Two views of a lopsided galaxy (w/ video)

May 04, 2011
This picture of the Meathook Galaxy (NGC 2442) was taken by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at La Silla, Chile. It shows a much broader view than the Hubble image, although less detailed. This view includes the whole galaxy and the surrounding sky, and clearly shows the asymmetric spiral arms. The longer of the two arms has intense star formation, which is visible here as a pink glow: this is due to the radiation of young stars ionising the gas they form from. The asymmetric shape and star formation are both thought to be caused by tidal disruptions from a near-miss with another galaxy at some point in its history. Credit: ESO

(PhysOrg.com) -- The Meathook Galaxy, or NGC 2442, has a dramatically lopsided shape. One spiral arm is tightly folded in on itself and host to a recent supernova, while the other, dotted with recent star formation, extends far out from the nucleus. The NASA/ESA Hubble Space Telescope and the MPG/ESO 2.2-metre telescope have captured two contrasting views of this asymmetric spiral galaxy.

The Meathook Galaxy, or NGC 2442, in the southern constellation of Volans (The Flying Fish), is easily recognised for its asymmetric spiral arms. The galaxy’s lopsided appearance is thought to be due to gravitational interactions with another galaxy at some point in its history — though astronomers have not so far been able to positively identify the culprit.

This close-up Hubble view of the Meathook Galaxy (NGC 2442) focuses on the more compact of its two asymmetric spiral arms as well as the central regions. The spiral arm was the location of a supernova that exploded in 1999. These Hubble observations were made in 2006 in order to study the aftermath of this supernova. Ground-based data from MPG/ESO 2.2-metre telescope were used to fill out parts of the edges of this image. Credit: NASA

A close-up image from the NASA/ESA focuses on the galaxy’s nucleus and the more compact of its two spiral arms. In 1999, a massive star at the end of its life exploded in this arm in a . By comparing older ground-based observations, previous Hubble images made in 2001, and these shots taken in late 2006, astronomers have been able to study in detail what happened to the star in its dying moments. By the time of this image the supernova itself had faded and is not visible.

A much broader view, taken by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at La Silla, Chile, very clearly shows the double hook shape that gives the galaxy its nickname. This image also captures several other close to NGC 2442 as well as many more remote galaxies that form a rich backdrop. Although the Wide Field Imager, on the ground, cannot approach the sharpness of images from Hubble in space, it can cover a much bigger section of sky in a single exposure. The two tools often provide complementary information to astronomers.

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This video zooms in on NGC 2442, which is nicknamed the Meathook Galaxy due to its distinctive shape. The distorted shape of the galaxy is thought to be due to a near-miss encounter between NGC 2442 and another galaxy. As well as stretching one of the spiral arms, this episode in its history also triggered intense star formation in the longer arm. This is visible here as the red/pink glow of gas bathed in the intense ultraviolet light from the newly formed stars. Credit: NASA, ESA

ESO’s observations also highlight the other end of the life cycle of stars from Hubble. Dotted across much of the galaxy, and particularly in the longer of the two , are patches of pink and red. This colour comes from hydrogen gas in star-forming regions: as the powerful radiation of new-born stars excites the gas in the clouds they formed from, it glows a bright shade of red.

The interaction with another galaxy that gave the Meathook Galaxy its unusual asymmetric shape is also likely to have been the trigger of this recent episode of . The same tidal forces that deformed the galaxy disrupted clouds of gas and triggered their gravitational collapse.

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LKD
not rated yet May 04, 2011
The asymmetric shape and star formation are both thought to be caused by tidal disruptions from a near-miss with another galaxy at some point in its history


Shouldn't it be easy to track down the culprit? It couldn't have gone far. It is a galaxy after all, and should have just as much of a deformation as NGC 2442.
Skeptic_Heretic
4 / 5 (1) May 04, 2011
The asymmetric shape and star formation are both thought to be caused by tidal disruptions from a near-miss with another galaxy at some point in its history


Shouldn't it be easy to track down the culprit? It couldn't have gone far. It is a galaxy after all, and should have just as much of a deformation as NGC 2442.

A passing SMBH wouldn't leave much of a trace. Then again, the existence of a SMBH without a galaxy is pure speculation.
yyz
5 / 5 (7) May 04, 2011
"Shouldn't it be easy to track down the culprit? It couldn't have gone far. It is a galaxy after all, and should have just as much of a deformation as NGC 2442."

NGC 2442 is the largest member of a compact group of galaxies (LGG 147) with about a dozen lower mass members. While several of these smaller galaxies have been proposed as possible actors, none of them presents unambiguous signs of having 'done the deed'. No galaxies adjoining LGG 147 seem to fit the bill either.

However, radio observations have revealed a large intergalactic cloud of neutral hydrogen (HI) near NGC 2442. This HI cloud is roughly a third the mass of HI in NGC 2442, equivalent to a billion solar masses.

A second scenario has been proposed by Ryder et al that invokes ram-pressure stripping as a possible mechanism responsible for the nearby HI cloud and NGC 2442's distorted appearance: http://iopscience....web.pdf

(Fig 2 has a deep pic of 7 small companions of NGC 2442)

LKD
5 / 5 (2) May 04, 2011
yyz,
Thank you, that link and your comments are very helpful and I appreciate the reading to understand this better.
ubavontuba
1 / 5 (1) May 04, 2011
Clouds of neutral hydrogen gas have been associated with dark matter galaxies. Could this be the fist evidence for a galaxy-dark matter galaxy collision?

"Neutral hydrogen gas streams between NGC 4254 and VIRGOH1 21. Image credit: Arecibo Observatory. Click to enlarge
New evidence that VIRGOHI 21, a mysterious cloud of hydrogen in the Virgo Cluster 50 million light-years from the Earth, is a Dark Galaxy, emitting no star light, was presented today at the American Astronomical Society meeting in Washington, D. C."

http://www.univer...-galaxy/

RobertKarlStonjek
not rated yet May 05, 2011
The Long Arm appears to be much closer than the other arm which is trailing off in the distance.
yyz
5 / 5 (4) May 05, 2011
@uba:

"Clouds of neutral hydrogen gas have been associated with dark matter galaxies. Could this be the fist evidence for a galaxy-dark matter galaxy collision?"

An interesting observation in light of early work published wrt VIRGOHI 21:

http://arxiv.org/...53v1.pdf
http://iopscience....web.pdf

But I (and apparently others) remain skeptical that VIRGOHI 21 is truly a massive dark matter-dominated galaxy ('dark galaxy'). More recent radio observations have revealed VIRGOHI 21 is likely an overdensity in a tidal tail drawn out by an earlier encounter with another Virgo Cluster galaxy: http://iopscience....web.pdf

This paper makes a strong case for VIRGOHI 21 being one of several overdensities in a tidal tail drawn from NGC 4254, similar to those seen in NGC 7252 and NGC 6872. VIRGOHI 21 may indeed be a proto-Tidal Dwarf Galaxy.

con't

yyz
5 / 5 (4) May 05, 2011
con't

But back to uba's question wrt the HI cloud found near NGC 2442. The Ryder paper mention the velocity field and asymmetric HI profiles observed in NGC 2442 are inconsistent with a rotating disk structure (such as those proposed for massive dark galaxies): http://iopscience....web.pdf

Most models of dark galaxies involve isolated systems instead of objects in groups or clusters of galaxies. The formation and evolution of dark galaxies in dynamical groups and clusters needs to be elucidated.

The Ryder paper also mentions the possibility the HI cloud could be related to low surface brightness galaxies like Malin 1. Deep optical and H-alpha imaging would be helpful in resolving this matter.

Don't get me wrong, dark galaxies may very well exist. I'm just unconvinced HIPASS J0731-69 or VIRGOHI 21 are examples of such systems. New corroborating evidence or other examples may be found in the future.

Interesting food for thought though.

yyz
5 / 5 (2) May 05, 2011
A correction. This sentence should read:

The Ryder paper mentions the velocity field and asymmetric HI profiles observed in HIPASS J0731-69 (the HI cloud near NGC 2442) are inconsistent with a rotating disk structure (such as those proposed for massive dark galaxies): http://iopscience....web.pdf
emsquared
not rated yet May 05, 2011
It's comment sections like these that keep me hangin' around here at physorg... (as opposed to the flame fests that errupt) Thanks for the info, all!
ubavontuba
1 / 5 (1) May 06, 2011
@yyz:

Thank you for your excellent and edifying response.

The Ryder paper mentions the velocity field and asymmetric HI profiles observed in HIPASS J0731-69 (the HI cloud near NGC 2442) are inconsistent with a rotating disk structure (such as those proposed for massive dark galaxies):
Sure, but the graphs indicate a highly organized structure.

As it's supposedly the nature of dark matter to be diffuse, would it be a particularly far stretch of the imagination to suppose dark matter galaxy densities and organization might differ from ordinary galaxies?

What I'm really asking is, might many (or most?) of the large, dark HI structures we "see" be associated dark matter densities?

And, supposing dark matter galaxies do exist, wouldn't the ideal test for them be to confirm a collision with a visible galaxy?

Skeptic_Heretic
5 / 5 (1) May 06, 2011
And, supposing dark matter galaxies do exist, wouldn't the ideal test for them be to confirm a collision with a visible galaxy?
That would be a strong affirmation of dark matter itself, but how would we quantify the structure of the mass?
yyz
5 / 5 (3) May 06, 2011
" The Ryder paper mentions the velocity field and asymmetric HI profiles observed in HIPASS J0731-69 (the HI cloud near NGC 2442) are inconsistent with a rotating disk structure (such as those proposed for massive dark galaxies):

Sure, but the graphs indicate a highly organized structure."

I'm not sure I would describe the HI observations revealing a *highly organized* system, but there is some. Of the papers I've looked at concerning dark galaxy theory, rotation is one of the few unique characteristics dark galaxies would present. The 2001 Ryder paper notes observations are "inconsistent with a uniformly rotating structure".

(There may be some newer radio studies of this cloud, though I haven't seen any)

As for the rest of your post, uba, I'll confess up front my limited knowledge on the topic of dark galaxies and their structure. I read a few other papers besides those I've linked.....and now I am even more confused on defining characteristics of dark galaxies:^)

yyz
5 / 5 (3) May 06, 2011
con't

Some papers described dark galaxies with masses up to 10E12 M_Sun with faint stellar haloes enclosing a starless rotating disk of HI, others described systems similar to LSB galaxies. I saw little consensus on this. I guess this is to be expected, as the shape of our own galaxy's dark matter halo is a hotly contested issue at this time.

However, dark matter-dominated *dwarf* galaxies have been discovered circling the MWG and Andromeda. The mass-to-light ratio is observed to be over 500:1 in some dGs. Several of these systems have been observed by astronomers for signs of possible DM annihilation signatures (w-null results at this time).

From what I've seen, there is little consensus on issues concerning dark galaxies (formation, evolution, appearance). I would point you back to some of the (newer) papers I've linked for further refs.
yyz
5 / 5 (3) May 06, 2011
con't

Here's a couple of (earlier) papers on dark galaxies that may be of interest:

http://articles.a...ype=.pdf

http://articles.a...ype=.pdf

[This second paper discusses the possibility of using gravitational lensing in the search for dark galaxies]

________________

"And, supposing dark matter galaxies do exist, wouldn't the ideal test for them be to confirm a collision with a visible galaxy?

That would be a strong affirmation of dark matter itself, but how would we quantify the structure of the mass?"

Ah, SH, that was what I was hoping to find out; what would these galaxies look like? Alas, to no (conclusive) avail.
ubavontuba
1 / 5 (1) May 06, 2011
That would be a strong affirmation of dark matter itself, but how would we quantify the structure of the mass?
Well, I would hope that the "visible" portion of the disturbance would give clues as to the structure of the generally undetectable portion of the disturbance. That is, if the normally detectable mass of the disturbance appears significantly smaller than the mass needed to create the disturbance, we might conclude that a significant portion of the mass is undetectable, save for its telltale gravity signature - which would strongly implicate "dark matter."

Essentially, "visible" analyses of the disturbance could offer clues as to the structure of dark matter galaxies.

We may even verify this effect in ordinary galaxy collisions (supposing they support large dark matter content). I'd recommend modeling galaxy collisions with and without significant DM, and comparing these results with actual observations.

ubavontuba
1 / 5 (1) May 06, 2011
@yyz:

Good stuff, as usual. At least you might now see why I'm suggesting the possibility that dark matter may be playing a significant role in the appearance of NGC 2442.

Anyhow, if I was writing a doctoral thesis, I might find this a worthy investigation...

Johannes414
1 / 5 (4) May 07, 2011
The cluster was deliberately created in an unusual shape to make astronomers look foolish.
Quantum_Conundrum
1 / 5 (1) May 08, 2011
Maybe it had a close encounter with an invisible Dark Matter SMBH with an invisible Dark Matter Accretion disk which cannot be detected by any direct observational method.

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