Molecular line probes of activity in galaxies
S. Garcia-Burillo, J.Gracia-Carpio, A. Usero, P. Planesas, A. Fuente, M. Krips
aa r X i v : . [ a s t r o - ph ] J a n Title : will be set by the publisher
Editors : will be set by the publisherEAS Publications Series, Vol. ?, 2018
MOLECULAR LINE PROBES OF ACTIVITY IN GALAXIES
S. Garc´ıa-Burillo , J. Graci´a-Carpio , A. Usero , P. Planesas ,A. Fuente and M. Krips Abstract.
The use of specific tracers of the dense molecular gas phasecan help to explore the feedback of activity on the interstellar medium(ISM) in galaxies. This information is a key to any quantitative as-sessment of the efficiency of the star formation process in galaxies. Wepresent the results of a survey devoted to probe the feedback of ac-tivity through the study of the excitation and chemistry of the densemolecular gas in a sample of local universe starbursts and active galac-tic nuclei (AGNs). Our sample includes also 17 luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs). From the analysisof the LIRGs/ULIRGs subsample, published in Graci´a-Carpio et al. (2007), we find the first clear observational evidence that the star for-mation efficiency of the dense gas, measured by the L
F IR /L HCN ratio,is significantly higher in LIRGs and ULIRGs than in normal galaxies.Mounting evidence of overabundant HCN in active environments wouldeven reinforce the reported trend, pointing to a significant turn upwardin the Kennicutt-Schmidt law around L
F IR =10 L ⊙ . This result hasmajor implications for the use of HCN as a tracer of the dense gas inlocal and high-redshift luminous infrared galaxies. Millimeter telescopes can provide a detailed picture of the distribution and kine-matics of molecular gas in galaxy disks through extensive CO line mapping. How-ever, the use of tracers more specific to the dense molecular gas phase, the onedirectly involved in the fueling of star formation and AGN episodes, is required toprobe the feedback of activity on the ISM. Provided that high spatial resolutionand high sensitivity requirements are met, the observation of complex molecularspecies can help to track down galaxy evolution. Extragalactic chemistry can beused to challenge current chemical models of molecular gas, as active galaxies can Observatorio de Madrid, OAN, Alfonso XII, 3, 28014-Madrid, SPAINe-mail: [email protected] CfA, SMA project, 60 Garden Street, MS 78 Cambridge, MA 02138, USAc (cid:13)
EDP Sciences 2018DOI: (will be inserted later)
Title : will be set by the publisherdrive chemical complexity on much larger scales compared to our Galaxy. Fur-thermore, this type of studies are a key to constrain conversion factors, which arerequired to derive gas masses from line luminosities. This is mostly relevant, asthe excitation and chemistry of some of the more routinely used dense gas tracers(e.g., HCN lines) can be heavily affected in active environments.Different processes can shape the evolution of molecular gas along the typicalevolutionary track of a starburst: large-scale shocks, strong UV-fields, cosmic-rays,and eventually X-rays, in the presence of an AGN. These actors are expected tobe at play at different stages but also at different locations in the disk and in thedisk-halo interface of galaxies showing activity. The use of interferometers makespossible to unveil a strong chemical differentiation in the molecular gas disks ofactive galaxies. The example of M82, a prototypical starburst, is paradigmatic inthis respect. Virtually all of the large-scale SiO emission detected in the PdBI mapof M 82, published by Garc´ıa-Burillo et al. (2001), traces the disk-halo interface ofthe galaxy where episodes of mass injection are building up the gaseous halo. Incontrast, widespread emission of the formyl radical, HCO, mapped in M 82 withthe PdBI, reveals the propagation of photo-dissociation region (PDR) chemistryinside the disk of this starburst (Garc´ıa-Burillo et al. + , CO + as wellas a set of small hydrocarbon chains (Fuente et al. There is increasing evidence that the excitation and the chemistry of molecular gasshow significant differences between starbursts and AGNs (e.g., Kohno et al. et al. et al. (2007) have recently shown thatstarbursts and AGNs populate different regions in a set of diagnostic diagramsthat make use of HCN and HCO + line ratios. LVG fits to the observed ratiospoint to lower H densities and larger HCN abundances in AGNs compared tostarburst galaxies. Overluminous HCN lines (possibly leading to overabundantHCN) seem to be the rule in the circumnuclear disks (CND) of many Seyferts.The 200 pc-radius CND of NGC 1068 is viewed as the first clear example of agiant X-ray dominated region (XDR) leading to enhanced abundances of somemolecular species like HCN (Usero et al. et al. et al. et al. : Molecular line probes of activity in galaxies 3 −1 S S F R [ M y r − k p c − ] S SFR (cid:181) S dense 1.12 – a) −0.50.00.5 S dense [ M pc −2 ] R e s i dua l s [ de x ] L FIR < L S SFR (cid:181) S dense 0.88 – b) S dense [ M pc −2 ] L FIR ‡ L S SFR (cid:181) S dense 1.14 – c) S dense [ M pc −2 ] Fig. 1. (a)
Surface density of star formation rate, Σ
SFR , against surface density of thedense molecular gas mass derived from HCN, Σ dense , for normal galaxies (open squares)as well as LIRGs, ULIRGs and high-z objects (filled squares). The solid line is theregression fit to the full sample of galaxies. In (b) and (c) we separate between normaland LIRGs/ULIRGs/high-z, respectively. The dashed (dotted) line is the fit to galaxieswith L FIR < L ⊙ ( L FIR > L ⊙ ). Lower panels show the residuals of the fits.(Adapted from Graci´a-Carpio et al. L ’ HCN ( - ) L ’ HC O + ( - ) L FIR [ L ]a) L FIR [ L ] L ’ HCN ( - ) L ’ HC O + ( - ) [ HCN ] [ HCO + ] ~ 1 − 6[ HCN ] [ HCO + ] ~ 5 − 20[ HCN ] [ HCO + ] ~ 15 − 60 Arp 220 b) Fig. 2. (a)
HCN(1–0) / HCO + (1–0) luminosity ratio as a function of L FIR in the sample ofLIRGs and ULIRGs of Graci´a-Carpio et al. (2007). The linear fit to the data (dashed line)shows the trend. (b)
Same as (a) but for the J=3–2 line. We indicate the [HCN]/[HCO + ]abundance ratios derived from multiline LVG analysis. Title : will be set by the publisher
The caveats on the use of HCN as a tracer of dense gas in active galaxies callfor the use of alternative tracers of dense gas in LIRGs and ULIRGs, galaxieswhere star formation and AGN activity are expected to be highly embedded. Thisquestion is paramount to disentangle the power sources of the infrared luminositiesof these galaxies. In a recent paper, Graci´a-Carpio et al. (2007) have presentedevidence that the L FIR /L HCN(1 − ratio, taken as a fair proxy for the star formationefficiency of the dense gas (SFE dense ), is a factor ∼ L FIR > L ⊙ ) compared to normal galaxies. Local universe LIRGsand ULIRGs populate a region in the SFE dense diagram that lies between thoseoccupied by normal and high- z IR luminous galaxies. The reported trend in theSFE dense derived from HCN data implies that there is a statistically significantturn upward in the Kennicutt-Schmidt law, Σ
SFR ∝ Σ N dense , at high L FIR : N changes from ∼ L FIR < L ⊙ ) to ∼ L FIR > L ⊙ )(See Fig. 1).Based on a multiline LVG analysis of HCN and HCO + data, that follows thesurvey published by Graci´a-Carpio et al. (2006), Graci´a-Carpio et al. (2007) findthat the the conversion factor between L HCN(1 − and M dense , X HCN , is ∼ L FIR (See Fig. 2). Of particular note, a significant overabundanceof HCN has also been reported in the IR luminous z ∼ et al. et al. dense could well be an order of magnitude higher in extreme LIRGs/ULIRGscompared to normal galaxies. References
Davies, R. I., Mueller-S´anchez, F., Genzel, R., Tacconi, L. J., Hicks, E. K. S., Friedrich,S., & Sternberg, A. 2007, ApJ, 671, 1388Fuente, A., Garc´ıa-Burillo, S., Gerin, M., Teyssier, D., Usero, A., Rizzo, J. R., & deVicente, P. 2005, ApJ, 619, L155Fuente, A., Garc´ıa-Burillo, S., Gerin, M., Rizzo, J. R., Usero, A., Teyssier, D., Roueff,E., & Le Bourlot, J. 2006, ApJ, 641, L105Garc´ıa-Burillo, S., Mart´ın-Pintado, J., Fuente, A., & Neri, R. 2001, ApJ, 563, L27Garc´ıa-Burillo, S., Mart´ın-Pintado, J., Fuente, A., Usero, A., & Neri, R. 2002, ApJ, 575,L55Garc´ıa-Burillo, S., et al. 2006, ApJ, 645, L17Graci´a-Carpio, J., Garc´ıa-Burillo, S., Planesas, P., & Colina, L. 2006, ApJ, 640, L135Graci´a-Carpio, J., Garc´ıa-Burillo, S., Planesas, P., Fuente, A., & Usero, A. 2007, ArXive-prints, 712, arXiv:0712.0582Kohno, K., Matsushita, S., Vila-Vilar´o, B., Okumura, S. K., Shibatsuka, T., Okiura, M.,Ishizuki, S., & Kawabe, R. 2001, The Central Kiloparsec of Starbursts and AGN:The La Palma Connection, 249, 672Krips, M., Neri, R., Garcia-Burillo, S., Martin, S., Combes, F., Gracia-Carpio, J., &Eckart, A. 2007, ArXiv e-prints, 712, arXiv:0712.0319 . Garc´ıa-Burillo et al. : Molecular line probes of activity in galaxies 5: Molecular line probes of activity in galaxies 5