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As one of the nearest and most dormant supermassive black holes (SMBHs), M31* provides a rare but promising opportunity for studying the physics of black hole accretion and feedback at the quiescent state. Previous Karl G. Jansky Very Large Array (VLA) observations with an arcsec resolution have detected M31* as a compact radio source over centimeter wavelengths, but the steep radio spectrum suggests optically-thin synchrotron radiation from an outflow driven by a hot accretion flow onto the SMBH. Aiming to probe the putative radio outflow, we have conducted milli-arcsec-resolution very long baseline interferometric (VLBI) observations of M31* in 2016, primarily at 5 GHz and combining the Very Long Baseline Array, Tianma-65m and Shanghai-25m Radio Telescopes. Despite the unprecedented simultaneous resolution and sensitivity achieved, no significant (& 3) signal is detected at the putative position of M31* given an RMS level of 5.9 μJy beam−1, thus ruling out a point-like source with a peak flux density comparable to that ( 30 μJy beam−1) measured by the VLA observations taken in 2012. We disfavor the possibility that M31* has substantially faded since 2012, in view that a 2017 VLA observation successfully detected M31* at a historically-high peak flux density ( 75 μJy beam−1 at 6 GHz). Instead, the non-detection of the VLBI observations is best interpreted as the arcsec-scale core being resolved out at the milli-arcsec-scale, suggesting an intrinsic size of M31* at 5 GHz larger than 300 times the Schwarzschild radius. Such extended radio emission may originate from a hot wind driven by the weakly accreting SMBH.
For most of their lifetime, supermassive black holes (SMBHs), commonly residing in galactic nuclei, obtain mass from the ambient gas at a rate well below the Eddington limit (Soltan 1982; Yu & Tremaine 2002; Marconi et al. 2004), which is generally thought to be mediated by a radiatively inefficient, hot accretion flow (Yuan & Narayan 2014). As such, most SMBHs in the local Universe manifest themselves as low-luminosity active galactic nuclei (LLAGNs; Ho 2008). Direct probes of these LLAGNs prove to be challenging and generally require high-resolution, high-sensitivity observations. Nevertheless, studies of LLAGNs are crucial for our comprehensive understanding of SMBH accretion and feedback over cosmic time (Fabian 2012; Kormendy & Ho 2013). It is now widely accepted that the hot accretion flow is symbiotic with outflows in the form of relativistic jets and/or hot winds (see review by Yuan & Narayan 2014). The prevalence of jets has long been established from radio interfer-ometric surveys of nearby LLAGNs, in which synchrotron cores, with or without elongated components, are frequently detected and conventionally interpreted as a highly collimated and magnetized relativistic outflow (e.g., Nagar et al. 2000; Ho & Ulvestad 2001). These jets can inject an enormous amount of mechanical energy and momentum into the environment, providing the so-called radio-mode or kinetic-mode feedback. Compelling evidence for radio-mode feed- back comes from the observation of radio bubbles (often spatially coincident with X-ray cavities) inflated by relativistic jets, which are typically found in massive elliptical galaxies, galaxy groups, and galaxy clusters (McNamara & Nulsen 2012). The hot wind, on the other hand, is a generic prediction of both theories (Blandford & Begelman 1999) and numerical simulations (Yuan et al. 2012; Narayan et al. 2012). Originating from over a wide radial range in the hot accretion flow, the wind may affect the accretion process of the black hole (Yuan & Narayan 2014). Moreover, the wind has a much larger opening angle compared to the jet, which facilitates the coupling of its momentum and kinetic energy to the ambient gas (Yuan & Narayan 2014). For these reasons, recent years have witnessed a growing interest in the hot wind as an efficient means of kinetic feedback, in addition to the conventional jet-driven feedback. In particular, in the influential cosmological simulations of galaxy formation and evolution, IllustrisTNG (Weinberger et al. 2017; Pillepich et al. 2018), a kinetic feedback mode mimicking an isotropic wind from weakly accreting SMBHs is invoked to quench star formation in intermediate- to high-mass galaxies. On the observational side, however, direct evidence for LLAGN-driven hot winds is still limited (Shi et al. 2021, 2022). Moreover, it remains unclear whether and how an efficient kinetic feedback is materialized by the most dormant SMBHs, such as the one hosted by our own Galaxy, commonly known as Sgr A*. Indeed, it remains an open question whether Sgr A* produces a jet (see Li et al. 2013; Zhu et al. 2019, and discussions therein).
In this work, we have presented VLBI observations of M31* combining the VLBA, Tianma-65m, and Shanghai-25m radio telescopes in four epochs in 2016, primarily at a central frequency of 4.9 GHz. Despite the unprecedented simultaneous resolution and sensitivity achieved, these VLBI observations do not detect the expected radio emission from M31*, which has been firmly detected by VLA observations since the 1990s. We consider two physical possibilities for this non-detection. The first possibility is that M31* experienced a temporary decrease (&40%) in its radio flux during our 2016 VLBI observations. As accretion-powered sources, LLAGNs are known to exhibit flux variation at nearly all wavelengths and on timescales from hours to years (Ho 2008). Specifically for M31*, radio variability at 8.4 GHz was first noticed by Crane et al. (1993), which was in fact taken as a strong argument for the radio emission coming from an LLAGN. Y17 found significant (up to a fractional amplitude of 70%) variability on timescales from days to months in the VLA 6 GHz observations between 2011–2012. Moreover, Y17 noticed that the mean flux density of M31* between 2011–2012 is 50% lower than that measured from historical VLA observations between 2002–2005, which was 60.0 ± 10.0 μJy beam−1 at 5 GHz (Garcia et al. 2010). Therefore, either a continued fading of M31* till 2016 or a sudden drop of flux in 2016 might explain the VLBI non-detection. However, both situations seem rather unlikely, in view of the 2017 VLA detection of M31* at 6.0 GHz, which exhibited a flux density comparable to its highest level recorded during 2002–2005, although the lack of sensitive 6.0 GHz observations between 2013–2016 prevents us from completely ruling out these two possibilities. A related possibility is short-term (i.e., daily) variability. The most significant such variability was recorded around Dec. 30th, 2012. On that day, the 6 GHz flux density of M31* was only 27.3 ± 3.7 μJy, having decreased by 60% compared to the flux density of 65.9 ± 5.1 μJy observed a week before (Y17). However, it is rather unlikely that all four epochs
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