APOD: The Tidal Tail of NGC 3628 (2012 Jul 06)

[APOD Robot]

The Tidal Tail of NGC 3628

Explanation: A mere 30 million light-years away, large spiral galaxy NGC 3628 (center left) shares its neighborhood in the local Universe with two other large spirals, in a magnificent grouping otherwise known as the Leo Triplet. In fact, fellow trio member M66 is near the center right of this deep cosmic group portrait, with M66 just above it and to the left. But, perhaps most intriguing is the spectacular tail stretching down and to the left for about 300,000 light-years from NGC 3628's warped, edge-on disk. Known as a tidal tail, the structure has been drawn out of the galaxy by gravitational tides during brief but violent past interactions with its large neighbors. Not often imaged so distinctly, the tidal tail is composed of young bluish star clusters and star-forming regions.

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[TerribleTadpole]

Can you imagine the night-sky from a planet orbiting a star in the tidal tail? Being above the galactic plane and outside of the galaxy, but so close...

[Beyond]

To me, the tidal tail is going downward and to the left. I guess the picture didn't get loaded according to the description. Still a nice picture though.

[Chris Peterson]

Can you imagine the night-sky from a planet orbiting a star in the tidal tail? Being above the galactic plane and outside of the galaxy, but so close...

It would look similar to the Milky Way in our own sky, but less impressive, since it wouldn't go from horizon to horizon.

[Flase]

I hope this isn't rude but that galaxy with the tail looks a lot like um a lady's sanitary protection..

[nstahl]

I'm with Beyond; the tail I see is pointing down. It was up and to the left in the April 8, 2005 APOD.

And it is a nice picture. However the version in the "Not often imaged" link is spectacular.

[Ann]

It's a very fine picture. I particularly like the color contrast between M65 and M66. M65 looks properly yellow, as this galaxy has almost completely given up star formation. M66, however, is forming many new stars, even though rather large parts of this galaxy can be described as "red and dead". The difference between M65 and M66 is marked, however, and M66 looks much bluer than M65 in this picture. Also note the "undisturbed" shape of M65, while M66, apart from its increased star formation, also displays a contorted shape due to its interaction with the two other galaxies.

It is harder to assess the stellar content of NGC 3628, due to its edge-on appearance. It is certainly possible that the galaxy is forming many new stars, and the puffed-up ends of its stellar disk are interesting. Its long tidal tail is pretty spectacular, and Thomas V. Davies did a good job capturing it!

Ann

[smitty]

I'm confused! The description at the image of NGC 3628 which appears at the top of this discussion section says (correctly I believe) that the long tail is going down. But the write-up which appears on the main apod website says the tail is going up. What gives? Why the different descriptions?

[Ann]

I'm confused! The description at the image of NGC 3628 which appears at the top of this discussion section says (correctly I believe) that the long tail is going down. But the write-up which appears on the main apod website says the tail is going up. What gives? Why the different descriptions?

When the map doesn't agree with the terrain, you should probably believe in the terrain.

The tail of NGC 3628 is going down in today's APOD, but since APOD Robot is probably human in spite of its android appearance, it made a mistake.

Lion bookend, tail pointing down. Artist: Edward Kemeys

The tail of NGC 3628 is pointing down in today's APOD!







Ann

P.S. The caption of today's APOD appears to have been corrected now!

[Canadain Grandma]

Thank you for clarifying "the tail"--now I see it. Question! How do you KNOW it is a spiral galaxy when we see it edge on? (I thought this yesterday too!) In very simple language, please, for someone very new to this.

[eltodesukane]

Thank you for clarifying "the tail"--now I see it. Question! How do you KNOW it is a spiral galaxy when we see it edge on? (I thought this yesterday too!) In very simple language, please, for someone very new to this.

An elliptical galaxy is a somewhat fat ellipsoid and would never be seen edge on from any direction. So if it's thin and flat then it's a spiral.

[Canadian Grandma]

Thank you--that is clear. I notice now my 2 typing digits got confused over who went first. I meant Canadian.

[merryjman]

To me, the tidal tail is going downward and to the left. I guess the picture didn't get loaded according to the description. Still a nice picture though.

No, the reason is that once again, they recycled an old description (http://apod.nasa.gov/apod/ap070727.html) and barely changed it for this new image. Lazy and frustrating.

[smitty]

P.S. The caption of today's APOD appears to have been corrected now!

The caption has not been corrected on my version of today's apod (and I just now reloaded it to be sure). Strangely, the caption is correct on the version which appears at the top of the discussion page. Fully concur, however, with your advice to believe the terrain rather than the map!

[Ann]

Thank you for clarifying "the tail"--now I see it. Question! How do you KNOW it is a spiral galaxy when we see it edge on? (I thought this yesterday too!) In very simple language, please, for someone very new to this.

We use the term "spiral galaxy" a bit loosely. Not all spiral galaxies have what we might call spiral arms.

NGC 7217. Photo: Adam Block.

This is galaxy NGC 7217. It is technically a spiral galaxy, although it doesn't have what we normally mean by spiral arms.

All "spiral galaxies" have flattened disks, however. NGC 3628 definitely has a flattened disk. We can't know if it really has spiral arms, and we can't know what the arms look like if it has any. We can see, however, that NGC 3628 displays the usual color distribution for spiral galaxies: that is, it has a long dark dust lane bisecting it, and the galaxy is yellow in the middle and bluer at the edges. We expect spiral galaxies to have that sort of color distribution, even though there are exceptions to that rule.

Click to view full size image

NGC 7331.
Credit & Copyright: Vicent Peris (OAUV / PTeam), Gilles Bergond, Calar Alto Observatory.

Spiral galaxy NGC 7331 is fairly unusual in that we can see how the spiral arms "end" on one side of the galaxy. In this picture, the "end of the spiral arms" is seen on the left side of the galaxy. Note the three bright background galaxies. The one at the upper left has beautiful, thin spiral arms, but the two other galaxies lack obvious arms. All three galaxies are disk galaxies with "organized shapes", however, and all three would be classified as spiral galaxies.

Ann

[500pesos]

What will happen to the stars in the tidal tail? Are they forever lost in space? Are they going to form a blob "galaxy" that people on APOD will mock?

[nstahl]

The tail direction here is corrected but the APOD description is still wrong; up and to the left.

[owlice]

To me, the tidal tail is going downward and to the left. I guess the picture didn't get loaded according to the description. Still a nice picture though.

No, the reason is that once again, they recycled an old description (http://apod.nasa.gov/apod/ap070727.html) and barely changed it for this new image. Lazy and frustrating.

You do know there are only two people who select images, write the descriptions, find relevant (and sometimes humorous) links, and maintain the whole APOD production and archiving system, yes? And that they have full-time jobs (which is not APOD), families, homes to maintain, etc.? And that APOD continues even when they go on vacation, are ill, travel for work, have the normal ups and downs of life to handle, and so on?

No?

Well, now you do.

[rstevenson]

What will happen to the stars in the tidal tail? Are they forever lost in space? Are they going to form a blob "galaxy" that people on APOD will mock?

If the galaxies remain well separated for long enough, most of those stars will gradually fall back towards their home galaxy. But if the next glancing blow by one or both of the other galaxies occurs soon enough and in just the right way (considering location and velocity), many of them could be lost to space or even picked up by one of the other galaxies. Since we're seeing just a snapshot in time, with no really firm idea of the relative velocities of everything involved, we can't predict with much accuracy what will happen to any part of the system.

Rob

[geckzilla]

Thank you for clarifying "the tail"--now I see it. Question! How do you KNOW it is a spiral galaxy when we see it edge on? (I thought this yesterday too!) In very simple language, please, for someone very new to this.

We use the term "spiral galaxy" a bit loosely. Not all spiral galaxies have what we might call spiral arms.

To confuse matters further, some galaxies with spiral dust arms and also some viewed edge on with dust lanes are lenticular. http://en.wikipedia.org/wiki/Lenticular_galaxy

[Craig Willford]

According to the description, "...the tidal tail is composed of young bluish star clusters and star-forming regions."

It is my understanding that those big blue stars live short (by stellar standards) lives and that we see them in our galaxies because sweeping material inside a galaxy causes compression of dust and gas, triggering new star formation all the time to offset the "burning out" of the previous batch of blue hot stars. It is my understanding that this is the cause of the appearance of the concentration of blue stars in the arms within spiral galaxies.

Ok, then, the tidal tail has to have been separated from its host galaxy for quite a time, again on a stellar scale. I would have thought that the blue hot stars formed when the galaxies made their near pass (or collision) would have run out of hydrogen to fuse and would have kicked the bucket by now. May I presume these are freshly formed? If so, what is the source of the interstellar pressure to cause the fresh collapse to new stars?

Craig Willford

[Canadian Grandma]

Thank you to all--particularly Ann--who helped my understanding of spiral galaxies.

[Flase]

You do know there are only two people who select images, write the descriptions, find relevant (and sometimes humorous) links, and maintain the whole APOD production and archiving system, yes? And that they have full-time jobs (which is not APOD), families, homes to maintain, etc.? And that APOD continues even when they go on vacation, are ill, travel for work, have the normal ups and downs of life to handle, and so on?

No?

Well, now you do.

Well that sounds cheap. A US government agency, even a cash-strapped one can afford to invest a little money in paying one or two staff members to work on this very public website. Tell the decision makers it's good PR.

Maybe a student or two studying astrophysics could have a part-time job doing it...

[Craig Willford]

I'm sorry. How can anyone criticize volunteers annotating the APOD pictures?

First and foremost: their commentary and hyperlinks to deeper pages for understanding is phenomenal. I congratulate them!
Second: The work just keeps coming, day after day. Who could ask for more?
Third (and this too is important): You don't criticize volunteers! Do you want them to keep volunteering? I for one do!

I thank you, all who work on putting up the APOD. It is my HomePage and it greets me each morning. It cheers me with the beauty and stimulates my mind with the intellectual content.

Please keep up the good work and just ignore critical comments, please.

Sincerely,

Craig Willford

[Flase]

I personally wouldn't criticise the volunteers themselves. I would say that although the APOD site does not have the same publicity value of the Moon landings etc., millions of people know about it and visit discretely. Compared with the half-a-billion bucks spent on each space shuttle launch etc., the pittance required to pay a couple of part-timers is negligible.

What's more, education about space, furthering human understanding, has a much more intrinsic social worth than war (cold or otherwise) and that's what a government is for.