A Galactic Collision, and the Sun’s Future

by Paul Gilster on May 19, 2007

I remember a startling painting from an astronomy book I once had when I was a kid. It showed two spiral galaxies much like the Milky Way in the process of collision, and I recall the caption saying that the stars in galaxies were so widely spaced that even in an event like this, few if any stars would collide individually. The galaxies, so the writer surmised, would simply pass through each other, leaving both relatively unscathed.

What made the picture interesting was reading in the same volume that the Milky Way is eventually going to collide with the Andromeda galaxy, so that I had the vivid image of a vast galaxy getting ever closer in the night sky until the entire view was consumed by cities of stars. It was a lovely image, but the idea of galaxies merging without notable disruptive effect is long gone. And new work from the Harvard-Smithsonian Center for Astrophysics has implications for our Solar System as well.

Andromeda collides with Milky Way

In two billion years, with the Sun still firmly on the main sequence — although considerably brighter and hotter than today — Andromeda will make its first close pass, the two galaxies beginning to intermingle their stars as they swing around each other. It will take some three billion years beyond that, say the CfA’s T.J. Cox and Avi Loeb, for the two galaxies to complete their merger. Far from passing through each other and going their own ways, they would now form a single elliptical galaxy. An animation of the event can be viewed here.

Image: An artist’s impression of the collision of the Milky Way and Andromeda. Credit: James Gitlin/STScI.

By that time, the Sun will have become a red giant, with conditions in our current habitable zone far too extreme for life, and the entire Solar System will have receded from the new galactic center, from the current 25,000 light years to 100,000. And what will be the fate of our species leading up to this era? Here’s an interesting speculation from the paper. Note that the Solar System becomes problematic long before the Sun actually reaches the red giant stage or the galactic collision occurs:

Current evolutionary models (see, e.g., Sackmann et al. 1993) predict that the Sun will steadily increase its size and luminosity for the next 7 Gyr as it slowly consumes all available hydrogen and evolves towards a red giant phase. While this places a strong upper limit to the extent of life on Earth, it is likely that much smaller changes (< 50%) in the Sun’s luminosity will significantly alter the Earth’s atmosphere and thus its habitability within the next 1.1-3.5 Gyr (Kasting 1988). Korycansky et al. (2001) suggested that the onset of these effects could be delayed by increasing the orbital radius of the Earth through a sequence of interactions with bodies in the outer Solar System, and we can not rule out the possible colonization of habitable planets in nearby stars, especially long-lived M-dwarfs (Udry et al. 2007) whose lifetime may exceed 1012 years (Adams et al. 2005). In short, it is conceivable that life may exists for as little as 1.1 Gyr into the future or, if interstellar travel is possible, much longer.

Another interesting note is this: Given sufficient time, and assuming that the cosmological constant does not evolve with time (in other words, no slowing of the universe’s accelerated expansion), all galaxies not part of the Local Group will eventually recede and exit our event horizon. At that point, write Cox and Loeb, “…the merger product of the Milky Way and Andromeda (with its bound satellites) will constitute the entire visible Universe.”

The paper is Cox and Loeb, “The Collision Between The Milky Way And Andromeda,” submitted to to the Monthly Notices of the Royal Astronomical Society, and available online. As to visual effects, any observers five billion years from now will see not a strip of stars like the current Milky Way from Earth, but a huge starry bulge that defines galactic center. I like Loeb’s comment: “This is the first paper in my publication record that has a chance of being cited five billion years from now.” It’s interesting to speculate where the citation might appear!

andy May 19, 2007 at 9:28

I wonder what the fate of M33 (Triangulum Galaxy) will be?

Administrator May 19, 2007 at 9:30

That’s a good question. M33 is considered gravitationally bound to Andromeda, is it not?

Marc May 19, 2007 at 12:59

I’m getting an 404: File Not Found error whenever I click on the animation link. ;)

Administrator May 19, 2007 at 13:40

Marc, sorry, that was my fault. Try the animation again; the link should be fixed.

Gregory Benford May 19, 2007 at 17:44

About our surviving our sun’s evolution:
What might humanity–however transformed by natural selection, or by its own hand–do to save itself? Sitting further from the fire might work. Temperature drops inversely with the square of distance, so Jupiter will be cooler by a factor of 2.3, Saturn by 3.1. But for a sun 500 times more luminous than now, the Jovian moons will still be 600 degrees Kelvin (K), and Saturn’s about 450 K. Uranus might work, 4.4 times cooler, a warm but reasonable 320 K. Neptune will be a brisk 255 K. What strange lives could transpire in the warmed, deep atmospheres of those gas giants?
Still, such havens will not last. When the sun begins helium burning in earnest it will fall in luminosity, and Uranus will become a chilly 200 K. Moving inward to Saturn would work, for it will then be at 300 K, balmy shirtsleeve weather — if we have arms by then.
To avoid this fate, intelligent life can tinker–at least for a while–with stellar burning. Our star will get into trouble because it will eventually pollute its core with the heavier elements that come from burning hydrogen. In a complex cycle, hydrogen fuses and leaves assorted helium, lithium, carbon and other elements. With all its hydrogen burned up at its core, where pressures and temperatures are highest, the sun will begin fusing helium. This takes higher temperatures, which the star attains by compressing under gravity. Soon the helium runs out. The next heavier element fuses. Carbon burns until the star enters a complex, unstable regime leading to swelling. (For other stars than ours, there could even be explosions (supernovas) if its mass is great enough.)
To stave off this fate, a cosmic engineer need only note that at least ninety percent of the hydrogen in the star is still unburned, when the cycle turns in desperation to fusing helium. The star’s oven lies at the core, and hydrogen is too light to sink down into it.
Envision a great spoon which can stir the elements in a star, mixing hydrogen into the nuclear ash at the core. The star could then return to its calmer, hydrogen-fusing reaction.
No spoon of matter could possibly survive the immense temperatures there, of course. But magnetic fields can move mass through their rubbery pressures. The sun’s surface displays this, with its magnetic arches and loops which stretch for thousands of kilometers, tightly clasping hot plasma into tubes and strands.
If a huge magnetic paddle could reach down into the sun’s core and stir it, the solar life span could extend to perhaps a hundred billion years. To do this requires immense currents, circulating over coils larger than the sun itself.
What “wires” could support such currents, and what battery would drive them? Such cosmic engineering is beyond our practical comprehension, but it violates no physical laws. Perhaps, with five billion years to plan, we can figure a way to do it. In return, we would extend the lifetime of our planet tenfold.
To fully use this extended stellar lifetime, we would need strategies for capturing more sunlight than a planet can. Freeman Dyson envisioned breaking up worlds into small asteroids, each orbiting its star in a shell of many billions of small worldlets. These could in principle capture nearly all the sunlight. We could conceivably do this to the Earth, then the rest of the planets.
Of course, the environmental impact report for such engineering would be rather hefty…

Administrator May 19, 2007 at 19:19

This is a fascinating scenario, and I for one had no idea that as much as 90 percent of the Sun’s hydrogen will remain when helium fusion gets active. The magnetic paddle perhaps does the trick for the species with a five billion year evolutionary head-start — cosmic engineers indeed! — but that kind of engineering could surely come up with interstellar flight alternatives as well. Still, the notion is just spectacular, especially as melded with Dyson’s ideas. As usual, Gregory, you have me thinking way, way out on the edge of things…

Bob Shaw May 19, 2007 at 20:00

I think that we have to accept that he ‘Habitable Zone’ is a variable feast – and that strictly speaking the Earth isn’t in the Habitable Zone of Sol at present, never mind the future. We inhabit the earth simply because the so-called ‘Big Whack’ reset our rock/volatile ratio – without that, we’d see a temperate Venus where the earth resides. If an earth-sized planet was in the asteroid belt. it’d be habitable – but normally one at 93 million miles from Sol wouldn’t be! The old RAND Corpration ‘Habitable Planets For Man’ study used a pre-Excel spreadsheet to calculate habitability, but based everything on the assumption that Earth is ‘normal’, whereas it ain’t!

Bob Shaw

Chris Wren May 20, 2007 at 2:42

More reason to suspect that any truly hyper-advanced civilizations will high-tail it out of not only their parent solar systems, but their home galaxies as well. Of course, even the relative stability of intergalactic space will just mean buying a little more time – ‘little’, if the lifespan of your species ends up being measured in billions of years.

andy May 20, 2007 at 7:37

Is the volatile content of Venus really all that different from Earth? On Venus the temperatures are high enough to drive carbonates out of the rocks to produce immense quantities of carbon dioxide – if we put the volatile content of the Earth’s crust (including the carbonates) into the atmosphere, you’d end up with something a lot thicker than we have today, probably comparable to Venus. The apparent difference in volatile content between the two planets is because on Venus the volatiles are in the atmosphere and the crust is probably dry, whereas on Earth, large quantities of volatiles are in the crust.

Furthermore, is the “Big Whack” really all that rare an occurrence, or is it just the subset of such impacts that result in moon formation rare? Most such collisions would cause volatiles to be lost from the planet system (due to the total amount of energy/momentum delivered by the impactor), but may not necessarily form a moon. Furthermore, a second such collision could potentially destroy existing moon systems by altering the rotation rate of the planet and changing the direction of tidal evolution of the system. Just because a terrestrial planet doesn’t have a moon doesn’t mean it hasn’t undergone a “Big Whack”.

As for engineering the Sun, what about the Earth itself? The magnetic field and tectonic activity are also important for habitability, and the Earth’s internal heat source is finite.

Adam May 21, 2007 at 3:02

Hi All

Robin Canup has extensively modelled the putative Moon-forming collision of Earth and “Theia”. She produced an Earth-Moon combination in about 1/4 of all solar systems forming via the usual oligarchic cascade that terrestrial planets are believed to form out of. About 1/3 of those combinations were very close to the Earth-Moon as we see them.

Earth has about the equivalent of 44 bars of CO2 in its carbonates, while Venus has about 100 bars, with who knows how much locked away. The current enhanced hydrogen/deuterium ratio could either have come from a lost ocean, or just from comets over the billennia. We don’t know if Venus formed wet or otherwise, but it’s not unreasonable as simulations show “wet” asteroids from the outer Main Belt accreting onto all the current terrestrial planets – but we still don’t know for sure.

JD May 21, 2007 at 4:58

The subject of elliptical galaxies intruegs me a bit. What I’ve read suggests that most dust has been swept from them and new stellar formation has basically ceased. What are the odds that previously established solar systems survived in a viable state after the elliptical forming mergers?

Adam May 21, 2007 at 6:10


We really don’t know yet. Perhaps when microlensing surveys become possible to the nearest large ellipticals then we might have real data.

andy May 21, 2007 at 6:44

Adam: cheers for the info on Venus.

For some info on the feasibility of extragalactic planet searches, see here – for the James Webb Space Telescope (at the time the article was written, it was the Next Generation Space Telescope), the giant elliptical galaxy M87 is apparently at an optimal distance for the pixel microlensing technique.

Steve Pease May 21, 2007 at 17:43

Re the earlier comment about the Earth not being the Sol’s habitable zone. Can you expand on that briefly??

Ronald May 22, 2007 at 5:23

Ref. Earth’s HZ: I understand from Kasting, Wetherill, and others, that the Earth *is* in our sun’s HZ, though rather near the inner edge, as a result of which we will move out of it within the next 1 billion years orso. Anyway, the Earth a bit (20% orso, at i.2 AU) futher out and/or a slightly less ‘aggressive’ sun (G3-G5 orso) might have been more ideal, but for the time being we’ll have to do with what we’ve got.

djlactin May 22, 2007 at 6:12

Attention everybody!
We’re Talking 5 BILLION years from now. Longer than life has existed on Earth!
I ain’t gonna worry about it.
And neither are the great great great grand child species of H. sap.

Let’s worry about our more immediate problems.

Paul Dietz May 22, 2007 at 9:11

> Temperature drops inversely with the square of distance,

Inversely with the square root of distance, assuming fixed emissivity.

MirrorStephen May 22, 2007 at 9:19

djlactin Says:

May 22nd, 2007 at 6:12
Attention everybody!
We’re Talking 5 BILLION years from now. Longer than life has existed on Earth!
I ain’t gonna worry about it.
And neither are the great great great grand child species of H. sap.

Let’s worry about our more immediate problems.

Isn’t that what the microbes said swimming around the little
ponds on the early Earth 4 billion years ago?

Gregory Benford May 22, 2007 at 13:05

The point is not to benefit humanity 5 gigayears from now, but to illuminate what other intelligences could have done to persist, from earlier eras in our galaxy.

A society that can megaengineer its star can survive to send SETI signals, or even visit.

Adam May 22, 2007 at 16:00

Greg, the disturbing thing is that there’s no obvious signs of stellar engineering out there in the Galaxy. Perhaps life in this Universe has come into being too late for us to see any yet?

ljk May 22, 2007 at 16:12

How obvious does an astroengineering project need to be?

A Dyson Shell as wide as Earth’s orbit would not be obvious –
and we’ve got 400 BILLION star systems in this galaxy alone
to look through.

And it would not surprise me a bit if there are vast cosmic structures
out there that we are too primitive to understand as artificial, like
ants crawling across a cement sidewalk.

Maybe the Milky Way IS the artificial construct.

Adam May 23, 2007 at 3:59

Hi Larry

If it’s a construct it’s not obviously so. There’s nothing blindingly obviously artificial about our galaxy nor the multitudes we can see.

Ronald May 23, 2007 at 7:30

OK, ‘our more immediate problems’ (the major ones):
– large meteorite impacts (civilization threatening: once in a couple of hundred thousand to a few million years, but may happen any time)
– mega-volcanic eruptions (civilization threatening: once in a couple of hundred thousand years, but may happen within a few centuries)
– next ice-age (within next 500-15,000 years ?)
– pandemic diseases (any time)
– ???
In other words, even within a timespan of centuries to a few millennia we have good reasons to want a few more planets to perpetuate humankind and not have all our eggs in one basket (almost literally). Terraforming of Mars is one option, prospecting for extrasolar real-estate another one (the latter may actually also involve terraforming). Hence the usefulness of this website.
BTW, even in the very long run, our don’t have planet does not have 5 gy, but rather 0.5 – 1 gy, because our sun is getting brighter and earth will move out of the HZ (on the inside that is) within that timeframe. About the same as there has been ‘higher lifeforms’ (Cambrian diversification).

ljk May 23, 2007 at 9:50

I recall reading in the book The Spike an interesting passage
that our galaxy and Universe are what is left AFTER a massive
space war. So that would be artificial, in a sense.

If we turn the Sol system into a Dyson Shell and inject our star
with helium and hydrogen, Sol should last far longer than our
current estimates. Why should we let Nature arbitrarily wipe us
out like it did to the dinosaurs when we have the means to change
our fate?

Stars called blue stragglers last far longer than astronomers
think they should. Perhaps they are being helped along. Perhaps
this is one reason why we can’t recognize an artificially cultivated
galaxy and Universe – it looks natural to us on the surface, but is
being run by intelligences underneath.

Plus no astronomer who wants to get tenure would dare to mention
aliens. Even more, we barely have a clue what is going on out there.
Every new exoplanet discovered surprises us, and we don’t even
know what dark matter and dark energy are yet, and they make
up most of the Universe!

ljk May 23, 2007 at 22:36

Observable Signatures of a Black Hole Ejected by Gravitational Radiation Recoil in a Galaxy Merger

Authors: Abraham Loeb (Harvard)

(Submitted on 28 Mar 2007 (v1), last revised 22 May 2007 (this version, v2))

Abstract: According to recent general-relativistic simulations, the coalescence of two spinning black holes (BHs) could lead to recoil speeds of the BH remnant of up to thousands of km/s as a result of the emission of gravitational radiation. Such speeds would enable the merger product to escape its host galaxy. Here we examine the circumstances resulting from a gas-rich galaxy merger under which the ejected BH would carry an accretion disk with it and be observable. As the initial BH binary emits gravitational radiation and its orbit tightens, a hole is opened around it in the disk which delays the consumption of gas prior to the eventual BH ejection. The punctured disk remains bound to the ejected BH within the region where the gas orbital velocity is larger than the ejection speed. For a ~10^7 solar mass BH the ejected disk has a characteristic size of tens of thousands of Schwarzschild radii and an accretion lifetime of ~10^7 years. During that time, the ejected BH could traverse a considerable distance and appear as an off-center quasar with a feedback trail along the path it left behind. A small fraction of all quasars could be associated with an escaping BH.


4 pages, Accepted for publication in Physical Review Letters


Astrophysics (astro-ph); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics – Phenomenology (hep-ph)

Cite as:


Submission history

From: Avi Loeb [view email]

[v1] Wed, 28 Mar 2007 14:05:19 GMT (9kb)

[v2] Tue, 22 May 2007 19:46:32 GMT (10kb)


Richard Kemp May 24, 2007 at 14:51

That`s a ridiculous question….which one of the billions of suns are you talking about?

Slippy May 24, 2007 at 22:38

About engineering being observable:

A complete Dyson Sphere, if built as described to fully surround a star to capture 100% of it’s energy output for use by a civilization, would, at the same time, make the star virtually 100% invisible to detection by visual means. Perhaps it could be seen by it’s gravitational effects on nearby stars, or thru micro-lensing, but not visibly. Gravitationally, it’s too small to be detected as a black hole might be detected.

Perhaps the life-forms, who would take the time required to construct a Dyson Sphere, are extremely long-lived, and a project that takes thousands of years is no big deal. Could they also, using some way to ‘anchor’ their star to the sphere, then use it as a huge spacecraft to travel and explore? Granted, accelleration and course changes would be very slow, but to a long-lived race, what’s the rush.

Just putting my brain thru it’s paces to work out some strange scenarios here.

ljk May 25, 2007 at 8:00

There have been searches for Dyson Shells sporadically since the
1970s. The latest is documented here:


And galaxy NGC 5907 seems to have far more red dwarves
than it should:



Sidney A. Morris May 25, 2007 at 10:50

I have often wondered why, of the many great minds talking about the far future are wasted on such a extreme futuristic event .
How about pulling our best together to find solutions to our problems today.
Wars that could make mankind extinct soon. Global warming , our atmosphere now. The earths dying oceans, (a big one since the sea produces most of the oxygen we need to exist at all.)
Let us spend much more time and effort on solving these problems now. If not all of this ridiculous long term speculation is like spitting into the wind.

ljk May 25, 2007 at 11:01

Most scientists and others ARE focused on those issues.
Not to mention tons of money thrown at these problems.

Says a lot that so much of it remains undone, doesn’t it?

So let a few scientists on the side speculate about the far
future. It doesn’t take away any “serious” resources, it
doesn’t hurt anyone, it makes us a wee bit smarter, and
maybe we’ll actually get something tangible out of it down
the road – and by that I don’t mean 5 billion years from now.

Those guys who played with electricty a few centuries ago
were also considered time wasters by others who only
thought of the moment and immediate needs.

Administrator May 25, 2007 at 12:52

We have many things to do in our short lives. Some of them include just what Sidney is talking about, problems that are immediate and involve issues like poverty and global warming and sectarian violence. But we also have an obligation to look farther into the future, to ensure that the planet does indeed survive, and to explore the universe around us. These are not incompatible tasks, and neither needs to be pursued in isolation. Nor is long-term speculation ‘ridiculous.’ Without it, we would have no understanding of slowly developing problems like global warming itself, and where it may lead if unchecked.

A.B. Atwater May 25, 2007 at 15:31

This is always very interesting reading for me! Regardless of the fact that I do not plan on being around five billion years from now, I have interest in what will happen to our beloved homeworld, and the galactic neighborhood. Please feel free to send details regarding further studies about any astronomy-related topics! Take care of yourselves, and continue your excellent work!

A.B. Atwater

andy May 25, 2007 at 18:56

Heaven forbid that all science must be directly and immediately useful! Have you no sense of wonder?

Adam May 25, 2007 at 23:51

Hi Sidney

We already have the means to combat global warming, but government wastes resurces on silly things like wars, oil subsidies, nuclear-waste repositories and massive military budgets.

astronomyfreak54 September 7, 2007 at 2:05

is it true that the antennae galaxy collided with another

Joseph September 12, 2007 at 17:56

I am Joey i do not know as much as you gentlemen but i am a free thinker.
I am Young of only 21 and have not taken any advanced science but i did love more then anything to read what you all had to say, tho i understand little, i know the concept of it all. I appologize in advance for my grammar and spelling errors.
I have some questions that may be interesting.
1) why is humanity so obessed with the fact that if there is life in the galaxy that they have to be more evolved and more intellegent then us? Their has to be a first species of intellegence, or does it? What if we were the first actual scientifically intellegent species to be created as the universe was? What if what we do or our mistakes will be a guiding lines for those newer species.
I know it would be a grand idea to stumble across another planet where another more intellegent species dwell inwhich we could learn or (as humanity so loves to do) rob that life of its hard work by force. Why is it so hard to believe that we are not the most intellegent of all that is out there? I believe most people would rather find a civilization in another planet that somehow vanished without a trace so but left behind clues of their existence inwhich we could learn from. So that we could all experience what its like to be adventurers excavating and problem solving.
We shouldnt give up on looking for other more intellegent life, but we shouldnt be so focused and so obessed on believing that we are the most ingorant and less intellegent of the intellegent life/beings.

2) Conserning our selfs with the future is the only way to have fun in science is it not? Many of our past Brilliant minds have had the pleasure of thinking about such inventions and ideas and sharing them or not sharing them to the fact that it would be fun to do. Even tho we are unable to do much of what is talked about, just thinking about it and sharing brings the idea into reality. I heard someone say that most sci fi movies are just predictions of the future. Without these possible ideas, then what would you have to shoot for? Even einstien had ideas that he knew that humanity wouldnt beable to do for a great many of years. But the fact that he and many like him have even thought of the likly hood of possibility would give a young scientific genius or brilliant mind something to shoot for. because its those who thinking outside the box are the ones with the huge breakthrews. Is it so bad to rule out the possibility of interstellar travel in the near future? you never know, but if you put the idea on the back burner in your mind, and never attempt to grasp the concept or allow another person with interest to take on the challange, you will procrastinate to the point where its to late or you will slow down the evolution of science. Never be so hard to life in the present and the past that you neglect the future, because its the future that will dictate how we precieve the present and the past. For the present will become the past. If you live in the past you will never see the future because you are too scared to accept that things will and could change. All brillant minds were inspired by people who had crazy ideas about this and that, are they not?

And all you people who are worried about todays problems; Is it not possible to infact think of the future on your free time? You can have a job and work on a problem like global warming but then on your free time are you not able to consider the possibilities of the future? Life is ment to be enjoyed. So allow others to enjoy it. If the future is your love and hobbie, as it is my interest, then you should be allowed to think how you wish. Because its you the thinkers of the future who will be revered in time as the great minds of the past. Those who thought outside the box. And infact thinking about greater engineering will always have by product that we the present can use.
Thinking about how we can terraform planets and creating a new atmosphere would be helpful in maybe…. strengthening our own? Would not considering the possibility of fixing the suns problem not help produce better ways of space travel? Did nasa not create many of our common use things today by trying to find better ways to do things in the future?

DO NOT rule out the future, because its the future that will eventually become today, and when today happens, lets hope its not too late.

I love all scientists and free thinkers. Because its you that makes a difference in the world and its only You that can save humanity from its mistakes. Don’t ever give up on your dreams they should be whats most important to you. (other then religon/family/friends)
My only regret (probably some of yours also) will be the inablity to see and experience the great advances of the future.
And its you that I, Joseph United States Marine, will die to proctect your freedom to think as you please.
If you have any other good related websites or want to contact me my email is Thejoecorps@hotmail.com

george scaglione September 13, 2007 at 13:24

joey i think i can speak for many of us here when i say thank you for the kind words. allow me to answer afew of your points. yes for some reason we do always imagine alien life as more intelligent than us. maybe that is because to have existed for so long as to be able to explore the galaxy then they must have been (no kiddin) pretty intelligent! and yes i have heard the theory too that we have found no other intelligence because… we ARE first!! maybe so.but considering the size of the galaxy and the universe and the time which has passed since the big bang,i find that unlikely! also, a funny thing i just said to someone else afew minutes ago…that if we wish to survive to explore the galaxy,then we HAVE to do better.that is about all i have to say for now and thank you very much…and…for cryin out loud don’t apologize for being young!!! my interests in these topics began a long time ago,long before i was 21 !!! all the very best to you joey,your friend george

ljk January 13, 2008 at 12:00

Space Cloud to Collide With Our Galaxy

Careening gas cloud headed for Milky Way to create celestial fireworks.


ljk January 31, 2008 at 12:05

Discovery of a Giant Stellar Tidal Stream Around the Disk Galaxy NGC 4013

Authors: David Martinez-Delgado (1,2), Michael Pohlen (3), R. Jay Gabany (4), Steven R. Majewski (5), Jorge Penarrubia (6), Chris Palma (7) ((1) Instituto de Astrofisica de Canarias E, (2) Max-Planck Institut fuer Astronomie D, (3) Cardiff University UK, (4) BlackBird Observatory USA, (5) University of Virginia USA, (6) University of Victoria CDN, (7) Penn State University USA)

(Submitted on 30 Jan 2008)

Abstract: We report the discovery of a giant, low surface-brightness loop-like stellar structure around the edge-on spiral galaxy NGC 4013. This arcing feature extends 6 arcmin (~26 kpc in projected distance) northeast from the center; likely related features are also apparent on the southwest side of the disk, extending to 4 arcmin (~17 kpc). The detection of this loop-like structure is independently confirmed in three separate datasets from three different telescopes. We estimate a surface brightness of muB = 28.6+0.6-0.4 magsqarcsec and muR = 27.0+0.3-0.2 magsqarcsec. The significantly redder colour of the stream material compared to the outer parts of the disk of NGC 4013 suggests that this loop did not originate from the disk itself, but rather is the tidal stream of a dwarf galaxy being destroyed in NGC 4013’s gravitational potential. Although its true three-dimensional geometry is unknown, the projected tidal loop displays a very good overall match with the external edge-on perspective of the Monoceros tidal stream in the Milky Way predicted by recent N-body simulations.

Our results demonstrate that NGC 4013, previously considered a prototypical isolated disk galaxy, is in fact undergoing a tidal encounter with a low-mass satellite. In this sense NGC 4013, with one of the most prominent HI warped disks known but showing no previously obvious indication of tidal activity, could be a Rosetta Stone for understanding disks that appear almost pristine in the optical but warped in HI maps and may provide key insights into the formation of warps in general.

Comments: LaTeX, ApJ preprint style, 21 pages, 5 figures (reduced quality), submitted to ApJ. A high resolution version of the paper in addition to a full version of the colour image of NGC 4013 can be found at this http URL

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.4657v1 [astro-ph]

Submission history

From: Michael Pohlen [view email]

[v1] Wed, 30 Jan 2008 12:22:38 GMT (984kb)


ljk February 28, 2008 at 6:56

Frequency and Impact of Galaxy Mergers and Interactions over the last 7 Gyr

Authors: S. Jogee (UT Austin), S. Miller (UT Austin), K. Penner (UT Austin), E. F. Bell (MPA), C. Conselice (Nottingham), R. E. Skelton (MPA), R. Somerville (MPA), H-W. Rix (MPA), F. D. Barazza (EPFL), M. Barden (Innsbruck), A. Borch (MPA), S. V. Beckwith (JHU), J. A. Caldwell (McDonald Observatory), B. Haussler (Nottingham), C. Heymans (UBC, AIP), K. Jahnke (MPA), D. McIntosh (UMass), K. Meisenheimer (MPA), C. Papovich (Arizona), C. Peng (NRC-HIA), A. Robaina (MPA), S. Sanchez (CAHA), L. Wisotzki (AIP), C. Wolf (Oxford)

(Submitted on 26 Feb 2008)

Abstract: We explore the history and impact of galaxy mergers and interactions over z~0.24 to 0.80, based on HST ACS, Combo-17, and Spitzer 24 mu data of ~4500 galaxies in the GEMS survey. Using visual and quantitative parameters,we identify galaxies with strong distortions indicative of recent strong interactions and mergers versus normal galaxies (E/S0, Sa, Sb-Sc, Sd/Irr).

Our results are: (1) The observed fraction F of strongly disturbed systems among high mass (M greater than = 2.5E10 Msun) galaxies is ~9% to 12% in every Gyr bin over z~0.24 to 0.80. The corresponding merger rate is a few times 10^-4 galaxies Gyr-1 Mpc-3. The fraction F shows fair agreement with the merger fraction of mass ratio greater than =1:10 predicted by several LCDM-based simulations. (2) For M greater than =1E9 Msun systems, the average star formation rate (SFR) of strongly disturbed systems is only modestly enhanced with respect to normal galaxies, in agreement with recent simulations. In fact, over z~0.24 to 0.80, strongly disturbed systems only account for a small fraction (less than ~30%) of the total SFR density. This suggests that the behaviour of the cosmic SFR density over the last seven billion years is predominantly shaped by normal galaxies.

Comments: To appear in Proceedings of “Formation and Evolution of Galaxy Disks”, held in Rome, 1-5 October 2007. 4 pages, 4 figures. Uses asp2006.sty, graphicx.sty psfig.sty

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.3901v1 [astro-ph]

Submission history

From: Shardha Jogee [view email]

[v1] Tue, 26 Feb 2008 22:49:07 GMT (234kb)


ljk February 28, 2008 at 7:04

Exploring the Impact of Galaxy Interactions over Seven Billion Years with CAS

Authors: Sarah H. Miller (UT Austin), S. Jogee (UT Austin), C. Conselice (Nottingham), K. Penner (UT Austin), E. Bell (MPIA), X. Zheng (PMO), C. Papovich (Arizona), R. Skelton (MPIA), R. Somerville (MPIA), H. Rix (MPIA), F. Barazza (EPFL), M. Barden (Innsbruck), A. Borch (MPIA), S. Beckwith (JHU), J. Caldwell (UT McDonald), B. Haeussler (Nottingham), C. Heymans (UBC IAP), K. Jahnke (MPIA), D. McIntosh (UMass), K. Meisenheimer (MPIA), C. Peng (NRC HIA STScI), A. Robaina (MPIA), S. Sanchez (CAHA), L. Wisotzki (AIP), C. Wolf (Oxford)

(Submitted on 26 Feb 2008)

Abstract: We explore galaxy assembly over the last seven billion years by characterizing “normal” galaxies along the Hubble sequence, against strongly disturbed merging/interacting galaxies with the widely used CAS system of concentration (C), asymmetry (A), and ‘clumpiness’ (S) parameters, as well as visual classification. We analyze Hubble Space Telescope (HST) ACS images of ~4000 intermediate and high mass (greater than 10^9 solar masses) galaxies from the GEMS survey, one of the largest HST surveys conducted to date in two filters. We explore the effectiveness of the CAS criteria [A greater than S and A greater than ~0.35] in separating normal and strongly disturbed galaxies at different redshifts, and quantify the recovery and contamination rate. We also compare the average star formation rate and the cosmic star formation rate density as a function of redshift between normal and interacting systems identified by CAS.

Comments: ASP conference proceedings of 2007 Bash Symposium. Latex with asp2006.sty. 4 pages, 4 figures

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.3917v1 [astro-ph]

Submission history

From: Sarah Miller [view email]

[v1] Tue, 26 Feb 2008 22:48:01 GMT (110kb)


ljk May 7, 2008 at 9:25

The cover story for the June, 2008 issue of Astronomy Magazine
is about the eventual merger of the Milky Way and Andromeda

The article is not online, but you can see some information here:


ljk January 6, 2009 at 14:39

The Milky Way galaxy is larger and more massive than the Andromeda
galaxy, of which the opposite was once thought:


ljk April 6, 2009 at 10:30

Messier 33 is not only bigger than astronomers used to think, it
is also headed our way, just like Messier 31:


So expect a whole lot of new neighbors in a few billion years –
unless we get flung out of the mix. Oh well, Earth probably won’t
be a major concern by then.

ljk April 12, 2009 at 23:32

A surviving disk from a galaxy collision at z=0.4

Authors: Y. Yang (1), F. Hammer (1), H. Flores (1), M. Puech (2,1), M. Rodrigues (1) ((1) GEPI, Observatoire de Paris, (2) ESO)

(Submitted on 9 Apr 2009)

Abstract: Spiral galaxies dominate the local galaxy population. Disks are known to be fragile with respect to collisions. Thus it is worthwhile to probe under which conditions a disk can possibly survive such interactions.

We present a detailed morpho-kinematics study of a massive galaxy with two nuclei, J033210.76–274234.6, at z=0.4. The morphological analysis reveals that the object consists of two bulges and a massive disk, as well as a faint blue ring. Combining the kinematics with morphology we propose a near-center collision model to interpret the object.

We find that the massive disk is likely to have survived the collision of galaxies with an initial mass ratio of ~4:1. The N-body/SPH simulations show that the collision possibly is a single-shot polar collision with a very small pericentric distance of ~1 kpc and that the remnant of the main galaxy will be dominated by a disk.

The results support the disk survival hypothesis. The survival of the disk is related to the polar collision with an extremely small pericentric distance.

With the help of N-body/SPH simulations we find the probability of disk survival is quite large regardless whether the two galaxies merge or not.

Comments: 7 pages, 8 figures, accepted by A&A

Subjects: Galaxy Astrophysics (astro-ph.GA)

Cite as: arXiv:0904.1621v1 [astro-ph.GA]

Submission history

From: Yanbin Yang [view email]

[v1] Thu, 9 Apr 2009 23:34:58 GMT (1450kb)


ljk July 1, 2009 at 15:14

Milky Way vs Andromeda: a tale of two disks

Authors: J. Yin (Shanghai Astronomical Observatory (SHAO)), J.L. Hou (SHAO), N. Prantzos (Institute d’Astrophysique de Paris), S. Boissier (Lab. d’Astrophysique de Marseille), R.X. Chang (SHAO), S.Y. Shen (SHAO), B. Zhangh (Hebei Normal University)

(Submitted on 26 Jun 2009)

Abstract: We study the chemical evolution of the disks of the Milky Way (MW) and of Andromeda (M31), in order to reveal common points and differences between the two major galaxies of the Local group.

We use a large set of observational data for M31, including recent observations of the Star Formation Rate (SFR) and gas profiles, as well as stellar metallicity distributions along its disk.

We show that, when expressed in terms of the corresponding disk scale lengths, the observed radial profiles of MW and M31 exhibit interesting similarities, suggesting the possibility of a description within a common framework. We find that the profiles of stars, gas fraction and metallicity of the two galaxies, as well as most of their global properties, are well described by our model, provided the star formation efficiency in M31 disk is twice as large as in the MW.

We show that the star formation rate profile of M31 cannot be fitted with any form of the Kennicutt-Schmidt law (KS Law) for star formation. We attribute those discrepancies to the fact that M31 has undergone a more active star formation history, even in the recent past, as suggested by observations of a “head-on” collision with the neighboring M32 galaxy about 200 Myr ago.

The MW has most probably undergone a quiescent secular evolution, making possible a fairly successful description with a simple model. If M31 is more typical of spiral galaxies, as recently suggested by Hammer et al. (2007), more complex models, involving galaxy interactions, will be required for the description of spirals.

Comments: 13 Pages, 7 figures, 2 tables, accepted for publication in Astronomy and Astrophysics

Subjects: Galaxy Astrophysics (astro-ph.GA); Cosmology and Extragalactic Astrophysics (astro-ph.CO)

Cite as: arXiv:0906.4821v1 [astro-ph.GA]

Submission history

From: Jinliang Hou [view email]

[v1] Fri, 26 Jun 2009 00:53:36 GMT (115kb)


ljk September 4, 2009 at 10:13

September 3, 2009

Andromeda Galaxy Eating the Neighborhood

Written by Nancy Atkinson

From Earth, the Andromeda Galaxy looks like a calm, bright galaxy, and is visible with the naked eye in our night sky. But astronomers have discovered things aren’t as tranquil as it seems over at M31. Andromeda is eating the neighbors.

The Andromeda Galaxy contains a trillion stars and lies only about 2.5 million light-years away, so it is a great object to observe and study. But recently astronomers observed wispy streams of stars on the outer fringes of Andromeda, and realized they were leftovers from a cannibalistic feeding frenzy of smaller galaxies it has absorbed.

“This is a startling visual demonstration of the truly vast scale of galaxies,” said Dr. Mike Irwin from the University of Cambridge. “The survey has produced an unrivalled panorama of galaxy structure which reveals that galaxies are the result of an ongoing process of accretion and interaction with their neighbours.”

The cannibalism continues and another victim lies in wait: M33 in the constellation of Triangulum, is destined for a future meal.

“Ultimately, these two galaxies may end up merging completely,” Dr. Scott also from the University of Cambridge. “Ironically, galaxy formation and galaxy destruction seem to go hand in hand.”

Full article here:


ljk September 4, 2009 at 13:17

The remnants of galaxy formation from a panoramic survey of the region around M31

Authors: Alan W. McConnachie, Michael J. Irwin, Rodrigo A. Ibata, John Dubinski, Lawrence M. Widrow, Nicolas F. Martin, Patrick Cote, Aaron L. Dotter, Julio F. Navarro, Annette M. N. Ferguson, Thomas H. Puzia, Geraint F. Lewis, Arif Babul, Pauline Barmby, Olivier Bienayme, Scott C. Chapman, Robert Cockcroft, Michelle L. M. Collins, Mark A. Fardal, William E. Harris, Avon Huxor, A. Dougal Mackey, Jorge Penarrubia, R. Michael Rich, Harvey B. Richer, Arnaud Siebert, Nial Tanvir, David Valls-Gabaud, Kimberly A. Venn

(Submitted on 2 Sep 2009)

Abstract: In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach $10 – 100$ times the radius of the central disk.

The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent.

Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31.

An improved census of their surviving counterparts implies that three-quarters of M31’s satellites brighter than $M_V < -6$ await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31.

This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

Comments: Published in Nature. Supplementary movie available at this https URL

Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); Galaxy Astrophysics (astro-ph.GA)

Cite as: arXiv:0909.0398v1 [astro-ph.CO]

Submission history

From: Alan McConnachie [view email]

[v1] Wed, 2 Sep 2009 12:51:25 GMT (295kb)


Stability Analysis December 15, 2009 at 21:43

Kind of sad for all those alien cosmologists in the Milky Way several billion years into the future when they won’t be able to see most other galaxies. They’re gonna have a real tough time explaining the origin of the universe.

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