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NGC 3628 and questions on dark matter models April 12, 2007

Posted by dorigo in astronomy, internet, personal, science.
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Today I posted on a forum of visual observers an invitation to an observing session for tomorrow evening in a dark site on the western italian alps, with a program of galaxy observation which included a very interesting object: NGC 3628 in LEO, a member of a very famous triplet which includes M65 and M66. It is a large galaxy, with a mass of 130 billion suns, located 25 million light years from us.

A colleague replied by declining the invitation with regret, and he pointed me to a picture taken by an amateur astronomer which had my jaw drop. Here is a small part of the picture, which includes NGC3628 and its tail:

 

The original picture, which I beg you to visit, includes the two other members of the trio, and deserves a look. Please find it at this site  .

The picture was taken by Jon Christensen with 220 minutes of exposure on a f/3 Takahashi Epsilon 210 astrograph which may be called an “amateur instrument” but is something that makes most of us astro-freaks drool: and in fact the picture you see is one of the very few around capable of showing in all its glory the fantastic plume of neutral hydrogen departing from the galaxy.

Now, what is the interest of that hydrogen structure besides its HUGE size, you might well ask. Well, an explanation which I do not elect to try and improve nor contest is in Eric Flesch’s quasars site. Let me quote it, since I find it quite interesting:

The HI contours tell a plain story of gas which is venting out of NGC3628 and filling up a gravitational bowl around the galaxy, then overflowing the lip of the bowl towards the left and flowing viscously into the intergalactic medium (IGM), free of the gravitational influence of its parent galaxy. There is also a smaller flow to the south.

The significance of this picture [one you can see in the above linked site – TD] is that NGC3628 has a clearly defined gravitational boundary not far from the visible disk. This contradicts a major tenet of “dark matter” astronomy that galaxies have enormous haloes of material which extend very far away from the visible galaxy.

A further significance is the commentary on the nature of gravity. Why does NGC3628’s gravitational influence terminate so close to the visible disk? After all, should not the gravitational influence taper off slowly with ever greater distance, as per the inverse square law? That is does not do so shows that there is an ambient gravitation to the IGM which is not modelled by conventional cosmology, a gravitational scalar which has no clear source. Furthermore, the absence of a large halo means that gravitational rotation profiles are more anomalous than ever; to recapitulate this, the outer disks of spiral galaxies rotate about their nucleus about as fast as the inner disks, in flagrant contradiction of normal gravitational models. Very large (unseen) matter haloes have been posited to account for the discrepancy, the idea being that the additional matter generates stronger gravitational fields in the outer disk. But the HI around NGC3628 bleeding off into the IGM shows us that there is no large halo.[…]

The author of the above text is Eric Flesch, who wrote a paper on the galaxy in question (astro-ph/9907219 ) with Halton Arp. They discuss the correlation between the features of the giant galaxy and the presence of a large number of quasars in the same position in the sky, and the significance of the observation.

I recently observed NGC3628 from the mountains with 25×100 binoculars. It is a very nice view, but after reading the above paper I am itching to aim my 16″ dob at that target. Maybe tomorrow… I will not be happy until I see at least the dark dust lane splitting the galaxy in two!

Comments

1. Kea - April 12, 2007

Wow!! The evidence just keeps on coming. I’m just amazed that they managed to get such a photo with a (relatively) lightweight instrument. There are some pretty cool people out there!

2. dorigo - April 12, 2007

Kea, you win the price for the fastest comment! I had just finished checking the post when I saw your comment🙂

And yes, you can do such cool things these days, even without the need to visit the Keck observatory… As Riccardo Rando once put it, “It is amazing how much you can do with some ingenuity and thousands of dollars of precision equipment”.

If you think that pictures such as the one linked above were at reach of only the largest observatories only twenty years ago, you can’t help feeling that the “evidence” you mention is bound to produce some revolution in the field. But, as I read in a blog today, “Cosmologists are often wrong, but never in doubt” (Lev Landau dixit).

Cheers,
T.

3. Thomas Dent - April 14, 2007

By the way, the paper cited is nothing to do with gravitation or dark matter.

Looking at the picture, how can one tell what the velocity and direction of motion of the gas in the plume is? Not knowing this, how can any deduction be made about gravitational forces on it? Why couldn’t it, for example, have been pulled off from the edge of the spiral galaxy by a glancing collision with a smaller galaxy, and currently be *recollapsing* towards it – rather than, apparently of its own accord, ‘escaping from the gravitational influence’ of the spiral?

Why should one believe that the gas is ‘venting’ or ‘departing’? Seems like a completely unwarranted assumption.

Flesch is making the mere shape of a single plume of gas do way too much work … it bears the entire burden of ‘disproving’ dark matter halos?!

I think one would do better to refer to more than one possible explanation of the shape! After all, it never does any good to give just one side of a story.

4. dorigo - April 14, 2007

Hi Thomas,

I gave a one-sided interpretation because it looked intriguing… I did not mean to be exhaustive! Furthermore, I have a debt of gratitude to Halton Arp for putting together such an amazing list of galaxies.

Please feel free to quote papers here which explain differently NGC3628’s strange appearance.

Cheers,
T.

5. Doug - April 14, 2007

A product like AstroMed from Harvard might be able to determine if NGC 3628, the tail and the other two galaxies in the uncropped photo ‘Trio of Galaxies in Leo’ actually do lie in the same plane.

http://astromed.iic.harvard.edu/

AstroMed is a medical imaging program modified for astronomy.

6. Tripitaka - April 15, 2007

Wow the linked photo is so cool… looks a bit like a low buget sci-fi film background!
Thanks for posting it Tommaso

7. dorigo - April 15, 2007

Hi Tripitaka,

you are welcome… And yesterday evening I had a great view of the dust lane in the bigger galaxy.

Doug, thanks for linking that program. Quite interesting!

Cheers,
T.

8. Thomas Dent - April 15, 2007

I’m not aware of any published work, by Flesch or anyone else, on the gravitational implications (if any) of this galaxy. But you don’t need a peer-reviewed journal article to understand my problem with the explanation.

The anti-dark matter argument relies exactly on assuming that the gas has been ‘ejected’ somehow from the galaxy and is escaping freely, as it were, under its own steam. But why believe that? Do they have any data on the motion of the gas? Why would one think that the gas is actually moving away at present?

Flesch says:
“The HI contours tell a plain story of gas which is venting out of NGC3628 and filling up a gravitational bowl around the galaxy, then overflowing the lip of the bowl towards the left and flowing viscously into the intergalactic medium (IGM), free of the gravitational influence of its parent galaxy.”

… What we see – the contours – are merely the two-dimensional distribution of the gas against the sky. In order to tell something useful about the dynamics we would need to know its velocity. Even then we couldn’t get the acceleration, and thus the force, without additional assumptions about the history of the system.

So how on earth can Flesch make these statements? How much of the ‘story’ of things ‘venting’ and ‘filling’ and ‘overflowing’ and ‘flowing viscously’ is supported by data, and how much by his imagination? All those words refer to MOTION, yet I don’t see any data that addresses how fast or in what direction the gas is moving.

What this is about is the degree of credulousness you have: do you believe just about anything these guys say, or do you think about where their claims come from and ask for evidence before endorsing them?

I’ll sketch, based on a few minutes’ guesswork, an alternative possible explanation. Namely: 1) the galaxy MAY have a large halo; 2) some time ago one or more massive, rapidly-moving massive objects – perhaps clumps of dark matter?? – hit the edge of the disc in a glancing collision 3) the object-whatever-it-was started accreting gas 4) on leaving, it pulled gas along with it into the halo 5) but much of the gas has been dropping off back into the gravitational well. The current pattern is like the trail of a firework which is falling back to Earth…

Now it’s up to someone else to punch holes in that!

9. dorigo - April 16, 2007

Thomas said:

“What this is about is the degree of credulousness you have: do you believe just about anything these guys say, or do you think about where their claims come from and ask for evidence before endorsing them?”

No, Thomas, it is not about my credulousness. What I believe and what I do not is not so easy to guess from my posts. Nor am I endorsing things I publish here, especially when I just comment, as above,

“an explanation which I do not elect to try and improve nor contest is in Eric Flesch’s quasars site. Let me quote it, since I find it quite interesting”

So, please respect everybody else’s opinions here, especially if you do not quote papers or overwhelming evidence in disproof of what is reported here. I may agree with your explanations but I agree more with the fact that having more options is better than having one, especially on such a controversial matter as dark matter models.

Cheers,
T.

10. Leo Triplet (mosaic) « Observatori Ventalló - May 10, 2009

[…] NGC 3628 and questions on dark matter models […]


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