aa r X i v : . [ a s t r o - ph ] S e p Astrophysical Masers and their EnvironmentsProceedings IAU Symposium No. 242, 2007J.M. Chapman & W.A. Baan c (cid:13) The irradiated ISM of ULIRGs
M. Spaans , R. Meijerink , F.P. Israel , A.F. Loenen , , W.A. Baan Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlandsemail: [email protected] Astronomy Department, University of California, Berkeley, CA 94720, United States Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands ASTRON, P.O. Box 2, 7990 AA Dwingeloo, The Netherlands
Abstract.
The nuclei of ULIRGs harbor massive young stars, an accreting central black hole, orboth. Results are presented for molecular gas that is exposed to X-rays (1-100 keV, XDRs) andfar-ultraviolet radiation (6-13.6 eV, PDRs). Attention is paid to species like HCO + , HCN, HNC,OH, H O and CO. Line ratios of HCN/HCO + and HNC/HCN discriminate between PDRs andXDRs. Very high J ( >
10) CO lines, observable with HIFI/Herschel, discriminate very wellbetween XDRs and PDRs. In XDRs, it is easy to produce large abundances of warm (
T > O and OH. In PDRs, only OH is produced similarly well.
Keywords.
ISM: molecules – galaxies: evolution
1. Introduction
The power that emanates from (ultra-)luminous infrared galaxies is believed to stemfrom active star formation and/or an accretion disk around a central super-massive blackhole (e.g. Sanders & Mirabel 1996). The unambiguous identification of the central energysource, or the relative contributions from stars and an active galactic nucleus (AGN),remains a major challenge in the study of active galaxy centers (Aalto et al. 2007).Molecular lines are ideal in this to penetrate deep into the large column density regionsof active galaxies (Israel 2005; Baan & Kl¨ockner 2005). The interested reader is referredto Meijerink & Spaans (2005) and Meijerink, Spaans & Israel (2006, 2007) for details onPhoton-Dominated Region (PDR) and X-ray Dominated Region (XDR) physics. PDRsrefer to the presence of a starburst that produces photons with energies of 6-13.6 eV,while XDRs indicate an accreting black hole with photon energies of 1-100 keV (Maloneyet al. 1996; Lepp & Dalgarno 1996). The most important thing to realize is that a 1 keVphoton penetrates a hydrogen column of about 10 cm − , while a UV photon (10 eV)is absorbed by dust after about 1 mag of visual extinction. This is a consequence ofthe fact that X-ray absorption cross sections scale roughly like energy − , allowing deeppenetration of X-rays into interstellar clouds.
2. Results
All presented models are plane-parallel slabs and are parameterized by a constantdensity and an impinging UV (in multiples of G = 1 . × − erg s − cm − ) or X-ray(energy − . ) radiation field F X in erg s − cm − . Note that a flux of F X = 100 erg s − cm − corresponds to a 10 erg s − Seyfert nucleus at 100 pc from an interstellar cloud.It is clear from figures 1, 2, 3 and 4 that OH is easily formed in PDRs and XDRs, whilewarm water is present mostly in XDRs. In PDRs, OH is the dissociation product of water,while it is formed indirectly through charge transfer of H + and O, rapid reactions with H Figure 1.
Depth dependence of a few important chemical species in the PDR, n = 10 cm − and 10 . G . Note the clear stratification. Figure 2.
Depth dependence of a few important chemical species in the XDR, n = 10 cm − and F X = 5 erg s − cm − . Note the different column density scale and warm water. SM in ULIRGs Figure 3.
Depth dependence of a few important species in the PDR, same as figure 1. to OH + and H O + , followed by dissociative recombination or directly through O+H → OH+H. The latter reaction is driven efficiently above 200 K. Additional reactions ofH O + with molecular hydrogen lead to H O + which dissociatively recombines to, amongothers, water or OH. Note that an elevated cosmic ray ionization rate also leads to largerabundances of H O + , but not to the same degree as in XDRs (Meijerink et al. 2006).In both PDRs and XDRs, vibrationally excited H is also present through, respectively,UV pumping and thermal collisions with electrons. This significantly lowers the effectiveenergy barrier of the O+H reaction. In XDRs, an internal UV radiation field is createdby collisional excitation of H and H that leads to Lyman α and Lyman-Werner photonsthrough radiative decay. However, this UV field is not strong enough to dissociate wateras efficiently as in PDRs. At the high temperatures reached in XDRs, because photo-ionization heating is more efficient than photo-electric heating by dust grains, the endo-ergic reaction OH+H also leads to water. Of course, shocks are similarly capable ofproducing OH and H O.Typically, the HCO + lines are stronger in XDRs than in PDRs by a factor of at leastthree. This is a direct consequence of the higher ionization degree in XDRs (Meijerink &Spaans 2005), leading to an enhanced HCO + formation rate. Depending on the incidentradiation field, HCN or HCO + is more abundant at the PDR edge of the cloud. At suffi-ciently large column and densities, the HCN(1-0)/HCO + (1-0) ratio becomes larger than1. In the XDR models, HCO + is chemically less abundant than HCN for very large H X /n (Meijerink & Spaans 2005). However, for larger columns HCO + always becomes moreabundant than HCN (Fig. 10 in Meijerink & Spaans 2005). These effects are summarizedin figure 2, 4 and 5, see also Loenen et al. (this volume).The critical densities of HCN and HNC are almost identical, so the only differencesin ratios can be due to differences in the abundances. It turns out that the HNC/HCNcolumn density ratio is quite close to that of HNC(1-0)/HCN(1-0) line intensity ratio. In Spaans et al. Figure 4.
Depth dependence of a few important species in the XDR, same as figure 2. Notethe enhancement of HNC with respect to HCN (factor 2).
PDRs, HCN is more abundant in the radical region, but deeper into the cloud the ratiobecomes about one. In XDRs, HCN is more abundant in the highly ionized part of thecloud. However, HNC is equally or even more abundant than HCN deep into the cloud.All this can be seen in figures 2 and 4.In XDRs at high densities CO is present all throughout the cloud, even when F X /n is large. This is a direct consequence of the fact that X-rays do not lead to strongdissociation of CO and thus C + , C and CO typically co-exist in an XDR at elevated( >
100 K) temperatures. Such warm CO gas produces emission originating from highrotational transitions. Contrary to XDRs, most CO in PDRs is produced after the H/H transition and has on average much lower temperatures ( T ∼ −
50 K). Future missionssuch as Herschel/HIFI will be able to distinguish between PDRs and XDRs by observinghigh rotational transitions such as CO(16-15), see figure 6.
3. Future work
In the future, observatories like Herschel and ALMA will revolutionize our understand-ing of the nuclear activity in (maser) galaxies by resolving the central regions of thesesystems spatially. In this light, it is important to stress that starburst activity typicallyoccupies a larger fraction of an active galaxy than accretion onto a central black hole.Consequently, single dish observations are likely to be dominated by a PDR signal evenwhen an XDR is present. This is particularly true for the very high J CO lines andemphasizes the need for high spatial resolution. ALMA can detect and resolve very high J CO emission from ULIRGs at redshifts beyond eight. Additional molecules that allowone to probe an accreting black buried inside ULIRGs, i.e. that are direct tracers of X-rayirradiation, are CO + , CH + and H +3 . SM in ULIRGs Figure 5.
The HCN(1-0)/HCO + (1-0) line intensity ratio for a grid of PDR (left) and XDR(right) models. Figure 6.
The CO(16-15)/CO(1-0) line intensity ratio for a grid of PDR (left) and XDR(right) models.
Acknowledgements
We are grateful to Dieter Poelman and Juan Pablo P´erez-Beaupuits for discussions.
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