I Would we know if the Milky Way were a quasar?

[swampwiz]

AIUI, a quasar is simply a black hole at the core of a galaxy that shoots out an enormous amount of energy along the axis of circulation. Since we are far from that axis, we cannot look into the Milky Way's core from that vantage point.

 

[DaveC426913]

We would see reflected radiation from the beam slamming into all the matter beyond the poles of the galaxy.
It's not like those tens of thousand of cubic light years above the poles are devoid of reflective material.

 

[Janus]

Energy/radiation escape from a quasar is not limited to the axis. (in fact, it is estimated that only ~10% of quasars even have polar jets). Polar jets are caused by charged particles that are redirected by magnetic fields.
On the other hand, a good deal of radiation produced by quasars is caused by interaction of material in the accretion disk, and this EM radiation is emitted in all directions and not just along the axis.

 

[stefan r]

We can indirectly detect the total energy being emitted without have a direct line of sight to the source. Quasars are hundreds or thousands of times more luminous than normal giant galaxies like the Milky Way.

 

[Ken G]

Also, we know that the central black hole has a fairly small mass, so if it were a quasar, it could not have been operating for very long. Typically, quasars are associated with much more massive black holes, because they shine by having material fall toward the black hole, and that builds a very massive black hole. So although the other answers tell you why we know it's not a quasar now, the low mass tells us it was never a quasar in its past either, and there's no reason it would have suddenly turned on at this late stage.

 

[stefan r]

... the low mass tells us it was never a quasar in its past either, and there's no reason it would have suddenly turned on at this late stage.

Wikipedia puts the mass of Sagetarius A* at 4.1 to 4.4 million solar mass. If an object accreted enough mass to go from 3 to 4 million solar masses it would radiate off a lot of energy. It is reasonable to suspect that this occurred at some time in the Milky Ways past.

 

[Ken G]

That is so, but the point is, quasars typically host black holes that are a hundred times that mass, and they are generally more luminous than the entire Milky Way galaxy as a result. Lesser black holes would have been far less bright than quasars, though they might graduate in their heyday to the level of an "active galactic nucleus." If one converts 4 million solar masses into a luminosity over, say, a billion years (a fraction of the Milky Way lifetime), one gets a luminosity less than a tenth of the Milky Way luminosity-- orders of magnitude less than what would be regarded as a quasar. What would be required is a way to add millions of solar masses on timescales like a million years, rather than a billion years, so there would need to be some evidence that the black hole built strangely quickly in order to think the Milky Way ever rose to the level of a quasar.

But perhaps this does raise an interesting question that might not yet be answered-- do many galaxies like the Milky Way have very much less massive black holes than do the quasars we observe because they accreted matter at a slow steady rate and so were never quasars, or does black hole accretion generally happen in short bursts such that the real issue is simply what fraction of the time is a galaxy a quasar? If the latter, then the Milky Way could only have been a quasar for perhaps 1/1000 of its lifetime, and so would only be observed as a quasar by perhaps 1/1000 of the alien astronomers out there looking at it. But it seems more likely that its present state of slow accretion describes better its history.

 

[Ratman]

What would be required is a way to add millions of solar masses on timescales like a million years

Hang on, there's a SMBH delivery scheduled from the Andromeda Galaxy. Just several bilion years ahead. And together we will shine.

 

[Ken G]

Yes, that might turn us into a quasar for a bit!