A&A 383, 65-70 (2002)
Y. Hagiwara1 - P. J. Diamond 2 - M. Miyoshi 3
1 - Max-Planck-Institut für Radioastronomie (MPIfR) Auf dem Hügel 69, 53121 Bonn, Germany
2 - Jodrell Bank Observatory, University of Manchester, Macclesfield, Cheshire SK11 9DL, UK
3 - National Astronomy Observatory, Osawa, Mitaka, Japan
Received 31 July 2001 / Accepted 26 November 2001
We report the result of an on-going survey for 22 GHz H2O maser emission towards infrared luminous galaxies. The observed galaxies were selected primarily from the IRAS bright galaxy sample. The survey has resulted in the detection of one new maser. The new maser was discovered towards the [U]LIRG/merger galaxy NGC6240, which contains a LINER nucleus. This is the first detection of an H2Omaser towards this class of galaxy, they are traditionally associated with OH megamaser sources. The detected maser emission is highly redshifted (260-300 km s-1) with respect to the adopted systemic velocity of the galaxy, and we identified no other significant emission at velocities 500 km s relative to the systemic velocity. The presence of high-velocity maser emission implies the possible existence of a rotating maser disk formed in the merging process. The large maser luminosity (40 ) suggests that an active galactic nucleus could be the energy source that gives rise to the water emission. Alternatively, the maser emission could be associated with the previously observed double radio source in the centre of the galaxy. Interferometric observations with high angular resolution will be able to clarify the origin of the new maser.
Key words: masers - galaxies: active - galaxies: individual (NGC 6240): radio lines - ISM: molecules
|(1950)||(1950)||(km s)||(km s)||(mJy)|
|UGC 05101||09 32 05||+61 34 37||11809||11580-12070||12||A|
|NGC 3310||10 35 40||+53 45 49||993||500-1520||21||A|
|ARP 299||11 25 38||+58 51 14||3111||2880-3370||18||C|
|NGC 3822f||11 39 36||+10 33 19||6120||5340-6610||16||A|
|NGC 4388||12 23 15||+12 56 20||2524||2290-2780||28||B|
|NGC 4438||12 25 14||+13 17 07||71||-160-340||32||B|
|NGC 4490||12 28 10||+41 55 08||565||330-820||14||A|
|NGC 4532||12 31 47||+06 44 39||2012||1780-2270||25||B|
|NGC 4631||12 39 42||+32 48 52||606||380-860||12||A|
|NGC 6052||16 03 01||+20 40 37||4716||4490-4990||14||A|
|NGC 6090||16 10 24||+52 35 04||8785||8560-9040||25||B|
|NGC 6240||16 50 28||+02 28 58||7339||6850-7870||4.5-7.0||B, C, D|
|NGC 6285||16 57 37||+59 01 47||5580||5190-6220||14||A|
|NGC 7674||23 25 24||+08 30 13||8671||8210-9230||28||A|
|UGC 12914||23 59 08||+23 12 58||4371||3890-4910||14||A|
a From NED database.
b Heliocentric velocity.
c Observed velocity range.
d Observing epoch; A(23-24 April), B(9-10 May), C(13-14 June), and D(16-17 July) all in 2001.
e Condon et al. (1990).
f The source was not selected from IRAS galaxies.
At this time, 24 extragalactic water masers inside AGN have been discovered with single-dish surveys (e.g. Moran et al. 1999). The detection rates of new masers remain quite low, less than 7% (e.g., Braatz et al. 1996; Braatz et al. 1997; Greenhill et al. 1997). From the presently known sample of sources we have already deduced some properties of H2Omegamasers. They prefer to lie in type 2 Seyfert AGN, suggesting that obscuring gas around an active nucleus coupled with long gain paths along an edge-on disk in the line of sight, plays a vital role in giving rise to strong maser emission. A number of H2Omegamasers are known to contain a parsec-scale radio "core-jet'' structure which the maser can amplify in the gaseous environment. Accordingly it is reasonable to search for H2Omegamasers towards type 2 Seyfert/LINER nuclei enshrouded by circumnuclear gas around a compact continuum nucleus.
Here we report the result of an on-going single-dish survey for H2Omegamasers towards AGN in infrared (IR)-luminous galaxies, and the discovery of the new maser in the galaxy NGC6240.
The observed galaxies were primarily selected from far-infrared (FIR) luminous galaxies (IRAS galaxies) with an apparent FIR luminosity ( (Soifer et al. 1989). Condon et al. (1990) carried out 1.49 GHz imaging with the VLA at arcsecond resolution for these objects. Due to the now well-known correlation of with radio continuum flux density for the IR-luminous galaxies, Condon et al.'s work has resulted in a significant number of detections of radio continuum sources towards such galaxies. The sample was selected from galaxies in which the ratio of 1.49 GHz continuum flux density to far-infrared (FIR) flux density ( ) was >0.003, where . About 25 IR-luminous galaxies were selected in this way, of which 14 sources have been observed to date. Here we present the initial results of the survey. The sample observed so far is listed in Table 1.
The observations of H2Oemission (rest frequency: 22.23508 GHz) were made with the MPIfR 100 m radio telescope at Effelsberg between April and July 2001, as part of an extragalactic H2Omaser survey towards infrared-luminous galaxies. 15 galaxies have been observed to date (Table 1). We used a K-band HEMT receiver with two orthogonal linear polarizations. The system temperature was typically 70-110 K, depending on atmospheric conditions. The amplitude calibration was based on a flux density at 22 GHz for 3C286 (Ott et al. 1994), and flux densities were estimated from the measured antennas temperature, . The observations were made in position-switching mode. Typical observing time for each source was about 50-80 min, yielding rms sensitivities of 15-30 mJy. Pointing calibrations using nearby calibrator sources were carried out during the observations every 1-1.5 hours. The resultant pointing accuracy was <8 i.e. within 20% of the antenna beam size (40 [FWHM]). Uncertainties in the amplitude calibration are nominally 10-20%. The back-end digital autocorrelation system provided 4 autocorrelators for each of the 2 polarisation signals. Each autocorrelator produced a spectrum in a 40 MHz band (corresponding to a rest-frame velocity range of 540 km s-1) with 512 spectral channels providing a frequency resolution of 78 kHz or 1.1 km s-1. The center frequencies of the four autocorrelators were arranged to have 10 or 20 MHz overlap (depending on source), resulting in a total velocity coverage of 500 km srelative to the galaxy's systemic velocity. After adding the two polarizations together, first or second order baselines were fitted to, and subtracted from, each of the spectra.
The survey of 15 sources resulted in the detection of one new H2Omegamaser. On 9 May 2001, we detected H2Oemission towards the LINER
galaxy NGC6240 (Fig. 1a), the detection was confirmed on 14 Jun. 2001
(Fig. 1b). We searched for water masers in the other 14 AGN but
obtained no detections at a 3
level of 40-90 mJy
(Table 1). The detection of one new maser among the 15 AGN in the
biased sample yields a detection rate of 6.7%, which is
comparable to those of 7.0% in Braatz et al. (1997) and
3.4% in Greenhill et al. (1997). The H2Omaser in the Seyfert
2 galaxy NGC5793 was originally selected from this sample, its
discovery stimulated us to continue the programme
(Hagiwara et al. 1997; Hagiwara et al. 2001). Our survey of IRAS galaxies is still ongoing,
however, if we include the previous detection of the maser in
NGC5793, the detection rate in our survey is significantly higher
than those mentioned above. Although the survey is incomplete, its
success seems to partially depend on the assumption that the masing
can be enhanced by "warm'' dust emission heated by a nuclear radio
|Figure 1: Spectra of water maser emission towards NGC6240 for different three epochs in 2001. The spectral channel spacing is 1.1 km s-1. is denoted by an arrow. Note that the flux density of the feature at km sclearly decreased at the last epoch c).|
|Open with DEXTER|
Figure 1 shows spectra of the H2Omaser towards NGC6240 taken with the
100 m telescope at Effelsberg. The spectra show a prominent H2Omaser
feature with a narrow velocity-width (4.3 km s[FWHM]) centered at
km s-1. The feature is redshifted by 261 km sfrom the
systemic velocity (
km swith respect to the local standard
of rest (LSR), based on HI observations; de Vaucouleurs et al. 1991). NGC6240
exhibits bright emission with a peak flux density of 40 mJy
(Gaussian-fitted intensity is 36 mJy). Figure 2 shows a spectrum averaged
over the two earlier observing epochs in Fig. 1 from which we estimate
the apparent isotropic luminosity of the prominent feature alone to be
at a distance of 97 Mpc. In addition, we detect
faint narrower (1.1 km s[FWHM]) emission at
level detection). Including both emission features, the total
luminosity of the maser is 40 .
Prior to our
observations no H2Oemission had been firmly detected
(e.g. Claussen & Lo 1986), although tentative detections had been reported
(Henkel et al. 1984; Braatz et al. 1994). The maser therefore appeared to be in a
flaring state during our observing epochs in 2001. The peak flux
density remained the same within the uncertainties of amplitude
calibrations, at both early observing epochs. However, a third
spectrum, taken on 12 July 2001 shows the strongest feature declining
in flux to 27 mJy while the feature at 7609 km sis more
prominent (Fig. 1c).
|Figure 2: Spectrum averaged over two observing epochs on 9 May and 14 June 2001. A downward arrow indicates the adopted systemic velocity of the galaxy, km s-1. Solid line indicates the result of 2-D Gaussian-fitting, assuming two Gaussian components in the observed velocity range. The fitted center velocities of each component are km s-1( FWHM = 4.3 km s) and 7609 km s( FWHM = 1.1 km s-1) with peak flux densities of 35 mJy and 15 mJy, respectively.|
|Open with DEXTER|
|Systemic Velocity (21 cm HI)b,c||kms-1|
|Inclination (optical) d||60|
|(2-10 keV)i||- erg s-1|
a Assuming H0= 75 kms-1 Mpc-1.
b Converted to LSR from helio centric velocity.
c de Vaucouleurs et al. (1991).
d Braatz et al. (1997).
e Heckman et al. (1987).
f VLA in B configuration (Colbert et al. 1994).
g Primary from NED.
h Sanders et al. (1988).
i Ikebe et al. (2000).
NGC6240 is categorized as an (ultra) luminous infrared galaxy ([U]LIRG) with a complex structure (Sanders et al. 1988) and is one of the most distant megamasers ever detected, rivaling the "Giga Maser'' in TXFS2226-184, which lies at a distance of 100 Mpc (Koekemoer et al. 1995). The basic characteristics of the galaxy are listed in Table 2. The large IR luminosity ( > 1010.8 ) is considered as having its origin in re-emission from a dust shell surrounding a hot core, suggesting the presence of an on-going starburst triggered by the merger of two galaxies (Sanders et al. 1988). However, recent hard X-ray measurements detected a strong iron-K emission line at 6.4 keV with an excess atomic hydrogen column density of 1022 cm-2 towards the nucleus, a phenomenon which appears to be common in the presence of an AGN (Iwasawa & Comastri 1998; Vignati et al. 1999; Ikebe et al. 2000). A 5 GHz high-resolution MERLIN continuum map (Beswick et al. 2001) revealed a double source with a separation of 1.5 (700 pc). Beswick et al concluded that the radio continuum flux found in the galaxy is a combination of a starburst and a weak AGN induced by a merger event. A molecular gas concentration, which lies between the radio sources, was imaged in CO emission on 100 parsec-scales (Bryant & Scoville 1999; Tacconi et al. 1999; Tecza et al. 2000). According to Tacconi et al. (1999), the CO emission is settling down into the two nuclei (which correspond to the double peaks in the MERLIN map) and will likely form a central thin disk in the final stage of evolution.
The detection of H2Omasers from NGC6240, which is exhibiting starburst activity, raises questions concerning the nuclear activity of the galaxy. The major energy source is considered to be a nuclear starburst (e.g., Tecza et al. 2000) since the ratio of the hard X-ray luminosity to the IR luminosity is small, 0.01-0.1 (Table 2). Such a ratio suggests that the hard X-ray source (i.e. the AGN) is not dominant (e.g., Ikebe et al. 2000). What is the energy source that gives rise to the water maser in NGC6240?
H2Omegamasers are often associated with a compact continuum source such as a radio core or jet. In the nuclear region of NGC6240 the two radio sources at 5, 8.4 and 15 GHz remain unresolved at 0.1 arcsec resolution. Each source has a brightness temperature upper limit () of -105 (K) (Colbert et al. 1994; Beswick et al. 2001). (There has been no reported detection of milliarcsecond-scale structure in the galaxy.)
Tacconi et al. (1999) imaged the CO(2-1) emission peak between the two radio sources with 0.7 resolution, the spectral profiles show wide line-widths spanning 500 km s with respect to the systemic velocity of 7339 km s-1. A similar result was obtained in HCN(1-0) emission, which is a more appropriate tracer of the warm dense molecular gas that can produce 22 GHz H2Omaser emission. Both CO and HCN cover the velocities of the maser but there are no distinct velocity peaks corresponding to those of the maser, weakening the case for an association between the CO/HCN gas and the masing cloud.
The location of the H2Omaser of NGC6240 is of great interest. MERLIN high-resolution HI absorption observations show that there exist velocity gradients against the two unresolved radio sources. The HI gas in front of the two sources differ in their velocity centroids by km s-1; the gradients range from -7200 km s and 7250-7350 km srespectively (Beswick et al. 2001). However, there is no significant detection of HI gas at the H2Omaser velocities of and 7609 km s-1, implying the masers are not associated with the observed HI absorbing gas. It is thus unlikely that the maser emission is associated with these two radio sources, which are widely accepted as being supernova remnants. This is consistent with the fact that total maser luminosity, which we estimate to be 40 is an order of magnitude larger than those of extragalactic H2Omasers outside AGN (0.1-1.0 ), i.e. kilomasers that are associated with star forming regions in nearby galaxies (Claussen et al. 1984; Ho et al. 1987; Greenhill et al. 1990). Consequently, it is likely that the maser in NGC6240 is directly associated with nuclear activity, the presence of which was probed by the hard X-ray measurements.
Given the detection of the features redshifted by 260-300 km sfrom and the narrowness of each line, it is reasonable to assume that the maser might originate from a well-defined region where the masing gas is receding relative to the molecular gas disk observed on 100 pc-scales. One can also speculate on the possible existence of an edge-on molecular gas disk rotating around a super massive object by analogy with other megamasers (Moran et al. 1999). Such a hypothesis is supported by the nature of the observed spectrum of the H2Omaser towards NGC 6240 and its similarity to that of the so-called disk masers. If such a scenario was true, the H2Omaser should lie in the dynamical center of the galaxy, where the observed dense molecular gas will eventually settle down between the two radio sources and form a disk.
NGC6240 has often been compared with the prototype ULIRG Arp220 (IC4553) from which hard X-ray radiation or other evidence of AGN activity has never been incontrovertibly detected (Kii et al. 1997; Genzel et al. 1998; Smith et al. 1998 and references therein). The galaxy is a merging system showing a large concentration of dense molecular gas between twin stellar/starburst nuclei (Scoville et al. 1997). H2Omaser emission has not been detected in Arp220 (Braatz et al. 1997) however, it is the prototype of the class of galaxies that exhibit OH megamaser emission. Recent surveys with Arecibo have revealed many new OH megamaser sources bringing the total number of detections to almost 100 (Darling & Giovanelli 2001). Most of the OH megamaser detections are in galaxies that are classified as [U]LIRGs, and all [U]LIRGs appear to be merging systems. The most remarkable thing is that H2Onuclear megamasers and OH megamasers are mutually exclusive: No galaxy observed to date contains both an OH megamaser and a nuclear H2Omegamaser. NGC6240 does not show OH emission, however OH gas is seen in intense absorption (e.g. Baan et al. 1998). This is similar to the case of the OH absorption towards the nucleus of the megamaser NGC5793 (Hagiwara et al. 2000). Until now there have been no detections of H2Omasers towards merging systems, NGC6240 appears to break the mould.
The presence of strong H2Omaser emission and the iron-K line in NGC6240 might mean that the galaxy is experiencing a different phase of merging from that of Arp220 i.e. compact single disk formation surrounding a "central engine'', thereby placing it the final evolution stage in galaxy-galaxy merging.
We have made the first detection of H2Omegamaser emission from a [U]LIRG/merger galaxy. We have detected emission in the LINER galaxy NGC6240, the maser features are observed to be by 260-300 km sredshifted with respect to the systemic velocity and to have a luminosity of (40 ). The maser could arise from a dense circumnuclear molecular cloud on a parsec scale or from a spatially compact maser disk inferred from the presence of the high velocity maser feature(s). The interpretation of the observed maser is not unique, though we favour the latter case. Interferometric observations of the new maser will provide an opportunity to investigate the kinematics of the ongoing merger.
YH would like to thank H. Imai for his assistance with a part of the observations. YH also appreciates R. Kawabe for helpful discussion on the source selection, and A. W. Sherwood and Chr. Henkel for able advice during the data analysis. We acknowledge M. J. Claussen for his suggestions on the manuscript. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.