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A&A
Volume 687, July 2024
Article Number A193
Number of page(s) 12
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/202450206
Published online 09 July 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1. Introduction

Hydroxyl megamasers (OHMs) are bright sources of OH maser emission found in luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs), which result from interactions and mergers of gas-rich disk galaxies (Sanders & Mirabel 1996). To date, ∼150 OHMs have been discovered, primarily on the 1667 MHz main line, while the 1665 MHz main line is generally weak or absent (see Zhang et al. 2024, and reference therein). OHM emission is typically thought to be stimulated by far-infrared radiation from their surroundings and to amplify an intense background of radio continuum emission.

Meanwhile, the relationship between OH megamasers and active galactic nucleus (AGN) processes remains unclear, as the OHM emission may arise from a central AGN, be contaminated by the emission from circumnuclear star-forming regions, or occur in an intermediate stage between a starburst and the emergence of an AGN (see Hekatelyne et al. 2018a, 2020).

High-resolution observations are crucial to determining whether OHM galaxies host an AGN or compact starburst and their associations with merging stages and environmental factors (Peng et al. 2020). However, due to the low flux densities of OH megamaser emission, only a limited number of bright OHM galaxies (typically around a dozen) have been examined using very long baseline interferometry (VLBI) techniques. Early VLBI observations of OHM galaxies have unveiled rotating, dusty, molecular structures in various galaxies, such as Mrk 231 (Klöckner et al. 2003), Arp 220 (Rovilos et al. 2003), III Zw 35 (Trotter et al. 1997), IRAS 17208−0014 (Momjian et al. 2006), and IRAS 12032+1707 (Pihlström et al. 2005). These findings indicate a rotating disc or torus surrounding the nuclear supermassive black holes. Meanwhile, these high resolutions of OH emission also show additional activities including inflow/outflow, two rotating discs, or turbulent nuclei (e.g., Pihlström et al. 2005; Momjian et al. 2006; Hekatelyne et al. 2018a). Furthermore, non-circumnuclear origins for OH maser emissions have also been identified in IRAS 20100−4156 (see Gowardhan et al. 2018) and Zw049-057 (Aalto et al. 2024). We also observe that the two OH emission regions of IIzw 096 reside in a dense gas environment, prompting speculation that these OH emission regions might originate from two or more systems (see Wu et al. 2022). Thus, the velocity structure of known OHM galaxies with high-resolution observations is complex, indicating that they are experiencing different merging activities, as OHM emissions are believed to be produced by major galaxy mergers (Roberts et al. 2021). Therefore, VLBI observations of bright OH masers from diverse environments are necessary to better understand the formation of OH megamasers.

IRAS 01298−0744 exhibits a peak flux density of 118 mJy in the 1667 MHz line (see Fig. 1) and is the brightest among all OHMs detected by the Green Bank Telescope (see Willett 2012). The OHM emission peak may be related to low-velocity outflows, as it is blueshifted from the systematic velocity (see Fig. 1). The optical image of IRAS 01298−0744 reveals two prominent tidal tails, indicating a recent merger (Veilleux et al. 2002). Based on the morphology and projected nuclear separation, Yuan et al. (2010) classified it as a compact merger. Moreover, Rigopoulou et al. (1999) suggested that it has completed the merger and is evolving into a more relaxed system. Several studies provide evidence of an AGN in the nuclear region. Yuan et al. (2010) classified this source as a starburst-AGN composite galaxy based on optical emission lines. According to infrared spectroscopy and bolometric luminosity, this galaxy can be classified as a buried AGN galaxy (see Imanishi et al. 2011; Nardini et al. 2009). Furthermore, Papaefthymiou et al. (2022) classified this source as a low-ionisation nuclear emission line region (LINER). Multi-frequency Very Large Array (VLA) observations of the radio continuum emission of this source reveal a gigahertz-peaked spectrum, indicating that the radio continuum emission arises from an AGN rather than a starburst (see Hayashi et al. 2021).

thumbnail Fig. 1.

Comparison of OH line with other molecular and atomic line profiles of IRAS 01298−0744. The left panel shows the line profiles of OH and HCN, while the right panel displays the line profiles of OH and Na I D. The OH line profile obtained from GBT observations by Willett (2012) is represented by the solid line. The red dash-dotted line represents the integrated VLBA OH line profile extracted from a square region measuring about 50 × 50 mas and centred at RA:01 32 21.415 Dec:−07 29 08.347, which has been smoothed to about 20 km s−1. The three downward arrows represent the three peaks of the OH emission profile. The HCN line profiles obtained from ALMA observations (project: 2017.1.00057.S) are depicted by the dashed black line, which was generated in a circular region (radius ∼150 mas) and has been smoothed to 45 km s−1. The upward arrow corresponds to the optical system velocity c * z (z = 0.1362; Soto & Martin 2012). The normalised Na I D spectra from Rupke et al. (2005) are represented by the black dotted line in the right panel. The y-axis of the OH lines is on the left, while the y-axis of the other lines is on the right in both panels.

The optical redshift of IRAS 01298−0744 is 0.1362 (Soto & Martin 2012). At the distance of this galaxy, 1 mas corresponds to approximately 2.3 pc, assuming H0 = 73 km s−1 Mpc−1, ΩM = 0.27, and ΩΛ = 0.73. The primary objective of this paper is to investigate the high-resolution properties of OH line and radio continuum emission from IRAS 01298−0744, and better understand the relationship between OHM emission and the nuclear region’s environment, including the merging phase, the presence of a radio AGN, dense gas, and possible low-velocity outflows. The observations and data reduction methods are detailed in Sect. 2, and the results and discussions are presented in Sects. 3 and 4. We summarise our results in Sect. 5.

2. Observations and data reduction

We conducted continuum and OH line observations of IRAS 01298−0744 using the Very Long Baseline Array (VLBA) for eight hours on August 2, 2022. Nine VLBA antennas participated in the observation, with the exception of the Kitt Peak antenna. The pointing position was from VLA images in the U band in the A configuration (Nagar et al. 2003), and we found that this position was within 10 mas of our L-band VLBA observation. We observed with two sub-bands centred at 1440.250 and 1696.172 MHz, with each sub-band having a bandwidth of 128 MHz. We employed J0127−0821 as the phase-referencing calibrator, situated at a separation of approximately 1.53 deg from the target. The observation durations for the phase-calibrator and target are roughly one minute and three minutes, respectively, in each cycle. To resolve the spectral line, a zoomed-in 16 MHz band with 1000 evenly spaced channels from the first sub-band centred at 1467.5 MHz, which corresponds to the redshifted frequency of the OH 1667 MHz line, was utilised. The remaining bandwidth of the first sub-band and the full bandwidth of the second sub-band were used for radio-continuum studies (see Table 1 for additional information on this observation).

Table 1.

Parameters of high-resolution observations.

To calibrate the VLBA data, we utilised the NRAO Astronomical Image Processing System (AIPS) package. The data-reduction process involved several steps, including ionospheric correction, amplitude calibration, editing, bandpass calibration, instrumental phase corrections, antenna-based fringe fitting of the phase calibrator, and application of the solutions to the target source. We also set the velocity using the heliocentric system with the optical definition for velocity. The calibrated data were imported into the DIFMAP package (Shepherd 1997) to generate continuum- and spectral-line images. Despite the weak radio-continuum emission compared to the OH line emission, we also subtracted the continuum emission using a CLEANed map of the continuum emission. Additionally, we utilised the ‘Spectral Profile’ tool in the Common Astronomy Software Application package (CASA; McMullin et al. 2007) to generate and fit OH line profiles at selected pixels.

In addition to the VLBA observations, we also analysed the sub-millimetre radio continuum and line emission of this source from the archival Atacama Large Millimeter Array (ALMA) project 2017.1.00057.S (PI M. Imanishi). The basic parameters of this project can be found in Table 1. To analyse the data of the sub-millimetre HCN and HCO+ lines, we initially restored the data calibrated by the pipeline and binned the channels to 14 km s−1 using the CASA package. These data were reimaged following the parameters outlined by Imanishi et al. (2019), specifically employing Briggs weighting with robust = 0.5 and gain = 0.1 settings. The two sub-millimetre emission lines exhibited similar properties, including the peak velocity, line width, and emission regions, which were consistent with the results reported by Imanishi et al. (2019). For our examination of the sub-millimetre radio-continuum emission, we utilised the primary-beam-corrected image file of Project 2017.1.00057.S, which is readily accessible in the ALMA archive.

3. Results

We generated the OH line profiles by integrating the flux densities over an area of approximately 50 mas squared in each channel image (see Figs. 1 and 2). The VLBA OH line profile displays multiple strong and narrow components, as well as broad, shallow OH emission spanning more than 1000 km s−1, similar to what is observed in the single-dish Green Bank Telescope (GBT) line profile. Moreover, the VLBA observation captures over 82% of the integrated flux densities across the velocity ranges presented in the single-dish GBT profile (see Fig. 1). We overlaid the OH emission onto an optical R-band Sloan Digital Sky Survey (SDSS) image and compared it to the HCN emission and sub-millimetre radio-continuum images (see Fig. 3). We can see that the OH emission spans a region of < 50 mas and is likely centred on the central region of the galaxy.

thumbnail Fig. 2.

OH line emission maps of IRAS 01298−0744 from VLBA observation. The top left image shows the integrated channel image of OH emission encompassing velocities from 40 554 to 40 937 km s−1. The other three images depict channel images at three peak velocities, each with a velocity width of approximately 3.8 km s−1, highlighting the brightest components A, B, C, and D. Their respective coordinates are as follows: A: 01 32 21.415, −07 29 08.346; B: 01 32 21.414, −07 29 08.336; C: 01 32 21.414, −07 29 08.348; D: 01 32 21.415, and −07 29 08.344. The red and black contours represent the OH line emission (40 554–40 937 km s−1), with contour levels of 0.72 mJy beam−1 × (1, 2, 4, 8), and the radio continuum emission, with contour levels of 0.06 mJy beam−1 × (1, 2), respectively. The colour bar on the right of each figure shows the OH line flux densities at each pixel in Jy beam−1. The synthesised beam of each channel image is shown in the bottom left corner.

thumbnail Fig. 3.

OH emission overlaid on multi-band images of IRAS 01298−0744. Left panel: blue contours indicate the Very Large Array Sky Survey (VLASS, Lacy et al. 2020) continuum emission (with contour levels of 0.33 mJy beam−1 × (1, 2, 4)) superposed on an SDSS R-band grey image. Right panel: dashed lines show continuum emission from ALMA observations at 235 GHz, with contour levels of 0.1 mJy beam−1 × (1,2,4), while the solid contours represent HCN line emission (J = 3 − 2) with levels of 0.23 mJy beam−1 × (1, 2, 4, 8, 16). As the OH emission region is relatively small when compared to the two images, the red star in both images denotes the location of the OH emission, identified by the coordinates RA: 01 32 21.415 and Dec: −07 29 08.347.

The emission of the OH line presents four distinct compact components, designated as A, B, C, and D, as depicted in Fig. 2. We employed Gaussian fitting for these components and determined that their sizes are smaller than the beam FWHM, as summarised in Table 1. Additionally, we selected a square region measuring 50 mas in size to encompass the broader OH emission. Within this region, we extracted spectra for each pixel, allowing the creation of a velocity field for OH emission (see Fig. 4). This figure clearly illustrates the presence of higher velocities, approximately within the range of 40–100 km s−1, in the western portion of the OH emission region. In particular, multiple peaks were observed in the OH lines, both within the D region and between components C and B. The OH spectra of these components, as well as eight additional locations labelled OH1-8, are shown in Figs. 5 and A.2. We performed line profile fitting for the four primary components (A, B, C, and D), as well as for OH5, which exhibited multiple components. The results of these fits are documented in Table 2. In particular, Component A displayed the highest emission intensity, featuring five peaks, including two strong peaks and three weaker ones. The other components also displayed multiple peaks. For OH1-8, with the exception of OH5, we used single-peak fitting, and the results are outlined in Table A.1.

thumbnail Fig. 4.

Velocity (left panel) and FWHM line-width distributions (right panel) of OHM emission from our VLBA observation. A, B, C, and D represent the locations of four bright OH components. The numbers 1–8 represent the example pixel from which we extracted OH line profiles. The ellipse at the bottom left of each panel represents the synthesised beam of each channel image. The contours indicate the OH emission channel image at V = 40709.3 km s−1 at levels of 0.005 Jy beam−1*(1,2,4,8...). The blue spots indicate where the extracted spectra show two or more peaks, roughly distributed in regions around component D and between B and C.

Table 2.

OH line spectrum of the components in IRAS 01298-07.

Our VLBA observation has detected radio-continuum emission with a signal-to-noise ratio (S/N) greater than nine, as illustrated in Figs. 3 and 2. Its peak brightness reached around 0.19 mJy beam−1. To measure the total continuum emission, we performed a direct fit of calibrated visibilities using an elliptical Gaussian model to mitigate potential deconvolution effects (Martí-Vidal et al. 2014). After obtaining the clean image, we further integrated over a 50 mas region to determine the radio-continuum flux. Both techniques provided a flux density close to 2 mJy. OH regions B and C lie near the continuum emission, but regions D and A are offset by roughly 22 mas (see Fig. 2). Generally, the uncertainties of the offset between continuum emission and line emission derived from the same data are only limited by the S/Ns with equation (Tarchi et al. 2011): σ rel = ( θ line / ( 2 ∗ SNR line ) ) 2 + ( θ cont / ( 2 ∗ SNR cont ) ) 2 $ \sigma_{\mathrm{rel}} = \sqrt{(\theta_{\mathrm{line}}/(\mathrm{2*SNR}_{\mathrm{line}}))^2+(\theta_{\mathrm{cont}}/(2*\mathrm{SNR}_{\mathrm{cont}}))^2} $, where θ denotes the beam FWHM of the map, and line and cont refer to line and continuum emission. Based on this equation, we estimate that the error σrel on the relative positions between component A/D and the continuum emission is about 0.8 mas, which supports the existence of apparent offset between the peak OHM emission and radio-continuum emission in this source.

4. Discussion

Our VLBA observations precisely identified the locations of compact OH maser sources in IRAS 01298−0744. These sources are situated close to the galaxy’s centre, as observed in the R-band SDSS image (see Fig. 3). The OH line emission is primarily concentrated in a region less than 50 mas in size, equivalent to about 116 pc. Within this area, we detected four distinct bright components and additional diffuse OH emission enveloping them.

4.1. The OH line profile and different OH components

In Fig. 1, the OH line profiles present three discernible peaks. Peaks 1 and 2 are the brightest, with central velocities of approximately 40 720 km s−1 and 40 760 km s−1, respectively. On the other hand, Peak 3 stands alone with a velocity around 40 870 km s−1. The profiles of the OH line at points A, B, C, and D are depicted in Fig. 5, along with the Gaussian components described in Table 2. In particular, Component A features its two most radiant line elements, A2 and A3. Components B and C both present a narrow line feature (B1 and C1 respectively, as indicated in Table 2) and a broader characteristic (B2 and C2, respectively). Component D unveils four line characteristics, with D4 being its most prominent narrow-line element. Furthermore, two specific areas, the region between C and B and the region D itself, display OH line profiles with two or more evident peaks, as shown in Fig. 4.

thumbnail Fig. 5.

OH emission lines extracted at four bright compact spots of IRAS 01298−0744 (see Fig. 4). The OH line profiles are fitted with two or more Gaussian components, and the upper spectrum of each panel shows the residual spectrum (data-model), which are plotted with an offset (+44 mJy) for clarity.

Based on refractive interstellar scintillation models, Darling & Giovanelli (2002) suggested that the size scales of narrow OH line features should be less than 2 pc if they exhibit variability and a few parsecs if they remain quiescent. By comparing the peaks in the single-dish profile (peaks 1–3 in Fig. 1) with the line profiles derived from different pixels in the VLBA image (see Figs. 5 and A.2), our results suggest that the narrow-line features observed in the single-dish OH spectrum may not originate from a single bright component. Instead, they may result from multiple compact components and may also include contributions from diffuse OH emissions. Moreover, the broad line feature is present in all four regions and their surroundings (see Fig. A.2). These broad line profiles at each pixel might mean that the beam size is not enough to resolve the OH clouds at a particular velocity. An alternate scheme is that the clouds of similar velocities cover a broad area. In that case, the multiple OH lines and the broad OH line profiles at various spots may be due to the overlapping of OH clouds at different velocities.

Generally, the brightness in a VLBI image is derived from measurements of antenna gains and system temperatures. These measurements carry uncertainties ranging from about ±10% to 15% (see ‘uncertainties in VLBI data’ in the EVN data reduction guide1). In Fig. 1, the OH line profile from our VLBA observation aligns fairly well with the GBT measurement. However, the VLBA spectrum exhibits values that are approximately 20% lower at the flux-density peaks. This discrepancy might be attributed to calibration uncertainties. Yet, it is also feasible that the OH emission has been slightly resolved in the VLBA channel images.

In addition to the distinct, narrow line peaks, the GBT OH line profile also exhibits a broad, shallow feature that spans over 1000 km s−1. The OH line profile from our VLBA observation aligns with the GBT profile throughout the velocity range. Our high-resolution findings indicate that the expansive OH line profiles in this source arise from a masing region, characterised by a confidence level of 3σ within a rectangular area that approximately measures 40 × 30 mas, as deduced from Fig. 4. This equates to an area of approximately 92 × 69 pc, which aligns with the idea that OH maser emissions are primarily localised within a radius of 100 pc in the nucleus (see Hess et al. 2021, and reference therein).

4.2. Comparing OH with sub-millimetre line emission

Generally, OHM formation appears to be related to dense molecular gas, as evidenced by dense gas tracers such as HCN and HCO+ (see Darling 2007; Huang et al. 2018). HCN and HCO+ line emissions from IRAS 01298−0744 have been detected by Imanishi et al. (2019). The full widths at half-maximum (FWHMs) of the two lines are 621 and 634 km s−1, respectively, making this galaxy among the broadest in a selected sample of 26 nearby ULIRGs at z < 0.15 in Imanishi et al. (2019).

Our VLBA observation shows that both OH-line and radio-continuum emissions coincide with dense HCN emission (see Fig. 3). However, a significant portion of the OH emission occurs at considerably lower velocities compared to the molecular line emission. The HCN and HCO+ lines likely contain a minor blueshifted component, while the OH emission is consistent with this dense gas component (see Figs. 1 and A.1). Therefore, the OH emission may share a similar origin with the blueshifted dense-gas component, possibly originating in a different region or displaying distinct kinetic properties in comparison to the dominant dense gases.

4.3. The velocity structure and merging activities

Generally, OHMs are products of gas-rich major mergers (see Roberts et al. 2021). The optical image of IRAS 01298−0744 shows two prominent tidal tails, which are indicative of a recent merger (Veilleux et al. 2002; Yuan et al. 2010). Optical emission lines suggest rotation in the central and northern regions of the disc (Soto & Martin 2012). Furthermore, Rigopoulou et al. (1999) speculated that this source has completed the merger and is gradually evolving into a more relaxed system.

We find that the OH emission is predominantly concentrated towards the central region of the nucleus, potentially slightly offset to the eastern portion of the nucleus (see Fig. 3). Additionally, the velocity structure of the OH emission in this source is depicted in Fig. 4. Notably, both the western section of the nucleus and the edge of its eastern section display marginally higher velocities, with the most luminous region around point A exhibiting the bluest peak velocity. The velocity maps derived from the HCN and HCO+ lines, peaking at velocities 200 km s−1 higher than the OH line (ranging from 40 900 to 41 100 km s−1), have been presented by Imanishi et al. (2019). These maps reveal that the edge of the nucleus’s western part possesses the highest peak velocity. Moreover, there is no clearly ordered velocity field (see Fig. 12 in Imanishi et al. 2019). Therefore, the velocity structures derived from the OH, HCN, and HCO+ lines appear to be intricate, different from the ordered velocity structures identified in other OHM galaxies in the existing literature (for more details, see Pihlström 2007, and the references therein). In conclusion, our findings suggest that the central nuclear region of this galaxy might still be experiencing a merging phase characterized by intense nuclear activities, such as outflows or inflows. This phase has yet to reach its final relaxed state.

4.4. The nature of the continuum emission and association of the OH line

Multi-frequency VLA observations of IRAS 01298−0744 display typical non-thermal emission features that are characteristic of AGNs, including a gigahertz-peaked spectrum steepening towards higher frequencies (see Hayashi et al. 2021). Our VLBA observation detected radio-continuum emission with a peak flux density of 0.19 mJy beam−1 and a total flux density of approximately 2 mJy (see Sect. 3). This accounts for only half of the total flux densities observed from the NRAO VLA Sky Survey (NVSS; Condon et al. 1998). The radio-continuum emission (> 3σ) is confined to a region with a diameter of less than 30 mas. Similarly, this emission fits a component size of roughly 50 × 20 mas with a position angle of −30°. The OH emission appears to be distributed within a boxy region measuring approximately 40 × 30 mas. Given that only half of the radio-continuum emission was detected in our VLBA observation, and over 80% of the OH line emission was captured, it is plausible that the radio-continuum emission is less compact than the OH emission, possibly spanning larger scales than the OHM emission.

The estimated brightness temperature of the radio continuum emission (∼3 × 106 K) is consistent with clustered supernova remnants and/or luminous radio supernovae, supporting a starburst origin for the emission (see Momjian et al. 2006; Pihlström et al. 2005, and references therein). We also tried to find compact radio-continuum emission from the archival VLBA project BH237D (see Table 1). The dirty images from the VLBA observations at S and X bands are presented in Fig. A.3. As the S-band VLBA radio-continuum emission could be comparable to our L-band VLBA observation (approximately 0.2 mJy beam−1), the non-detection in the S-band image is likely due to the high noise level (0.19 mJy beam−1). The X-band image likely shows a feature at the 4σ level with a peak of about 0.1 mJy beam−1 (the prominent feature near the western edge of the VLBA L-band radio contour, see the right panel of Fig. A.3); we estimate that the brightness temperature is about 8.7 × 105 K, which is consistent with the results from our L-band VLBA observation. These results support the starburst origin of OHMs.

Generally, the radio emission from (U)LIRGs at low frequencies may experience significant optical depth effects due to free-free absorption and/or synchrotron self-absorption (e.g., Leroy et al. 2011; Hayashi et al. 2021). Consequently, the observed brightness temperature at 1.5 GHz might be underestimated, representing only a lower limit. Additionally, the characteristics of radio AGN cores and jet components in nearby (U)LIRGs are likely compact, with physical scales of several parsecs, or even sub-parsec scales (e.g., Lonsdale et al. 2003; Pérez-Torres et al. 2010; Romero-Cañizales et al. 2017). These scales might correspond to less than 4 mas for IRAS 01298−0744. Although VLBA X-band observations have suitable resolution to identify potential compact features, the S/N of the compact feature is low (approximately 4), which may also stem from calibration errors (see the right panel of Fig. A.3). Thus, further high-sensitivity and high-resolution VLBI observations of the radio-continuum emission are essential to conclusively confirm or reject the existence of the compact component as a plausible radio core or jet component associated with the radio AGN in IRAS 01298−0744.

4.5. The possible physical scheme of OH emission in IRAS 01298−0744

4.5.1. The outflowing activities and OHM emission

Generally, OH line profiles often exhibit multiple components that can be attributed to a combination of gas-rotation, inflow, and outflow processes (see Pihlström et al. 2001; Momjian et al. 2006). Alternatively, they may be primarily related to outflow and inflow activities near the central AGN (see Gowardhan et al. 2018; Hekatelyne et al. 2018a). Our results suggest that the dominant OH emission in IRAS 01298−0744 may originate from outflows, based on the following evidence. First, the peak OH emission is blueshifted compared to the systemic velocity, inferred from optical and dense gas tracers. Also, the velocity range of the brightest OH line components is similar to the blueshifted component seen in dense gas tracers (see Figs. 1 and A.1). Second, Rupke et al. (2005) identified a broad and blueshifted Na I D interstellar absorption feature in IRAS 01298−0744, which is likely attributed to the presence of a superwind in this infrared-luminous galaxy. The Na I D absorption feature is indicative of neutral gas, and its presence is believed to unambiguously indicate the outflow of gas (see Rupke et al. 2005). Given that the OH velocities align with the Na I D absorption feature (as depicted in Fig. 1), it suggests that the OH emission might share a common origin with the Na I D absorption feature. This supports the scheme that the OH gas is also moving outward from the central region.

The third factor is that Imanishi et al. (2019) found that there are regions with enhanced HCN-to-HCO+ flux ratios near the western nucleus of IRAS 01298−0744, which may be due to shocks related to the outflow. We find a broad component of OH emission across all bright OH regions, and this component has an FWHM of 200 km s−1 (see Fig. 5 and Table 2), consistent with the shock component from optical line profiles of the nuclear region (see Soto & Martin 2012), which displays an FWHM of 177 km s−1. In addition to the shock component, the optical line emission also shows a broad line feature with an FWHM of about 447 km s−1, and the full velocity ranges are more than 1000 km s−1, which is similar to the OH, HCO+, and HCN line profiles present in Figs. 1 and A.1. Soto & Martin (2012) attribute the broad profiles to H II emission or shocks.

4.5.2. Comparison with the knowledge of OHM emission in the literature

VLBI observations of about a dozen bright OHM galaxies have revealed that the OHM maser emission regions are generally compact, typically ∼100 pc in their extent (see Hess et al. 2021; Wu et al. 2023, and reference therein). Two different types of OH emission (diffuse and compact emission) are commonly observed. Our high-resolution VLBA observations of IRAS 01298−0744 show that the OH maser emission is also spread over a nearly 100 pc region centred on the nucleus. Although a small fraction of the emission might be diffuse, most are found in several compact components < 6.8 × 2.5 mas (1 mas ∼ 2.3 pc) in size (see Sect. 3). These properties of OH emission agree well with those of other OHMs with VLBI observations in the literature.

The prevailing model for OH megamasers is that they are pumped by far-infrared radiation and are low-gain unsaturated amplifiers of the background radio continuum (Baan 1985). This model can generally account for diffuse OH megamaser emission, and the compact OH maser can be further explained by a refined standard model with clumpy-ring maser geometry, such that compact OH masers are caused by an overlapping effect along the line of sight (Parra et al. 2005; Lockett & Elitzur 2008). The observational findings indicate that IRAS 01298−0744 may exhibit shocks, with the bright compact OH emission possibly stemming from the outflowing gas (see Sect. 4.5.1). Typically, galaxies hosting outflowing sources tend to exhibit a more face-on orientation compared to others (see Heckman et al. 2000). As a result, the face-on alignment of the disc/torus in the central region could lead to a lack of prominent high optical depth tangent points within the inclined disc/torus (see Lockett & Elitzur 2008), potentially explaining the absence of bright OH emission associated with systemic velocity in IRAS 01298−0744.

Based on the velocity structure of IRAS 01298−0744, we propose a simplified scheme regarding the OHM emission in IRAS 01298−0744. It is suggested that merging or outflow activities have generated clouds with varying velocities distributed within a region approximately 100 pc in size. These clouds become mixed together, leading to the observation of broad, shallow-line profiles consistent with dense gas tracers. Meanwhile, the narrow, bright, compact OH maser emission may arise from denser gas clouds along our line of sight, either due to a longer gain path or the overlapping of denser gas clouds that are slowly moving along our line of sight. Thus, while the OH line emission is associated with dense gas, the brightest OH emission may not only be correlated with dense gas emission, but it may also be influenced by the possible overlapping geometry of the OH clouds, which can provide sufficient gain length, such as inclined discs/tori, inflows, and outflows. A hypothesis proposed that the OH megamaser traces a phase in a galaxy where the AGN is being triggered (see Hekatelyne et al. 2020, 2018a,b; Sales et al. 2019, and references therein); it is possible that a particular geometry for the formation of bright compact OHM emission is more likely, such as the inclination of the disc/torus and the inflows or outflows along our line of sight.

Approximately 80% of (U)LIRGs show no OHM activity (Roberts & Darling 2024a). There is no optical nuclear distinction between masing and non-masing (U)LIRGs (Roberts & Darling 2024a; Darling & Giovanelli 2006), and there are also no discernible differences in radio-continuum properties between masing and non-masing (U)LIRGs (Wu et al. 2023). Darling (2007) demonstrated that OHMs have significantly higher average dense gas fractions than non-masing ULIRGs, while Roberts & Darling (2024b) scrutinised this hypothesis with a sample size three times larger than the original sample and showed that it fails to differentiate between masing and non-masing ULIRGs in a diverse population. This suggests that the mechanism of OH megamaser emission and dense gas emission may not be entirely the same. Dense gas emissions must be concentrated and massive to achieve the mean density required to form an OHM in a galactic nucleus (Darling 2007). The spatial alignment of OHM emission and dense gas emission in IRAS 01298−0744 suggests a correlation between them. While compact OH megamasers may also rely on particular geometries that provide a sufficient gain path along our line of sight (Parra et al. 2005), the differing peak velocities of the OH emission and dense gas in IRAS 01298−0744 may support distinct geometries for OH megamaser emission and dense gas emission. It is noteworthy that high-resolution observations of OHM galaxies and dense gas emissions are scarce in the literature, highlighting the importance of expanding the sample size to ascertain their correlations.

5. Summary

We present our findings from the VLBA observations of IRAS 01298−0744, focusing on its radio-continuum and OH line emission. Our results are summarised as follows.

First, we detected OH emission on parsec scales, consisting of four compact components (A, B, C, and D) and diffuse emission distributed in a region smaller than 50 mas (i.e., less than 116 pc). All of these components are smaller than the observation beam. The total OH emission recovered in our VLBA observation is at least 82% of the flux density of single-dish GBT observations. The ’missing’ 18% of the single-dist flux density could be accounted for with absolute flux-scale calibration differences and/or some large-scale emission resolved by our shortest interferometer baselines. Each bright narrow peak in IRAS 01298−0744 in the single-dish OH spectrum might not only be related to one single compact feature, but it is also likely caused by an accumulation of more than one compact component, as well as diffuse components.

Second we found a slightly higher peak OH velocity at the western and eastern edges of the source. The velocity range of the dominant OH emission (from peak 1 to 2) is consistent with the deepest absorption Na I D profiles. Taking into account these characteristics, we propose that the compact OH emission in this galaxy may be indicative of a low-velocity outflow, similar to what has been observed in Zw049-057 (Aalto et al. 2024), IRAS 20100−4156 (Gowardhan et al. 2018), and IRAS F23199+023 (Hekatelyne et al. 2018a). This suggests the potential existence of a subgroup of OHM galaxies where compact OH emissions primarily stem from nuclear outflows rather than from a disc or torus-ring structure.

Third, we measure the brightness of the 1.4 GHz radio continuum emission of approximately 3 × 106 K. Our VLBA observations reveal that the total radio continuum emission is approximately 2 mJy. These results are consistent with previous observations of OHM galaxies using VLBI, which suggest a starburst origin for radio continuum emission in OHM galaxies.

Acknowledgments

We thank an anonymous referee for useful suggestions and comments that helped to improve the paper. This work is supported by NSFC grants (Grant No. 12363001,U1931203). The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under a cooperative agreement with the National Science Foundation.

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Appendix A: Additional material

thumbnail Fig. A.1.

Line profiles of OH and HCO+. The OH line profile obtained from GBT observations by Willett (2012) is represented by the solid line. The red, dash-doted line represents the integrated VLBA OH line profile extracted from a square region measuring about 50 × 50 mas centred at RA:01 32 21.415 DEC:-07 29 08.347, which has been smoothed to about 20 km s−1. The three downward arrows represent the three peaks of the OH emission profile. The HCO+ line profiles obtained from ALMA observations (project: 2017.1.00057.S) are depicted by the dashed black line, which was generated in a circular region (radius ∼ 150 mas) and has been smoothed to 45 km s−1. The upward arrow corresponds to the optical system velocity c*z (z = 0.1362 Soto & Martin (2012)). The y-axis of the OH lines is on the left, whereas the y-axis of the other lines is on the right in both panels.

thumbnail Fig. A.2.

OH emission lines extracted at various points of IRAS 01298−0744. The positions of these points are presented in Fig. 4 and the line profiles are fitted with one Gaussian component.

thumbnail Fig. A.3.

VLBA S- and X-band dirty images of IRAS 01298−0744. The colour bar on the right of each figure shows the radio-continuum flux densities in mJy beam−1. The noise levels of the two images is 0.22mJy beam−1 and 0.025mJy beam−1, respectively. The beam FWHM of the two images: 2.9 ×1.2 mas at 17°, 8.2 × 2.3 mas at −4°. The contours are for radio-continuum emission with levels of 0.06 mJy beam−1 ×(1, 2) from VLBA L-band observation (see Table 1).

Table A.1.

OH line spectrum of the components in IRAS 01298-07.

All Tables

Table 1.

Parameters of high-resolution observations.

In the text
Table 2.

OH line spectrum of the components in IRAS 01298-07.

In the text
Table A.1.

OH line spectrum of the components in IRAS 01298-07.

In the text

All Figures

thumbnail Fig. 1.

Comparison of OH line with other molecular and atomic line profiles of IRAS 01298−0744. The left panel shows the line profiles of OH and HCN, while the right panel displays the line profiles of OH and Na I D. The OH line profile obtained from GBT observations by Willett (2012) is represented by the solid line. The red dash-dotted line represents the integrated VLBA OH line profile extracted from a square region measuring about 50 × 50 mas and centred at RA:01 32 21.415 Dec:−07 29 08.347, which has been smoothed to about 20 km s−1. The three downward arrows represent the three peaks of the OH emission profile. The HCN line profiles obtained from ALMA observations (project: 2017.1.00057.S) are depicted by the dashed black line, which was generated in a circular region (radius ∼150 mas) and has been smoothed to 45 km s−1. The upward arrow corresponds to the optical system velocity c * z (z = 0.1362; Soto & Martin 2012). The normalised Na I D spectra from Rupke et al. (2005) are represented by the black dotted line in the right panel. The y-axis of the OH lines is on the left, while the y-axis of the other lines is on the right in both panels.
In the text
thumbnail Fig. 2.

OH line emission maps of IRAS 01298−0744 from VLBA observation. The top left image shows the integrated channel image of OH emission encompassing velocities from 40 554 to 40 937 km s−1. The other three images depict channel images at three peak velocities, each with a velocity width of approximately 3.8 km s−1, highlighting the brightest components A, B, C, and D. Their respective coordinates are as follows: A: 01 32 21.415, −07 29 08.346; B: 01 32 21.414, −07 29 08.336; C: 01 32 21.414, −07 29 08.348; D: 01 32 21.415, and −07 29 08.344. The red and black contours represent the OH line emission (40 554–40 937 km s−1), with contour levels of 0.72 mJy beam−1 × (1, 2, 4, 8), and the radio continuum emission, with contour levels of 0.06 mJy beam−1 × (1, 2), respectively. The colour bar on the right of each figure shows the OH line flux densities at each pixel in Jy beam−1. The synthesised beam of each channel image is shown in the bottom left corner.
In the text
thumbnail Fig. 3.

OH emission overlaid on multi-band images of IRAS 01298−0744. Left panel: blue contours indicate the Very Large Array Sky Survey (VLASS, Lacy et al. 2020) continuum emission (with contour levels of 0.33 mJy beam−1 × (1, 2, 4)) superposed on an SDSS R-band grey image. Right panel: dashed lines show continuum emission from ALMA observations at 235 GHz, with contour levels of 0.1 mJy beam−1 × (1,2,4), while the solid contours represent HCN line emission (J = 3 − 2) with levels of 0.23 mJy beam−1 × (1, 2, 4, 8, 16). As the OH emission region is relatively small when compared to the two images, the red star in both images denotes the location of the OH emission, identified by the coordinates RA: 01 32 21.415 and Dec: −07 29 08.347.
In the text
thumbnail Fig. 4.

Velocity (left panel) and FWHM line-width distributions (right panel) of OHM emission from our VLBA observation. A, B, C, and D represent the locations of four bright OH components. The numbers 1–8 represent the example pixel from which we extracted OH line profiles. The ellipse at the bottom left of each panel represents the synthesised beam of each channel image. The contours indicate the OH emission channel image at V = 40709.3 km s−1 at levels of 0.005 Jy beam−1*(1,2,4,8...). The blue spots indicate where the extracted spectra show two or more peaks, roughly distributed in regions around component D and between B and C.
In the text
thumbnail Fig. 5.

OH emission lines extracted at four bright compact spots of IRAS 01298−0744 (see Fig. 4). The OH line profiles are fitted with two or more Gaussian components, and the upper spectrum of each panel shows the residual spectrum (data-model), which are plotted with an offset (+44 mJy) for clarity.
In the text
thumbnail Fig. A.1.

Line profiles of OH and HCO+. The OH line profile obtained from GBT observations by Willett (2012) is represented by the solid line. The red, dash-doted line represents the integrated VLBA OH line profile extracted from a square region measuring about 50 × 50 mas centred at RA:01 32 21.415 DEC:-07 29 08.347, which has been smoothed to about 20 km s−1. The three downward arrows represent the three peaks of the OH emission profile. The HCO+ line profiles obtained from ALMA observations (project: 2017.1.00057.S) are depicted by the dashed black line, which was generated in a circular region (radius ∼ 150 mas) and has been smoothed to 45 km s−1. The upward arrow corresponds to the optical system velocity c*z (z = 0.1362 Soto & Martin (2012)). The y-axis of the OH lines is on the left, whereas the y-axis of the other lines is on the right in both panels.
In the text
thumbnail Fig. A.2.

OH emission lines extracted at various points of IRAS 01298−0744. The positions of these points are presented in Fig. 4 and the line profiles are fitted with one Gaussian component.
In the text
thumbnail Fig. A.3.

VLBA S- and X-band dirty images of IRAS 01298−0744. The colour bar on the right of each figure shows the radio-continuum flux densities in mJy beam−1. The noise levels of the two images is 0.22mJy beam−1 and 0.025mJy beam−1, respectively. The beam FWHM of the two images: 2.9 ×1.2 mas at 17°, 8.2 × 2.3 mas at −4°. The contours are for radio-continuum emission with levels of 0.06 mJy beam−1 ×(1, 2) from VLBA L-band observation (see Table 1).
In the text

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