Can a massive black hole cause lensing?
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hallojatin
Hi,
In some of the Images there is only blue arc but no galaxy to cause lensing. Cant it be done by a massive black hole which is not visible in front?
Thanks
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Tom_Collett
scientist
in response to hallojatin's comment.
Hi,
As far as we know super-massive black holes are only found in the centers of galaxies, so there should always be a galaxy visible as the lens. Some blue objects can be slightly curved, or it can be two galaxies in a close alignment. Stellar mass black holes are too small to cause a strong lensing event.
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anupreeta
scientist, admin
Great question! Lensing by black holes or dark matter halos is not theoretically impossible. Free floating black holes are unusual occurrences and there haven't been any reliable detections of them. Dark matter halos may have very faint or small population of stars but these usually, have low mass making them inefficient lenses. So, if you found an arc by itself, its most likely an artefact or chance alignment of some distant spiral galaxies seen edge-on (See the "False positives" section under Spotter's Guide for more info).
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hallojatin
But what happened if black hole eat up every star, planets and everything which belongs to a galaxy and stand alone. In this case that black hole is not the part of galaxy but it become a giant and massive object in the universe and can able to make a lensing effect.
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Tom_Collett
scientist
It's actually very hard for a black hole to eat things. In our solar system we have a really massive object (the sun) pulling us towards it, but the earth doesn't fall in because it's already moving perpendicular to line between us and the sun; the sun's gravity pulls the earth round into a circular orbit (it's actually an ellipse, but pretty close to a circle). Whilst much more massive a black hole does the same thing: if you have any velocity perpendicular to the direction of the black hole you want fall in, but will orbit it. Black holes in galaxies tend to have 'eaten' about 1% of the host galaxy's mass.
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drphilmarshall
scientist, admin
And that mass is (was?!) mostly gas. In fact, these quasars we keep talking about are themselves we think supermassive black holes in the distant universe, "accreting" gas which heats up and glows very brightly as it falls into the hole. In the last few years there have been a few claims of supermassive black holes that have been ejected from galaxies, via some sort of gravitational slingshot mechanism. I haven't seen any calculations of what sort of lensing effect we might get from such a free-floating black hole, might be interesting to try and make (or first find!) a quantitative prediction based on @hallojatin 's suggestion?
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drphilmarshall
scientist, admin
Another possibility is that of a stellar-mass black hole wandering around our own galaxy: what sort of lensing effect would that have on one of the many many background galaxies in our images? You see some crazy simulations out there on teh internets, but how likely are lensing events like these to happen? Should we be on the lookout here?
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psaha
scientist
in response to drphilmarshall's comment.
Tian and Mao wrote a paper arguing that you could find nearby neutron stars by looking for their lensing effect on galaxies! You'd need a bigger telescope than CFHT though...
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Budgieye
moderator
I keep hoping to find a ring of bright light from a lensed galaxy, and the central lensing galaxy has been redshifted out of detection. Still looking though.
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psaha
scientist
@Budgieye how about a new cloverleaf? 😃
In the old cloverleaf quasar the lensing galaxy is indeed redshifted out of the R and I bands, but in the H infrared band, you can see the galaxy.
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popedarren
Budgieye, I was trying to get a handle on a conversation about lensing and I asked the question, "If gravity only bends light, and doesn't accelerate it, how does one explain gravitational lensing? Doesn't lensing allow an observer to see light from sources farther away (and farther back in time) than what we could see without lensing?" And was met with several angry responses. People are so passionate about science! ... or about being right. Whatever. Your statement about looking for a ring of bright light from a lensed galaxy with a central lensing galaxy redshifted out of detection made me wonder again if my question is somewhat true. Could lensing potentially be used to peer even further back in time? Is that why they are so cool?
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JasonJason
this shows a Black Hole lensing effect
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drphilmarshall
scientist, admin
in response to popedarren's comment.
Well hello there, Your Eminence! 😃
It's true that lensing can help you detect galaxies that are further away than you can normally see - some of the most distant galaxies we know about were only detected because they had been magnified by a gravitational lens! And yes, the more distant the object, the earlier the time at which you are seeing it. The light is not being accelerated (its already going at the speed limit!), its just that the lens focuses (sort of) more of the light from the source into our telescope than would have reached us if the lens wasn't there - that's all magnification is. So I think you had it right - but maybe the word acceleration threw your audience off a bit?
Anyway, if people are truly passionate about science, they will enjoy being wrong - because that's exactly when you get to learn something new! 😃
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drphilmarshall
scientist, admin
in response to JasonJason's comment.
Neato! I knew I'd seen something like this before. OK good: let's see if we can extend this prediction to the regime we are dealing with. What would a stellar-mass black hole in front of a distant faint blue galaxy look like? Let's take the distance to the lens to be 30,000 light years, th emass of the lens to be 1 Msun, and imagine a source 300 million light years away. What's the Einstein radius? That's where the ring would form...
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Tom_Collett
scientist
in response to drphilmarshall's comment.
I get 2 milliarcseconds if the black hole is a kiloparsec away. The resolution limit for CFHT is about 0.6 arcseconds, so it's a non starter sadly. If we had a solar mass black hole within 2000 AU it would produce a detectable lensing effect at CFHT resolution if aligned with a background source, but the planets' orbits would be perturbed by a solar mass that close.
Black holes are weird; the Einstein radius grows the closer you are to them; this is because they are point masses. With galaxies the opposite tends to happen, because the mass looks more spread out as you get closer to it. That being said, the cross section for lensing by galaxies shrinks if you put the galaxy too far away, since there is less volume left for potential sources to be behind the potential lens.
I'll shut up now before this becomes a geometry class...
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Budgieye
moderator
in response to psaha's comment.
@Budgieye how about a new cloverleaf? 😃
In the old cloverleaf quasar the lensing galaxy is indeed redshifted out of the R and I bands, but in the H infrared band, you can see the galaxy.
Great! Just what I wanted! An invisible lensing galaxy!
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popedarren
in response to drphilmarshall's comment.
Good morrow. Kiss the ring. Jokes. It's my last name.
I think that is the most clear and concise answer I have ever received about lensing. I can't thank you enough. I think you're right about the word "acceleration". I even thought of changing it before I posted so people wouldn't get hung up on that one word. Instead, I thought, "Eh, they get the idea." shrugs I can't please everyone, and at that time, everyone was reading my comment.
Thanks again! Peace, Love, Pope
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drphilmarshall
scientist, admin
in response to Tom Collett's comment.
Interesting! I hadn't thought about the solar system constraint. Nice to know that we'd know from the planets if a black hole got close!
OK, what combination of black hole mass and us-lens distance would be need to get an observable, obvious lensing effect on a background faint blue galaxy? You're allowed to consider rogue (i.e free floating) black holes up to 10^9 Msun in mass provided they are in intergalactic space (and the more distant those ones are the better!). 10^4 Msun black holes could plausibly have been ejected from globular clusters around our own galaxy I guess, so they're on the table too.Posted
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Tom_Collett
scientist
in response to drphilmarshall's comment.
A 10^4 solar mass black hole (IMBH) would need to be within 200 parsecs (~650 ly), to produce a lensing effect visible with CFHT. That's probably too close to expect an IMBH to exist, and it would probably be sufficiently bright in the X-ray that we would know about it (unless it wasn't accreting any gas at all.)
A 10^9 solar mass black hole (SMBH) would have the potential to be a lens at CFHT resolution out to about z=0.01; the only question would be if a source is aligned sufficiently well to see a lensing event. Oh and if free floating black holes actually exist in that (relatively small) volume!
This had me thinking about if lensing can give any constraints on free-floating black holes. We can't say anything interesting for IMBHs, but we're close to saying somthing about SMBHs if we rule out any the presence of invisible lenses. But the absence of evidence is not evidence for absence - we haven't really been looking for systems with invisible lenses; so we'd probably miss them if they exist. At a resolution of 0.1 arcseconds (like Hubble Space Telescope has) we could detected free-floating SMBH lenses out to z=0.2, so maybe this could become an interesting measurement in a decade or so.
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psaha
scientist
in response to Tom Collett's comment.
Some free-floating SMBHs are expected, because mergers of spinning black holes give recoils that can kick the merged BH out of a galaxy. Our gravitational-wave friends have probably calculated how many to expect.
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Budgieye
moderator
Oh, that is interesting!
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drphilmarshall
scientist, admin
Thanks @Tom_Collett! To estimate observability, did you just compare the Einstein radius of the black hole lens with the typical size of a background source galaxy (or the CFHTLS point spread function)? Seems like spotting a black hole lens is a bit of a long shot. Agreed about not looking for them makes them even less likely to be spotted.
But yes, I believe @hallojatin had free-floating black holes in mind when they first asked their question! 😃
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Tom_Collett
scientist
in response to drphilmarshall's comment.
In my estimates 'observable' means an Einstein radius greater than 0.6 arcseconds, which is the median seeing (the resolution limit due to looking through our atmosphere) for CFHT. NB/ I rounded the numbers a bit.
I didn't include the probability of being sufficiently well aligned with a background source that strong lensing actually occurs. So it's just an estimate of 'potentially detectable', given a sufficiently well aligned bright compact source.
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Mizaru
in response to Tom Collett's comment.
What if in ASW0000w4j is an orphaned black hole that is big enough? Could you tell, with a large enough telescope, whether these are lensed objects, or not?
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