Accretion disks around compact objects. Compact low-luminous massive objects are expected to lurk in the CGM of galaxies. From lone low-mass stars (e.g., Helmi 2020), to black holes (BHs) arising from stellar evolution (e.g., Fender et al. 2013), including remnants of Population III stars (e.g., Madau & Rees 2001; Filho & Sánchez Almeida 2018), and primordial BHs (e.g., Carr & Hawking 1974; Clesse & García-Bellido 2015). The number density of these objects is assumed to be high; for example, there are claims that primordial BHs account for all the dark matter in the universe (Clesse & García-Bellido 2015). Even if this is not the case (e.g., Carr & Kühnel 2020), the expectations clearly overwhelm the density required to account for all the emission signals that we observe, of the order of one clump per central galaxy (Section 4.1). When these compact objects are surrounded by accretion disks, they should emit in Hα, with the rotation of the disk giving rise to two-horn Hα profiles when observed with the appropriate viewing angle (Smak 1969). This kind of double-peak Hα emission is observed in X-ray binaries (Grundstrom et al. 2007; Zamanov et al. 2013; Casares & Torres 2018; Monageng et al. 2020) and cataclysmic variables (e.g., Zolotukhin & Chilingarian 2011), with peak separations of up to hundreds of km s−1 (Zamanov et al. 2013). At a completely different mass scale, double-peak Hα emission is sometimes observed in the broad-line region of AGN, where it is also supposed to trace a rotating disk (e.g., Eracleous & Halpern 1994; Ho 2008). The double-peak Hα emission observed around stellar-mass compact objects comes together with continuum emission (e.g., McSwain et al. 2010; Zamanov et al. 2019). There are hints of continuum emission in our stacks of observed spectra (Figure 8), but the question arises as to whether this very faint continuum is consistent or not with an accretion disk around a compact object. A number of arguments show that the observed lack of continuum emission does not discard the accretion disk scenario. First, large Hα equivalent widths (EWs) are sometimes observed in X-ray binaries. Hα EWs larger than 100 Å are not rare during quiescence (Fender et al. 2009; Casares & Torres 2018), with maxima reaching 2000 Å (Muñoz-Darias et al. 2016), which implies line to continuum flux ratios enough to account for our observations (Figure 8). Second, Hα photons are to be produced by recombination of H atoms photoionized by the accretion disk (e.g., Matthews et al. 2015). Under particular circumstances, the nebular emission produced by a photoionizing source can emit an Hα line with very little underlying continuum. For example, young starbursts produce Hα with an EW in excess of 103 Å (Leitherer et al. 1999), and galaxy-integrated spectra with EW of hundreds of angstrom are not uncommon (e.g., Morales-Luis et al. 2011). The main physical ingredient for the photoionization source to produce little continuum is presenting a hard spectrum with an ionization flux greatly exceeding the flux in the optical. Accretion disks can easily match this requirement since they can be extremely hot with spectra peaking in the far-UV and X-ray.
Citation: Smak 1969
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Citation: Eracleous & Halpern 1994
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