CN and HNC Line Emission in IR Luminous Galaxies

We have observed HNC 1-0, CN 1-0 and 2-1 line emission in a sample of 13 IR luminous (LIRGs, L_IR > 10E11 Lo) starburst and Seyfert galaxies. HNC 1-0 is detected in 9, CN 1-0 is detected in 10 and CN 2-1 in 7 of the galaxies. We also report the first detection of HC3N (10-9) emission in Arp220. The excitation of HNC and CN emission requires densities n > 10E4 cm-3. We compare their intensities to that of the usual high density tracer HCN. The I(HCN)/I(HNC}) and I(HCN)/I(CN) 1-0 line intensity ratios vary significantly, from 0.5 to >6, among the galaxies. This implies that the actual properties of the dense gas is varying among galaxies who otherwise have similar I(CO)/I(HCN) line intensity ratios. We suggest that the HNC emission is not a reliable tracer of cold (10 K) gas at the center of LIRGs, as it often is in the disk of the Milky Way. Instead, the HNC abundance may remain substantial, despite high gas temperatures, because the emission is emerging from regions where the HCN and HNC formation and destruction processes are dominated by ion-neutral reactions which are not strongly dependent on kinetic temperature. We find five galaxies (four AGNs and one starburst) where the I(HCN)/I(HNC) intensity ratio is close to unity. In other AGNs, however, I(HCN)/I(HNC}) is >4. The CN emission is on average a factor of two fainter than HCN, but the variation is large and there seems to be a trend of reduced relative CN luminosity with increasing IR luminosity. One galaxy, NGC3690, has a CN luminosity twice that of HCN and its ISM is thus strongly affected by UV radiation. We discuss the I(HCN)/I(HNC) and I(HCN)/I(CN) line ratios as indicators of starburst evolution.Comment: 12 pages, 4 figures. Accepted for publication in Astronomy and Astrophysic

Arp 220 - IC 4553/4: understanding the system and diagnosing the ISM

Arp220 is a nearby system in final stages of galaxy merger with powerful ongoing star-formation at and surrounding the two nuclei. Arp 220 was detected in HI absorption and OH Megamaser emission and later recognized as the nearest ultra-luminous infrared galaxy also showing powerful molecular and X-ray emissions. In this paper we review the available radio and mm-wave observational data of Arp 220 in order to obtain an integrated picture of the dense interstellar medium that forms the location of the powerful star-formation at the two nuclei.Comment: 9 pages, 4 figures, to appear in: IAU Symposium 242 Astrophysical Masers and their Environment

A Molecular Tidal Tail in the Medusa Minor Merger

We have detected CO 1-0 emission along the tidal tail of the NGC 4194 (the Medusa) merger. It is the first CO detection in the optical tail of a minor merger. Emission is detected both in the centre of the tail and at its tip. The molecular mass in the 33'' Onsala 20m beam is estimated to be >= 8.5 x 10^7 M_{sun} which is at least 4% of the total molecular mass measured so far in this system. We suggest that the emission is a molecular tidal tail which is part of the extended structure of the main body, and that the molecular gas was thrown out by the collision instead of having formed in situ from condensing atomic material. We find it unlikely that the emission is associated with a tidal dwarf galaxy (even if the future formation of such an object is possible), but high resolution HI, CO and optical observations are necessary to resolve the issue. The Medusa is very likely the result of an elliptical+spiral collison and our detection supports the notion that molecular gas in minor mergers can be found at great distances from the merger centre.Comment: 4 Pages, 2 figures included, accepted for A&A letter

Variation of Molecular Line Ratios and Cloud Properties in the Arp 299 Galaxy Merger

High resolution observations of 12CO (2.''3), 13CO (3.''9), and HCN (5.''4) J=1--0 in the galaxy merger Arp 299 (IC 694 and NGC 3690) show the line ratios vary dramatically across the system. The 12CO/13CO ratio is unusually large, 60 +- 15, at the IC 694 nucleus, where 12CO emission is very strong, and much smaller, 10 +- 3, in the southern extended disk of that galaxy. Elsewhere, the 12CO/13CO line ratio is 5-20, typical of spiral galaxies. The line ratio variation in the overlap between the two galaxies is smaller, ranging from 10 +- 3 in the east to 20 +- 4 in the west. The 12CO/HCN line ratio also varies across Arp 299, although to a lesser degree. HCN emission is bright towards each galaxy nucleus and in the extranuclear region of active star formation; it was not detected in the IC 694 disk, or the eastern part of the overlap region, leading to lower limits of 25 and 20 respectively. By contrast, at the nuclei of IC 694 and NGC 3690 the ratios are 9 +- 1 and 14 +- 3 respectively. In the western part of the overlap region it is 11 +- 3.Comment: 16 pages, 4 postscript figures, to appear in ApJ Letter

Formation and evolution of dusty starburst galaxies I. A new method for deriving spectral energy distribution

We present a new numerical code which is designed to derive a spectral energy distribution (SED) for an arbitrary spatial distribution of stellar and gaseous components in a dusty starburst galaxy. We apply a ray tracing method to numerical simulations and thereby estimate extinction and reemission of stellar light by dusty gas in an explicitly self-consistent manner. By using this code, we can investigate simultaneously dynamical and photometric evolution of a dusty galaxy based on stellar and gaseous dynamical simulations. As an example, we demonstrate when and how a major galaxy merger with dusty starburst becomes an ultra-luminous infrared galaxy owing to strong internal dust extinction. We furthermore discuss advantages and disadvantages of the present new code in clarifying the nature and the origin of low and high redshift dusty starburst galaxies.Comment: 44 pages 19 figures (11 color), accepted by Ap

Radio continuum and X-ray emission from the most extreme FIR-excess galaxy NGC 1377: An extremely obscured AGN revealed

Galaxies which strongly deviate from the radio-far IR correlation are of great importance for studies of galaxy evolution as they may be tracing early, short-lived stages of starbursts and active galactic nuclei (AGNs). The most extreme FIR-excess galaxy NGC1377 has long been interpreted as a young dusty starburst, but millimeter observations of CO lines revealed a powerful collimated molecular outflow which cannot be explained by star formation alone. We present new radio observations at 1.5 and 10 GHz obtained with the Jansky Very Large Array (JVLA) and Chandra X-ray observations towards NGC1377. The observations are compared to synthetic starburst models to constrain the properties of the central energy source. We obtained the first detection of the cm radio continuum and X-ray emission in NGC1377. We find that the radio emission is distributed in two components, one on the nucleus and another offset by 4$"$.5 to the South-West. We confirm the extreme FIR-excess of the galaxy, with a $q_\mathrm{FIR}\simeq$4.2, which deviates by more than 7-$\sigma$ from the radio-FIR correlation. Soft X-ray emission is detected on the off-nucleus component. From the radio emission we estimate for a young ($<10$ Myr) starburst a star formation rate SFR$<$0.1 M$_\odot$ yr$^{-1}$. Such a SFR is not sufficient to power the observed IR luminosity and to drive the CO outflow. We find that a young starburst cannot reproduce all the observed properties of the nucleus of NGC1377. We suggest that the galaxy may be harboring a radio-quiet, obscured AGN of 10$^6$M$_\odot$, accreting at near-Eddington rates. We speculate that the off-nucleus component may be tracing an hot-spot in the AGN jet.Comment: 14 pages, accepted for publication on Astronomy and Astrophysics on 08/07/201

Molecules as tracers of galaxy evolution: an EMIR survey. I. Presentation of the data and first results

We investigate the molecular gas properties of a sample of 23 galaxies in order to find and test chemical signatures of galaxy evolution and to compare them to IR evolutionary tracers. Observation at 3 mm wavelengths were obtained with the EMIR broadband receiver, mounted on the IRAM 30 m telescope on Pico Veleta, Spain. We compare the emission of the main molecular species with existing models of chemical evolution by means of line intensity ratios diagrams and principal component analysis. We detect molecular emission in 19 galaxies in two 8 GHz-wide bands centred at 88 and 112 GHz. The main detected transitions are the J=1-0 lines of CO, 13CO, HCN, HNC, HCO+, CN, and C2H. We also detect HC3N J=10-9 in the galaxies IRAS 17208, IC 860, NGC 4418, NGC 7771, and NGC 1068. The only HC3N detections are in objects with HCO+/HCN<1 and warm IRAS colours. Galaxies with the highest HC3N/HCN ratios have warm IRAS colours (60/100 {\mu}m>0.8). The brightest HC3N emission is found in IC 860, where we also detect the molecule in its vibrationally excited state.We find low HNC/HCN line ratios (<0.5), that cannot be explained by existing PDR or XDR chemical models. Bright HC3N emission in HCO+-faint objects may imply that these are not dominated by X-ray chemistry. Thus the HCN/HCO+ line ratio is not, by itself, a reliable tracer of XDRs. Bright HC3N and faint HCO+ could be signatures of embedded starformation, instead of AGN activity

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&

Canopy uptake dominates nighttime carbonyl sulfide fluxes in a boreal forest

Nighttime vegetative uptake of carbonyl sulfide (COS) can exist due to the incomplete closure of stomata and the light independence of the enzyme carbonic anhydrase, which complicates the use of COS as a tracer for gross primary productivity (GPP). In this study we derived nighttime COS fluxes in a boreal forest (the SMEAR II station in Hyytiälä, Finland; 61°51′ N, 24°17′ E; 181 m a.s.l.) from June to November 2015 using two different methods: eddy-covariance (EC) measurements (FCOS-EC) and the radon-tracer method (FCOS-Rn). The total nighttime COS fluxes averaged over the whole measurement period were −6.8 ± 2.2 and −7.9 ± 3.8 pmol m−2 s−1 for FCOS-Rn and FCOS-EC, respectively, which is 33–38 % of the average daytime fluxes and 21 % of the total daily COS uptake. The correlation of 222Rn (of which the source is the soil) with COS (average R2  =  0.58) was lower than with CO2 (0.70), suggesting that the main sink of COS is not located at the ground. These observations are supported by soil chamber measurements that show that soil contributes to only 34–40 % of the total nighttime COS uptake. We found a decrease in COS uptake with decreasing nighttime stomatal conductance and increasing vapor-pressure deficit and air temperature, driven by stomatal closure in response to a warm and dry period in August. We also discuss the effect that canopy layer mixing can have on the radon-tracer method and the sensitivity of (FCOS-EC) to atmospheric turbulence. Our results suggest that the nighttime uptake of COS is mainly driven by the tree foliage and is significant in a boreal forest, such that it needs to be taken into account when using COS as a tracer for GPP