DISCOVERY OF CANDIDATE H₂O DISK MASERS IN ACTIVE GALACTIC NUCLEI AND ESTIMATIONS OF CENTRIPETAL ACCELERATIONS

In a survey of 40 inclined (i > 70°) AGNs with v_sys< 18,000 km s⁻¹ selected from the NASA/IPAC Extragalactic Database (NED; Table 1), we have detected three new H₂O maser sources in Seyfert 2 systems: NGC 1320, NGC 17,³ and IRAS 16288+3929 (Figure 1). The three discoveries were subsequently confirmed with the Very Large Array (VLA) of the NRAO using a 6.25 MHz observing bandwidth and 97.7 kHz channels (NGC 1320, IRAS 16288+3929) or with the GBT on a different day but using the same spectrometer configuration (NGC 17, IRAS 16288+3929). Each of the masers is probably associated with nuclear activity. The NGC 1320 and IRAS 16288+3929 maser positions measured with the VLA lie ≪1 σ from the optical positions of the nuclei (Table 1), or less than 300 pc. The NGC 17 maser is 3–4 times more distant than the farthest H₂O maser known to be associated with star formation (Arp 244; Darling et al. 2008), yet it is comparably bright.

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Spectroscopic classification of sources as disk maser candidates is most suggestive when three line complexes are detected, one close to the systemic velocity and two bracketing it, with similar but opposite velocity offsets. Approximate symmetry in this respect is indicative of a disk, and a minimum offset on the order of 200 km s⁻¹ is reasonable to exclude misidentification of emission driven by nonnuclear star formation (Greenhill 2007). However, detailed geometry of the underlying accretion disks, orientation with respect to the line of sight, and sensitivity limitations may result in fewer than three detectable line complexes. In these cases, interpretation of the spectra depends critically on estimates of systemic velocity.

The spectrum of NGC 1320 is relatively symmetric about a mean of ∼2790 km s⁻¹ (Figure 1), with emission extending outward ∼170 km s⁻¹ from peaks at 2614 and 2969 km s⁻¹. However, the mean velocity is offset ∼70 km s⁻¹ from the most precise optical systemic velocity (Huchra et al. 1993), though agreement appears to be good with the estimate of Bottinelli et al. (1992). The approximate mirror symmetry and breadth of the line complexes are suggestive of 150–180 km s⁻¹ rotation velocity, though it would be twice as great if one of the detected line complexes were to correspond to systemic emission. Time monitoring might distinguish between scenarios if a third, blueshifted, variable line complex is detected or if secular drift in velocity is detected in one of the known complexes.

Classification of NGC 17 is somewhat less certain because of a spread in systemic velocity estimates that spans the separation of the two detected line complexes ∼300 km s⁻¹ (Figure 1). However, we note that the most precise estimate (Rothberg & Joseph 2006), obtained from dynamical modeling of the galaxy, corresponds well to the peak of the stronger of the two line complexes. We infer from this circumstance that NGC 17 is likely to be a disk maser, with rotation velocity ∼300 km s⁻¹. As for NGC 1320, monitoring may increase certainty should a blueshifted line complex or secular drift be detected. Nonetheless, because NGC 17 is a merger (e.g., Vorontsov-Vel'Yaminov 1977; Mazzarella & Boroson 1993; Hunt et al. 1999; Schweizer & Seitzer 2007), interpretation of the spectrum requires some caution. The velocity offset between the two spectral features is comparable to the stellar velocity dispersion, σ_○ = 201 ± 8 km s⁻¹ (Rothberg & Joseph 2006), and the maser lines may arise in different compact structures within the system.

For IRAS 16288+3929, classification is problematic because only a single spectral feature is evident (∼8920 km s⁻¹), and it is displaced ∼200 km s⁻¹ from available optical systemic velocities (Figure 1). The emission may represent high-velocity blueshifted emission or activity unrelated to any accretion disk. An alternative origin could be jet activity driven by the central engine. We note comparable velocity offsets for known "jet masers" (NGC 1068, NGC 1052, Mrk 348, and M 51; Gallimore et al. 1996, 2001; Claussen et al. 1998; Kameno et al. 2005; Peck et al. 2003; Hagiwara 2007). On the other hand, in these cases, spectral features are characteristically at least 50 km s⁻¹ wide, at least an order of magnitude broader than the linewidth for the known maser feature in IRAS 16288+3929.

NGC 1320 and NGC 17 were targeted but not detected in previous surveys for maser emission. Braatz et al. (1996) and Kondratko et al. (2006b) reported observation of NGC 17 with 1σ noise levels of 73 and 16 mJy in 0.66 and 1.3 km s⁻¹ spectral channels, respectively. These correspond to signal-to-noise ratios (S/N) of ≲1 for the observed line flux of 11 mJy (peak) over 1.5 km s⁻¹. In the case of NGC 1320, these earlier studies obtained 1σ noise levels of 61 and 19 mJy in 0.84 and 1.3 km s⁻¹ spectral channels, respectively. The S/N for the observed line flux of 37 mJy, at 1.5 km s⁻¹ resolution, would have been less than 2.

Through deep integration, we detected new features in the spectra of masers not previously recognized to have high-velocity emission and evidence for origins in accretion disks (Figures 2–4; Table 2). Noise levels were 2–4 mJy (1σ) referenced to outside the atmosphere. We comment on individual systems below. The first three we discuss are the new disk maser candidates.

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NGC 449 (Mrk 1) (Figure 2). We detected an emission complex at ∼4860 km s⁻¹, a ∼20 mJy component at ∼4600 km s⁻¹, and a new weak line complex (7 mJy peak in a 1.5 km s⁻¹ spectral channel) at ∼4720 km s⁻¹. The complex at ∼4860 km s⁻¹ has been present in the spectrum of the source since its discovery (Braatz et al. 1996, 2003), while the component at ∼4600 km s⁻¹ was marginally detected first by Braatz et al. (2003). The newly detected emission is approximately halfway in velocity between the two previously known spectral complexes. Although the position of the maser emission with respect to the nucleus has not been measured, the spectrum shows the archetypical spectral signature of emission from a highly inclined accretion disk. In particular, considering the uncertainty on the systemic velocity of the galaxy (Figure 2; Table 2), we associate the complex near 4720 km s⁻¹ with low-velocity disk emission and infer an orbital velocity (which we take to be the maximum velocity with respect to the systemic velocity) of ∼150 km s⁻¹. It is notable that the suggested systemic velocity of the galaxy would be blueshifted by a few times 10 km s⁻¹ from optical measures and ∼100 km s⁻¹ from the estimate obtained from H i data.

NGC 2979 (Figure 2). Emission was newly detected at a level of ∼3 mJy at 2390 km s⁻¹ (cf. Greenhill et al. 2003a), which is blueshifted by ∼300 km s⁻¹ from estimates of the systemic velocity. The new spectrum is approximately centered on the optical systemic velocity estimate of 2720 ± 15 km s⁻¹ (Fisher et al. 1995), which is suggestive of emission from a highly inclined disk with ∼300 km s⁻¹ orbital velocity, though extended line wings may suggest a somewhat more rapid rotation. This interpretation is consistent with the known coincidence of the maser source position and the active nucleus (Greenhill et al. 2003a). Asymmetry in the maser spectrum is pronounced but difficult to explain without imaging data that would enable geometric modeling of the disk. However, we note that the archetypal disk maser in NGC 4258 exhibits about a 10 times imbalance in the flux densities of red- and blueshifted emission. In the new spectrum, the velocity range of emission ≳10 mJy is consistent with a weak plateau observed in the 6 times lower sensitivity discovery spectrum (Greenhill et al. 2003a).

NGC 3735 (Figure 2). We detected narrow lines at 2444, 2484, 2784, and 2896 km s⁻¹ that were not observed in previous spectra (Greenhill et al. 1997b; Braatz et al. 2003). An additional complex of weak lines may be present between 2830 and 2870 km s⁻¹. Doppler shifts from systemic velocities derived from H i data are ±200–250 km s⁻¹ (Staveley-Smith & Davies 1988). This bracketing of systemic emission and known angular coincidence of the maser and nucleus (Greenhill et al. 1997b) are suggestive of emission from a close-to edge-on accretion disk.

UGC 4203 (Mrk 1210) (Figure 3). We detected line complexes bracketing the systemic velocity, at 3755–3861 km s⁻¹ and 4190–4350 km s⁻¹ (Figure 3), as well as isolated narrow lines distributed between these, at 4106 km s⁻¹ (∼70 mJy), 4134 km s⁻¹, and 3992 km s⁻¹ (5–10 mJy). The peaks of the red- and blueshifted complexes were detected with lower S/N previously (Braatz et al. 1996, 2003), though the symmetry of the spectrum around the systemic velocity (Table 2) and suggestion of emission from an inclined disk were less pronounced. The inferred orbital velocity is ∼300 km s⁻¹, depending on the adopted systemic velocity.

Arp 299 (NGC 3690) (Figure 3). The spectrum contains a broad feature with FWHM ∼230 km s⁻¹ and overlying narrow Doppler components that were not evident in the previous spectra of the source obtained with 4.3 km s⁻¹ channels (Henkel et al. 2005). Maser emission is known to arise from three regions in this merging system, the nuclei of NGC 3690 and IC 694 and an internuclear radio continuum "hotspot" (Tarchi et al. 2007). Both nuclear maser sources are ∼200 km s⁻¹ wide, which is notable because large widths have been associated at least circumstantially with jet activity (see Tarchi et al. 2007), as exemplified in three AGNs where maser and radio continuum emission have been mapped relative to one another interferometrically: NGC 1052, NGC 1068, and Mrk 348 (Claussen et al. 1998; Gallimore et al. 2001; Peck et al. 2003; Kameno et al. 2005, also review by Greenhill 2007). Notable time variability has been observed toward these masers, and position-resolved monitoring of Arp 299 may offer useful diagnostics.

NGC 4293 (Figure 3). The spectrum exhibits emission distributed over ∼290 km s⁻¹ but that unevenly brackets the most precise estimate of systemic velocity (di Nella et al. 1995). An additional emission feature at ∼1200 km s⁻¹ may be present at the ∼3σ level. The case for origin in a disk is marginal due to uncertainty in the full velocity range of emission and the degree of symmetry.

NGC 0235A (Figure 4). Dominated by a single spectral feature redshifted by ∼300 km s⁻¹ from the systemic velocity, interpretation of the spectrum is difficult. Goals of further monitoring should include a search for blueshifted emission.

NGC 2824 (Figure 4). As observed in the case of Arp 299, the NGC 2824 maser exhibits narrow Doppler components overlying a broad component (FWHM ∼ 150 km s⁻¹). This is consistent with the discovery spectrum (Greenhill et al. 2003a), though the newly recognized zero-intensity width is substantially broadened, ∼450 km s⁻¹. The maser emission is associated in position with the AGN (Greenhill et al. 2003a), and the spectrum is suggestive of jet activity, but the interpretation is uncertain.

IRASF 19370 − 0131 (Figure 4). Observed emission is limited to a range ∼50 km s⁻¹ centered on the systemic velocity estimated by Strauss et al. (1992). Detection of secular drift in feature velocities, as well as emission over a broader range, would be useful goals for future monitoring.

Deep integrations were also used in monitoring three masers reported by Kondratko et al. (2006a), which display among the largest Doppler shifts for high-velocity emission: CG 211, NGC 6264, and VV 340A (Table 3; Figure 5). We detected secular drifts in low-velocity features, which probably reflect centripetal acceleration due to disk rotation (Table 4; Figures 6 and 7). For each source, we used an iterative least-squares technique, adopted from Humphreys et al. (2008), to decompose the time-series spectra into Gaussian components, solving simultaneously for the light curve, time-varying line width, velocity at a reference epoch, and velocity drift of each component. (Each component was assumed to have its own fixed drift rate.) We constrained the widths of most Gaussian components to be <5 km s⁻¹, motivated by empirical observation. A first-order polynomial and a wide (∼25 km s⁻¹) stationary Gaussian component were used to model the broad low-velocity plateaus in VV 340A and CG 211, respectively. The fitted accelerations were not sensitive to inclusion of the broad components, though reduced χ² estimates were improved. We obtained drifts of 0.1–1.9 km s⁻¹ yr⁻¹ with uncertainties of 0.005–0.09 km s⁻¹ yr⁻¹, 1.2–2.1 km s⁻¹ yr⁻¹ with uncertainties of 0.1–0.6 km s⁻¹ yr⁻¹, and 3.8 ± 0.1 km s⁻¹ yr⁻¹ for CG 211, NGC 6264, and VV 340A, respectively (Table 4).

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Mean measured centripetal acceleration, a, combined with mean orbital velocity inferred from single-dish spectra, v, can be used to estimate central-engine gravitational masses and accretion disk radii. The mean acceleration among low-velocity features may be expressed as a_l = v²_l/R_l, where v_l is an effective mean rotational velocity and R_l is the effective mean radius, both for material contributing low-velocity emission. The mean rotational velocity inferred from spectra may be expressed as v²_h = GM/R_h, where G is the gravitational constant, M is black hole mass, and R_h is the effect mean radius for material contributing high-velocity emission. For edge-on orientation, we obtain M = v²_hv²_l(R_h/R_l)/Ga_l. Unfortunately, without VLBI, we cannot determine R_h and R_l independently or, therefore, v_h and v_l independently. Hence, for the purpose of calculation, we assume that the low- and high-velocity emission arise at the same effective mean radii (R_h ∼ R_l ≡ R) and that the effective mean rotation velocity of accelerating low-velocity material is the same as the measured mean rotational velocity, v_h ∼ v_l ≡ v. We then obtain M = v⁴/Ga_l and R = v²/a. These values are reported in Table 5. VV 340A may have the largest central mass among known maser galaxies.

We note that the scatter in the velocity drift measurements for CG 211 is substantially larger than the individual formal uncertainties. Since a = GM/r² for low-velocity material and hence Δa/a = −2 Δr/r, the scatter in the measured accelerations of Δa/a ∼ 1.0 might be due to low-velocity emission that populates a wide range of radii within the accretion disk, i.e., Δr/r ∼ 0.5. In our simple model, the radial spread among high-velocity material might be expected to be Δv/v = 0.5 Δr/r ∼ 0.25. The observed value of Δv/v in the high-velocity components is about 0.3 (see Figure 5). This suggests that the radial spread is about the same for both high- and low-velocity masers. For NGC 6264, Δa/a ∼ 0.4, so Δr/r ∼ 0.2, and Δv/v for the high-velocity masers is expected to be about 0.1. However, inspection of the spectrum (Figure 5) suggests that Δv/v ∼ 0.5. Hence, the high-velocity masers probably occupy a larger range of radii than the low-velocity masers. Note that the case of NGC 6264 may be similar to that of NGC 4258, where the radial range of the high-velocity features is also greater than that of the low-velocity features (Herrnstein et al. 2005).

In each case where we report centripetal acceleration, it has been positive, i.e., spectral components drift toward larger velocities. This is consistent with the emission being from material on the near side of the disks. Material behind a dynamical center, which would exhibit negative centripetal accelerations, has not been detected toward any known disk maser or disk maser candidate. Although the relatively low S/N achieved for CG 211, NGC 6264, and VV 340A would make recognition of "back-side" emission relatively difficult, we can exclude the possibility that flux densities are equivalent to the front-side emission. It is difficult to know whether the apparent absence of back-side emission may be due to a maser radiation propagation effect (e.g., competitive gain diminishes inward-directed beams) or differential absorption (e.g., inward-directed beams pass through regions of enhanced scattering or absorption in the vicinity of a central engine; inward-directed beams cross the full breadth of a disk and are attenuated by high emission-measure material at large disk scale heights, as in Herrnstein et al. 1996).

The three masers in Table 5 are priority targets for interferometric programs that are intended to estimate geometric distances (e.g., Herrnstein et al. 1999) and (in combination with distances for other maser targets) a combined Hubble constant, H₀. For a sample of N anchor galaxies, uncertainty in H₀ scales as ∼N^−0.5 (for uniform uncertainties) because errors for individual distance measurements are largely uncorrelated; they depend principally on the distribution of Doppler components across the disk face and details of disk geometries, which differ from galaxy to galaxy.

To provide substantive independent constraint on additional cosmological parameters, such as a time-variable equation of state (EOS) for dark energy and curvature, estimates of H₀ need to achieve on the order of 1% uncertainty, matching the growing accuracy of other measurements, e.g., cosmic microwave background (CMB) fluctuations and baryon acoustic oscillations (Hu 2005; Ichikawa & Takahashi 2008; Olling 2007; Greenhill et al. 2009, and references therein). In combination with CMB data alone, a 1% estimate of H₀ would enable a 5% constraint on a presumed-constant EOS, for a flat universe (Figure 14, Riess et al. 2009). A sample on the order of 100 disk masers would be required to achieve 1% accuracy in H₀, if each distance is accurate to 10%. At present, this is at least an order of magnitude beyond the number of disk masers apparently suitable for distance measurement. However, a three-year survey of 3000 galaxies at the GBT, achieving rms noise ∼3 mJy over 1 km s⁻¹ (4 sources per hour for ∼750^h), would result in the detection of 150 masers, assuming a conservative detection rate of one-half of that reported by Kondratko et al. (2006a) for AGNs at 10,000–15,000 km s⁻¹. For a disk maser fraction of one-third, and a further one-quarter that appear suitable for distance measurement (as suggested in the currently known population), it would be possible to assemble a sample on the order of 25 objects. For 10% distance uncertainty per maser, this would enable estimation of H₀ with 2% accuracy in the near term. Prospects are improved by better individual distances as may be achievable with the VSOP-2 space-VLBI mission and larger target samples made accessible by the Square Kilometer Array (Greenhill 2004; Morganti et al. 2004).

However, peculiar radial motions of galaxies increase scatter in the Hubble relation and introduce partial correlation among recessional velocities. The estimate of the Hubble constant from any particular galaxy measurement is D/v. The sample size needed to achieve a given total uncertainty grows as the quadrature sum of ΔD/D and fractional peculiar motion, v_pec/v_sys, so that the uncertainty in a sample of N galaxies is ΔH₀/H₀ ∼ [(v_pec/v_sys)² + (ΔD/D)²]^1/2N^−1/2. If ΔD/D = 0.1, and we require that the uncertainty in peculiar motion raises the error by less than 10%, then v_pec/v_sys < 0.05. Outside of clusters, v_pec ≲ 500 km s⁻¹ is typical (Springob et al. 2007). We take this to be an rms value and infer that v_sys ≳ 10⁴ km s⁻¹ is needed. For maser hosts inside rich clusters, barycenter velocities can be estimated with uncertainty <200 km s⁻¹ (e.g., Crook et al. 2007) and used in place of individual galaxy recessional velocities, though cluster peculiar motions would still deserve attention.

It would be desirable to target relatively nearby galaxies that are in "quiet" portions of the Hubble flow, to reduce statistical and nonstatistical errors (e.g., UGC 3789; Masters 2005; Reid et al. 2009). However, a broad distribution of sample members in velocity and angle on the sky is needed. Subtraction of cosmic flow models to at least partially correct for v_pec reduces the lower limit on acceptable v_sys and admits galaxies in not so quiet regions. Using Tully–Fisher distances to estimate peculiar velocities and construct a multi-attractor model inside 6000 km s⁻¹, Masters (2005) obtained a 160 ± 20 km s⁻¹ rms residual. Erdoğdu et al. (2006) reported a somewhat less direct error estimate, a scatter of 100–150 km s⁻¹ around a flow model derived from galaxy recessional velocities (as opposed to peculiar velocities), up to ∼16, 000 km s⁻¹. Adopting a characteristic 160 km s⁻¹ uncertainty, the lower limit in v_sys is 3200 km s⁻¹, for maser distances good to 10% and the nominal limit of ∼10% contribution by peculiar motions to the error budget. The technique works for maser hosts away from the Great Attractor and the galactic plane (|b|>20°; Masters 2005), where flow models are not well constrained.

In studies of the relative orientation of AGN central engines and host galaxies, radio jet position angles at parsec and kiloparsec scales are used to infer the orientations of the accretion disks that launch them. Ionization cone position angles are used to infer the orientations of larger scale, dusty, geometrically thick structures at larger radii. Comparisons of position angles suggest that accretion disks are not aligned with galactic spiral disks (Ulvestad & Wilson 1984; Nagar & Wilson 1999; Kinney et al. 2000; Schmitt et al. 2003; Middelberg et al. 2004). At the same time, the accretion disks appear reasonably well aligned with the structures that collimate ionization cones (Schmitt et al. 2003), though the sample is small.

Because interferometric mapping has demonstrated that disk maser emission marks molecular structures that are oriented close to edge-on, the emission provides a relatively precise indicator of orientation, via inclination rather than position angle, as is otherwise the case principally for radio jets and ionization cones. Thus, estimation of stellar disk inclination in spiral galaxies that host disk masers enables immediate estimation of (mis)alignment between parsec-scale and kiloparsec-scale structures. The approach complements other techniques, in terms of orientation angle (position angle versus inclination) and of radius, where the range of radii for maser emission lies outside the zone where accretion disks act to collimate radio jets. Perhaps the fullest case studies would be possible for galaxies with observable radio jets, disk maser emission, well-resolved ionization cones, accurate B-band isophotal radii (for detection of spiral arms), and a reasonably firm morphological type. Among nearby galaxies, inclinations could be obtained from detailed modeling of spectroscopic data for well-resolved stellar disks and circumnuclear molecular rings (e.g., Curran 2000).

A suitable sample for study may be compiled directly from lists of known disk maser candidate hosts that are spirals. The spectroscopic identification of masers that trace underlying disk structure is sufficient. Galactic inclinations, to first order, may be inferred directly from axial ratios, i = cos⁻¹(b/a), though detailed dynamical modeling would be preferred to obtain the most accurate values.

Assuming the spectroscopic identifications are accurate, then there is no apparent preponderance of edge-on galactic disks among disk maser candidate hosts. For inclinations <70°–80°, the distribution of orientations for the disk maser candidate matches what would be anticipated for random galaxy orientations (Figure 8). To make the comparison, we assembled a sample of axial ratios for published disk maser candidates, defined with respect to surface brightness of B = 25.0 per s⁻², for S0 and later systems (Table 6). Axial ratios were taken from the Third Reference Catalogue of Bright Galaxies (de Vaucouleurs et al. 1991).

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Table 6. Inclinations of Galaxies that have Candidate or Confirmed Disk Masers

Sourcea	T b	cos−1(b/a)c	imodd	Evidence for Disk Masere
		(°)	(°)	Spectrum Ref.	Map Ref.
NGC 4945f	6	79	86	1	1
Circinusf	3	64	69	2, 3	16
NGC 4258f	4	67	71	4	17
NGC 1386f	−0.6	68	(87)	5	18
NGC 1068f	3	32	33	6	19
NGC 3079f	7	80	85	7	20
NGC 4388	3	77	(87)	8	...
NGC 3735	5	78	(86)	9	...
NGC 1320	1	70	85	9	...
NGC 5728	1	54	60	8	...
IC 2560f	3.3	51	53	5	21
NGC$\dot{5}$793	3	70	77	10	...
NGC 3393f	1	24	26	11	22
NGC 591	0	34	37	8	...
Mrk 1419	1	46	50	12	...
NGC 6926	4	46	48	13	...
NGC 5495	4.6	37	38	11	...
NGC 6323f	2	71	81	8	23
NGC 2979	1	51	55	9	...
UGC 3789f	2	29	31	14	24
UGC 3193	2.3	68	74	14	...
UGC 4203	1	0	0	9	...
NGC 449	5	52	54	9	...
ESO 269-G012	−2	39	41	13	...
NGC 6264	...	51	...	15	...

Notes. ^aPublished disk maser candidates, in order of increasing recessional velocity. Missing isophotal sizes or merger activity exclude IRASF 22265 − 1826; CG 211, VV 340A, 3C 403, NGC 17. ^bMorphological classification. ^cInclination from axial ratio b/a. ^dModified inclination (Fouque et al. 1990). Numbers in parentheses are obtained by adding 1σ to T to obtain real inclinations. ^eReferences for identification of disk maser candidates via spectroscopy: (1) Greenhill et al. 1997c; (2) Nakai et al. 1995; (3) Greenhill et al. 1997a; (4) Nakai et al. 1993; (5) Braatz et al. 1996; (6) Claussen & Lo 1986; (7) Hagiwara et al. 2002; (8) Braatz et al. 2004; (9) this work; (10) Hagiwara et al. 1997; (11) Kondratko et al. 2006b; (12) Henkel et al. 2002; (13) Greenhill et al. 2003a; (14) Braatz & Gugliucci 2008; (15) Kondratko et al. 2006a. References for confirmation of disk maser identification via VLBI study: (16) Greenhill et al. 2003b; (17) Argon et al. 2007; (18) Braatz et al. 1997a; (19) Greenhill & Gwinn 1997; (20) Kondratko et al. 2005; (21) Ishihara et al. 2001; (22) Kondratko et al. 2008; (23) Braatz et al. 2007; (24) Reid et al. 2009. ^fDisk maser status confirmed by VLBI observations.

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The apparent undercount among high-inclination systems in Figure 8 is probably due to overestimation of disk minor axes due to galactic bulges, which is anticipated to be greatest for nearly edge-on massive systems (i.e., morphologies from S0 to Sa). Fouque et al. (1990) described an empirical correction that is dependent on galaxy morphology, quantified in the T parameter (e.g., de Vaucouleurs et al. 1991). Determination of morphologies is strongly dependent on image quality and may be affected by subjective bias (e.g., see methodology discussion of Fukugita et al. 2007). Corrected inclinations are particularly sensitive to uncertainty in T for highly inclined systems, where forbidden values of cos i may result. We estimate corrected inclinations using cataloged morphological types (de Vaucouleurs et al. 1991; Malkan et al. 1998; Lauberts & Valentjin 1989). For galaxies with forbidden values of cos i (NGC 1386, NGC 4388, NGC 3735), we increment T by 0.6, which is approximately the cataloged uncertainty for these systems. We obtain inclinations that are within a few degrees of 90°. (In light of the chosen coarse binning, the precise values are not critical.) As a result, agreement with what is expected for random galactic disk orientations is obtained over the whole range (Figure 8). For an intensively studied sample that includes well-characterized inclination uncertainties for each object, formal statistical comparison would be possible using a Monte Carlo analysis.

No correlation of misalignment with morphological type or mass is apparent. Disk maser candidate hosts span a range of almost 2 orders of magnitude in black hole mass and a broad range of apparent morphological type, from S0 (T = −2) to Sc (T ⩽ 5), excluding the merger system NGC 17. The median type is 2.7, with 1 < T < 4 for the middle quartiles (Table 6). Masses are inferred from VLBI or acceleration measurements: NGC 4258 (e.g., Miyoshi et al. 1995; Herrnstein et al. 2005), NGC 1068 (Greenhill & Gwinn 1997), Circinus (Greenhill et al. 2003a), NGC 3079 (Yamauchi et al. 2004; Kondratko et al. 2005), UGC 3789 (Reid et al. 2009), NGC 6323 (Braatz et al. 2007), IC 2560 (Ishihara et al. 2001), Mrk 1419 (Henkel et al. 2002), and CG 211, NGC 6264, and VV 340A (this work).

Comparison of galactic plane misalignments inferred for disk masers and for accretion disks at smaller radii thus far is statistical. Direct comparison would require disk masers that have been mapped with VLBI, but that sample is comparatively small. In several examples, the position angles for jets (or lobes) and ionization cones are well aligned with disk maser rotation axes, projected on the sky: NGC 3393 (Kondratko et al. 2008), NGC 4258 (Cecil et al. 2000), and NGC 4945 (Greenhill et al. 1997c). However, misalignment between a jet and disk maser has also been observed in the case of NGC 3079 (Kondratko et al. 2005), and skewing of ionization cone orientation by shadowing from a warped disk has been observed in Circinus (Greenhill et al. 2003b), both at radii ≲1 pc. Misalignment on the order of 30° is also observed in NGC 1068, between the position angle of the disk maser and the major axis of ionized material just inside the inner radius for molecular emission (Gallimore et al. 2004). If these examples are representative, counterparts may contribute to scatter in the alignment of jets and ionization cones noted by Schmitt et al. (2003) but ascribed to S/N and projection effects. Warping as observed among disk masers and larger nuclear structures (e.g., Schinnerer et al. 2000) may also be responsible, where fueling of central engines does not depend on conveyance of material along galactic planes to ever smaller radii and vertical heights until it is concentrated into a coplanar accretion disk.