Where Can I Find Fluxes for Strong Radio Galaxies?

Question: I am trying to calibrate New Horizons radiometry data and need flux of strong radio sources (Cas A, Cyg A, Tau A, and Vir A) at 7182 MHz.  Is there a good catalog of this basic information (flux at nearby frequencies plus spectral index) readily available on the web, such as for amateur radio astronomers?  My searches so far have turned up lots of specialized survey results and catalogs but not much I can apply to this simple task.  — Dick

Answer:  You might try the NASA/IPAC Extragalactic Database (NED).  As the name implies, this database contains a plethora of information on galaxies, including photometric information.  Select the “photometric data point(s) and SED” link within the entry for each galaxy and search for the flux entry corresponding to the frequency you are looking for.  For example, Cas A has a flux at 8250 MHz of 620 Jy (with an accuracy of +-4.8%).

Jeff Mangum

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When will the Andromeda Galaxy Collide with the Milky Way Galaxy?

Question: Do you have an accurate estimate on when Andromeda will be colliding with our Milky Way galaxy?  — Charlie

Answer: About 4 billion years.  There is a nice simulation of this collision that you should check-out.  Gives one a nice idea as to what this collision will look like.

Jeff Mangum

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What if the Big Bang Started as a Gigantic Supernova and Associated Black Hole?

Question: A cosmological “what if ”
Here’s my question:

suppose for the sake of argument, the big bang happened somewhat differently, that it was like a huge mega supernova, and created super-duper massive black hole. A universal black hole. THE black hole of the entire known universe!

slowing down all the stuff that blew away from it.
the stuff coalesced into galaxies which are all still expanding away from each other.

such a scenario would have all the galaxies at different heights in a huge potential well. now…if our galaxy were further up such a potential well, all the light from all the galaxies further down would be red shifted.

also photons emitted from those galaxies would not only come ‘straight’ up the potential well but also would spiral up the potential well.

If our galaxy were near the ‘top’, light from one galaxy would reach us from many directions, all such photons would be red-shifted, and would have taken different lengths of time to reach us because of the many different spiral geodisics they could have taken. In other words, many of the galaxies we see could in fact be the SAME galaxy seen from a different direction in the sky and at vastly different times in its evolution as well as from its different orientations.

pretty much all the galaxies would appear to be receeding from ours (whether they are or not)

furthermore, between our galaxy and the more red-shifted ones further down the well,  the space-time would become more and more stretched the further away from our galaxy you’d go.
And therefore it would appear that the expansion of the universe was therefore ‘speeding up’.

Thus explaining the embarassing ‘dark’ energy issue.

In other words, all the distant galaxies might not be ‘spread out’ over the night sky as they appear to be, but instead be all more or less in the same ‘direction’ (downwards), in one and the same huge potential well of “THE” black hole of the entire Universe, that would make a galactic supermassive black hole look like an electron neutrino!

There would be no ‘center’ because any such center would be in all directions, it would therefore be ‘spread out’ as the surface of a sphere.

So maybe therefore, our view of the universe has been ‘inside out’ as it were.

This view seems consistent with general relativity.

How would we know? observationally, how could we tell the difference?
(it sure would explain the ‘dark’ energy /cosmic acceleration issue, plus it’s a lot less absurd)

— Tom


Answer: I think that your scenario has one basic flaw in that if the giant supernova which led to the “central” black hole did exist, we would observe a “source” or center for the overall expansion of the universe.  In fact, what we see are all galaxies (excluding local gravitational interactions between galaxies located near each other) moving away from each other rather than moving away from a common point.  You might want to take a look at some of the questions and answers that have been posted to the cosmology section of this blog for further information.


Jeff Mangum

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Availability of VLA Measurements of M87?

Question: I am contacting you on behalf of the MAGIC collaboration. I was wondering whether you took with VLA any further data on M87. We are trying to get an MWL coverage for our observation period spanning from 2012 to 2015. What we would need is the lightcurve for these years and a skymap. — Cornelia

Answer: In fact, you can search the VLA data archive, which contains all of the data acquired by the VLA since it started observations in 1976, for observations of M87.  Go to https://archive.nrao.edu/archive/advquery.jsp to access the NRAO science data archive (which contains the VLA science data).  You will need to generate the images from these measurements yourself, and also any lightcurve derived from these measurements.

Jeff Mangum

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How Many Earths Can Fit in the Volume of the Milky Way?

Question: How many earths can fit in one galaxy and if the earth were about the size of galaxy how many planets, moons, etc. would be able to fit on earth.  — Xondra

Answer: To answer this question you need to know the volume of the Earth and a galaxy.  Let’s take the Milky Way as our galaxy example.  The Earth’s volume is a little over 10^(12) km^3.  To calculate the volume of the Milky Way, we assume that it can be approximated by a disk with a thickness of 1000 light years and a radius of 50,000 light years.  A light year is about 9.5 x 10^(12) km, while the volume of a disk is pi*(thickness)*(radius)^2.  Plugging in our thickness and radius we get about 6.7 X 10^(51) km^3.  Dividing the volume of the Milky Way by the volume of the Earth, you get (6.7 X 10^(51))/(10^12) =~ 6.7 X 10^(39) Earths that can fit in the volume of the Milky Way galaxy.

Jeff Mangum

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How Long Does it Take the Sun to Orbit the Center of our Galaxy?

Question: How long does it take our sun to complete a full orbit around the super blackhole at the center of the Milky Way?  — Dustin

Answer: We can calculate this value by noting that the Sun is about 8 kilo-parsecs, or about 2.5×10^(17) km, from the center of our galaxy and travels at a speed of about 225 km/sec around the center of the galaxy.  Assuming that the Sun’s orbit about the center of the galaxy is circular, we know that the circumference of that circular orbit is 2*pi*r, where r is the distance from our Sun to the galactic center.  Since distance = rate * time, we know that time = (2*pi*r)/rate = (2*pi*2.5×10^(17) km)/220 km/sec =~ 225 million years.

Jeff Mangum

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The Eventual Collision of the Andromeda and Milky Way Galaxies

Question: If I understand the various articles I’ve read and programs I’ve seen on TV and the internet.  It has been determined that the Andromeda and Milky Way galaxies are on a collision course and are destined to “collide” in about 4 billion years. My question is this. If the collision will happen, how can it?  Given the observations thus far that all of the galaxies are receding from each other due to the expansion of space-time itself.  — Guy

Answer: You are right in that on the large scale all galaxies are expanding away from each other.  On smaller scales, though, there are what are called “peculiar motions” where galaxies can have motions relative to each other that are due to local gravitational attraction.  The Andromeda and Milky Way galaxies have peculiar motions whereby they are moving toward each other, which results in the prediction that they will eventually collide.

Jeff Mangum

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How Do Galaxies Stay Intact?

Question: Do black holes spin? What makes galaxies stay semi organised while precessing around it? I don’t know how else to word that. Physics is hard.  —  Nathan

Answer: Yes, it is believed that black holes do spin.  The stars in galaxies stay organized over long periods of time while the galaxy rotates due to the mutual gravitational attraction of the stars, gas, and dust in galaxies.  That gravitational force balances any forces that might pull a galaxy apart, making the galaxy maintain a stable configuration over many millions of years.

Jeff Mangum


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How Can Gas Particles Stick Together to Ultimately Form Stars and Planets?

Question: There is a lot of acceptance that when the gases of the universe began to join and spin because of the impact of those collisions. That would mean that all particles hit in the same direction, like using your hand to spin a basketball on your finger. When one questions this I just get blank stares as if I’m suggesting some sort of heresy. But, apart from the difficulty I have with the Big Bang being possible on it’s own (Quantum Physics suggests you need an outside force) the answers I read about the formation of planets and gravity seems absurd yet I can’t find anyone to explain it without theory and conjecture. Can someone help me. Just to make it clear what I’m asking, please help me understand how gas particles flying in every possible direction as can form into a rotating ball that turns into rock because physics itself suggests that is impossible. I use the theory that is used for the formation of planets as….

“…there was a massive cloud of hydrogen gas left over from the Big Bang. Some event, like a nearby supernova explosion triggered a gravitational collapse of the cloud, causing the hydrogen atoms to attach to one another through mutual gravity. Each individual hydrogen atom had its own momentum, and so when the atoms collected together into larger and larger clumps of gas, the conservation of momentum across all the particles set these clumps of gas spinning.”

Now that is a lot of conjecture – considering collisions from every possible direction…let alone how those hydrogen gas particles, once collected together formed everything we have on and in the planet.

— Brent

Answer: I think that the slight misunderstanding in your logic is the equate “sticking” with the gravitational attraction between particles such as atoms, molecules, and dust particles, which ultimately results in the formation of more massive objects like asteroids, comets, planets, and stars.  It is not necessary for objects to collide and “stick” to each other immediately.  A stable cloud of massive particles that is affected by a nearby event that “pushes” on it, such as a nearby supernova, will potentially be pushed in such a way that gravity causes objects to slowly move closer to each other and ultimately coalesce.  This slow collapse of massive objects toward other massive objects ultimately builds on itself, collecting larger and larger massive objects.  This ultimately is a mechanism for forming objects like planets, stars, and galaxies.

Jeff Mangum

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Inconsistency Between the Age and Diameter of the Universe?

Question: I have read that the diameter of the universe is 96 billion light years.  How can that be if the universe is a mere 14 billion years old?  Am I to conclude that the 96 billion figure is some extrapolation based on rapid inflation?  — Carlton

Answer: One of my physicist colleagues, Frank Heile from Stanford University, has provided an excellent answer to this question.  First of all, the 13.8 billion light years is derived from the radius of a sphere of the Cosmic Microwave Background (CMB) radiation that is being observed by the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck satellites.  These satellites have mapped the structures which are precursors to galaxies and clusters of galaxies.  Now, the radiation from the CMB measured by the WMAP and Planck satellites has, in the meantime, continued to expand so that the structures measured in the CMB signal would be 46.5 billion light years from us at this time.  Second, the 93 billion light year diameter estimate refers to the “observable” universe.

Now, if we waited another 46.5 – 13.8 = 32.7 billion years, we should actually be able to see the light emitted right now from those superclusters of galaxies which formed from the CMB structures.  The light is already on its way towards us but it will take a while to reach us since it will have to come from a sphere with a diameter of 93 billion light years.  This is the explanation for the difference; the “observable” universe is larger than we can see it today.

This is only, at best, a theoretical estimate of the diameter of the universe, though.  We now know that due to dark energy the expansion of the universe is accelerating.  This acceleration will not allow us to see those superclusters which are now 46.5 billion light years from us.  In fact, if we wait the requisite 32.7 billion years those superclusters will be receding from us at a rate that is greater than the speed of light.  We just will not be able to observe those galaxies and clusters of galaxies which have formed at the edge of the universe.

Jeff Mangum

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