# Why is the Milky Way's SMBH inactive?

Our galaxy is not an active galactic nucleus (AGN). With so many stars and so much gas orbiting it, as explained here Central kiloparsec of Seyfert and inactive host galaxies: a comparison of two-dimensional stellar and gaseous kinematics:

The ionized gas distributions show a range of low-excitation regions, such as star formation rings in Seyfert and inactive galaxies, and high-excitation regions related to photoionization by the active galactic nucleus (AGN). The stellar kinematics of all galaxies in the sample show regular rotation patterns typical of disc-like systems, with kinematic axes that are well aligned with those derived from the outer photometry and provide a reliable representation of the galactic line of nodes.

why is the MW not an AGN? In terms of the orbiting gas and stars, and apart from the obvious observational differences, what sets an inactive galaxy like the MW apart from and AGN?

Links to scientific papers would be much appreciated.

• I would like to upvote this question. But your number of points is currently 1216, which is the wavelength of my favorite photon, Lyman α, and I don't want to ruin that.
– pela
Jun 24 at 10:23
• @pela - oops, looks like some physics constants just changed, since the Lyman $\alpha$ photon just shifted to 1226. I'm not sure Jim421616 knew he controlled the universe in that way before now. Jun 24 at 12:50
• @JonCuster Oh damn, guess I have to upvote then…
– pela
Jun 24 at 12:52
• Redshift I suppose... Jun 24 at 14:12
• @ProfRob But redshift alone doesn't "set an inactive galaxy like the MW apart from an AGN", since we do have local AGNs.
– pela
Jun 25 at 10:27

I think the answer to your question is 1) because of the general smaller probability of hosting an AGN with time, and 2) stochasticity.

In general, the fraction of galaxies with active galactic nuclei (AGN) activity has decreased at least over the last 10 billion years, and is also lower for late type spirals, so the fact that the Milky Way and other local galaxies don't host AGN is not very surprising. Still, however, AGN activity in local galaxies is seen. What makes one galaxy host an AGN, and others not, is not entirely understood.

AGN require not only a supermassive black hole, but also quite a lot of gas. In order to reach the central parts of the galaxy, gas must either cool, so as to slowly settle down in the center, or it must be "forced" in more violent events, such as galactic merging.

### Galaxy mergers

Because major mergers (i.e. with roughly 1:1 mass ratios) are so efficient at removing the angular momentum of gas (e.g. Barnes 1988; D'Onghia et al. ), they usually initiate starbursts (Mihos & Hernquist 1996; Cox et al. 2008). AGN was therefore previously thought be be associated mostly with mergers, and has indeed been found to be (Canalizo & Stockton 2001; Di Matteo et al. 2005).

However, the major merger rate peaks around $$z\sim1.3$$ (e.g. Ryan et al. 2008, albeit with a broad distribution in $$z$$, whereas AGN activity peaked slightly earlier, around $$z\sim2$$ (e.g. Lan & Guinevere 2008; Mason & Biermann's chapter in Gordon & Sharov 2018). And more recent work actually shows no statistically significant difference in the fraction of mergers and non-merger with AGN activity (Gabor et al. 2009; Cisternas et al. 2011; Schawinski et al. 2011; Vilforth et al. 2017; Marian et al. 2019; Sharma et al. 2021), except perhaps for the most luminous AGN (Treister et al. 2012), especially around $$z\sim2$$ (Hewlett et al. 2017), and in post-merger epochs (Ellison et al. 2013).

### Other mechanisms

The main mechanism of powering AGN is thus not settled, I think (as is also discussed in the article you link to). Other "secular" channels are possible, such as disk instabilities (Bournaud et al. 2011), bars (Oh et al. 2000, though Hwang et al. 2011 argue that this correlation is just consequence of AGN-host galaxies being on average more massive and redder than non-AGN galaxies), and minor mergers, with mass ratios of the order of 1:10 (Kaviraj 2014).

### The Milky Way was an AGN recently

The fraction of galaxies with AGN activity increases toward $$z\simeq1\text{–}2$$, but lies around some tens of percent (Silva et al. 2021), lower in the local Universe (Martini et al. 2009), and with variation between galaxy type (late- vs. early type), environment (e.g. distance to nearest neighbor, Hwang et al. 2011), and, to some extend, mass (Argudo-Fernández et al. 2018). I think, but am not sure, that it might just be stochastic accretion of cold gas that sometimes happens to be funneled toward the central part of the galaxy (Hopkins & Hernquist 2006). If so, then the Milky should sometimes exhibit AGN activity, and in fact there is evidence that it did quite recently, just a few million years ago (Bland-Hawthorn et al. 2019).