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Megamaser Physics Revealed by the Starburst Arp

C J Skinner H A Smith E Sturm M J Barlow R J Cohen G J

Stacey

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Space Telescope Science Institute San Martin Drive Baltimore MD USA

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Smithsonian Institution Lab for Astrophyiscs Ro om National Air and Space Museum

Mail Stop Washington DC USA

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MaxPlanckInstitut fur Extraterrestrische Physik Postfach Garching Germany

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Dept of Physics Astronomy University College London Gower Street London WCE BT

Great Britain

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Nueld Lab oratories Jo drell Bank Maccleseld Cheshire SK DL Great

Britain

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Cornell University Dept of Astronomy Space Sciences Building Ithaca NY USA

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Based on observations with ISO an ESA pro ject with instruments funded by ESA Member States esp ecially

the PI countries France Germany the Netherlands and the United Kingdom and with the participation of ISAS

and NASA

UltraLuminous are a class of galaxies with very large farinfrared

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whose p ower source is controversial In one theory a quasarlike Active

Galactic Nucleus is surounded by a dense dust torus which absorbs the p owerful

nuclear emission is warmed and reemits at farinfrared wavelengths and hides the

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nucleus from direct view In another in dense dusty clouds close to the nucleus a

huge burst of Starburst o ccurs heating the dust by starlight OH

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Megamasers very luminous up to L microwave emitted by

OH radicals are extraordinarily p owerful radio sp ectral lines found in some of these

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galaxies The pumping scheme for OH Megamasers is not certain but many mo dels

suggest that absorption of farinfrared pumps the masers radiatively Arp

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is the prototype OH Megamaser galaxy and we rep ort here the rst detection of

its m OH absorption We nd that the p ower absorb ed in this line alone can

pump the masers providing the rst direct observational supp ort for radiative pump

mo dels and that the IR pumping continuum probably arises in a warm extended

region more consistent with a Starburst than with an mo del

Details of the mechanism generating the emission in astrophysical sources Megamaser

galaxies and in our own galaxy star formation regions and winds from evolved stars are still

somewhat uncertain For the Megamaser galaxies a go o d correlation exists b etween OH maser

and the square of the FIR luminosity esp ecially that measured in the IRAS

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m band consistent with an IR radiative pump In fact many OH Megamaser galaxies are

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UltraLuminous InfraRed Galaxies ULIRGs to m luminosities greater than L

where L is the and all have huge farinfrared luminosities It has b een

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suggested that ULIRGs may b e created by the merger of two galaxies and Arp indeed

89 1011

app ears to b e the result of such a pro cess VLBI radio images of it show that the OH

Megamaser emission arises in regions only a few in size in the core of the galaxy and have

b een used to supp ort Active Galactic Nucleus AGN mo dels for Arp

OH has strong transitions at m and m and it is susp ected that the masers may b e pump ed

radiatively via absorption in these lines exploiting the p owerful infrared emission of the host

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galaxy Such mo dels p osit that absorption of photons via the ab ove two transitions excites the

OH radicals There follows a radiative cascade to lower levels via other farIR transitions

principally those at and m with transitions from the m pump ed level

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J inverting the ground state doublet levels resp onsible for the MHz maser

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These mo dels suggest for Arp that the maser emission arises predominantly in very small

dense clouds near the nucleus

We observed the m sp ectrum of Arp with the Infrared Space Observatorys Short

Wavelength Sp ectrometer SWS After pro cessing the sp ectrum has a resolving p ower of ab out

and is presented in Figure At m the SiI I line rest wavelength m is seen

in emission The OH rotational transition is split into two doublet states with wavelengths of

and m Hyp erne splitting of the doublet comp onents is small a few kmsec and

safely ignored in this analysis The mean multiplet rest wavelength is m The blended OH

lines are apparent as a single resolved feature with maximum optical depth at m

yielding a velocity of kmsec

To analyse the OH absorption we derived the ux and shifted the data into the rest

510

frame adopting a redshift of kmsec We assumed that the Megamaser emission and the

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We determined a IR absorption are isotropic and adopted a distance to Arp of Mp c

baseline for the OH absorption by tting a linear continuum and then inverted the absorption to

obtain the sp ectrum presented in Figure showing the photon absorption rate in the OH

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transition Integrating across the line the total absorption is  photonssecond

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The luminosity of the MHz OH Megamaser line in Arp is  ergsecond

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assuming isotropic emission or  photonssecond The MHz line accounts for

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ab out of the total OH Megamaser emission from Arp the rest b eing in lines at

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and MHz The radiative pump mo dels so far constructed can attain pump eciencies

as high as so the m absorption in Arp is consistent with the mo dels and in fact could

p ower the observed OH masers on its own In practise the m transition should also play a

role This is the rst time a farIR OH transition has b een observed in a Megamaser galaxy and

provides the rst direct observational supp ort for the radiative pump mo dels From the strength

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of the OH absorption we can estimate an OH column of  cm just slightly lower

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than predicted by some mo dels For a single absorbing cloud of radius p c the OH density is

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ab out cm much higher than predicted by mo dels From the OH column density we

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determine the OH excitation temp erature to b e K for an isothermal OH column very similar

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to the estimate from radio maser lines For an OHH abundance ratio of appropriate for

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312 23 2

chemical equilibrium in warm gas this yields a total gas column density of  cm

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which assuming the mean Galactic reddening law yields a reddening A  and a m

v

optical depth close to Other observations have b een used to estimate A  for Arp

v

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The ab ove calculations are very crude and sub ject to numerous simplications namely

the absorbtion originates from a single cloud with uniform temp erature and the dusttogas ratio

and dust absorbtion prop erties for the absorbing cloud are similar to those for the interstellar

medium in our galaxy However the consistency of the results with those of other indep endent

studies implies that they are at least of mo derate accuracy

From the continuum ux and the absorption depth we can make some general inferences ab out the

absorbing regions Mo dels have presumed the pumping IR radiation to come from warm dust

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embedded in the maser cloud itself and derived dust temp eratures around K for a p c cloud

At the line center Jy of continuum is absorb ed Because the maximum blackbo dy ux at

m from a K p c cloud at the distance of Arp is only mJy compared with the

observed Jy absorption this mo del clearly do es not work a much larger or warmer dusty region

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is needed to supply the m photons for absorption VLBI results in fact reveal maser

regions with sizes of  p c with spatially coincident continuum hotsp ots of comparable size

surrounded by a larger p c diuse source The ISO data allow us to determine the minimum

parameters of such regions If most of the observed Jy comes from the diuse region at a level

of Jy and the remaining Jy represents the ux from a hot inner pump region which is then

absorb ed by OH in the diuse region the latter region must b e at least as hot as K if it

emits as eciently as a blackbo dy in order to supply the required Jy If the IRpumping cloud

itself is as small as p c the Jy it contributes at m which is fully absorb ed by the OH cloud

implies its temp erature is at least K if it emits as a blackbo dy if its size were the p c

suggested ab ove the corresp onding temp erature would b e K The high colour temp erature

and compact nature of the former would imply an obscured AGN while the latter temp erature and

size would b e more consistent with a starburst The complete lack of any high ionization sp ectral

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lines which are AGN diagnostics in the sp ectra of Arp render an AGN mo del unlikely

unless the is very much higher even than we estimate ab ove We therefore conclude

that a more consistent picture emerges from an extended starburst scenario The maser emission

may b e b eamed that is the coherence is directional in order to make the region app ear small in

OH and so the OH cloud may b e as large as the entire starburst or very much smaller An AGN

is not required nor is one indicated by these data though we cannot rule its presence out

If the OH radicals absorbing at m are pumping the radio maser lines then the absorption

should in principle have the same Doppler broadened prole as the maser lines The m OH

multiplet comprises six hyperne split lines The separation of the dominant multiplet members is

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m or kmsec The FWHM of the line core is kmsec m and that of the

underlying p edestal kmsec m The MHz maser line in Arp has a core with a

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FWHM of kmsec and a p edestal with a FWHM of ab out kmsec Convolution of the

SWS sp ectral resp onse with the intrinsic line width as indicated by the maser lines indicate that

we should exp ect a core FWHM of ab out kmsec and a p edestal FWHM of ab out kmsec

Thus the p edestal width is ab out larger than might b e exp ected from the radio observations

but the core which contains the ma jority ab out of the line ux is as exp ected The

p edestal is roughly symmetric which is consistent with the radio maser line prole except for the

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discrepant width The maser gains should b e greater than in Arp In a given line

comp onent either the p edestal or the core the maser amplication will b e preferentially

strengthened at the strongest velocities narrowing the OH maser line This might explain the

comparison b etween our ISO absorption p edestal and the maser line p edestal Alternatively

p erhaps not all of the absorbing cloud generates masers or do es so along our lineofsight and the

extra velocity width in absorption corresp onds to unseen masers or unamplied comp onents

Losing half the absorb ed photons to unseen masers would not aect our conclusions

Alternatively Keplerian velocities in the IR source may b e sucient to explain the width of the

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absorption p edestal

Our ongoing ISO program will observe OH transitions at and m in

several Megamaser galaxies and OHIR stars These observations should provide a more

comprehensive test of the accuracy of radiative pump mo dels by charting the course of the

radiative cascade through the OH energy levels and by determining the p ower absorb ed in all the

pumping lines These results are unlikely to qualitatively change the results presented here but

may rene them numerically by as much as several tens of p ercent The primary diculty

remaining regarding the origin of the farIR luminosity of Arp is the determination of the

extinction to the p ower source Other studies have shown that if the extinction is as high as we

have determined then the strengths of low excitation sp ectral lines are consistent with a

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starburst while the high excitation lines required by an AGN are so far undetected An

unambiguous determination of the extinction can thus conrm the presence of a starburst If

however the extinction can b e shown to b e much lower than we have determined this would

argue against the presence of either a starburst or an AGN leaving the origin of the farIR

luminosity of Arp highly mysterious

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Corresp ondence and requests for materials to Chris Skinner STScI

Figure Captions

Figure ISO SWS sp ectrum of Arp The emission feature at ab out m is due to SiI I

and the strong absorption centered around m is the OH m multiplet Both the observed

wavelength b ottom scale and the rest frame wavelength corrected for a kmsec redshift

top scale are indicated

Figure the strength of the OH absorption as an absolute ux at the source assuming isotropic

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emission The integrated absorption strength from this sp ectrum is  photonssecond