NGC 3628 and questions on dark matter models April 12, 2007Posted by dorigo in astronomy, internet, personal, science.
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!