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The Saturation States of Compact and Diffuse Components of OH Megamaser Galaxies

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The Saturation States of Compact and Diffuse Components of OH Megamaser Galaxies A&A 404, 513–518 (2003) Astronomy DOI: 10.1051/0004-6361:20030565 & c ESO 2003 Astrophysics The saturation states of compact and diffuse components of OH megamaser galaxies R. A. Kandalyan?;?? V. A. Ambartsumian Byurakan Astrophysical Observatory, 378433 Byurakan, Armenia and Isaac Newton Institute of Chile, Armenian Branch, 378433 Byurakan, Armenia Received 22 January 2003 / Accepted 27 March 2003 Abstract. A sample of 9 OH megamaser galaxies detected in the soft X-ray domain was compiled. Using available OH and X-ray data a striking correlation was found between the X-ray luminosity and the width of the OH line. This correlation may indicate that the X-ray heating of a molecular gas may increase the collisional excitation of the maser emission. However, this result should be considered as a tentative one because of the insufficient number of galaxies. An analysis of the saturation states of compact and diffuse components of OH emission was performed. The results of the analysis support the assumption that both the compact and diffuse OH maser emissions in the megamaser galaxies are saturated. The diffuse component might show unsaturated masing under certain conditions, such as the appropriate relation between the intensities of compact and diffuse components and a relevant number of the IR photons to pump the maser emission. Key words. masers – galaxies: general – X-rays: galaxies 1. Introduction example, IRAS 21077+3358, 22116+0437 etc.) as the actual detections, and it is not indicated what formula was used in the The OH megamasers are found among powerful infrared galax- calculations of the FIR luminosity. A linear dependence indi- ies. Nearly 110 OH megamaser galaxies have been discovered cates the saturation of the compact OH component, although (e.g. Martin 1989; Martin et al. 1989; Kandalyan 1996 and ref- the extended component might have an unsaturated masing. erences therein; Baan et al. 1998; Darling & Giovanelli 2000, Furthermore, the global VLBI observations of the megamaser 2001, 2002a; Kent et al. 2002). The megamaser emission has galaxies (III Zw 35: Trotter et al. 1997; Diamond et al. 1999 been interpreted as a low-gain, unsaturated amplification of the and Pihlstrom et al. 2001; Arp 220: Lonsdale et al. 1998 and nuclear radio continuum by the extended molecular cloud (e.g. Rovilos et al. 2003; IRAS 17208-0014: Diamond et al. 1999; Bann & Haschick 1984). As a pumping source, the powerful in- Mkn 273: Yates et al. 2000; Mkn 231: Richards et al. 2003) frared radiation was suggested. This scenario was derived from showed that the continuum emission is not associated with the observed quadratic relation between the OH and FIR lu- the OH masers (except Mkn 273, Yates et al. 2000, and per- minosities (Martin et al. 1988; Bann 1989). However, recently, haps, Arp 220, Rovilos et al. 2003), which indicates that mega- Lonsdale et al. (1998), Diamond et al. (1999) and Darling & masers have a high amplification factor (>100). The VLBI re- Giovanelli (2002a) have suggested that the unsaturated low- sults of these galaxies suggest a dual component structure of gain and IR-pumping model may explain the OH emission of the maser cloud: a diffuse component, that fits the low-gain the diffuse component. The collisional pumping and saturated unsaturated amplification model (quadratic relationship) and a amplification may account for the OH emission of the compact compact component, produced by saturated masers in compact component (Kandalyan 1999; Diamond et al. 1999; Darling masing clouds (linear relationship). Pihlstrom et al. (2001) ar- 2002). gued that the observed compact and diffuse components in III The analysis of the OH FIR luminosity-luminosity rela- ∼ Zw 35 lie in a ring of 22 kpc radius. The compact and diffuse tion showed that the slope is close to linear (Kandalyan 1996; masers appear to be different because of the geometry of the Darling & Giovanelli 2002a; Zhi-yao 2003). However, Zhi-yao ring and the path length effects. It is argued that in III Zw 35 (2003) used many upper limits of the IRAS data points (for the OH emission is an unsaturated low-gain maser. ? e-mail: [email protected] The OH line observations of IRAS 21272+2514 have ?? Present address: Institute of Astronomy and Space Sciences of revealed the first variable megamaser galaxy (Darling & Al Al-Bayt University, PO Box 130040, Mafraq 25113, Jordan Giovanelli 2002b). The observed variability is partly due e-mail: [email protected] to the interstellar scintillation. There is evidence that Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20030565 514 R. A. Kandalyan: The saturation states of OH megamaser galaxies IRAS 21272+2514 is composed of both the extended and com- 2. Results pact components. 2.1. A sample of OH megamaser galaxies detected Thanks to the Arecibo Observatory OH megamaser sur- in the soft X-ray vey (Darling & Giovanelli 2000, 2001, 2002a), the number of the known megamasers has doubled compared to that in In order to investigate the X-ray properties of OH megamaser 1996 (Kandalyan 1996). Our previous analysis of the OH galaxies, we have extracted from the literature all the mega- FIR luminosity-luminosity relation of megamaser galaxies was∼ masers detected in the soft X-ray (0.1–2.4 keV). There are based on a sample of 49 galaxies (Kandalyan 1996). Darling 9 OH megamaser galaxies that have been observed with the & Giovanelli (2002a) analyzed the OH FIR luminosity- ROSAT satellite in the soft X-ray (Rigopoulou et al. 1996; luminosity relation using a sample of 95∼ megamasers, while Wang et al. 1996; Bauer et al. 2000). The X-ray flux den- Zhi-yao’s (2003) analysis was based on a sample of 67 galax- sities for IRAS 00509+1225 (Wang et al. 1996) and IRAS ies. Kandalyan (1999) argued that in the OH megamasers the 12112+0305 (Bauer et al. 2000) have been converted from the collisional pumping and saturated amplification provide the (0.1–2.4 keV) band to the 1 keV (Schmidt & Green 1986). unit slope of the OH FIR relation. The possible causes which The X-ray flux densities at 1 keV for the remaining 7 galax- may deviate the slope∼ from unity are discussed by Kandalyan ies have been taken from Rigopoulou et al. (1996). The IRAS (1999). Several OH megamasers have been recently observed 12112+0305, IRAS 15250+3609 and IRAS 22491-1808 galax- both in the OH line and radio continuum with the VLBI tech- ies have marginal detections in the soft X-ray at 2:5σ,2σ and nique (Yates et al. 2000; Pihlstrom et al. 2001; Richards et al. 2:5σ levels, respectively (Rigopoulou et al. 1996). 2003; Rovilos et al. 2003). The new VLBI observations, as The relation between the observed flux density and lumi- well as the derived luminosity function (Darling & Giovanelli nosity is given by Schmidt & Green (1986) as: 2 2002c) and optical observations (Baan et al. 1998; Darling L(E1; E2) = 4πC(z)A (z)F(E1; E2) for an energy band E1 2002) of megamasers have enlarged our knowledge in this field. to E2,whereC(z) is the K-correction term, A(z)isthelumi- nosity distance term, F(E ; E ) is the observed flux density, It is not clear whether the unsaturated low-gain and IR- 1 2 and z is a red shift. For the power-law spectra with the en- pumping model may explain the OH emission of all diffuse ergy index α and Friedman cosmology with the Hubble con- components in megamaser galaxies. It cannot be ruled out stant H = 75 km s 1 Mpc 1 and q = 0:5 the K-correction that the collisional pumping and saturated amplification may o − − o term C(z) and the luminosity distance term A(z)aregivenby account for the OH emission of both compact and diffuse C(z) = (1 + z) (1+α), A(z) = 2(c=H )[(1 + z) (1 + z)0:5]. components. − o The integrated OH flux density is approximated− by the Both starburst and monsters can produce IR, X-ray and ra- product of the peak flux density and the rest frame width of dio emission. Both energy sources may be present in the same the line. galaxy, however quantitative data are needed to decide which is In our calculations we accept the following values of α: energetically dominant (Smith et al. 1998). According to Smith 0:5 (FIR band) (Rigopoulou et al. 1996; Boller et al. 1998), et al. (1998) a starburst can explain the bolometric luminosity −0:7 (radio continuum) (Smith et al. 1998) and 1:3 (X-ray) of Arp 220. The hard X-ray sources in the nuclei of Arp 220 (Boller− et al. 1998). − (Clements et al. 2002) and Mkn 273 (Xia et al. 2002) suggest The FIR and red shift data have been extracted from that there is an active nucleus in these galaxies. Neufeld et al. the NED1 databases. The radio continuum data at 1.4 GHz (1994) have modelled the physical conditions present within are selected from the VLA-A observations of these galaxies dense circumnuclear gas, which is irradiated by X-ray emis- (Kandalyan et al. 2003) except for IRAS 00509+1225 (Condon sion from an active galactic nucleus, and it is argued that the et al. 1998). X-ray heating may give rise to a collisional excitation of masers The above formulae have been used to calculate the within a warm dusty medium. Therefore, if we suggest that the corresponding monochromatic luminosity (L60, LOH, L1:4 and collisions may pump the OH maser levels, then observation- LX).
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