A&A manuscript no. (will be inserted by hand later) ASTRONOMY AND Your thesaurus codes are: ASTROPHYSICS 03(11.01.2, 11.05.2, 11.09.2, 11.12.2, 13.09.1, 13.19.1) 18.11.2018 Cosmologically distant OH megamasers: A test of the galaxy merging rate at Z 2 and a contaminant of blind HI surveys in the 21cm line≈ F. H. Briggs Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands Received 30 September 1997; accepted 20 May 1998 Abstract. Bright OH megamaser galaxies, radiating 1. Introduction 1667/1665 MHz lines, could be detected at redshifts from z 1 to 3 in moderate integration times with existing The detectability of OH megamaser and gigamaser galax- radio≈ telescopes. The superluminous FIR galaxies that ies at cosmological distances has been noted by several host the megamasers are relatively rare at z 0, but authors (Baan 1989, Baan et al 1992a, Norman & Braun they may have been more common at high redshift,≈ if the 1996, Baan 1997). So far, the sample of known mega- galaxy merger rate increases steeply with redshift. There- masers has been assembled from targeted observations fore, blind radio spectroscopic surveys at frequencies of of luminous galaxies for which redshifts are known (cf. 400 to 1000 Mhz can form an independent test of the Baan et al 1992b, Martin et al 1989, Stavely-Smith et al galaxy merger rate as a function of time over the redshift 1992). The receiving systems are tuned to the predicted interval z = 4 to 0.7. frequencies of the redshifted OH lines whose rest frequen- The redshift range z =0.17 to 0.4 will be difficult to ∼ cies fall at 1665 and 1667 MHz. The greatest success survey for OH masers, since spectroscopic survey signals rate for discovery of megamasers occurs in galaxy sam- will be confused with HI emission from normal galaxies at ples that are selected for having the highest far infrared redshifts less than 0.3. In fact, the signals from OH masers (FIR) 60 µm luminosity, with the result that 50% of are likely to dominate over 21cm line emission from nor- 11.2 −2 ∼ galaxies with L60µ > 10 h L⊙ are strong OH masers mal galaxies at frequencies below 1200 MHz (i.e. large −1 (h = Ho/(100kms ). The odds decline to 5% for sam- redshifts zHI > 0.18 and zOH > 0.4). Surveyors of nearby 9 ∼ ples where L60µ 10 L⊙. There is a striking correlation galaxies in the 21cm line may find that OH masers form ≈ 2 between OH luminosity LOH and L60µ, with LOH L60µ a contaminant to deep, blind HI surveys for redshift ve- (Baan et al 1992a, Baan 1989, Martin et al 1989).∝ locities less than a few hundred kilometers per second. At frequencies just above 1420 MHz, sensitive sky surveys A picture explaining both the probability of detection might detect OH masers, which could be mistaken for a and the relative strengths of megamaser emission from ob- arXiv:astro-ph/9710143v2 12 Jun 1998 population of “infalling, compact High Velocity Clouds” jects of different L60µ has the maser strength depending but would ultimately be traced to luminous FIR back- on strength of the starburst activity in the galaxy (Baan ground galaxies at z 0.17 once optical and IR follow-up 1989, Henkel et al 1991). Major merger and interaction has been performed. ≈ events are the strongest stimulants of star formation and also produce the most turbulent interstellar gas distribu- tions; these conditions provide the strongest IR fluxes for pumping the maser levels, as well as the largest covering factors of the star forming regions, leading to the greatest probability that views from random directions will see OH Key words: Galaxies: active – Galaxies: evolution – maser activity. Galaxies with milder star formation have Galaxies: interaction – Galaxies: luminosity function, more placid disks, and maser emission is then observed mass function – Infrared: galaxies – Radio lines: galaxies only by observers who view the galaxies nearly edge-on to their planes. The strong association between the ultra- luminous FIR galaxies and strongly-interacting and merg- ing systems has been recently discussed by Clements et al Send offprint requests to: [email protected] (1996). 2 F.H. Briggs: OH MegaMasers At the present epoch, the superluminous FIR galax- tion, based on complete samples of galaxies from the IRAS ies that host the most luminous masers are relatively rare 60 µm survey. Their paper analyzes the sample in the con- galaxies. This means that radio spectroscopic surveys of text of several cosmological models; here, parameters for the sky would need to cover large solid angles before they the “p = 1” case, which is the conventional cosmology, would begin to detect OH masers at random from the lo- are adopted to describe the number density of luminous, cal galaxy population. On the other hand, this paper will FIR-selected galaxies. show that, since OH emission can be so very strong, it The 60 µm differential luminosity function is given by becomes probable that surveys with large spectral band- widths and covering large depths are likely to identify cos- α β Φ(L60)= + ∗ Ψ(L60) (1) mologically distant megamasers with moderate integra- L60 L60 + L60 tion times. The most luminous infrared sources known are the ob- with Ψ(L60), the cumulative luminosity function, defined jects detected at high redshifts, z 0.4 to 4.7, (Rowan- as ≈ 12 Robinson 1996, and references therein) with LFIR 10 −α −β 14 −2 ∼ L60 L60 to 10 h L⊙. There is general consensus that the merg- Ψ(L60)= C ∗ 1+ ∗ . (2) ing and interaction rate of galaxies was greater in the past L60 L60 m with merging rate rising in proportion to (1 + z) . Values For the case of conventional cosmology, Koranyi & Strauss for m as high as 4 (Carlberg 1992, Lavery et al 1996) are ∗ 9.68 fit values of α = 0.49, β = 1.81 and L60 = 10 L⊙. mentioned in the literature, although there is some obser- The normalization constant, C, is related to their pa- vational evidence pointing to m 1.2 for z < 1 in the rameter n = 0.058 Mpc−3, through the relation n = HST Medium Deep Survey (Neuschaefer≈ et al 1997) and 1 1 Ψ[Lmin(zs)], where Lmin(zs) is the minimum luminosity m = 2.8 0.9 from faint galaxy catalogs compiled from detectable in the sample, whose members are restricted CFHT surveys± (Patton et al 1997). The comoving number to lie at redshifts greater than zs. For Koranyi & Strauss’ density of bright QSOs (MB < 24.5 +5 log(h)) increases ∗ −3 6 − analysis, Lmin(zs)/L = 8.8 10 , which leads to C = as (1+ z) from z = 0 to 2 (Hewett et al 1993), and, if the −3 −3 × −1 −1 5.8 10 Mpc (Ho = 100 km s Mpc ). QSO phenomenon is tied to galaxy interactions, this steep × ∗ In the L >> L regime, which will characterize the evolution could point to a steep rise in the interaction rate. 60 60 hosts of the brightest OH line emitters, Φ(L60) may be Surveys in the range 400 to 1000 MHz may provide an approximated as independent measure of the merging rate by determining the density of merging galaxies as a function of redshift. ∗−1 ∗ −(α+β+1) Φ(L60) C(α + β)L60 (L60/L60) . (3) Since some of the currently most viable models of galaxy ≈ formation require the bulk of the construction of the larger 2.2. The effective OH luminosity function Θ(L ) galaxies to occur by merging throughout this range of red- OH shift (z 0.5 to 3), surveys for OH masers will form an The strong correlation between OH and 60µm luminosity, ≈ 2 important test of the principal processes of galaxy forma- LOH L60 (cf. Baan et al, 1992a) provides a way to recast tion. the 60∝µm luminosity function for luminous FIR galaxies to obtain an OH megamaser luminosity function Θ(LOH ) as 2. Detection rate of cosmologically distant OH shown in Fig. 1. In this paper, this has been accomplished Masers in two complementary ways: one by numerical integration of Φ(L60) to obtain the number of OH sources falling in The detection rate for OH megamasers can be estimated logarithmically spaced bins of LOH , and the second by by combining 1) knowledge of the density of prospective noting that the masers that are detectable at cosmological FIR luminous host galaxies, 2) the probability that galax- distances inhabit host galaxies that are far more luminous ∗ ies of each luminosity will be seen as megamaser emitters, than L60 and then using the approximation in Eq.(3) to 3) the dependence of megamaser strength on FIR lumi- derive an analytic expression. nosity, and 4) the sensitivity of radio telescope receiving The advantages of computing Θ(LOH ) numerically are systems. The steps in the estimation of the detection rate that it (1) simplifies inclusion of the spread of OH lumi- ±0.7 2 are outlined in the following subsections. A parallel line of nosities in a band of width 10 about the LOH L60 reasoning applies to the detectability of normal galaxies relation (Baan et al 1992a),∼ and (2) allows for varying∝ the in the 21cm line of neutral hydrogen, as discussed in Sect. probability fm that luminous FIR galaxies will appear to 2.5. be megamasers as a function of L60. Here, the range of FIR luminosities that are considered to contribute OH megamasers is conservatively restricted to the range where 2.1. The FIR luminosity function Φ(L60µ) strong OH emission has actually been detected (cf.