Star-formation in the central kpc of the starburst/LINER galaxy NGC1614

A high angular resolution, multi-wavelength study of the LINER galaxy NGC1614 has been carried out. OVRO CO 1-0 observations are presented together with extensive multi-frequency radio continuum and HI absorption observations with the VLA and MERLIN. Toward the center of NGC1614, we have detected a ring of radio continuum emission with a radius of 300 pc. This ring is coincident with previous radio and Paschen-alpha observations. The dynamical mass of the ring based on HI absorption is 3.1 x 10E9 Msun. The peak of the integrated CO 1-0 emission is shifted by 1" to the north-west of the ring center and a significant fraction of the CO emission is associated with a crossing dust lane. An upper limit to the molecular gas mass in the ring region is 1.7 x 10E9 Msun. Inside the ring, there is a north to south elongated 1.4GHz radio continuum feature with a nuclear peak. This peak is also seen in the 5GHz radio continuum and in the CO. We suggest that the R=300 pc star forming ring represents the radius of a dynamical resonance - as an alternative to the scenario that the starburst is propagating outwards from the center into a molecular ring. The ring-like appearance probably part of a spiral structure. Substantial amounts of molecular gas have passed the radius of the ring and reached the nuclear region. The nuclear peak seen in 5GHz radio continuum and CO is likely related to previous star formation, where all molecular gas was not consumed. The LINER-like optical spectrum observed in NGC1614 may be due to nuclear starburst activity, and not to an Active Galactic Nucleus (AGN). Although the presence of an AGN cannot be excluded.Comment: Accepted by Astronomy and Astrophysics, 12 pages, 10 figure

An Origin of the Huge Far-Infrared Luminosity of Starburst Mergers

Recently Taniguchi and Ohyama found that the higher $^{12}$CO to $^{13}$CO integrated intensity ratios at a transition $J$=1--0, $R = I(^{12}$CO)$/I(^{13}$CO) $\gtrsim 20$, in a sample of starburst merging galaxies such as Arp 220 are mainly attributed to the depression of $^{13}$CO emission with respect to $^{12}$CO. Investigating the same sample of galaxies analyzed by Taniguchi & Ohyama, we find that there is a tight, almost linear correlation between the dust mass and $^{13}$CO luminosity. This implies that dust grains are also depressed in the high-$R$ starburst mergers, leading to the higher dust temperature ($T_{\rm d}$) in them because of the relative increase in the radiation density. Nevertheless, the average dust mass ($M_{\rm d}$) of the high-$R$ starburst mergers is higher significantly than that of non-high $R$ galaxies. This is naturally understood because the galaxy mergers could accumulate a lot of dust grains from their progenitor galaxies together with supply of dust grains formed newly in the star forming regions. Since $L$(FIR) $\propto M_{\rm d} T_{\rm d}^5$ given the dust emissivity law, $S_\nu \propto \lambda^{-1}$, the increases in both $M_{\rm d}$ and $T_{\rm d}$ explain well why the starburst mergers are so bright in the FIR. We discuss that the superwind activity plays an important role in destroying dust grains as well as dense gas clouds in the central region of mergers.Comment: 10 pages (aaspp4.sty), 3 postscript figures (embedded). Accepted for publication in Astrophysical Journal Letter

Luminous HC3N line emission in NGC4418 - buried AGN or nascent starburst?

IRAM 30m observations reveal that the deeply obscured IR-luminous galaxy NGC4418 has a rich molecular chemistry - including unusually luminous HC3N line emission. We furthermore detect: ortho-H2CO 2-1, 3-2; CN 1-0, 2-1; HCO+, 1-0. 3-2, HCN 3-2, HNC 1-0, 3-2 (and tentatively OCS 12-11). The HCN, HCO+, H2CO and CN line emission can be fitted to densities of n=5 x 10E4 - 10E5 cm-3 and gas temperatures Tk=80-150 K. Both HNC and HC3N are, however, significantly more excited than the other species which requires higher gas densities - or radiative excitation through e.g. mid-IR pumping. The HCN line intensity is fainter than that of HCO+ and HNC for the 3-2 transition, in contrast to previous findings for the 1-0 lines where the HCN emission is the most luminous. We tentatively suggest that the observed molecular line emission is consistent with a young starburst, where the emission can be understood as emerging from dense, warm gas with an additional PDR component. We find that X-ray chemistry is not required to explain the observed mm line emission, including the HCN/HCO+ 1-0 and 3-2 line ratios. The luminous HC3N line emission is an expected signature of dense, starforming gas. A deeply buried AGN can not be excluded, but its impact on the surrounding molecular medium is then suggested to be limited. However, detailed modelling of HC3N abundances in X-ray dominated regions (XDRs) should be carried out. The possibility of radiative excitation should also be further investigatedComment: 7 pages, one eps figure, uses aa.cls, submitted to Astronomy and Astrophysic

Policies to promote sustainable consumption: framework for a future-oriented evaluation

Governments are today developing policies to promote sustainable consumption, yet policy makers face many uncertainties about policy impacts. These include uncertainties about how policy instruments influence consumption patterns and about the impact of changes in consumption patterns on ecological, social and economic sustainability. An assessment of such impacts must account for the fact that consumer action is interlinked with the dynamic activities of other market players and the path-creating effects of technologies and systems of consumption and provision

CI emission in Ultra Luminous Infrared Galaxies as a molecular gas mass tracer

We present new sensitive wide-band measurements of the fine structure line 3^P_1 -> 3^P_0 (J=1-0, 492GHz) of neutral atomic carbon (CI) in the two typical Ultra Luminous Infrared Galaxies NGC6240 and Arp220. We then use them along with several other CI measurements in similar objects found in the literature to estimate their global molecular gas content under the assumption of a full CI-H_2 concomitance. We find excellent agreement between the H_2 gas mass estimated with this method and the standard methods using 12^CO. This may provide a new way to measure H_2 gas mass in galaxies, and one which may be very valuable in ULIRGs since in such systems the bright 12^CO emission is known to systematically overestimate the gas mass while their 13^CO emission is usually very weak. At redshifts z>=1 the CI J=1-0 line shifts to much more favorable atmospheric windows and can become a viable alternative tracer of the H_2 gas fueling starburst events in the distant Universe.Comment: 11 pages, 2 figures. Accepted for publication in ApJ Letter

A high-resolution mm and cm study of the obscured LIRG NGC 4418 - A compact obscured nucleus fed by in-falling gas?

The aim of this study is to constrain the dynamics, structure and feeding of the compact nucleous of NGC4418, and to reveal the nature of the main hidden power source: starburst or AGN. We obtained high spatial resolution observations of NGC4418 at 1.4 and 5 GHz with MERLIN, and at 230 and 270 GHz with the SMA very extended configuration. We use the continuum morphology and flux density to estimate the size of the emitting region, the star formation rate and the dust temperature. Emission lines are used to study the kinematics through position-velocity diagrams. Molecular emission is studied with population diagrams and by fitting an LTE synthetic spectrum. We detect bright 1mm line emission from CO, HC3N, HNC and C34S, and 1.4 GHz absorption from HI. The CO 2-1 emission and HI absorption can be fit by two velocity components at 2090 and 2180 km s-1. We detect vibrationally excited HC3N and HNC, with Tvib 300K. Molecular excitation is consistent with a layered temperature structure, with three main components at 80, 160 and 300 K. For the hot component we estimate a source size of less than 5 pc. The nuclear molecular gas surface density of 1e4 Msun pc-2 is extremely high, and similar to that found in the ultra-luminous infrared galaxy (ULIRG) Arp220. Our observations confirm the the presence of a molecular and atomic in-flow, previously suggested by Herschel observations, which is feeding the activity in the center of NGC4418. Molecular excitation confirms the presence of a very compact, hot dusty core. If a starburst is responsible for the observed IR flux, this has to be at least as extreme as the one in Arp220, with an age of 3-10 Myr and a star formation rate >10 Msun yr-1. If an AGN is present, it must be extremely Compton-thick.Comment: 18 pages, 11 figures, Accepted for publication by A&A on 10/6/201

A Survey of HC3N in Extragalactic Sources - Is HC3N a Tracer of Activity in ULIRGs?

Context. HC3N is a molecule that is mainly associated with Galactic star-forming regions, but it has also been detected in extragalactic environments. Aims. To present the first extragalactic survey of HC3N, when combining earlier data from the literature with six new single-dish detections, and to compare HC3N with other molecular tracers (HCN, HNC), as well as other properties (silicate absorption strength, IR flux density ratios, C II flux, and megamaser activity). Methods. We present mm IRAM 30 m, OSO 20 m, and SEST observations of HC3N rotational lines (mainly the J = 10-9 transition) and of the J = 1-0 transitions of HCN and HNC. Our combined HC3N data account for 13 galaxies (excluding the upper limits reported for the non-detections), while we have HCN and HNC data for more than 20 galaxies. Results. A preliminary definition "HC3N-luminous galaxy" is made based upon the HC3N/HCN ratio. Most (~80 %) HC3N-luminous galaxies seem to be deeply obscured galaxies and (U)LIRGs. A majority (~60 % or more) of the HC3N-luminous galaxies in the sample present OH mega- or strong kilomaser activity. A possible explanation is that both HC3N and OH megamasers need warm dust for their excitation. Alternatively, the dust that excites the OH megamaser offers protection against UV destruction of HC3N. A high silicate absorption strength is also found in several of the HC3N-luminous objects, which may help the HC3N to survive. Finally, we find that a high HC3N/HCN ratio is related to a high dust temperature and a low C II flux.Comment: 17 pages, 12 figures, to be published in Astronomy & Astrophysic

Sub-arcsecond imaging of the radio continuum and neutral hydrogen in the Medusa merger

We present sub-arcsecond, Multi-Element Radio Linked Interferometer (MERLIN) observations of the decimetre radio continuum structure and neutral hydrogen (HI) absorption from the nuclear region of the starburst galaxy NGC 4194 (the Medusa Merger). The continuum structure of the central kiloparsec of the Medusa has been imaged, revealing a pair of compact radio components surrounded by more diffuse, weak radio emission. Using the constraints provided by these observations and those within the literature we conclude that the majority of this radio emission is related to the ongoing star-formation in this merger system. With these observations we also trace deep HI absorption across the detected radio continuum structure. The absorbing HI gas structure exhibits large variations in column densities. The largest column densities are found toward the south of the nuclear radio continuum, co-spatial with both a nuclear dust lane and peaks in $^{12}$CO (1->0) emission. The dynamics of the HI absorption, which are consistent with lower resolution $^{12}$CO emission observations, trace a shallow north-south velocity gradient of ~320km/s/kpc. This gradient is interpreted as part of a rotating gas structure within the nuclear region. The HI and CO velocity structure, in conjunction with the observed gas column densities and distribution, is further discussed in the context of the fuelling and gas physics of the ongoing starburst within the centre of this merger.Comment: 12 pages, 5 figures, to appear in A&

Extragalactic H3O+: Some Consequences

We discuss some implications of our recent detection of extragalactic H3O+: the location of the gas in M82, the origin of energetic radiation in M82, and the possible feedback effects of star formation on the cosmic ray flux in galaxies.Comment: Five pages, one figure; contribution to proceedings of conference "Far-infrared observations of the interstellar medium", December 2007, Bad Honne

COLA. III. Radio Detection of Active Galactic Nucleus in Compact Moderate Luminosity Infrared Galaxies

We present results from 4.8 GHz Very Large Array (VLA) and global very long baseline interferometry (VLBI) observations of the northern half of the moderate FIR luminosity (median L_(IR) = 10^(11.01) L_☉) COLA sample of star-forming galaxies. VLBI sources are detected in a high fraction (20/90) of the galaxies observed. The radio luminosities of these cores (~10^(21) W Hz^(–1)) are too large to be explained by radio supernovae or supernova remnants and we argue that they are instead powered by active galactic nuclei (AGNs). These sub-parsec scale radio cores are preferentially detected toward galaxies whose VLA maps show bright 100-500 parsec scale nuclear radio components. Since these latter structures tightly follow the FIR to radio-continuum correlation for star formation, we conclude that the AGN-powered VLBI sources are associated with compact nuclear starburst environments. The implications for possible starburst-AGN connections are discussed. The detected VLBI sources have a relatively narrow range of radio luminosity consistent with models in which intense compact Eddington-limited starbursts regulate the gas supply onto a central supermassive black hole. The high incidence of AGN radio cores in compact starbursts suggests little or no delay between the starburst phase and the onset of AGN activity