxy (LIR). This trend casts doubts on the use of HCN as an unbiased quantitative tracer of the dense molecular gas content in LIRGs and ULIRGs. A plausible scenario explaining the observed trend implies that X-rays coming from an embedded active galactic nucleus may play a dominant role in the chemistry of molecular gas at LIR ? 1012 L?. We discuss the implications of this result for the understanding of LIRGs, ULIRGs, and high-redshift gas-rich galaxies.We present the results of the first HCO+ survey probing the dense molecular gas content of a sample of 16 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). Previous work, based on HCN(1-0) observations, had shown that LIRGs and ULIRGs posses a significantly higher fraction of dense molecular gas compared to normal galaxies. While the picture issued from HCO+ partly confirms this result, we have discovered an intriguing correlation between the HCN(1-0)/HCO+(1-0) luminosity ratio and the IR luminosity of the galaxy (L(IR)). This trend casts doubts on the use of HCN as an unbiased quantitative tracer of the dense molecular gas content in LIRGs and ULIRGs. A plausible scenario explaining the observed trend implies that X-rays coming from an embedded AGN may play a dominant role in the chemistry of molecular gas at L(IR) > 1e12 Lsun. We discuss the implications of this result for the understanding of LIRGs, ULIRGs and high redshift gas-rich galaxies.

A complete

The mechanisms governing the star formation rate in spiral galaxies are not yet clear. The nearby, almost face-on, and interacting galaxy M51 offers an excellent opportunity to study at high spatial resolutions the local star formation laws. In this first paper, we investigate the correlation of H2, HI, and total gas surface densities with the star forming activity, derived from the radio continuum (RC), along radial averages out to radii of 12kpc. We have created a complete map of M51 in 12CO 2-1 at a resolution of 450kpc using HERA at the IRAM-30m telescope. These data are combined with maps of HI and the radio-continuum at 20cm wavelength. The latter is used to estimate the star formation rate (SFR), thus allowing to study the star formation efficiency and the local Schmidt law. The velocity dispersion from CO is used to study the critical surface density and the gravitational stability of the disk. The critical gas velocity dispersions needed to stabilize the gas against gravitational collapse in the differentially rotating disk of M51 using the Toomre criterion, vary with radius between 1.7 and 6.8 km/s. Observed radially averaged dispersions derived from the CO data vary between 28 km/s in the center and 8 km/s at radii of 7 to 9 kpc. They exceed the critical dispersions by factors Q_gas of 1 to 5. We speculate that the gravitational potential of stars leads to a critically stable disk.

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We have carried out a survey of reactive ions (CO + , HOC + , HC 1 8 O + , SO + ) and cyclopropenylidene (C 3 H 2 ) in three prototypical photodissociation regions (PDRs), the reflection nebula NGC 7023, the Orion Bar and the planetary nebula (PN) NGC 7027. The reactive ion CO + has been detected in all the targets with fractional abundances ranging from ∼ 10 - 1 1 to ∼a few 10 - 9 . Its spatial distribution in NGC 7023 and the Orion Bar show that CO + arises in the innermost part (A, < 2 mag) of the PDR. In NGC 7027, the CO + lines shows an expansion velocity higher than that of the CO lines. This high expansion velocity is consistent with the CO + emission arising in the high velocity layer of neutral gas which is being accelerated by the ionized gas. Photochemistry determines the chemical composition of this layer. The reactive ions HOC + and SO + have been detected in NGC 7023 and the Orion Bar. In both sources, the fractional abundance of HOC + is enhanced by a factor of ∼ 10 towards the PDRs, with typical abundances, X H O C + = 0.7-3 x 10 - 1 1 . This enhancement produces a decrease of the [HCO + ]/[HOC + ] abundance ratio towards the PDR. In fact, we have derived [HCO + ]/[HOC + ] ∼ 50-120 in NGC 7023, which is the lowest ratio measured thus far. HOC + and SO + have not been detected in NGC 7027. Interestingly, this is the source with the highest CO + abundance, X C O + = 5 × 10 - 9 . This lack of detection is interpreted as due to the peculiar chemistry of C-rich PNs, in which the abundance of oxygenated molecules, in particular H 2 O, is low. We have detected cyclopropenylidene (C 3 H 2 ) in the three target PDRs. Similarly to the reactive ions, the abundance of C 3 H 2 in NGC 7023 and the Orion Bar is a factor of 10-100 higher in the PDRs than in the foreground molecular cloud with peak values ranging from 10 - 1 0 to 10 - 9 . In NGC 7027, we measured the maximum C 3 H 2 abundance with a value of ∼10 - 8 . Similarly to the case of CO + , the high expansion velocities of the C 3 H 2 lines in NGC 7027 suggests that its emission arises in the neutral gas which is being accelerated by the ionized gas. Photodestruction of Polycyclic Aromatic Hydrocarbons (PAHs) is proposed to explain the enhanced C 3 H 2 abundance in these PDRs.

CO 2-1 map of M 51 with HERA - I. Radial averages of CO, H I, and radio continuum

We report ALMA observations of CO(3-2) emission in the Seyfert 2 galaxy NGC 1433 at the unprecedented spatial resolution of 0: 5 = 24 pc. Our aim is to probe AGN (active galactic nucleus) feeding and feedback phenomena through the morphology and dynamics of the gas inside the central kpc. The galaxy NGC 1433 is a strongly barred spiral with three resonant rings: one at the ultra-harmonic resonance near corotation, and the others at the outer and inner Lindblad resonances (OLR and ILR). A nuclear bar of 400 pc radius is embedded in the large-scale primary bar. The CO map, which covers the whole nuclear region (nuclear bar and ring), reveals a nuclear gaseous spiral structure, inside the nuclear ring encircling the nuclear stellar bar. This gaseous spiral is well correlated with the dusty spiral seen in Hubble Space Telescope images. The nuclear spiral winds up in a pseudo-ring at 200 pc radius, which might correspond to the inner ILR. Continuum emission is detected at 0.87 mm only at the very centre, and its origin is more likely thermal dust emission than non-thermal emission from the AGN. It might correspond to the molecular torus expected to exist in this Seyfert 2 galaxy. The HCN(4-3) and HCO + (4-3) lines were observed simultaneously, but only upper limits are derived, with a ratio to the CO(3-2) line lower than 1/60 at 3 , indicating a relatively low abundance of very dense gas. The kinematics of the gas over the nuclear disk reveal rather regular rotation only slightly perturbed by streaming motions due to the spiral; the primary and secondary bars are too closely aligned with the galaxy major or minor axis to leave a signature in the projected velocities. Near the nucleus, there is an intense high-velocity CO emission feature redshifted to 200 km/s (if located in the plane), with a blue-shifted counterpart, at 2 00 (100 pc) from the centre. While the CO spectra are quite narrow in the centre, this wide component is interpreted as an outflow involving a molecular mass of 3.6 10 6 M and a flow rate 7 M /yr. The flow could be in part driven by the central star formation, but is mainly boosted by the AGN through its radio jets.

Observational study of reactive ions and radicals in PDRs

The observational study of star formation relations in gala xies is central to unraveling the related physical processe s that are at work on both local and global scales. It is still debated whether s tar formation can be described by a universal law that remains valid in different populations of galaxies. We wish to expand the sample of extreme starbursts, represented by local luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs), with high quality observations in the 1‐0 line of HCN, which is taken as a proxy for the dense molecular gas content. The new data presented in this work allow us to enlarge in particular the number of LIRGs studied in HCN by a factor 3 compared to previous works. The chosen LIRG sample has a range of HCN luminosities that partly overlaps with that of the normal galaxy population. We study if a universal law can account for the star formation relations observed for the dense molecular gas in normal star forming galaxies and extreme starbursts and explore the validity of different theoretical prescriptions of the star formation law. We have used the IRAM 30m telescope to observe a sample of 19 LIRGs in the 1‐0 lines of CO, HCN and HCO + . The galaxies have been extracted from a sample of local LIRGs with available high-quality and high-resolution images obtained at optical, near and mid IR wavelengths, which probe the star formation activity. We have thus derived the star f ormation rates using different tracers and determined the sizes of the star forming regions in all the targets. The analysis of the new data proves that the effi ciency of star formation in the dense molecular gas (SFEdense) of extreme starbursts is a factor 3‐4 higher compared to normal galaxies. Kennicutt-Schmidt (KS) power laws have also been derived. We find a dua lity in KS laws that is further reinforced if we account for the likel y different conversion factor for HCN (α HCN ) in extreme starbursts and for the unobscured star formation rate in normal galaxies. This result extends to the higher molecular densities probed by HCN lines the more extreme bimodal behavior of star formation laws, derived from CO molecular lines by two recent surveys. We have confronted our observations with the predictions of theoretical models in which the effi ciency of star formation is determined by the ratio of a constant star formation rate per free-fall time (SFRff) to the local free-fall time (tff). We find that it is possible to fit the observed differences in the SFEdense between normal galaxies and LIRGs/ULIRGs using a common constant SFRff and a set of physically acceptable HCN densities, but only if SFRff∼0.005‐0.01 and/or ifα HCN is a factor of∼a few lower than our favored values. Star formation recipes that explicitly depend on the galaxy global dynamical time scales do not significantly improve th e fit to the new HCN data presented in this work.

ALMA observations of feeding and feedback in nearby Seyfert galaxies: an AGN-driven outflow in NGC 1433

This paper has been partially funded by the Spanish MCyT under projects DGES/AYA2000-927, ESP2001-4519-PE and ESP2002-01693, and European FEDER funds.