Is gravitational lensing responsible for obscuring (figuratively) the supposed supermassive black holes at the center of our own and other galaxies? By "see" I mean relative to the measuring tools we are using. A radio telescope "sees" in radio waves.

In Hubble photos of galaxies, no black holes appear to be present in galaxy centers, at least not visually. How can this be explained?

  • 1
    $\begingroup$ Welcome to Astronomy SE. See eventhorizontelescope.org for attempts to image the milky way black hole at radio wavelengths. Regarding your question. Please edit to use standard grammar and orthography. Capitalise the first word of sentences. Also clarify what you mean by "visually" Do you want to restrict this question to observations in visible light. Clarify what you mean by "supposedly". Should we address the question of whether such black holes exist? Clarify why you think gravitational lensing may be responsible. Is this your own theory. Add some more appropriate tags. $\endgroup$
    – James K
    Apr 6 '17 at 9:24

The main reason supermassive black holes aren't visible in images of our galaxy is because of all the dust and stuff in between the center of the galaxy and us. The Milky Way's bulge is ~16000 light-years thick. There are millions of stars, and huge clouds of dust in between us and the black hole, which block visible light. Most of our measurements of the center of our galaxy are made in radio wavelengths.

In addition, compared to galaxies, black holes are small. A supermassive black hole might have a radius of up to 400 AU (for the really large ones). The nearest spiral galaxy to us, Andromeda, is 778,000 parsecs away. This corresponds to an angular size of 0.0005 arcseconds. Even the Hubble Space Telescope only has a resolution of 0.043 arcseconds in the optical. That means that Hubble can only see things that are 100 times this size. So we wouldn't even be able to see any black holes in another galaxy.

That being said, we can make observations of black holes using indirect means. For example, one of the ways we are able to precisely measure the black hole in our galaxy is by watching stars orbit around it. We take pictures of these stars in the infrared and radio wavelengths, which pass through the dust. By measuring how fast these stars are moving, we can calculate the mass of the black hole.

In other galaxies, we can observe other effects of black holes, such as active galactic nuclei, and quasars.

So despite being unable to get an optical picture of a black hole, we still know that they're there.

  • 1
    $\begingroup$ Ok so correct me if my understanding is way off here. Aren't radio waves and infrared also forms of light and subject to the same laws ? So if observing in there bands , would or wouldn't we be able to effectively see through(around) these SMBs because of gravitational lensing ? So as to make them impossible to be directly observed as an empty space since the radio and infrared waves would bend around the SBHs showing us the image we would see if we were on the far side? $\endgroup$
    – marcticus
    Apr 6 '17 at 7:40
  • 2
    $\begingroup$ The second paragraph is really the main reason. The first paragraph obviously doesn't apply to many galaxies, especially if observing at IR or radio wavelengths. $\endgroup$
    – ProfRob
    Apr 6 '17 at 8:09
  • $\begingroup$ @Marticus The light-bending is limited to light that passes quite close to the black hole, so it is still the case that limited spatial resolution is the problem. Attempts are underway at radio wavelengths to "image the event horizon" of our own BH in the Milky Way. $\endgroup$
    – ProfRob
    Apr 6 '17 at 8:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.