APOD: The Colliding Spiral Galaxies of Arp 271 (2013 Aug 25)

[APOD Robot]

The Colliding Spiral Galaxies of Arp 271

Explanation: What will become of these galaxies? Spiral galaxies NGC 5426 and NGC 5427 are passing dangerously close to each other, but each is likely to survive this collision. Typically when galaxies collide, a large galaxy eats a much smaller galaxy. In this case, however, the two galaxies are quite similar, each being a sprawling spiral with expansive arms and a compact core. As the galaxies advance over the next tens of millions of years, their component stars are unlikely to collide, although new stars will form in the bunching of gas caused by gravitational tides. Close inspection of the above image taken by the 8-meter Gemini-South Telescope in Chile shows a bridge of material momentarily connecting the two giants. Known collectively as Arp 271, the interacting pair spans about 130,000 light years and lies about 90 million light-years away toward the constellation of Virgo. Recent predictions hold that our Milky Way Galaxy will undergo a similar collision with the neighboring Andromeda Galaxy in a few billion years.

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[SouthEastAsia]

My two questions would be:

Assuming 'new stars form' out of the 'bunching of gas' caused by 'gravitational tides', will there be an actual net-gain in total stars member to these two galaxies after the collision?

And also, will the two surviving galaxies continue on there indepenent ways in differenent trajectories? Or will they do some form of gravitational dance and be pullled back into repeating near-collisions, etc?

Thanks in advance for any insight.

[bayareajohn]

So... if we know the distance to a galaxy from the red shift because the universe is expanding, and the more red shift, the farther way a galaxy is.... if a galaxy is coming toward us, like Andromeda is supposed to be, would there not be a BLUE shift instead of RED, and consequently we could not estimate distance?

[Chris Peterson]

My two questions would be:

Assuming 'new stars form' out of the 'bunching of gas' caused by 'gravitational tides', will there be an actual net-gain in total stars member to these two galaxies after the collision?

And also, will the two surviving galaxies continue on there indepenent ways in differenent trajectories? Or will they do some form of gravitational dance and be pullled back into repeating near-collisions, etc?

Thanks in advance for any insight.

Yes, there will be more stars, at least for a while.

The two galaxies are already in orbit around each other, a situation that is unlikely to change. Their dance, and occasional collisions, will continue.

[Chris Peterson]

So... if we know the distance to a galaxy from the red shift because the universe is expanding, and the more red shift, the farther way a galaxy is.... if a galaxy is coming toward us, like Andromeda is supposed to be, would there not be a BLUE shift instead of RED, and consequently we could not estimate distance?

Andromeda and our galaxy are gravitationally bound, so there is no cosmological redshift at all (because space between us isn't expanding). The only spectral shift we observe is Doppler, not cosmological, so there is no distance information present, only velocity.

[Ann]

So... if we know the distance to a galaxy from the red shift because the universe is expanding, and the more red shift, the farther way a galaxy is.... if a galaxy is coming toward us, like Andromeda is supposed to be, would there not be a BLUE shift instead of RED, and consequently we could not estimate distance?

All galaxies have intrinsic, individual motions, but all galaxies are also carried away by the expansion of the universe. For nearby galaxies, the net effect of the individual motion of a galaxy (which may carry the galaxy in any direction in relation to its neighbours) and the expansion of the universe (which will always impart a receding effect on the galaxy) may in fact be a blueshift, i.e., the galaxy is moving toward us. The best-known blueshifted galaxy is Andromeda, but a few galaxies in the Virgo cluster are also blueshifted in relation to us (for example M90).

It is important to remember that only nearby galaxies can be blueshifted. For more distant galaxies, the expansion of the universe always makes the galaxies move away from us. The more distant a galaxy is, the less important is its individual motion in relation to the expansion of the universe, and soon the individual motions become negligible.

For nearby galaxies, redshift is not always a good tool to establish distance. But for nearby galaxies there are many other tools available for distance estimation, for example Cepheids, red clump stars and the tip of the red giant branch.

Ann

[Chris Peterson]

It is important to remember that only nearby galaxies can be blueshifted. For more distant galaxies, the expansion of the universe always makes the galaxies move away from us.

That's really a bit of a misconception. Distant galaxies are not really moving away from us, which would result in a Doppler redshift. Rather, the space between us has grown while their light traveled, resulting in cosmological redshift. A distant galaxy is just as likely to be Doppler blueshifted as it is Doppler redshifted. We simply can't detect that because the cosmological redshift dominates, and there is no way to separate the two effects.

The problem with treating cosmological expansion as physical velocity is that it introduces all sorts of problems and paradoxes with respect to energy, exceeding the speed of light, etc.

[K1NS]

Think about the billions of astronomers, on the hundreds of billions of planets orbiting stars in ARP 271. The sky they see must be beautiful, with another nearby galaxy crossing the sky behind the "milky way" of their own galaxy. I wonder if they realize that their galaxies are passing through each other?

[Chris Peterson]

Think about the billions of astronomers, on the hundreds of billions of planets orbiting stars in ARP 271. The sky they see must be beautiful, with another nearby galaxy crossing the sky behind the "milky way" of their own galaxy. I wonder if they realize that their galaxies are passing through each other?

Depending on your location, I think you'd see something like two Milky Ways; a sort of cross structure as opposed to the linear structure we see here.

[neufer]

Think about the billions of astronomers, on the hundreds of billions of planets orbiting stars in ARP 271. The sky they see must be beautiful, with another nearby galaxy crossing the sky behind the "milky way" of their own galaxy.

I wonder if they realize that their galaxies are passing through each other?

• Are some of them radio astronomers?

[b][color=#CF00CF]A scan of our Milky Way galaxy at 21 cm shows that the distribution of neutral hydrogen is concentrated in the spiral arms. The Sun is marked as the yellow arrow, Galactic center is a blue dot. Notice how there is a cone of avoidance behind the Galactic center due to confusion in the HI signal.[/color][/b]

---

<<The hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 MHz, which is equivalent to the vacuum wavelength of 21.10611405413 cm in free space. This wavelength or frequency falls within the microwave radio region of the electromagnetic spectrum, and it is observed frequently in radio astronomy, since those radio waves can penetrate the large clouds of interstellar cosmic dust that are opaque to visible light.

During the 1930s, it was noticed that there was a radio 'hiss' that varied on a daily cycle and appeared to be extraterrestrial in origin. After initial suggestions that this was due to the Sun, it was observed that the radio waves seemed to be coming from the centre of the Galaxy. These discoveries were published in 1940 and were seen by Dutch Professor J.H. Oort who knew that significant advances could be made in astronomy if there were emission lines in the radio part of the spectrum. He referred this to Dr Hendrik van de Hulst who, in 1944, predicted that neutral hydrogen could produce radiation at a frequency of 1420.4058 MHz due to two closely spaced energy levels in the ground state of the hydrogen atom.

The 21 cm line (1420.4 MHz) was first detected in 1951 by Ewen and Purcell at Harvard University, and published after their data was corroborated by Dutch astronomers Muller and Oort, and by Christiansen and Hindman in Australia. After 1952 the first maps of the neutral hydrogen in the Galaxy were made and revealed, for the first time, the spiral structure of the Milky Way. Assuming that the hydrogen atoms are uniformly distributed throughout the galaxy, each line of sight through the galaxy will reveal a hydrogen line. The only difference between each of these lines is the Doppler shift that each of these lines has. Hence, one can calculate the relative speed of each arm of our galaxy. The rotation curve of our galaxy has also been calculated using the 21-cm hydrogen line. It is then possible to use the plot of the rotation curve and the velocity to determine the distance to a certain point within the galaxy.>>

[jmcgericault]

Is there any evidence of gravitational lensing in this photograph? I see lots of other background galaxies in the photo, some even directly behind Arp 271 itself. Wouldn't the gravity of these two massive galaxies in Arp 271 distort the light coming from these background galaxies?

[neufer]

Is there any evidence of gravitational lensing in this photograph? I see lots of other background galaxies in the photo, some even directly behind Arp 271 itself. Wouldn't the gravity of these two massive galaxies in Arp 271 distort the light coming from these background galaxies?

You need something at least an order of magnitude further away & larger (; and, perhaps, two orders of magnitude more massive?).

<<Abell 1689 is a galaxy cluster in the constellation Virgo nearly 2.2 billion light years away. The lensing zone itself is 2 million light years across. It is one of the biggest and most massive galaxy clusters known and acts as a gravitational lens, distorting the images of galaxies that lie behind it. It has the largest system of gravitational arcs ever found.>>

<<Arp 271 is a pair of similarly sized interacting spiral galaxies, NGC 5426 and NGC 5427. Located 90 million light-years away, the Arp 271 pair is about 130,000 light-years across. It was originally discovered in 1785 by William Herschel.>>

[Chris Peterson]

Is there any evidence of gravitational lensing in this photograph? I see lots of other background galaxies in the photo, some even directly behind Arp 271 itself. Wouldn't the gravity of these two massive galaxies in Arp 271 distort the light coming from these background galaxies?

These galaxies are probably too close to the Earth, and too diffuse to deflect any distant galaxies enough to be visible. I think the nearest lensing galaxy is a massive condensed elliptical, more than a half billion light years away. Any lensing in this case would be very close to one of the galactic cores, and the lensed object would be blocked from view by the galactic disc.

[FloridaMike]

It looks more like cell division to me.

[JuanAustin]

Just curious, how long would it take using conventional technology to travel perpendicular to the plane of our galaxy before a reasonably good view could be had to confirm that the milky way is a barred galaxy?
Conversely, if we had a starship that could erase the limitations of speed and time, how far would one have to go to confirm the same?

[Boomer12k]

"War of the Shuriken"....no....literally.....Throwing STARS.....at each other.

Really nice photo. Still very much spiral galaxies, even though they look like they are just beginning to intersect. What awesome beauty of a tragedy about to happen...and yet, a destructive event with so much creation....awesome...mind boggling...

:---[===] *

[ta152h0]

WOW, just when you know everything, here comes another APOD. between my ears both blue shift and red shift are happening at the same time.

[jambo]

What is the star with its apparent mirror image immediately below it, located about halfway between the core of the lower galaxy and the right edge of the picture?

[Ann]

What is the star with its apparent mirror image immediately below it, located about halfway between the core of the lower galaxy and the right edge of the picture?

Epsilon Lyrae.
Source: http://www.themcdonalds.net/richard/ind ... ilon_Lyrae

This object could be a nearby Milky Way binary star. A fine example of a double star, indeed a double-double star, is Epsilon Lyrae.

Another famous double star is Mizar and Alcor in the Big Dipper.

Ann

[neufer]

What is the star with its apparent mirror image immediately below it, located about
halfway between the core of the lower galaxy and the right edge of the picture?

USNOA2 0825-081911928 [Mag. ~18.5]

(Aren't you glad you asked.)

[Beyond]

Another famous double star is Mizar and Alcor in the Big Dipper.

Thanks Ann. I didn't know the middle of the handle was a binary. It does make sense though. That's a very big dipper and the handle could use all the strength it can get.

[jambo]

Thanks Ann & Neufer. Yes, I AM glad I asked!

[Ron-Astro Pharmacist]

Its not something we will need to do but what would they call the new collection of the two galaxies?

So going with today's APOD task why don't we give the interaction process a name? And why not Beattle song names (slightly altered). There's plenty of those. Such as:
Here Comes the Suns
The Long and Blinding Show
Twist & Merge
Come Together (no need to change that one)
From Across the Universe
Goin'ta be Something New

I'm sure I missed many. Ron

[Star Geezer]

How do we know that these two galaxies have not already collided and are now emerging into their post collision states?

[ta152h0]

would it not be a gas if we find out they actually bounced off each other, like a billiards ball. the big poobah in the sky would be laughing so hard..