Molecular properties of (U)LIRGs: CO, HCN, HNC and HCO+
aa r X i v : . [ a s t r o - ph ] S e p Proceedings Astrophysical Masers and their EnvironmentsProceedings IAU Symposium No. 242, 2007J.M. Chapman & W.A. Baan, eds. c (cid:13) Molecular properties of (U)LIRGs:CO, HCN, HNC and HCO + A.F. Loenen , , W.A. Baan and M. Spaans Kapteyn Astronomical Institute, University of Groningen,P.O. Box 800, 9700 AV Groningen, The Netherlandsemail: [email protected], [email protected] ASTRON, P.O. Box 2, 7990 AA Dwingeloo, The Netherlandsemail: [email protected]
Abstract.
The observed molecular properties of a sample of FIR-luminous and OH megamaser(OH-MM) galaxies have been investigated. The ratio of high and low-density tracer lines is foundto be determined by the progression of the star formation in the system. The HCO + /HCN andHCO + /HNC line ratios are good proxies for the density of the gas, and PDR and XDR sourcescan be distinguished using the HNC/HCN line ratio. The properties of the OH-MM sources inthe sample can be explained by PDR chemistry in gas with densities higher than 10 . cm − ,confirming the classical OH-MM model of IR pumped amplification with (variable) low gains. Keywords. galaxies: nuclei, galaxies: ISM, galaxies: active, galaxies: starburst, ISM: molecules,ISM: evolution, radio lines: ISM
1. Introduction
High-density molecular gas plays an important role in the physics of (Ultra-)LuminousInfrared Galaxies ((U)LIRGs), giving rise to spectacular starbursts and possibly provid-ing the fuel for an active galactic nucleus (AGN). The emission lines emanating from thenuclear gas provide information about the physical properties of the nuclear environmentin these systems, e.g. the (column) density, temperature and chemical composition of thegas, and the type and strength of the central energy source. It can also provide us aninsight into the processes influencing the (gas in the) nuclei: the star formation rate andhistory, fuelling of a possible central black hole and feedback processes.Baan et al. (2007) present data of the CO, HCN, HNC, HCO + , CN, and CS lineemissions of a representative group of 37 FIR-luminous and OH megamaser (OH-MM)galaxies and 80 additional sources taken from the literature. In this work, the molecularcharacteristics of this sample are explained using several models. First, the properties ofthe different (density) components of the nuclear gas are explained in terms of starburstevolution (see Baan et al. et al. (2007). Here only the J=1-0 transitions of the molecules areconsidered. A more detailed analysis will be presented in Loenen, Baan & Spaans (2007).
2. Starburst evolution
The gas in galaxies is build up from multiple components, each with different densitiesand temperatures. From our data we can derive information about these different com-ponents. The CO(1-0) line traces the large scale low-density (critical density n cr =3 × cm − ) component (LD), whereas the lines of HCN, HNC, and HCO + (all n cr > cm − )1 Loenen, Baan & Spaans RIF ⊙ )L(L ) − ( O C / ) − ( N C H RIF ⊙ )L(L ) − ( O C / ) − ( C NH RIF ⊙ )L(L ) − ( O C / ) − ( + O C H Figure 1. left, from topto bottom:
Integrated lineratios HCN(1-0)/CO(1-0),HNC(1-0)/CO(1-0), andHCO + (1-0)/CO(1-0) versusFIR luminosity. Squaresrepresent reliable values andtriangles upper or lowerlimits. Filled symbols aresources with OH-MM activ-ity. right, from top to bottom: Three FIR luminosity curvesused in the simulations withdifferent maximum lumi-nosities, the luminosity ofa high-density component,and the variation of thehigh- versus low-densityratio during the outburststarting at the upper part ofthe curve. trace the high-density component (HD) which mostly resides in the cores of the galaxies,at the sites of the star-formation activity.On the left hand side of Fig. 1, the relative contributions of these two density compo-nents are shown, by plotting the ratios of the high-density and CO(1-0) line strengths.The figure shows that the distribution of line ratios for all molecules increases with FIRluminosity, which gives an upwardly curved lower boundary for the distribution at higher L FIR . The highest values are found at L FIR > . L ⊙ . The figure also shows that ingeneral the OH-MM sources have a much larger spread in HD/CO(1-0).This behavior can be explained as the result of ongoing star formation. The FIRluminosity of the ULIRG during the evolution of the outburst reflects energy generatedby the star formation activity. The FIR luminosity integrated over the course of theoutburst would reflect the amount of high-density molecular material consumed by thestar formation process and destroyed or removed by feedback.In the following simplified scenario (see Baan et al. t as a response to a starburst starting at t =0 defined as: L FIR ( t ) = 1 . L FIR (0) (cid:18) tT (cid:19) . e − t/T , (2.1)where L F IR (0) is the maximum luminosity of the burst and T is the timescale of theoutburst. We note that a diffusion curve may not be the most appropriate representation olecular properties of (U)LIRGs L FIR , but this curve does resemble the outcome of starburst-driven FIR evolutionsimulations (Loenen, Baan & Spaans 2006).The high-density component HD can be defined as β LD, the low-density component.As a result, the high-low-density ratio varies with time during the FIR outburst as:HD( t )LD = β (cid:20) − γ R L FIR ( t )d tL FIR , int (cid:21) , (2.2)where γ is the fraction of the initial HD component that is consumed during the wholeoutburst, and L FIR , int the FIR luminosity integrated over the whole course of the out-burst. The large-scale low-density component LD is assumed to remain unchanged.The results of these simulations have been presented on the right hand side in Fig. 1.The top panel shows the FIR light curve of the outburst for three peak luminosities.The middle panel shows the luminosity of a representative high-density component forthe three L FIR curves. The bottom panel shows the high-low-density ratio for these sameFIR light curves. Combining the results of this simulation with the data shows that theHD/CO(1-0) data points in the left panels are a measure of the evolution of the starburst.This implies that the OH MM sources are galaxies in an early stage of star formation,which is consistent with OH MM sources being found in starburst-dominated galaxies(Genzel et al.
3. Chemistry
The model presented in the previous section does well in explaining the evolution ofthe different gas components, but it makes no distinction between the different high-density tracers. Even though the emission of the different molecules originates in thesame regions, the line strengths are influenced by the environmental properties like the(column) density, temperature, and the type and strength of the prevailing radiationfield. In order to study the effects of these parameters on the emission characteristics ofthe sources, we remove the intrinsic difference in line strength between the galaxies inour sample and use line ratios to find diagnostic properties. Fig. 2 presents the ratios ofthe integrated lines of HCO + /HCN, HNC/HCO + and HNC/HCN against each other.In order to interpret the behavior of the sources in this diagram, we compare the data tothe theoretical models that treat the chemistry and radiative transfer of molecular clouds,including all the relevant heating, cooling and chemical processes (see Meijerink & Spaans2005; Meijerink et al. et al. in these proceedings). A large grid of mod-els was created by Meijerink et al. (2007), varying the strength of the radiation field, itstype (UV and X-ray), the gas density and the column density. These results are comparedto our observational data in Fig. 2 (note: not all models are shown, some fall out of therange of our figure). 3.1. PDR models
The results of the PDR models (UV radiation field) are shown in Fig. 2 with heavy lines,where the line styles indicate different gas densities (solid: n =10 . , dashed: n =10 . , andstriped: n =10 . cm − ). The tracks vary as a function of column density, which rangesfrom N =10 cm − (the column density below which the strength of the emission linesdecreases rapidly) to N =10 , N =10 . and N =10 cm − for the n =10 . , n =10 . ,and n =10 . cm − models, respectively (corresponding to a cloud size of 1 pc; indicatedby the symbol at one end of the tracks). Two different radiation field strengths are shown: F UV =1 . − s − , indicated by a plus symbol at the highest column density point,and F UV =160 erg cm − s − , indicated with a circle. Loenen, Baan & Spaans -0.6 -0.4 -0.2 0.0 0.2 0.4 )0 − / )0 − -1.0-0.8-0.6-0.4-0.20.00.20.40.6-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-1.0-0.8-0.6-0.4-0.20.00.20.40.6 ) − ( N C H / ) − ( + O C H go l -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 )0 − + OCH / )0 − -0.6-0.4-0.20.00.20.4 ) − ( N C H / ) − ( C NH go l Figure 2. top left
Integrated HCO + (1-0)/HCN(1-0) versus HNC(1-0)/HCO + (1-0) ratios. topright Integrated HCO + (1-0)/HCN(1-0) versus HNC(1-0)/HCN(1-0) ratios. bottom left Inte-grated HNC(1-0)/HCN(1-0) versus HNC(1-0)/HCO + (1-0) ratios. Explanation about the plotsymbols and line styles is provided in Sect. 3. Two observations can be made, when comparing the data and the models. First of allone can see that the models are separated based on the density ( n ) in the HCO + /HCNand HCO + /HNC line ratios. The HNC/HCN line ratio shows no differentiation. Thiscan be explained in terms of the critical density of the individual molecules. HCO + hasa critical density of about 3 × cm − , whereas the critical density of HCN and HNCis around 3 × cm − . Therefore the excitation of HCO + will differ from HCN andHNC for different densities. A second observation that can be made is that most of theOH MM sources cluster together in an area traced by PDR models that have a high-density ( n > . cm − ), and a high column density N > cm − . This points to theclassical OH MM model of IR (UV radiation reprocessed by the surrounding gas anddust) pumped, low (and variable) gain amplification (Baan 1989).3.2. XDR models
The results of two XDR simulations are also shown in Fig. 2, using thin lines. Again, theline styles indicate different gas densities (solid: n =10 . , and dashed: n =10 . cm − );column densities range from N =10 to N =10 -10 . ; and the radiation field strengthsare F X =1 . F X =160 erg cm − s − (circle).The XDR models are not as well differentiated as the PDR models. Because X-rayphotons penetrate the molecular cloud much easier, the XDRs do not show the strongdensity dependency seen for the PDRs, making the distinction between different XDRmodels very difficult. The addition of higher transitions and other molecules (e.g. CN,CO + , HOC + ) will most likely break this degeneracy. Another problem with trying toidentify XDR sources is that they are in general smaller that PDR sources and thus aremore affected by beam dilution, especially in single dish observations like ours. This will olecular properties of (U)LIRGs > Terra Incognita
Not all our observational data in Fig. 2 is covered by the models. The few sources inour sample with known H O MM activity (indicated by plot symbols surrounded bycircles) are also located in this area, which is characterized by lower HNC and higherHCO + line strengths compared to HCN. The fact that this area is not covered by themodels suggests that other processes influence the line ratios, such as strong shocks.Shocks are not treated in the models and can have profound effects on the chemistry inmolecular clouds as they may selectively destroy HNC (Schilke et al. + relative to HCN (Dickinson et al. + anda decrease in HNC shifts the PDR models to the uncovered region, implying that theH O MM sources in our sample are UV driven systems with strong shocks. This wouldsuggest that these H O MM sources are similar to shock-induced Galactic H O maserspots (see other contributions in these proceedings).
4. Conclusions
Molecular line emissions of multiple species (and transitions) provide excellent diag-nostics for understanding the status of the nuclear gas in extra-galactic sources. Thedifferent molecules trace different (density) components and the ratio of high and low-density tracer lines follows the star formation activity in the system. Comparing differenthigh-density tracers tells a lot about physical characteristics of the gas. The HCO + /HCNand HCO + /HNC line ratios are good proxies for the density of the gas, due to the differ-ent critical densities of the species. PDR and XDR sources can be distinguished using theHNC/HCN line ratio: PDR sources all have ratios lower than unity and XDRs have ra-tios larger than 1. OH MM sources cluster in a particular part of the diagnostic diagram,which is only traced by PDR models with densities higher than 10 . cm − , confirmingthe classical OH MM model of FIR (UV radiation reprocessed by the surrounding gasand dust) pumped amplification with low but variable gains. References
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