Nasa just released some new Hubble photos.  One of them
(http://oposite.stsci.edu/pubinfo/pr/2002/11/pr-photos.html) shows a
long stream of stars strectching out from a large galaxy toward the
lower right.  The claim is that a small galaxy colliding with the
large galaxy created the stream of stars.  Further, it claims that the
small galaxy is now in the upper left of the large galaxy which means
that it entered from the lower right.

My question is: How does a galaxy entering from the lower right create
a stream of stars extending to the lower right?

Interesting question. :)

First off, the "Tadpole" galaxy is properly named UGC 10214. I think
the "Tadpole" moniker came up recently in relation to these Hubble
photos.

Knowing that, we can pull up a couple of references:

"They noticed that a galaxy called UGC 10214 has a stream of material
flowing out of it, as if it is interacting with another galaxy. But in
this
case, the stream of material is apparently flowing towards nothing."
[ http://groups.google.com/groups?q=UGC10214&hl=en&safe=off&selm=3A54AB70.7867CAF1%40nova.astro.utoronto.ca&rnum=1
]

It's interesting to hear that this article discusses the possible
presence of an invisible "dark galaxy" that is causing an interaction,
but I don't believe this to be the case.

I was also able to pull up another, far less detailed image:
http://www.ast.cam.ac.uk/~trentham/ugc10214.html

Keeping this and the new Hubble image visible, you should be able to
see that there is a slight, but definite, "arc" to the stream of
stars. How is this relevant, you ask?

First, the official report reads "Its distorted shape was caused by a
small interloper, a very blue, compact galaxy visible in the upper
left corner of the more massive Tadpole." and "Strong gravitational
forces from the interaction created the long tail of debris,
consisting of stars and gas that stretch out more than 280,000
light-years."

There is no evidence, in other words, of which direction the colliding
galaxy originated from, nor is this particularly relevant.

Here's my hypothesis: Each galaxy is getting pulled into the other's
gravitational force, and matter is being "whipped" around the other
galaxy's center. Much like the way space probes are "slingshot" around
planets, this causes a substantial increase in velocity, and would
cause the matter in question to be rapidly ejected out of the
vicinity. The trail is still within the gravitational field of the two
galaxies, however, and so is gradually pulled back in. Hence the arc
-- the trail is question in actually in a very eccentric elliptical
orbit.

In fact, you can see that matter from the larger galaxy is also being
influenced by the smaller galaxy's gravitational field -- on the
left-hand side, an arm is being "pulled up" and spun around the
smaller galaxy.

Search terms used: "UGC 10214", others difficult due to timeliness of
the subject.

---

Request for Answer Clarification by steveg-ga on 30 Apr 2002 16:00 PDT

I realize it is only 4 measly dollars, but I was hoping for a more
definitive answer :-).  In one place, the article has the phrase
"'tidal' tail of stars", so maybe like the Earth's tide (which bulges
on both the moon and non-moon side), this is a "galactic tide".  But
more than speculating - like I just did - I was hoping for a clear
description of galactic collisions and stellar streams.

Anyone up for another swing at the problem?

---

Clarification of Answer by alexander-ga on 30 Apr 2002 17:23 PDT

Indeed, a tide is a "distortion on one celestial body caused by the
gravitational attraction of another." The interactions between
colliding galaxies are largely due to the interactions of their
gravitational forces, as I described.

There is some more information about colliding galaxies at:
http://csep10.phys.utk.edu/astr162/lect/galaxies/colliding.html
, including a very good (if somewhat emotionally disturbing)
simulation animation at the very bottom of the page.

The answer was a hypothesis based on the available information on the
web.  However, I was hoping for an informed answer based on more
innate knowledge.

These references might be more technical than you want, but I'll
provide them anyway.  I did a quick look for some journal articles on
this object, UCG 10214, and only found one recent one.  A copy can be
found online at http://arXiv.org/abs/astro-ph/?0110115 .  The print
version reference is Briggs, F.H., Moller, O., Higdon, J.L., Trentham,
N., Ramirez-Ruiz, E.  2001, Astronomy and Astrophysics, 380, 418. 
It's titled "Did VV 29 collide with a dark Dark-Matter halo?".  I'm
not sure it has quite what you're looking for, but it contains a good
reference in there to a long paper (44 pages) titled "Galactic Bridges
and Tails" -- Toomre, A. and Toomre, J.  1972, Astrophysical Journal,
178, 623.  An online version should be availble at
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1972ApJ...178..623T&db_key=AST&high=3d08f73d8625917
.  I'm not sure if this web site requires your school/workplace to
have electronic access to the journal normally.  If you can't get it
online and are interested, hopefully a local university/college
library will carry it.  The Astrophysical Journal is one of the major
journals in astronomy research.  It may be more technical than you
want though, I'm not sure.  I'm just skimming it, but it looks like it
will explain a lot of the galactic dynamics involved in creating
things like tidal tails for you if you don't mind reading a
professional level journal article.

There's quite a lot of physics involved in modelling things like UGC
10214 and unless we have some of the input parameters, we're going to
have a difficult time making the models match reality with any level
of confidence.  As alexander-ga notes, there's really nothing in that
Hubble press release that says the small galaxy had to come from the
lower right.  It might have, but it could have started in the upper
left area and completed an orbit around the major galaxy already, too.
 It can be very difficult to determine these sorts of details and
without them, it can be hard to know exactly what's going on.  After
all, we basically get a snapshot photo of a work in progress and are
trying to determine the history and future of it from that one photo. 
Not an easy task I can assure you.  I can give you a basic idea of how
you can get a tidal tail on the side away from the object though.

Tidal effects like this and ocean tides on the Earth are caused by the
difference in the gravitational pull on the object.  The small galaxy
in the upper left exerts a gravitational pull on the larger galaxy
that goes as 1/(distance^2).  So it pulls harder on the side of the
big galaxy near it than the center.  That pulls the near side away
from the center.  However, in similar fashion it pulls on the center
of the big galaxy harder than it does the far side of it.  This pulls
the center away from the far side.  This is why we have tides on
opposite sides of the Earth at once -- the Moon pulls on the near side
harder than the center pulling the water up there, but also pulls
harder on the center than the far side making the center pull away
from the water which lags behind to form the other tide.  Depending on
the orbital parameters of the galaxies involved in the merger, this
same general effect can create tidal tails on both sides of the larger
galaxy.  Different orbital parameters have different effects on the
merger remnants and getting those parameters from a single snapshot
isn't easy.  That's why we don't always know exactly what's going on
in situations like this.

Does that help answer your original question?  The galaxies are
obviously going to evolve as the merger progresses, but that's the
basic idea on how you can get a tail on the side the smaller object
entered from.  That Toomre and Toomre paper has many figures showing
galaxies in different orbits and how that effects the way the merger
evolves.  Something like their Figure 4 shows an example of a tidal
tail forming on the side of the large galaxy that the smaller one
entered from.  I hope this helps and the journal articles aren't too
technical for you.

Hello, I appreciate this is all a bit late for this discussion but I think
you might find some of our current thoughts in Cambridge interesting.

What happened is that my colleagues and I were working on ways to find
galaxies made only of dark matter -- something for which I believe is
strongly motivated by theoretical astrophysics -- and simultaneously
I came upon whis galaxy in an image for a project where I was studying
the little background galaxies.  So we wondered if a dark galaxy could
be pulling the plume (we call this VV29b) out of the main galaxy (called
VV29a).  We though this was possible since we didn't see evidence for a
second galaxy (the little blue galaxy -- VV29c, that is probably taking
part in the interaction wasn't very clear in the images we had at the
time).  The idea was plausible but I didn't like it too much since
the hypothesized dark galaxies are small and you need quite a big
thing too pull this much material out of a regular galaxy like VV29a.
Now I don't really believe this idea, due to more recent data.

Following this, radio imaging of this galaxy was done at the Westerbork
Telescope in the Netherlands.  This is what is described in the article
by Frank Briggs.  This data told us two main things --
[what the radio HI data measures is the gas -- loose atoms -- not stars
in the galaxies so we one other main component of the galaxies]
1) There is very little velocity gradient *along* the plume VV29b.  This
   doesn't look to me to be material being pulled out of VV29a at speed
   or anything else.
2) In the HI gas image, VV29c is a big galaxy and has the potential
   to do quite a bit of damage to VV29a.  You couldn't tell that if
   you just looked at the stars and not the gas  We hardly noticed VV29c
   at all in the original optical data, and even in the new Hubble
   Space Telescope data -- one of the deepest and most impressive images
   ever taken -- you have to look pretty hard to see it.  But in the
   radio image it's obviously a major galaxy.

Armed with this information, and some velocity information from the
radio data, we're now trying to piece together a description of what
actually happened to produce this odd system.  The problem to me seems
one of uniqueness -- we know from Toomre & Toomre's 1972 article -- a
result confirmed by big computer simulations -- that interactions 
between two galaxies can produce shapes a bit like this.  A couple
scenarios are described in this forum and by Briggs, based on the plume
VV29b resulting from an interaction between the galaxies VV29a and VV29c.

Another idea I like is that we are looking at an interaction between
what was originally *three* galaxies, and VV29b started out as a regular
galaxy, just like VV29a and VV29c that somehow got flattened by
gravitational forces during the interaction.  I have no idea as to
details -- this is something we need to simulate with computers.

So in my opinion the bottom line is that at present the data are not
really good enough to tell us exactly what is going on.  Lots of models
might work -- based on 2 or 3 interacting galaxies -- but we can't
really say which is the right one.

Mostly what seems to be needed is very-high resolution spectroscopy so 
we know exactly what each bit of each of the 3 components is doing.  We
can then use a computer simulation to go back in time and figure out
all this.

Hope this of interest,

Neil Trentham, trentham@ast.cam.ac.uk