Dense Molecular Gas Tracers Research Articles

Continuum and spectral-line observations of the OH megamaser galaxy IRAS 01298−0744

We present Very Long Baseline Array observations of radio continuum and OH line emission toward the OH megamaser (OHM) galaxy IRAS 01298−0744. We recover over 82% of the OH line emission seen in single-dish observations, including multiple strong OH line components and the broad shallow line profile spanning over 1000 km s−1. The OH emission spatially has four compact components, as well as diffuse emission, and the 3σ signals are distributed in a rectangular region with size of ∼40 × 30 mas. The spatial-velocity structure is complex and is consistent with that from HCN and HCO+ lines in the literature, indicating that the nuclear region of this source is likely still in the merging process characterized by intense nuclear activities. The multiple strong OH peaks are blue-shifted compared to dense millimetre-wave molecular gas tracers, but similar to optical Na I absorption. This suggests that the compact masers might be in an outflow driven by an active galactic nucleus or starburst. We have detected compact radio continuum emissions adjacent to the maser positions with a brightness temperature of around 3 × 106 K, similar to other OHM galaxies with very-long-baseline interferometry observations in the literature, which is consistent with a starburst origin.

Dense gas star formation efficiency and the LHCN (4−3)′/LHCO+(4−3)′ ratio: insights from a statistical study of infrared bright star-forming galaxies

We present a statistical study on dense molecular gas tracers of HCN (4–3), HCO+ (4–3) lines and molecular tracers of [C i], and CO observations for a sample of 26 infrared bright star-forming (SF) galaxies. We investigate the dependence of dense gas star formation efficiency traced by HCN (4–3), HCO+ (4–3) (that is LIR/LHCN (4-3)′, and LIR/LHCO+(4−3)′), and luminosity ratio of LHCN (4-3)′/LHCO+(4−3)′ on [C i]-CO ratios of L[C I] (1-0)′/LCO (1-0)′, L[C I] (2-1)′/LCO (1-0)′ and L[C I] (2-1)′/L[C I] (1-0)′ (hereafter R[CI]) which are sensitive to interstellar medium conditions. Our findings show that both LIR/LHCN (4-3)′ and LIR/LHCO+(4−3)′ have moderate correlations with L[C I] (2-1)′/LCO (1-0)′ and R[CI], while LHCN (4-3)′/LHCO+(4−3)′ does not show any significant correlations with any of the [C i]-CO ratios. We compare the LHCN (4-3)′/LHCO+(4−3)′ ratios of AGN and SF galaxies, and find that although the higher LHCN (4-3)′/LHCO+(4−3)′ ratios are mainly found in AGN, the majority of the LHCN (4-3)′/LHCO+(4−3)′ values in SF galaxies are comparable to those in AGN. Based on our findings, it appears that the LHCN (4-3)′/LHCO+(4−3)′ ratio may not be a reliable indicator of the presence of an AGN, although further investigation is needed to confirm this conclusion.

Dense Molecular Gas Properties of the Central Kiloparsec of Nearby Ultraluminous Infrared Galaxies Constrained by ALMA Three Transition-line Observations

We report the results of Atacama Large Millimeter/submillimeter Array (ALMA) 1–2 kpc resolution, three rotational transition-line (J = 2–1, J = 3–2, and J = 4–3) observations of multiple dense molecular gas tracers (HCN, HCO+, and HNC) for 10 nearby (ultra)luminous infrared galaxies ((U)LIRGs). Following the matching of beam sizes to 1–2 kpc for each (U)LIRG, the high-J-to-low-J transition-line flux ratios of each molecule and the emission-line flux ratios of different molecules at each J transition are derived. We conduct RADEX non-LTE model calculations and find that, under a wide range of gas density and kinetic temperature, the observed HCN-to-HCO+ flux ratios in the overall (U)LIRGs are naturally reproduced with enhanced HCN abundance compared to HCO+. Thereafter, molecular gas properties are constrained primarily through the use of HCN and HCO+ data and the adoption of fiducial values for the HCO+ column density and HCN-to-HCO+ abundance ratio. We quantitatively confirm the following: (i) molecular gas at the (U)LIRGs’ nuclei is dense (≳103–4 cm−3) and warm (≳100 K), (ii) the molecular gas density and temperature in nine ULIRGs’ nuclei are significantly higher than those of one LIRG’s nucleus, (iii) molecular gas in starburst-dominated sources tends to be less dense and cooler than ULIRGs with luminous AGN signatures. For six selected sources, we also apply a Bayesian approach by freeing all parameters and support the above main results. Our ALMA 1–2 kpc resolution, multiple transition-line data of multiple molecules are a very powerful tool for scrutinizing the properties of molecular gas concentrated around luminous energy sources in nearby (U)LIRGs’ nuclei.

A Complete HCN Survey of the Perseus Molecular Cloud

We present a survey of the Perseus molecular cloud in the J = 1 → 0 transition of HCN, a widely used tracer of dense molecular gas. The survey was conducted with the CfA 1.2 m telescope, which at 89 GHz has a beamwidth of 11′ and a spectral resolution of 0.85 km s−1. A total of 8.1 deg2 was surveyed on a uniform 10′ grid to a sensitivity of 14 mK per channel. We compared the survey with similar surveys of CO and dust in order to study and calibrate the HCN line as a dense-gas tracer. We find the HCN emission to extend over a considerable fraction of the cloud. We show that the HCN intensity remains linear with H2 column density well into the regime where the CO line saturates. We use radiative-transfer modeling to show that this likely results from subthermal excitation of HCN in a cloud where the column and volume densities of H2 are positively correlated. To match our HCN observations the model requires an exponential decrease in HCN abundance with increasing extinction, consistent with HCN depletion onto grains. The modeling also reveals that the mean volume density of H2 in the HCN-emitting regions is ∼104 cm−3, well below the HCN critical density. For the first time, we obtain a direct measurement of the ratio of dense-gas mass to HCN luminosity for an entire nearby molecular cloud: α(HCN) = 92 M ⊙/(K km s−1 pc2).

DENSE GAS PROPERTIES IN ARP 220

Studying dense gas is of fundamental importance for understanding star formation and galaxy evolution. We present data from the Atacama Large Millimeter/Submillimeter Array (ALMA) for the dense molecular gas tracers HNC (hydrogen isocyanide) with transition J = 2-1, CS (carbon monosulfide) with transition J = 4-3, and CO (carbon monoxide) with transition J = 6-5. We studied some properties of dense gases to find their effect on star formation in Arp 220 galaxy. We identify Arp 220 into two regions in which we reveal emission from every one of the three molecules, including the east and west nuclei. We calculate the ratios of LHNC/LCO, LCS/LCO, and LCS/LHNC at a sub-arcsecond scale. The ratios of LHNC / LCO and LCS / LCO represent fractions of the dense molecular gas. The values of these ratios are found equal to (0.31, 0.67) and (0.15, 0.31) in the west and east nuclei, respectively. We detect that these ratios are greater in the east nucleus than in the west nucleus. We also calculated the star formation efficiency (SFEdense), which is the rate of star formation per unit dense gas mass at a sub-arcsecond scale. We found that this galaxy has a long depletion time and low SFEdense compared to the global scale.

ArH+ and H2O+ absorption towards luminous galaxies

Context. Along several sight lines within the Milky Way ArH+ has been ubiquitously detected with only one detection in extragalactic environments, namely along two sight lines in the redshift z = 0.89 absorber towards the lensed blazar PKS 1830-211. Being formed in predominantly atomic gas by reactions between Ar+, which were initially ionised by cosmic rays and molecular hydrogen, ArH+ has been shown to be an excellent tracer of atomic gas as well as the impinging cosmic-ray ionisation rates. Aims. In this work, we attempt to extend the observations of ArH+ in extragalactic sources to examine its use as a tracer of the atomic interstellar medium (ISM) in these galaxies. Methods. We report the detection of ArH+ towards two luminous nearby galaxies, NGC 253 and NGC 4945, and the non-detection towards Arp 220 observed using the SEPIA660 receiver on the APEX 12 m telescope. In addition, the two sidebands of this receiver allowed us to observe the NKaKc = 11,0 − 10,1 transitions of another atomic gas tracer p-H2O+ at 607.227 GHz with the ArH+ line, simultaneously. We modelled the optically thin spectra of both species and compared their observed line profiles with that of other well-known atomic gas tracers such as OH+ and o-H2O+ and diffuse and dense molecular gas tracers HF and CO, respectively. Results. Assuming that the observed absorption from the ArH+, OH+, and H2O+ molecules are affected by the same flux of cosmic rays, we investigate the properties of the different cloud layers. Based on a steady-state analysis of the chemistry of these three species and using statistical equilibrium calculations, we estimate the molecular fraction traced by ArH+ to be ∼​10−3 and find that ArH+ resides in gas volumes with low electron densities. We further study the ortho-to-para ratio of H2O+ and find that the derived ratios do not significantly deviate from the equilibrium value of three with spin temperatures greater than 15 and 24 K.

Multi-line modelling in nearby galaxies: The link between dense gas and star formation in different environments

TheJ=1–0 lines of HCN and HCO+have become the default tracers of dense molecular gas in external galaxies. To study the relation between HCN and HCO+emission and density, we have mapped theirJ=3–2 lines with the ALMA Compact Array (ACA) in the nearby star-forming galaxies NGC3351, NGC3627, and NGC4321. Combined with archival line maps of HCN(1–0), HCO+(1–0), CO(1–0), and CO(2–1), we use this data set to: (1) compare the excitation of dense gas tracers in normal and starburst/AGN galaxies; (2) explore how line ratios vary across galaxy disks; (3) infer the density distribution and other cloud properties on a pixel-by-pixel basis through a physically-motivated radiative transfer model.

Dynamically regulated star formation in the strongly interacting Taffy galaxies

The Taffy system stands out as one of the strongest gas-rich galaxy mergers with little star formation (SF) activity. We study the causes of this SF inefficiency by observing with the IRAM 30m telescope the line emission of several species in 6 positions. We report clear detections of tracers of bulk (12CO and 13CO) and dense (HCN and HCO+) molecular gas in all regions, as well as evidence for shocks (SiO J=2–1) over the intergalactic bridge. Our observations not only confirm the SF inefficiency of the bulk gas, but they also show that the SF efficiency of the dense gas phase is abnormally low too (∼1 dex below ULIRGs). The dense gas fraction (∝ HCN/CO) only shows small variations across the entire system, with a typical value of 4%. Although this fraction is somewhat low compared to other interacting/merging systems, it is similar to the values measured in the disk of nearby, normal star-forming galaxies. Finally, we use the outstanding properties of the Taffy system to place constraints on some of the turbulence-regulated SF models from the literature.

Cause and effects of the massive star formation in Messier 8 East

Context. Messier 8 (M8), one of the brightest H II regions in our Galaxy, is powered by massive O-type stars and is associated with recent and ongoing massive star formation. Two prominent massive star-forming regions associated with M8 are M8-Main, the particularly bright part of the large-scale H II region (mainly) ionized by the stellar system Herschel 36 (Her 36) and M8 East (M8 E), which is mainly powered by a deeply embedded young stellar object (YSO), the bright infrared (IR) source M8E-IR. Aims. We study the interaction of the massive star-forming region M8 E with its surroundings using observations of assorted diffuse and dense gas tracers that allow quantifying the kinetic temperatures and volume densities in this region. With a multiwavelength view of M8 E, we investigate the cause of star formation. Moreover, we compare the star-forming environments of M8-Main and M8 E, based on their physical conditions and the abundances of the various observed species toward them. Methods. We used the Institut de Radioastronomía Millimétrica 30 m telescope to perform an imaging spectroscopy survey of the ~1 pc scale molecular environment of M8E-IR and also performed deep integrations toward the source itself. We imaged and analyzed data for the J = 1 → 0 rotational transitions of 12CO, 13CO, N2H+, HCN, H13CN, HCO+, H13CO+, HNC, and HN13C observed for the first time toward M8 E. To visualize the distribution of the dense and diffuse gas in M8 E, we compared our velocity-integrated intensity maps of 12CO, 13CO, and N2H+ with ancillary data taken at IR and submillimeter wavelengths. We used techniques that assume local thermodynamic equilibrium (LTE) and non-LTE to determine column densities of the observed species and constrain the physical conditions of the gas that causes their emission. Examining the class 0/ I and class II YSO populations in M8 E, allows us to explore the observed ionization front (IF) as seen in the high resolution Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) 8 μm emission image. The difference between the ages of the YSOs and their distribution in M8 E were used to estimate the speed of the IF. Results. We find that 12CO probes the warm diffuse gas also traced by the GLIMPSE 8 μm emission, while N2H+ traces the cool and dense gas following the emission distribution of the APEX Telescope Large Area Survey of the Galaxy 870 μm dust continuum. We find that the star-formation in M8 E appears to be triggered by the earlier formed stellar cluster NGC 6530, which powers an H II region giving rise to an IF that is moving at a speed ≥0.26 km s−1 across M8 E. Based on our qualitative and quantitative analysis, the J = 1 → 0 transition lines of N2H+ and HN13C appear to be more direct tracers of dense molecular gas than the J = 1 → 0 transition lines of HCN and HCO+. We derive temperatures of 80 and 30 K for the warm and cool gas components, respectively, and constrain the H2 volume densities to be in the range of 104–106 cm−3. Comparison of the observed abundances of various species reflects the fact that M8 E is at an earlier stage of massive star formation than M8-Main.

ALMA Spatially Resolved Dense Molecular Gas Survey of Nearby Ultraluminous Infrared Galaxies

Abstract We present the results of our Atacama Large Millimeter Array HCN J = 3–2 and HCO+ J = 3–2 line observations of a uniformly selected sample (>25) of nearby ultraluminous infrared galaxies (ULIRGs) at z < 0.15. The emission of these dense molecular gas tracers and continuum are spatially resolved in the majority of observed ULIRGs for the first time with achieved synthesized beam sizes of ≲0.″2 or ≲500 pc. In most ULIRGs, the HCN-to-HCO+ J = 3–2 flux ratios in the nuclear regions within the beam size are systematically higher than those in the spatially extended regions. The elevated nuclear HCN J = 3–2 emission could be related to (a) luminous buried active galactic nuclei, (b) the high molecular gas density and temperature in ULIRG’s nuclei, and/or (c) mechanical heating by spatially compact nuclear outflows. A small fraction of the observed ULIRGs display higher HCN-to-HCO+ J = 3–2 flux ratios in localized off-nuclear regions than those of the nuclei, which may be due to mechanical heating by spatially extended outflows. The observed nearby ULIRGs are generally rich in dense (>105 cm−3) molecular gas, with an estimated mass of >109 M ⊙ within the nuclear (a few kpc) regions, and dense gas can dominate the total molecular mass there. We find a low detection rate (<20%) regarding the possible signature of a vibrationally excited (v 2 = 1f) HCN J = 3–2 emission line in the vicinity of the bright HCO+ J = 3–2 line that may be due, in part, to the large molecular line widths of ULIRGs.

Dense Molecular Gas in the Nearby Low-metallicity Dwarf Starburst Galaxy IC 10

Abstract Dense molecular gas and star formation are correlated in galaxies. The effect of low metallicity on this relationship is crucial for interpreting observations of high-redshift galaxies, which have lower metallicities than galaxies today. However, it remains relatively unexplored because dense molecular gas tracers like HCN and HCO+ are faint in low-metallicity systems. We present Green Bank Telescope observations of HCN(1–0) and HCO+(1–0) on giant molecular cloud (34 pc) scales in the nearby low-metallicity ( ) starburst IC 10 and compare them to those in other galaxies. We detect HCN and HCO+ in one and three of five pointings, respectively. The values are within the range seen in other galaxies, but are most similar to those seen in other low-metallicity sources and in starbursts. The detections follow the fiducial – and – relationships. These trends suggest that HCN and HCO+ can be used to trace dense molecular gas at metallicities of 1/4 Z ⊙, to first order. The dense gas fraction is similar to that in spiral galaxies, but lower than that in U/LIRGs. The dense molecular gas star formation efficiency, however, is on the upper end of those in normal galaxies and consistent with those in U/LIRGs. These results suggest that the CO and HCN/HCO+ emission occupy the same relative volumes as they do at higher metallicity, but that the entire emitting structure is reduced in size. Dense gas mass estimates for high-redshift galaxies may need to be corrected for this effect.

High Dense Gas Fraction in Intensely Star-forming Dusty Galaxies

Abstract We present ALMA and VLA observations of the dense molecular gas tracers HCN, HCO+, and HNC in two lensed, high-redshift starbursts selected from the Herschel-ATLAS survey: H-ATLAS J090740.0−004200 (SDP.9, ) and H-ATLAS J091043.1−000321 (SDP.11, ). In SDP.9 we have detected all transitions and also HCN(1–0) and HCO+(1–0). In SDP.11 we have detected HCN(3–2) and HCO+(3–2). The amplification factors for both galaxies have been determined from subarcsecond-resolution CO and dust emission observations carried out with NOEMA and the SMA. The HNC(1–0)/HCN(1–0) line ratio in SDP.9 suggests the presence of photon-dominated regions, as happens in most local (U)LIRGs. The CO, HCN, and HCO+ spectral line energy distribution (SLEDs) of SDP.9 are compatible to those found for many local, IR-bright galaxies, indicating that the molecular gas in local and high-redshift dusty starbursts can have similar excitation conditions. We obtain that the correlation between total IR ( ) and dense line ( ) luminosity in SDP.9 and SDP.11 and local star-forming galaxies can be represented by a single relation. We argue that the scatter of the correlation, together with the lack of sensitive dense molecular gas tracer observations for a homogeneous sample of high-redshift galaxies, prevents us from distinguishing differential trends with redshift. Our results suggest that the intense star formation found in some high-redshift, dusty, luminous starbursts is associated with more massive dense molecular gas reservoirs and higher dense molecular gas fractions.

HC3N observations of nearby galaxies

Aims. We aim to systematically study the properties of the different transitions of the dense molecular gas tracer HC3N in galaxies. Methods. We have conducted single-dish observations of HC3N emission lines towards a sample of nearby gas-rich galaxies. HC3N(J=2-1) was observed in 20 galaxies with Effelsberg 100-m telescope. HC3N(J=24-23) was observed in nine galaxies with the 10-m Submillimeter Telescope (SMT). Results. HC3 N 2-1 is detected in three galaxies: IC 342, M 66 and NGC 660 (> 3 {\sigma}). HC3 N 24-23 is detected in three galaxies: IC 342, NGC 1068 and IC 694. This is the first measurements of HC3N 2-1 in a relatively large sample of external galaxies, although the detection rate is low. For the HC3 N 2-1 non-detections, upper limits (2 {\sigma}) are derived for each galaxy, and stacking the non-detections is attempted to recover the weak signal of HC3N. But the stacked spectrum does not show any significant signs of HC3N 2-1 emission. The results are also compared with other transitions of HC3N observed in galaxies. Conclusions. The low detection rate of both transitions suggests low abundance of HC3N in galaxies, which is consistent with other observational studies. The comparison between HC3N and HCN or HCO+shows a large diversity in the ratios between HC3N and HCN or HCO+. More observations are needed to interpret the behavior of HC3N in different types of galaxies.

Dense Molecular Gas Tracers in the Outflow of the Starburst Galaxy NGC 253

Abstract We present a detailed study of a molecular outflow feature in the nearby starburst galaxy NGC 253 using ALMA. We find that this feature is clearly associated with the edge of NGC 253's prominent ionized outflow, has a projected length of ∼300 pc, with a width of ∼50 pc, and a velocity dispersion of ∼40 km s−1, which is consistent with an ejection from the disk about 1 Myr ago. The kinematics of the molecular gas in this feature can be interpreted (albeit not uniquely) as accelerating at a rate of 1 km s−1 pc−1. In this scenario, the gas is approaching an escape velocity at the last measured point. Strikingly, bright tracers of dense molecular gas (HCN, CN, HCO+, CS) are also detected in the molecular outflow: we measure an HCN(1–0)/CO(1–0) line ratio of in the outflow, similar to that in the central starburst region of NGC 253 and other starburst galaxies. By contrast, the HCN/CO line ratio in the NGC 253 disk is significantly lower ( ), similar to other nearby galaxy disks. This strongly suggests that the streamer gas originates from the starburst, and that its physical state does not change significantly over timescales of ∼1 Myr during its entrainment in the outflow. Simple calculations indicate that radiation pressure is not the main mechanism for driving the outflow. The presence of such dense material in molecular outflows needs to be accounted for in simulations of galactic outflows.

THE DENSE GAS IN THE LARGEST MOLECULAR COMPLEXES OF THE ANTENNAE: HCN AND HCO+ OBSERVATIONS OF NGC 4038/39 USING ALMA

ABSTRACT We present observations of the dense molecular gas tracers , , and in the transition using the Atacama Large Millimeter/submillimeter Array. We supplement our data sets with previous observations of , which trace the total molecular gas content. We separate the Antennae into seven bright regions in which we detect emission from all three molecules, including the nuclei of NGC 4038 and NGC 4039, five super giant molecular complexes in the overlap region, and two additional bright clouds. We find that the ratio of , which traces the dense molecular gas fraction, is greater in the two nuclei ( ) than in the overlap region ( ). We attribute this to an increase in pressure due to the stellar potential within the nuclei; a similar effect to what has been seen previously in the Milky Way and nearby spiral galaxies. Furthermore, the ratio of does not vary by more than a factor of 1.5 between regions. By comparing our measured ratios to photon dominated region (PDR) models including mechanical heating, we find that the ratio of is consistent with mechanical heating contributing ≳5%–10% of the PDR surface heating to the total heating budget. Finally, the ratio of varies from ∼1 in the nucleus of NGC 4038 down to ∼0.5 in the overlap region. The lower ratio in the overlap region may be due to an increase in the cosmic ray rate from the increased supernova rate within this region.

Dense molecular gas tracers in high mass star formation regions

We report the FCRAO observations that mapped HCN (1–0), CS (2–1), HNC (1–0) and HCO+ (1–0) in ten high-mass star forming cores associated with water masers. We present velocity integrated intensity maps of the four lines for these dense cores, compare their line profiles, and derive physical properties of these cores. We find that these four tracers identify areas with similar properties in these massive dense cores, and in most cases, the emissions of HCN and HCO+ are stronger than those of HNC and CS. We also use the line ratios of HCO+/HCN, HNC/HCN and HNC/HCO+ as the diagnostics to explore the environment of these high-mass star forming regions, and find that most of the cores agree with the model that photodominated regions dominate the radiation field, except for W44, for which the radiation field is similar to an X-ray dominated region.

VARIATIONS IN THE STAR FORMATION EFFICIENCY OF THE DENSE MOLECULAR GAS ACROSS THE DISKS OF STAR-FORMING GALAXIES

We present a new survey of HCN(1-0) emission, a tracer of dense molecular gas, focused on the little-explored regime of normal star-forming galaxy disks. Combining HCN, CO, and infrared (IR) emission, we investigate the role of dense gas in Star Formation (SF), finding systematic variations in both the apparent dense gas fraction and the apparent SF efficiency (SFE) of dense gas. The latter may be unexpected, given the popularity of gas density threshold models to explain SF scaling relations. We used the IRAM 30-m telescope to observe HCN(1-0) across 29 nearby disk galaxies whose CO(2-1) emission has previously been mapped by the HERACLES survey. Because our observations span a range of galactocentric radii, we are able to investigate the properties of the dense gas as a function of local conditions. We focus on how the IR/CO, HCN/CO, and IR/HCN ratios (observational cognates of the SFE, dense gas fraction, and dense gas SFE) depend on the stellar surface density and the molecular/atomic ratio. The HCN/CO ratio correlates tightly with these two parameters across a range of 2.1 dex and increases in the high surface density parts of galaxies. Simultaneously, the IR/HCN ratio decreases systematically with these same parameters and is ~6-8 times lower near galaxy centers than in the outer regions. For fixed line-mass conversion factors, these results are incompatible with a simple model in which SF depends only on the gas mass above some density threshold. Only a specific set of environment-dependent conversion factors can render our observations compatible with such a model. Whole cloud models, such as the theory of turbulence regulated SF, do a better job of matching our data. We explore one such model in which variations in the Mach number and in the mean density would respectively drive the trends within galaxy disks and the differences between disk and merging galaxies (abridged).

Molecular line emission in NGC 1068 imaged with ALMA

We investigate the fueling and the feedback of star formation and nuclear activity in NGC1068, a nearby (D=14Mpc) Seyfert 2 barred galaxy, by analyzing the distribution and kinematics of the molecular gas in the disk. We have used ALMA to map the emission of a set of dense molecular gas tracers (CO(3-2), CO(6-5), HCN(4-3), HCO+(4-3) and CS(7-6)) and their underlying continuum emission in the central r ~ 2kpc of NGC1068 with spatial resolutions ~ 0.3"-0.5" (~ 20-35pc). Molecular line and dust continuum emissions are detected from a r ~ 200pc off-centered circumnuclear disk (CND), from the 2.6kpc-diameter bar region, and from the r ~ 1.3kpc starburst (SB) ring. Most of the emission in HCO+, HCN and CS stems from the CND. Molecular line ratios show dramatic order-of-magnitude changes inside the CND that are correlated with the UV/X-ray illumination by the AGN, betraying ongoing feedback. The gas kinematics from r ~ 50pc out to r ~ 400pc reveal a massive (M_mol ~ 2.7 (+0.9, -1.2) x 10^7 Msun) outflow in all molecular tracers. The tight correlation between the ionized gas outflow, the radio jet and the occurrence of outward motions in the disk suggests that the outflow is AGN-driven. The outflow rate estimated in the CND, dM/dt ~ 63 (+21, -37) Msun yr^-1, is an order of magnitude higher than the star formation rate at these radii, confirming that the outflow is AGN-driven. The power of the AGN is able to account for the estimated momentum and kinetic luminosity of the outflow. The CND mass load rate of the CND outflow implies a very short gas depletion time scale of <=1 Myr.

Shock structure and shock heating in the Galactic central molecular zone

AbstractWe present maps of a large number of dense molecular gas tracers across the central molecular zone of our Galaxy. The data were taken with the CSIRO/CASS Mopra telescope in Large Projects in the 1.3 cm, 7 mm, and 3 mm wavelength regimes. Here, we focus on the brightness of the shock tracers SiO and HNCO, molecules that are liberated from dust grains under strong (SiO) and weak (HNCO) shocks. The shocks may have occurred when the gas enters the bar regions and the shock differences could be due to differences in the moving cloud masses. Based on tracers of ionizing photons, it is unlikely that the morphological differences are due to selective photo-dissociation of the molecules. We also observe direct heating of molecular gas in strongly shocked zones, with high SiO/HNCO ratios, where temperatures are determined from the transitions of ammonia. Strong shocks appear to be the most efficient heating source of molecular gas, apart from high energy emission emitted by the central supermassive black hole Sgr A* and the processes within the extreme star formation region Sgr B2.

Dense molecular gas: a sensitive probe of stellar feedback models

We show that the mass fraction of GMC gas (n>100 cm^-3) in dense (n>>10^4 cm^-3) star-forming clumps, observable in dense molecular tracers (L_HCN/L_CO(1-0)), is a sensitive probe of the strength and mechanism(s) of stellar feedback. Using high-resolution galaxy-scale simulations with pc-scale resolution and explicit models for feedback from radiation pressure, photoionization heating, stellar winds, and supernovae (SNe), we make predictions for the dense molecular gas tracers as a function of GMC and galaxy properties and the efficiency of stellar feedback. In models with weak/no feedback, much of the mass in GMCs collapses into dense sub-units, predicting L_HCN/L_CO(1-0) ratios order-of-magnitude larger than observed. By contrast, models with feedback properties taken directly from stellar evolution calculations predict dense gas tracers in good agreement with observations. Changing the strength or timing of SNe tends to move systems along, rather than off, the L_HCN-L_CO relation (because SNe heat lower-density material, not the high-density gas). Changing the strength of radiation pressure (which acts efficiently in the highest density gas), however, has a much stronger effect on L_HCN than on L_CO. We predict that the fraction of dense gas (L_HCN/L_CO(1-0)) increases with increasing GMC surface density; this drives a trend in L_HCN/L_CO(1-0) with SFR and luminosity which has tentatively been observed. Our results make specific predictions for enhancements in the dense gas tracers in unusually dense environments such as ULIRGs and galactic nuclei (including the galactic center).