Can Gravitationally-Lensed Objects be Studied at Any Wavelength?

Question:  As per definition, Gravitational lensing refers to bending of light rays from a distant source around a massive object (Galaxy cluster) which tends to magnify the background light source. If visible light rays bend around those massive objects then X-rays, Gamma rays, UV and IR rays which forms a part of electromagnetic spectrum must also bend around those massive objects. If true, Are there any initiative to detect those distant lensed invisible objects using the space observatories(Chandra X-ray, Spitzer IR etc.)?  – Vinod

Answer:  Yes.  In fact, just a few days ago there was a press release announcing the detection of the spin of a black hole using the gravitationally-lensed x-ray emission from a black hole in a distant quasar.  Gravitational lensing has also been used to study galaxies using the Spitzer Space Telescope.  In fact, as their is no wavelength dependence to the gravitational lens effect it is possible to study gravitationally-lensed objects at all wavelengths.

Jeff Mangum

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Top Achievements for the NRAO Very Large Array

Question:  I’m embarrassed to ask this, as I feel after working here for so long I should already know the answer. But here it is: What are the top 3 discoveries/achievements of the VLA? And why?  — Doug

Answer:  As you can imagine, an actually ranking of the Very Large Array (VLA) achievements would be open to debate.  Kind of “beauty in the eye of the beholder” problem.  Dave Finley, NRAO Public Information Officer, provided the following links listing what NRAO believes are the VLA’s top scientific achievements:

Jeff Mangum

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Are Quasars and Radio Galaxies the Same Thing?

Question:  I’m interested in the differences and similarities between radio galaxies and quasars. Different sources say different things. Are radio galaxies just quasars with the “jets” pointed away from us, or are there other differences between them?  – Steve

Answer:  It turns out that even though quasars, which is short for “quasi-stellar radio source”, were first discovered as sources of radio emission that appeared to be point-like (like a “star”) in the optical, only about 10% of the known quasars today are also sources of radio emission.  You can think of radio galaxies and quasars as just different kinds of galaxies.  There are lots of radio galaxies, some of which are also quasars.  There are also lots of quasars, only a fraction of which are also sources of radio emission.

Jeff Mangum


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How can a Telescope Image Both a Galaxy and the Stars within it?

Question:  How can Hubble give pictures with different zoom levels?  If telescopes are constructed with a fixed focal length and cannot “zoom”.  How come that Hubble can see individual stars in the Andromeda Galaxy  But can also see the whole galaxy  – Nikola

Answer:  Many telescopes, the Hubble Space Telescope being one of them, uses array detectors to capture the light from stars and galaxies.  Array detectors are composed of individual pixels which determine the telescope’s ability to separate objects that are very close to each other or to detect objects that are very small.  By putting many of these pixels together as an array, we can stitch together many pixels to make a picture, or “image” of larger objects while still retaining the ability to see small objects.  This is how we are able to see both galaxies and the individual stars within those galaxies.

Jeff Mangum

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Detecting a Galaxy Cluster With a Radio Telescope

Question:  Is it possible for a 7-m radio telescope to detect a galaxy cluster of redshift below 0.03? – Ungku

Answer:  Yes, as long as the cluster is big and bright enough at the radio frequency at which you want to detect the cluster.  The sensitivity of a radio telescope is proportional to the noise temperature of your detector system divided by the product of the effective area of your antenna, the square root of the bandwidth of your detector system, and the square root of the amount of time that you integrate on the source of interest.  Since the effective area of your antenna contains a term which is proportional to the area of the source in your antenna resolution element, you can see that as the size of the source gets bigger the sensitivity of your measurement improves.  Your measurement sensitivity also improves if your system noise is lower, and if your integration time and bandwidth of your detector system is larger.

Jeff Mangum

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How Did the First Galaxies Form?

Question:  From where did this initial cloud of gases and dust come? In other words, how did the cloud form? Why were these gases and this dust together in a cloud?
I read on another website that the cloud began to collapse because it was rotating. You say that the cloud began to rotate as it collapsed. I assume the discrepancy is because of either your or the other writer’s uncertainty. As the cloud collapsed, why would it have begun to rotate? What would have caused the cloud to collapse in your scenario?  – John Paul

Answer:  I presume that you are referring to the formation of galaxies.  Starting from the beginning, the first galaxies are believed to have formed as a result of primordial fluctuations in the early universe that gravitationally attracted gas and dark matter to the denser areas.  These structures became the first galaxies.  These first galaxies began to collapse due to self gravity, with rotation being part of this process in order to dissipate energy via angular momentum.  Remember too that that this early stage the universe was almost exclusively composed of hydrogen, helium, and dark matter.  Soon after these first galaxies formed, the hydrogen and helium gas within them formed the first stars, many of which were quite massive, evolved quickly, and ended their short lives as supernovae.  The supernova process expels processed matter into the interstellar medium of the young galaxies, matter that contains heavier elements (produced by the fusion process in those first young stars) and some dust.  That process of gravitational collapse, stellar evolution, and enrichment of the interstellar medium in galaxies then continues until we get the universe filled with galaxies we see today.

Jeff Mangum

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The Andromeda Galaxy’s Orientation Relative to Us

Question: Do you know if there is current agreement about which edge of the Andromeda galaxy (M31) is closest to the Milky Way?  I don’t believe present distance measurements are precise enough to distinguish the near and far sides with enough certainty (also not the intent of these measurements anyway).  Radial velocity measurements are very good, but we also need the spiral structure information – are the arms trailing or leading.  I know M31′s high inclination angle makes this hard to assess, but more recent observations and models appear to be converging on trailing arms for M31, thus the northwest edge is closer.  Is this consensus generally true today?  – Jon

Answer: In fact, we can measure very accurately which parts of M31 are moving towards or away from us.  It turns out that the NE side of the galaxy is moving towards us (i.e. the emission from the gas in this part of the galaxy is blueshifted relative to the average recession velocity of the galaxy).  But, this does not necessarily tell us which part of the galaxy is closest to us.  After reviewing this question with some of my colleagues, I found out that this is not such an easy question to answer.  Our best estimate is that the lower-left (SE) side of M31 is the part that is closest to us.  We believe that this side is the one closest to us due to the fact that in the image of the galaxy shown below (which is an image that is sensitive to ultraviolet light from M31) the SE side of the galaxy appears to have brighter stars than the NW side.  This is likely due to more dust obscuring the NW side due to its position on the far-side of M31 in relation to us.

UV image of M31 from Thilker etal. 2005 (ApJ 610, L67)

UV image of M31 from Thilker etal. 2005 (ApJ 610, L67)

Jeff Mangum

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Can a Black Hole Swallow a Galaxy?

Question:  I know that black holes are very dense and their gravitational pull is so strong no one really knows what they look like because they don’t come out alive. I have read that there might be one in the center of galaxies well why doesn’t it suck the Milky way galaxy in?   – Lily

Answer: It is a matter of relative scale.  Galaxies are quite large; many kiloparsecs in size, while black holes are quite small, only a few millionths of a parsec (a parsec is about 206265 Astronomical Units, where an Astronomical Unit is the distance from the Earth to the Sun).  A single Black Hole, even one at the center of our Milky Way galaxy, is just too small to eat an entire galaxy.

Jeff Mangum

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The Solar System’s Rotation About the Center of the Milky Way

Question:  I have a question about galaxies, how do scientist know about the galactic year and how long it takes? Well people think that are universe is only 14 billion years old has our solar system gone through a galactic year?  – Lily

Answer:  Well, we can measure the rotation speed of the Milky Way Galaxy, which we find to be about 270 kilometers per second (or about 168 miles per second).  It takes about 200 million years for the Milky Way to complete one rotation.  I believe that this is what you are referring to as the “galactic year” (I have also seen it referred to as the “Cosmic Year”).  Since the universe is about 14 billion years old, our solar system has gone around the center of the Milky Way galaxy a few times.  Note, though, that the solar system is quite a bit younger than the universe: its age is about 4.6 billion years.  So, the solar system has gone around the center of the Milky Way galaxy about 4.6 billion / 200 million = 23 times.

Jeff Mangum

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The Properties of Spiral Galaxies, Including Our Own

Question:  Me and the rest of our fifth grade class is learning about galaxies, but I have a couple questions. Why is the middle of a spiral galaxy bigger than the arms? How fast is our Galaxy moving, is it even moving at all? Do you think that there is life on other galaxies?  – Anthony

Answer:  Just like people, galaxies come in a lot of different shapes and sizes.  As you probably already know, there are galaxies that look round, called “elliptical galaxies”, and galaxies that look more like a spiral, called “spiral galaxies”.  For the spiral galaxies there are some that have small centers (or “nuclei”) and large/wide arms, while others have the opposite: big nuclei and rather small arms.  The NOAO Spiral Galaxy image gallery gives you a good idea as to the range of different sub-types of spiral galaxy.

As for your second question about the rotation speed of our galaxy, we have measured that speed to be about 270 kilometers per second (or about 168 miles per second).  Finally, is there life in other galaxies?  Well, there are approximately a hundred-billion stars in our galaxy, and approximately a hundred-billion galaxies in the universe.  It would be difficult to believe that our single star, which we call the Sun, would be the only star in the universe that possesses life.

Jeff Mangum

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