A&A 436, 75-90 (2005)
C. Henkel1 - A. B. Peck2 - A. Tarchi3,4 - N. M. Nagar5,6,7 - J. A. Braatz8 - P. Castangia4,9 - L. Moscadelli4
1 - Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
2 - Harvard-Smithsonian Center for Astrophysics, SAO/SMA Project, 654 N. A'ohoku Pl., Hilo, HI 96720, USA
3 - Istituto di Radioastronomia, CNR, via Gobetti 101, 40129-Bologna, Italy
4 - INAF - Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy
5 - INAF, Arcetri Observatory, Largo E. Fermi 5, 50125 Florence, Italy
6 - Kapteyn Instituut, Postbus 800, 9700 AV Groningen, The Netherlands
7 - Astronomy Group, Departamento de Física, Universidad de Concepción, Casilla 160-C, Concepción, Chile
8 - National Radio Astronomy Observatory, PO Box 2, Green Bank, WV 24944, USA
9 - Universitá di Cagliari, Dipartimento di Fisica, Cittadella Universitaria, 09012 Capoterra (CA), Italy
Received 14 October 2004 / Accepted 16 February 2005
Using the Effelsberg 100-m telescope, detections of four extragalactic water vapor masers are reported. Isotropic luminosities are 50, 1000, 1 and 230 for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 and Arp 299, respectively. Mrk 34 contains by far the most distant and one of the most luminous water vapor megamasers so far reported in a Seyfert galaxy. The interacting system Arp 299 appears to show two maser hotspots separated by approximately 20 . With these new results and even more recent data from Braatz et al. (#!Bra04!#, ApJ, 617, L29), the detection rate in our sample of Seyferts with known jet-Narrow Line Region interactions becomes 50% (7/14), while in star forming galaxies with high ( Jy) far infrared fluxes the detection rate is 22% (10/45). The jet-NLR interaction sample may not only contain "jet-masers'' but also a significant number of accretion "disk-masers'' like those seen in NGC 4258. A statistical analysis of 53 extragalactic H2O sources (excluding the Galaxy and the Magellanic Clouds) indicates (1) that the correlation between IRAS Point Source and H2O luminosities, established for individual star forming regions in the galactic disk, also holds for AGN-dominated megamaser galaxies; (2) that maser luminosities are not correlated with 60 m/100 m color temperatures; and (3) that only a small fraction of the luminous megamasers ( ) detectable with 100-m sized telescopes have so far been identified. The H2O luminosity function (LF) suggests that the number of galaxies with 1 , the transition range between "kilomasers'' (mostly star formation) and "megamasers'' (active galactic nuclei), is small. The overall slope of the LF, -1.5, indicates that the number of detectable masers is almost independent of their luminosity. If the LF is not steepening at very high maser luminosities and if it is possible to find suitable candidate sources, H2O megamasers at significant redshifts should be detectable even with present day state-of-the-art facilities.
Key words: masers - galaxies: active - galaxies: jets - galaxies: Seyfert - galaxies: starburst - radio lines: galaxies
Extragalactic water vapor masers, observed through the 22 GHz ( cm) line of ortho-H2O that traces warm ( K) and dense (n(H cm-3) molecular gas (e.g. Kylafis & Norman 1987, 1991; Fiebig & Güsten 1989), are primarily seen as a means to probe nuclear accretion disks in active galaxies. The best known source, NGC 4258, shows a thin, slightly warped, nearly edge-on Keplerian disk of subparsec scale enclosing a central mass of (e.g. Greenhill et al. 1995; Miyoshi et al. 1995; Herrnstein et al. 1999).
There is evidence, however, for additional classes of extragalactic H2O masers. There are sources in which at least a part of the H2O emission appears to be the result of an interaction between the nuclear radio jet and an encroaching molecular cloud (e.g. Mrk 348; Peck et al. 2003).
Most of the nuclear water vapor sources are characterised by (isotropic) and are classified as "megamasers''. H2O masers associated with prominent star forming regions similar to those seen in the Galaxy (e.g. in M 33; Greenhill et al. 1993) are less luminous and comprise the majority of known "kilomasers'' ( ).
Table 1: "Jet-maser'' observations.
Providing bright, almost point-like hotspots, H2O masers are ideal probes for Very Long Baseline Interferometry (VLBI). A broad variety of astrophysical studies is possible. This includes the determination of geometric distances and 3-dimensional velocity vectors of galaxies, masses of nuclear engines, maps of accretion disks and physics of nuclear jet-molecular cloud interaction (for recent reviews, see Greenhill 2002, 2004; Maloney 2002; Henkel & Braatz 2003; Morganti et al. 2004; Henkel et al. 2005).
So far, almost 1000 active galaxies have been surveyed. In order to detect a large number of strong maser sources that could help to elucidate the nuclear environment of their parent galaxies and their geometric distance, typical detection limits were several 10 mJy or more, resulting in detection rates between zero (e.g. Henkel et al. 1998) and a few percent (e.g. Henkel et al. 1984; Braatz et al. 1996; Greenhill et al. 2002). The low detection rates are probably the result of the limited sensitivity of the surveys, rather than an intrinsic lack of extragalactic H2O masers. The technology exists to do deeper searches; what is required is a set of criteria to narrow the list of candidates from all nearby galaxies to a manageable few. Here we present the results of two quite different, but equally successful, deep searches (to estimate distances, H0 = 75 km s-1 Mpc-1 is used, whenever possible, throughout the paper).
Table 2: "FIR-maser'' observations.
The target sources of the two samples were measured in the 616-523 line of H2O (rest frequency: 22.23508 GHz) with the 100-m telescope of the MPIfR at Effelsberg on various occasions between June 2001 and April 2004. The full width to half power beamwidth was 40 and the pointing accuracy was in most cases better than 10 (see also Sect. 4.2.3). A dual channel HEMT receiver provided system temperatures of 130-180 K on a main beam brightness temperature scale. The observations were carried out in a dual beam switching mode with a beam throw of 2 and a switching frequency of 1 Hz. The autocorrelator backend was split into eight bands of width 40 or 80 MHz and 512 or 256 channels each that could individually be shifted in frequency by up to 250 MHz relative to the recessional velocity of the galaxy. This yielded channel spacings of 1 or 4 km s-1. A few spectra were also taken with two bands of 20 MHz and 4096 channels each. The resulting channel spacing was then 0.07 km s-1. Flux calibration was obtained by measurements of W3(OH) (for the flux, see Mauersberger et al. 1988). Gain variations as a function of elevation were taken into account (see Eq. (1) of Gallimore et al. 2001) and the 1 flux calibration error is expected to not exceed 10%.
Table 3: Line parameters of newly detected masers.
From the jet-maser sample, we have detected two new megamasers, Mrk 1066 and Mrk 34. The FIR-maser sample also yields two new detections, a megamaser (Arp 299) and a kilomaser (NGC 3556). Line profiles are shown in Figs. 1-6. Line parameters including recessional velocity and (isotropic) H2O luminosity are given in Table 3. Properties of the detected galaxies are discussed below.
|Figure 1: 22 GHz H2O megamaser profiles toward Mrk 1066 with a channel spacing of 1.08 km s-1. = 025958 6, 4914 . Velocity scales are with respect to the Local Standard of Rest (LSR) and use the optical convention that is equivalent to cz. km s-1 (NASA/IPAC Extragalactic Database (NED)). km s-1.|
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Mrk 1066 is a FIR luminous ( ) SB0+ galaxy, containing a double nucleus (e.g. Gimeno et al. 2004). It is one of the few early-type galaxies that have been detected in CO (see Henkel & Wiklind 1997, note that their FIR luminosity (their Table 2) is too low). Its systemic velocity is 3605 km s-1 (see Table 1), corresponding to a distance of 50 Mpc. The inclination angle is 42 (Whittle 1992). Hubble Space Telescope (HST) imaging of the nuclear region (Bower et al. 1995) shows a jet-like feature in a narrow-band image which includes [O III] and H. The distribution is bipolar, oriented at 315, and extending to an angular radius of 1 5, with emission from the north-western side being dominant. In H and [N II], the jet is equally prominent on both sides of the nucleus. The 3.6 cm radio continuum emission (Nagar et al. 1999) is extended along the same axis over 2 5.
There is a strong narrow maser spike at 3636 km s-1 with a full width to half maximum linewidth of less than 2 km s-1 (Figs. 1 and 2) and a peak flux density of 80 mJy. The spike becomes narrower between March 7 and 10, 2002, appears to be unresolved in frequency in May and reaches only 50 mJy in September. It seems that a gradual decrease of the linewidth is finally accompanied by a decrease in peak flux density. At a distance of 50 Mpc, isotropic luminosities reach 10 . A second much wider component, at 3550 km s-1, has a peak flux density of 10-20 mJy and an isotropic luminosity of 40 .
|Figure 2: High spectral resolution profiles of the narrow maser spike in Mrk 1066 (see Fig. 1). The channel spacing is 0.067 km s-1.|
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Mrk 34 (IRAS 10309+6017), another luminous (1011 ) FIR source, is a distant Seyfert 2 galaxy (z=0.0505, Mpc; Falcke et al. 1998). The optical galaxy is characterized as having an inclination angle of 57 in Whittle (1992), although a second generation Digital Sky Survey (DSS) image shows the galaxy to be compact, with poorly defined outer isophotes (Nagar & Wilson 1999). The radio emission has an extended structure (2 5; Ulvestad & Wilson 1984), and strong evidence for an interaction between the radio jet and NLR clouds has been found by Falcke et al. (1998).
Mrk 34 is one of the most distant and most luminous H2O megamasers ever detected. The maser shows two or three distinct spectral features (Figs. 3 and 4). One is centered at a velocity of 14 840 km s-1, another at 15 770 km s-1, and a third one is tentatively seen at 14 665 km s-1. Peak flux densities are up to 10 mJy and total isotropic luminosities are 1000 .
|Figure 3: Low velocity H2O megamaser profiles toward Mrk 34 with a channel spacing of 4.65 km s-1. = 103408 6, = 600152 . c = 15145 km s-1 (de Grijp et al. 1992). km s-1.|
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|Figure 4: High velocity megamaser feature toward Mrk 34 with a channel spacing of 18.65 km s-1.|
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|Figure 5: H2O kilomaser spectra toward NCG 3556 ( 1131 2, 4025 ). Channel spacings are 4.2 km s-1 ( upper panel) and 1.06 km s-1. cz = 700 km s-1 (NED). km s-1.|
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NGC 3556 is an edge-on spiral galaxy located at a distance of 12 Mpc. Its FIR luminosity, , is similar to that of the Milky Way. Radio continuum, H I and X-ray data indicate a violent disk halo interaction, including a prominent radio halo (e.g. de Bruyn & Hummel 1979), large H I extensions possibly delineating expanding supershells (King & Irwin 1997), compact radio continuum sources, likely representing supernova remnants (Irwin et al. 2000), and extraplanar diffuse X-ray emission (Wang et al. 2003). 12CO and HCN observations (Gao & Solomon 2004) indicate a substantial molecular gas content. No OH maser was detected (Unger et al. 1986).
With a peak flux density of 20-40 mJy, the H2O maser has an isotropic luminosity of 1 . One or two velocity components are seen. The profiles are shown in Fig. 5.
Arp 299 is a merging system at Mpc, composed of two main sources, IC 694 and NGC 3690 (for an alternative nomenclature, see Sect. 4.2.3), that are separated by 22 in east-west direction (e.g. Sargent & Scoville 1991). A FIR luminosity of several 1011 (Casoli et al. 1992) places Arp 299 near the boundary between luminous (LIRGs) and ultraluminous (ULIRGs) infrared galaxies. Supporting the merging scenario, two highly extended H I tails have been identified by Hibbard & Yun (1999). Radio and infrared observations reveal three main regions of activity (e.g. Gehrz et al. 1983; Aalto et al. 1997; Casoli et al. 1999), the nuclear regions of IC 694 and NGC 3690 and an interface where IC 694 and NGC 3690 overlap. NGC 3690 contains a deeply enshrouded active galactic nucleus (AGN), while the situation with respect to the similarly obscured nuclear region of IC 694 is less clear (e.g. Della Ceca et al. 2002; Ballo et al. 2004; Gallais et al. 2004). 12CO and HCN J=1-0 emission line peaks are strongest toward these most active regions, indicating the presence of large amounts of molecular gas. The positions of strongest 13CO J =1-0 line emission are, however, displaced from these hotspots (Aalto et al. 1997).
In Arp 299, water maser profiles are extremely broad (200 km s-1), with peak flux densities of 30 mJy (Fig. 6). Adopting a distance of 42 Mpc, the total isotropic luminosity is 250 , placing the object among the more luminous H2O megamaser sources. The maser line is centered at a velocity of 3100 km s-1, i.e. close to the systemic velocity of the entire complex of sources constituting Arp 299.
We also observed the previously detected H2O maser sources IC 342 and NGC 2146. IC 342 was not detected in June 2001 and March 2002, indicating that the flaring component observed at 16 km s-1 (Tarchi et al. 2002a) has been quiescent since June 2001. Spectra from NGC 2146, obtained in March 2002, show no significant variations with respect to profiles observed two years earlier (see Tarchi et al. 2002b).
As indicated in Sect. 1, the surveys presented here have been targeted to detect two classes of extragalactic water masers, "jet-masers'' and "FIR-masers''.
Jet-masers provide insight into the interaction of nuclear jets with dense warm molecular gas in the central parsecs of galaxies. All jet-masers known to date arise from the innermost regions of active galaxies and yield important information about the evolution of jets and their hotspots. If continuum emission from the core of the radio source is responsible for variations in maser intensity, monitoring of continuum and line emission can provide estimates, through reverberation mapping, of the speed of the material in the jet, particularly in sources where the jet appears to lie close to the plane of the sky. If, on the other hand, the continuum flare is caused by the brightening of the hotspot or working surface in the jet as it impacts a denser molecular cloud, then the onset of the continuum and maser flares should be nearly simultaneous (Peck et al. 2003).
|Figure 6: H2O megamaser profile toward Arp 299 ( 2831 9, 3345 ). The channel spacing is 4.3 km s-1. km s-1. For radial velocities, see Sect. 4.2.3.|
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In view of these implications we need to investigate the true nature of the megamasers detected in Mrk 1066 and Mrk 34. Are these really jet-masers as suggested by the selection criteria (Sect. 1)? While only VLBI observations can provide a definite answer, a detailed look at well studied jet-masers and the maser sources recently discovered by Braatz et al. (2004) can provide relevant information.
The four jet-maser sources known prior to this survey are NGC 1068 (Gallimore et al. 1996), the Circinus galaxy (Greenhill et al. 2001, 2003a), NGC 1052 (Claussen et al. 1998) and Mrk 348 (Peck et al. 2003). The first two sources show both maser emission from a circumnuclear disk and emission arising along the edges of an ionization cone or outflow in the jet. In NGC 1068, the jet-maser velocities are blue-shifted by 160 km s-1 from systemic and the feature is broad (FWHP (full width to half power) linewidth 60 km s-1; e.g. Gallimore et al. 2001). In Circinus, both red- and blue-shifted features are seen at velocities up to 160 km s-1 from systemic (Greenhill et al. 2003a). In NGC 1052 and Mrk 348, all the maser emission may arise along the jet (Claussen et al. 1998; Peck et al. 2003). As in NGC 1068, this is accompanied by relatively large linewidths (90 and 130 km s-1) and significant shifts relative to the systemic velocity (+150-200 and +130 km s-1), respectively.
To summarize, jet-maser features tend to be broader (a few 10 km s-1) than those typically seen in disk-maser sources like NGC 4258 (a few km s-1) and are usually displaced from the systemic velocity. In Mrk 1066, it is the component at c km s-1 that shows the properties expected in the case of a nuclear jet-type H2O maser (see Fig. 1). The intense narrow spike near the systemic velocity would have a different origin. We also note, however, that the broad blue- and the narrow red-shifted features bracket the systemic velocity (3605 km s-1). Thus a masing disk like in NGC 4258 cannot be excluded.
Toward Mrk 34, the main components at c and 15 770 km s-1 are wide enough for characteristic jet-maser emission. However, the intrinsic weakness of the features requires smoothing which could hide individual narrow components that might represent a significant fraction of the maser emission. Furthermore, Figs. 3 and 4 show two or three velocity components that bracket the systemic velocity ( km s-1; de Grijp et al. 1992). The velocity displacements may not be symmetric; the red-shifted emission (Fig. 4) appears to show a larger displacement than the blue-shifted emission (Fig. 3) from systemic, which would argue against the possibility of a circumnuclear disk. However, the uncertainty in c is large so that an accretion "disk-maser'' scenario is also possible. Among the three "jet-maser'' sources detected by Braatz et al. (2004; see also Table 1), Mrk 1157 (NGC 591), Mrk 3 and NGC 4151, the first one also shows a profile reminiscent of a disk-maser source. We thus conclude that our jet-maser sample does not provide exclusively jet-maser sources. Having selected sources with jets that appear to be oriented close to the plane of the sky (Sect. 1), this is apparently also an excellent selection criterion to find disk-masers that are characterized by nuclear disks viewed edge-on. Disk-masers may constitute a significant fraction of the newly discovered `jet-maser' sources and some of these may even show both signatures (like NGC 1068) of nuclear activity.
Including all sources that fulfill the selection criteria of our jet-maser sample (Table 1), the detection rate becomes an (almost incredible) 7/14 or 50%. This is the first survey undertaken to look specifically for jet-masers. The number of sources and detections is still too small for a detailed statistical analysis. While it remains to be seen whether the masers are jet- or disk-masers, the unprecedented success rate suggests that both types of masers have been found and that a tilt of >55 between nuclear and large scale disk is a highly favorable configuration for the occurrence of H2O masers in Seyfert galaxies.
FIR emission commonly arises from dust grains heated by newly formed stars. In the Milky Way, 22 GHz H2O masers are associated with sites of (mostly massive) star formation. Therefore our sample of FIR bright galaxies (Table 2) is a suitable tool to detect extragalactic H2O masers associated with young massive stars. Such masers have the potential to pinpoint the location of prominent star forming regions and to estimate their distance through complementary measurements of proper motion and radial velocity (e.g. Greenhill et al. 1993). Monitoring such masers and determining their three dimensional velocity vectors allows us to derive the gravitational potential of galaxies or groups of galaxies and to improve our understanding of the evolution of such groups with time (for the Local Group, see Brunthaler et al. 2005).
Including the early part of our survey (Tarchi et al. 2002a,b), we detected with IC 342, NGC 2146, NGC 3556 and Arp 299 four new H2O masers in a total of 45 sources (see Table 2). The new detections are a consequence of higher sensitivity (1 noise levels of 10 mJy for a 1 km s-1 channel), highly improved baselines and luck (in the case of the short-lived flare observed toward IC 342). Including all previously detected sources in the complete sample shown in Table 2, we find a detection rate of 10/45 or %. For sources with 100m fluxes in excess of 100 Jy, the detection rate becomes even higher: 7/19 or %. Detection rates for the jet-maser and the FIR-maser samples lie far above the corresponding rates deduced from other carefully selected samples (see e.g. Henkel et al. 1984, 1986, 1998; Haschick & Baan 1985; Braatz et al. 1996; Greenhill et al. 2002).
|Figure 7: H2O megamaser profiles toward Arp 299, taken on Jan. 30, 2004, during a night with excellent pointing conditions. The reference position is 2831 9, 3345 . Offsets in arcsec ( , ) are given in the upper right corner of each box. Spectra with offsets trace emission from the "overlap'' region (north) and NGC 3690 (south). Spectra with are sensitive to emission from IC 694 (see Sect. 4.2.3). The channel spacing is 16.8 km s-1.|
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To find out why the FIR-maser sample contains numerous 22 GHz H2O maser sources and to elucidate the nature of the sources in NGC 3556 and Arp 299, we have to classify the properties of the previously studied masers of this sample. Two of the sources, those in NGC 1068 and NGC 3079, are luminous megamasers (e.g. Gallimore et al. 2001; Trotter et al. 1998). The weaker "kilomasers'' in IC 10, IC 342, NGC 2146 and NGC 3034 (M 82) are associated with sites of massive star formation (Argon et al. 1994; Baudry & Brouillet 1996; Tarchi et al. 2002a,b). There are also two known weak nuclear kilomasers, in NGC 5194 (M 51) (Hagiwara et al. 2001b) and in NGC 253 (Henkel et al. 2004). Whether they are related to the nearby AGN or to star formation remains, however, unclear.
The position of the H2O kilomaser in NGC 3556 is not yet accurately known. From the relative number of nuclear versus non-nuclear masers of similar luminosity, the most likely interpretation is an association with a site of massive star formation. In view of NGC 253 and NGC 5194, however, there is a small chance for a nuclear maser in NGC 3556.
As indicated in Sect. 3.4, the merging system Arp 299 is composed of two galaxies, NGC 3690 in the west, IC 694 in the east, and a star-bursting interface or overlap region 10 north of NGC 3690. Since NGC 3690 and IC 694 are only half a beam size (20 ) apart in our observations and since the separation between NGC 3690 and the overlap region is even smaller, extremely good pointing conditions were needed to map the region. Three maps were made. Figure 7 shows the most accurate (pointing accuracy 4 ) and extended, albeit also the most noisy one. In spite of the rather low signal-to-noise ratios we note that (1) H2O emission may originate from more than one hotspot; (2) one of the potential sources, the one in the east, is close to IC 694, where an OH megamaser was already reported (Baan & Haschick 1990); (3) there appears to be a western peak of emission that is located near the center of the second dominant galaxy of the system, NGC 3690; (4) a broader feature near the center is likely caused by blending of the two main hotspots associated with IC 694 and NGC 3690; (5) the vigorously star forming overlap region appears to be devoid of H2O megamaser emission.
When we compare these results with the CO velocity field observed by Casoli et al. (1999), we find that the H2O velocity of the eastern peak, 2980 km s-1, is consistent with the CO velocity of the south-eastern part of IC 694 (i.e. offset w.r.t. the nucleus of IC 694 that has a velocity of cz = 3110 km s-1). Although the location of the western hotspot is close to the core of NGC 3690, the velocities of the maser emission, the CO lines and the systemic velocity of NGC 3690 do not match perfectly (3100 km s-1 from CO, c = 3121 km s-1 (NASA/IPAC Extragalactic Database (NED)), versus 3150 km s-1 from H2O). Interestingly, velocities near 3150 km s-1 as seen in H2O are consistent with those of the overlap region.
A comparison of the profile shown in Fig. 6 with the central one in Fig. 7 suggests a slight offset in position (a few arcsec in east-west direction) and weaker peak emission in the latter case. This is within the uncertainties of pointing and calibration, but maser variability can also explain the differences.
With the H2O emission likely originating from IC 694 and NGC 3690, Arp 299 is the fourth extragalactic system beyond the Magellanic Clouds that is known to exhibit = 18 cm OH and = 1.3 cm H2O maser emission (in NGC 253, NGC 1068 and M 82, such masers are also observed; see Weliachew et al. 1984; Turner 1985; Baudry & Brouillet 1996; Gallimore et al. 1996; Henkel et al. 2004). In these other galaxies, however, either H2O or OH or both lines only reach kilomaser luminosities. IC 694 may thus be the first known galaxy with both an OH and an H2O megamaser (for OH, see Baan & Haschick 1990). The global OH and H2O line profiles appear to be similar, except for a weak OH feature at 3500 km s-1that is not seen in H2O.
Arp 299 is the second most luminous FIR source with a known H2O megamaser. While OH megamasers are closely associated with ultraluminous infrared galaxies (ULIRGs; see e.g. Darling & Giovanelli 2002a), the only ULIRG with a luminous H2O maser was so far NGC 6240 (Hagiwara et al. 2002, 2003a; Nakai et al. 2002; Braatz et al. 2003). In accordance with the observed anticorrelation between the occurence of OH and H2O megamasers (OH megamasers may arise from low density molecular gas, while H2O megamasers originate from gas of much higher density; e.g. Kylafis et al. 1991; Randell et al. 1995), this is one of those ULIRGs in which no OH emission is seen. The second detection of an H2O megamaser in a luminous FIR galaxy (although Arp 299 is not quite as luminous as NGC 6240) makes a dedicated survey of such luminous FIR galaxies worthwhile.
|Figure 8: Detection rate of the H2O FIR-maser sample (see Table 2 for the targets and Sect. 1 for selection criteria) including all galaxies exceeding a given IRAS Point Source Catalog flux density.|
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The high rate of maser detections in our sample of FIR luminous galaxies (Table 2) strongly suggests that a relationship between FIR flux density and maser phenomena exists, consistent with the assessment of HWB. The detection rate for masers in accretion disks is dictated by tight geometric constraints. The cumulative maser output of a star forming region may not be so narrowly confined.
Figure 8 shows the cumulative detection rate above a given 100 m IRAS Point Source Catalog flux for the parent galaxy. The detection rate strongly declines with decreasing FIR flux. For fluxes 1000 Jy, 100-300 Jy, and 50-100 Jy, we find detection rates of 2/2 or 100%, 5/17 or 29% and 3/26 or 12% (unlike in Fig. 8, these are not cumulative detection rates but detection rates related to their specific FIR flux density interval). Maffei 2 and NGC 5236 (M 83) show no detectable maser emission near their nuclei but have Jy. In view of the statistical properties of the sample, frequent monitoring of these sources would likely reveal H2O maser emission, possibly a short-lived flare like that seen in IC 342 (Tarchi et al. 2002a).
Figure 8 shows a detection probability of 50% for sources with Jy. If the two brightest FIR sources, NGC 253 and M 82 (NGC 3034), were at Mpc (i.e. three times their estimated distance), this would imply Jy and H2O peak fluxes of 5 mJy (broad emission feature) and 10 mJy (narrow emission feature), respectively (for the line profiles, see Ho et al. 1987; Baudry et al. 1994; Henkel et al. 2004). Thus the two sources would be just below the detection limit, consistent with the detection probability at the corresponding 100 m flux.
While there is significant scatter (among the sources of Table 2, the most extreme source by far is NGC 3079, whose H2O maser would be detectable even at a distance corresponding to Jy), we conclude that at present sensitivities, there is for most sources a detection threshold near Jy. It appears that and H2O peak fluxes are roughly proportional, as was already suggested by HWB on the basis of a smaller number of detected sources. Such a result is reminiscent of the - correlation found by Jaffe et al. (1981) for galactic star forming regions and is readily explained if most of the detected sources in our FIR sample are associated with sites of massive star formation. Four of the ten detected sources are indeed related to star formation, two to AGN, while the nature of the remaining four is uncertain. While the scatter is large, nevertheless even the AGN related megamaser galaxies roughly follow the correlation found for galactic sources, i.e. / 9 (see Fig. 9). This is difficult to explain and might be caused by a spatially extended cascade of nuclear bars that contains warm dust and that is needed to fuel the very nuclear region (e.g. Shlosman & Heller 2002).
Given the correlation between FIR and H2O maser flux densities, an improvement in sensitivity by one order of magnitude to detect H2O masers would lower the 100 m flux threshold from 100 Jy to 10 Jy, and would provide a 25 times richer sample of detectable targets (250 sources at ). This enlarged sample might then also include some of the brighter ULIRGs that are not part of this study because of too large distances and correspondingly low infrared flux densities.
|Figure 9: IRAS Point Source FIR luminosity versus total H2O luminosity of H2O detected galaxies (cf. Table 4). For NGC 2146, the total logarithmic H2O luminosity is 0.9 in solar units (Tarchi et al. 2002a). Stars denote the ten sources belonging to the FIR selected maser sample. The diagonal line shows the correlation found by Jaffe et al. (1981) for individual galactic star forming regions.|
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Table 4: Extragalactic H2O masers beyond the magellanic clouds.
Table 4 lists the 53 galaxies with a total of 57 groups of H2O masers detected beyond the Magellanic Clouds. The separation between megamasers (AGN environment) and kilomasers (mainly star formation) is not entirely clear. NGC 2782, an maser, might not possess an AGN (e.g. Braatz et al. 2004), while the masers in NGC 2146 (a total of 8 ; see Table 5) are known to be related to star formation (Tarchi et al. 2002b). A clear separation between jet and accretion disk masers would also be appropriate. The lack of high resolution data toward most sources, however, makes such a classification elusive. We thus group the masers according to their isotropic luminosity, assuming that all sources with (the megamasers) are nuclear.
The H2O detections presented in Table 4 were collected from various surveys
with different sensitivities and even
within a survey, noise levels may differ from source to source. Furthermore,
the masers are time variable and it is always
more difficult to detect a broad weak feature than a stronger but narrower spectral
component. In view of this highly heterogeneous data base, we adopt a characteristic
linewidth of the dominant spectral feature of 20 km s-1 and a
detection threshold of 50 mJy (this sensitivity is inferior to that in our
surveys (Sects. 4.1 and 4.2) and reflects the
higher noise levels of most other data, the exception being the Braatz et al.
2004, survey). We should then be able to detect
masers with 1, 10, 100, 1000, and 10 000
out to maximal distances of
We can check how consistent this is with the sample of detected masers listed in Table 4. The total H2O luminosity per galaxy is taken. Table 5 shows the results. The number of detections in the most likely distance bin is given in italics. IC 342 was not included because of the intrinsic weakness of its maser. It is apparent that for the kilomaser galaxies (here defined to show luminosities <10 ) either the number of sources in the expected bin is by far the highest or there are additional detections at both lower and higher distances (the latter a consequence of the fact that the sensitivity of the surveys is not uniform). This provides a picture that is approximately consistent with expectations.
For the more luminous megamasers the situation is different. The distance distribution of the 20 masers with 10 is still consistent. Four are located at D <21 Mpc, and four at a distance higher than the estimated limiting distance of 65 Mpc. Most of the detections are obtained in their most likely distance bin. Among the 18 masers with 100 , however, 13 are closer or near Mpc, the inner limit of the most likely distance range, while among the four galaxies with 1000 , three are closer than the corresponding Mpc limit. The fourth megamaser, 3C 403, surpasses this limit by only a small amount. None of the masers in the last two groups has a distance larger than the estimated maximum distance.
Table 5: Number of detected maser galaxies per luminosity and distance interval (see Table 4)a.
The surveys do not cover the entire sky and are therefore incomplete by an unknown amount. The fact that distances of masers with lower luminosity are consistent with expected values indicates that a bias related to the distance of these sources is negligible. Most of the luminous megamasers ( ), however, are observed at distances that are smaller than expected. Is this an effect of different lineshapes or a consequence of the observed sample of sources? Considering the four most luminous targets with , only TXS2226-184 has an unusually wide profile, while the others show "normal'' lineshapes. Without going into any detail, we note that the situation is similar for sources with 100 . We thus conclude that the bias towards "nearby'' sources in the sample of luminous water masers is caused by the selection of galaxies so far observed. The entire megamaser sample is dominated by the surveys of Braatz et al. (1994, 1996, 1997, 2003, 2004) that are mostly confined to recessional velocites 7000 km s-1, i.e. out to Mpc (this also holds for the two surveys discussed in Sects. 4.1 and 4.2). Sources with significantly larger distances were rarely observed.
From the number of sources at "near'' distances we may extrapolate to the larger volumes to estimate the percentage of missing detections in this larger volume. This may provide lower limits because detections at the `nearby' distances may be incomplete as well. For the more luminous megamasers with 100 , four sources are observed within D=21 Mpc, so that detectable targets may be expected within D=210 Mpc. This has to be compared with 16 known such objects. With 11 known sources within 65 Mpc, we still expect objects within D=210 Mpc, a factor of 20 above the detected number. Among the most luminous four sources, those with , three are detected inside of 210 Mpc, so we would expect detectable targets out to D=650 Mpc, of which so far only four have been identified.
While the large errors in the predicted numbers of detectable sources may raise scepticism, an analysis of the distances of the galaxies belonging to the two most luminous H2O luminosity bins (100 ) yields a clear result. 14 of the 22 sources in these bins are not in the expected most distant shell (between D=65 and 210 or between D=210 and 650 Mpc, respectively) but are located more nearby. Two additional sources are located at the inner boundary of the most likely shell, while no source is detected beyond the estimated distance limit. This implies that the majority of sources, 73%, is located within a volume that encompasses only 3.2% of the volume in which the masers would be detectable. Assuming an isotropic spatial distribution and applying the Bernoulli theorem, a deviation of 8% from 73% corresponds to 1. The discrepancy between the expected (3.2% of the detections in the inner, 96.8% in the outer shell) and observed (73% in the inner, 27% in the outer shell) spatial distributions is therefore significant. We conclude that statistical evidence strongly indicates that only a tiny fraction of the luminous megamaser sources detectable with presently available instrumentation has been discovered to date.
So far we have not yet considered that the maser luminosities are not necessarily at the upper edge of their respective bin. This has the consequence that not all of them should be detectable out to the upper limit of the corresponding most likely distance interval. To quantify this we have to determine the H2O luminosity function.
The luminosity function is the number density of objects with luminosity per logarithmic interval in . An unbiased direct measurement of would require that all objects with a given luminosity be detected within the survey volume, which is not possible in flux limited surveys like those presented in Sects. 4.1 and 4.2. Instead, each object in a survey has an effective volume in which it could have been detected and the sum of detections weighted by their available volumes determines the luminosity function.
Not accounting for the incompleteness of the detected H2O megamaser sample and ignoring the possibility that in different luminosity bins the fraction of detected sources may be different, we can derive a zeroth order approximation to the luminosity function (LF) of extragalactic H2O maser sources. Such a computation is not only limited by the effects mentioned above but there are additional factors, the main three being:
Figure 10 demonstrates that the H2O luminosity function does not strongly depend on the detection limit used. From the overall slope of the luminosity function we derive which is steeper than the LF for OH megamasers (see Darling & Giovanelli 2002b). Noteworthy is the fast decay in the number of sources at the upper end of the maser luminosity function that could indicate that ultraluminous H2O "gigamasers'' are rare. Obvious is also a low number of sources in the 1-10 bin. This bin marks the upper end of the luminosity distribution of known star forming regions and is located slightly below the luminosity of the "weak'' megamaser sources. So there might exist a minimum of H2O emitting targets just below the megamaser luminosity threshold. Both results, the minimum at and the fast decline at highest maser luminosities, are, however, of questionable significance. The number of sources in the bins is not yet large enough to make a convincing case. And Fig. 10 does not account for the distance bias discussed for the most luminous sources in Sect. 4.3.1.
|Figure 10: The 22 GHz H2O luminosity function (LF) for maser galaxies beyond the Magellanic Clouds (Table 4). The filled circles show the LF for a detection limit of 0.2 Jy km s-1. The dashed line shows a fit with a slope of -1.3 (for uncertainties in the slope, see Sect. 4.3.2). Error bars of individual points are derived from Poisson statistics following Condon (1989). Not included in the diagram are IC 342 (too low maser luminosity) and UGC 3255, Mrk 3, Mrk 78, NGC 4151, NGC 5256 and NGC 6240 (below the adopted detection limit). The number of masing galaxies in each bin are also shown. To illustrate the effect of changing the sensitivity, we also show the LF for a detection limit of 1 Jy km s-1 (empty circles; in this case 19 of the 53 galaxies fall below the detection limit).|
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Can we hope to see H2O megamaser emission at cosmological distances? The most luminous megamaser known at present, that in TXS2226-184 with a redshift of z=0.025 (Koekemoer et al. 1995), a peak flux density of 400 mJy, and a linewidth of 80 km s-1 would be detectable by a 100-m telescope out to . With the Square Kilometer Array (SKA) detections at significant redshifts will thus be possible.
We may also estimate the number of detectable H2O megamasers with the LF shown in Fig. 10. The lower limit to the H2O luminosity of a source at distance D varies as
In Sect. 4.3.1 evidence was found that only a small fraction of the detectable luminous H2O maser sources is known to date. This may be the main cause for the comparatively steep slope in the LF at highest maser luminosities (see Fig. 10). Ignoring therefore the two bins with highest maser luminosities in Fig. 10, the slope of the LF becomes -1.3 instead of -1.6. This provides a realistic estimate of systematic errors but does not qualitatively change our conclusion.
At the end of Sect. 4.3.1 it was mentioned that the LF is also needed to quantify the deficit of distant high luminosity masers.
Adopting the result (see above) that the number of detectable masers is almost independent of the maser luminosity, we can
This article presents a search for 22 GHz ( cm) H2O masers towards two classes of objects, i.e. galaxies that (1) either contain nuclear jets that are oriented close to the disk of the galaxy and the plane of the sky or that (2) are bright in the far infrared. The main results are:
We wish to thank M. Elitzur and L. J. Greenhill for useful discussions during the conception of this project and an anonymous referee for carefully reading the draft and making useful suggestions. AP and AT wish to thank the MPIfR for their hospitality during the observing run. NN was partially supported by the Italian Ministry for University and Research (MURST) under grant Cofin00-02-36 and the Italian Space Agency (ASI) under grant 1/R/27/00. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with NASA. This research has also made use of NASA's Astrophysics Data System Abstract Service.