Turbulence, Eddies and Star Nucleation
The near absolute coldness brought on by dust IR emissions is insufficient to condense dust, helium, and “metal” laden hydrogen molecules into a dense, incompressible star nucleus. For star nucleation to occur, a position of high pressure must also exists. Turbulent eddies in the flow of molecular clouds present an excellent explanation for these points of pressure build-up. However, the naturally stable configuration presented by a spiral galaxy is set up to minimize such eddies and their disapative effects keeping star formation at a leisurely pace.
The pull of other bodies in the form of other galaxies will disturb the structure of the galaxy, causing an increase in the eddies and turbulence found in the molecular clouds that constitute the spiral arms. Along with it, come an increase in star nucleation and formation in galaxies that have had their messed up by a galactic neighbour. This can be illustrated by using the three spiral galaxies that make up the Leo triplet of galaxies – namely M65, M66, and NGC3628 – that are caught up in a giant 3-body promlem some 35 million light years away, give or take. One of these galaxies – namely NGC3628 or “The Hamburger Galaxy” is seen by us edge on.
NGC3628 - The Hamburger Galaxy member of the Leo Triplet
Spiral Arms Pulled out of Plane
The Hamburger Galaxy, one of the Triplet of Galaxies in Leo
CDK 12.5in; AP 1100GTO AE; QHY600M, – Antlia Broadband and 3.5nm Ha Filters
L: (41 x 180s, Bin 1, Gain 100); H: (18 x 720s Bin 1, Gain 100); R,G,B: (22,26,23 x 210s, Bin 1, Gain 100)
Total integration time = 9.8 hrs (Apr 15-17, 2025) Vancouver Island, BC, Canada
The Hamburger Galaxy member of the Leo triplet of galaxies, presents to us edge on. Most edge-0n spirals look like a sliver with all the spiral arms in a single plane – creating a line of dust and stars. Edge-on spirals present themselves to us with the same geometry as our own Milky Way looks to us. We can’t see the core of the galaxy from the side as it is blocked by the dust lanes and the brightness of inner stars.
For the Hamburger, we can see that the spirals have been bent to form separate planes – particularly visible on the left and right extremities of the galaxy. The self-gravity of the galaxy attempts to keep the spiral arms in the same plane, while an external gravitation body or bodies below the image itself attempt to pull the arms out of plane.
One aside about the image, it is worth noting the double star that exists just above the very centre of the image. One of the star glows red while the larger star is much whiter. I believe that the majority of stars actually come as doubles, but it is rare to image and resolve the two stars of a double with such very different colours.
Full resolution, downloadable versions available at the Victoria RASC Zenfolio Site or at Astrobin.
We can readily seen from our edge-on view, that the dust-lanes that form the meat of the Hamburger Galaxy are not aligned in a single plane. Instead the are bent our of alignment by the pull of the other two galaxies in the triplet, that are off the page. Fighting this pull are the gravitational force of the arms themselves and viscous drag or friction forces that want to keep all the arms in a single plane. The tidal effects are particularly acute on the far left and right of the galaxy where single “dust lane” gets pulled downward. In this case, the tidal effects are caused by the gravitational pull of the other two members of the triplet that are off the image at the bottom.
The dust lanes are indicative of the presence of molecular clouds – actually hydrogen diatomic gas – laden with dust. Viscous drag tries to keep the molecular cloud material – cohesive and a continuous unit. Eddies are minimized when the spiral arms exists on the same plane, but tidal effects can cause the spiral arms to warp as they orbit the galaxy. In these cases, viscous forces get overwhelmed by turbulence and the resulting eddies can be seen in the fine strands of dust within the dust lanes themselves.
While such eddies are normal, even within a galaxy without major disturbance, when tidal effects mess things up the turbulence within the arms is amped up too. In turn this increased turbulence created many more high pressure opportunities for stars to nucleate. More new stars also creates more stellar nurseries that emit red Halpha light and this close-up reveals many of the tell-tale emission patches seen within the dusty spiral arms.
The small but densely packed Halpha emission sources at direct indicators of many stellar nurseries – almost like red relish amongst the dusty meat of our hamburger. As a result of its prolific star production, NGC 3628 is classified as a starburst galaxy. The classification also appears to go hand in hand with tidal disturbances. Many of the stars in the galaxy – appear blue or very young – again testament to rapid new star creation – despite the overall core appearing Halpha red.
In the image above, stars play the role of the buns for our Hamburger. Stars are far less subject to the viscous forces that partly control the flow of H2 molecules and dust. At the same time, they are subject to the same gravitational and centrifugal forces. While the individual arms or dust lanes stay cohesive in distinct planes, the stars lack that cohesion and form a unresolvable fog of glow around the arms.
It is tough to tell whether the disturbance of NGC 3628 is ongoing, or the result of a previous closer encounter with another memeber of the triplet. In the image at left taken from Wikipedia (unlisted creator, either assembled from more integration time, or more adeptly processed than my own), a faint trail of stars can be leading away from the galaxy. These were either pulled off or on to NGC3628 during that encounter.
The faint trail also contains a subcompact dwarf galaxy that may be having a tidal influence over the Hamburger. Indeed dwarf galaxies influence impact the structure of both the Andromeda galaxy (that looks like its been in an accident as a result) and our own Milky Way too.
At the danger of sounding like Lemony Snicket, there have actually been five galaxies identified within the Leo Triplet. Only two are large spirals, however, and we will turn to these two fact on galaxies to learn more about tidal disturbances.
A Tale of Two Spirals; Messiers M65 & M66
M65 and M66 Face-on Spirals round out the Leo Triplet
Planewave CDK 12.5 – AIS6200MM; A-P 1100 GTO AE, Antlia Pro LRGB filters
L (18 x 180s exposures, RGB 3x18x210s) = 4 hours int time, May 2020, Vancouver Island, BC
The two galaxies in the image are about the same distance from us, and about the same apparent size – yet they are different. M65 on the right is classified as an Sa galaxy – it is tightly wound spiral, not very colourful, and has a bulge in the middle. M66 on the left is also a spiral, but is classified as SBb – having a barred centre, emitting strong Halpha red and blue colours and fewer spiral windings. I think it is a great classification, but it doesn’t really say much about what is going on with these two galaxies.
Full resolution, downloadable versions available at the Victoria RASC Zenfolio Site or at Astrobin.
If you like things well behaved and ordered, I am pretty sure you are more likely drawn to the beauty and symmetry of the wonderfully formed M65 galaxy (right). However, if you like things spicy – almost revolutionary and battle hardened, then perhaps the left M66 fits your aesthetic. I feel drawn to the saying – you go to heaven for the weather, but you go to hell for the company.
M65 doesn’t look like it has had much interaction with other galaxies in the last few eons, let alone continued tidal effects. The stars look generally smaller and orange-yellow (ie. older). At the same time, M66 looks like it just left the battle field. The central bulge associated with orderly circulation of M65 has been replaced with a barred centre in M66. M66’s spirals look far less well defined and are absolutely looser. Furthemore, while we get the same dust brown in the spiral arms, there are a lot of bright red Halpha signal in and around the spiral arms – crowded with stellar nurseries. Some of the stars are also bright blue in colour – indicating that that they are larger and young in M66.
A closer look at M66 also indicates whisps of stars that have been pulled away from the galaxy – similar to the star trail left behind by NGC3628. If I had to wager, I would say the M66 is the more likely of the two galaxies to have had a fairly recent interaction with NGC 3628. While proximity can’t be determined from our 2-D image alone, definitely M66 has the most signs of tidal disruption by a close encounter.
Sticking to our theory, we would expect the tidal effect that mess up galactic structure to create more turbulence and larger eddies. This would lead to more stellar nurseries, heightened levels of star production, and the presence of younger blue stars. I would say M66 fits this bill to a tee and while not classified as a “starburst” galaxy, it is definitely described as having a lot of starburst regions. Most description are silent about star production on M65.
Cleanup in the Nursery
You may have missed a slight of hand I made in discussing stellar nurseries along with star formation. I will not clear this up for now, other than to leave a big clue to some future postings.
Indeed star nucleation is generally the rate-limiting step in creating new stars. In terms of nucleation, it is likely the formation of turbulence with sufficient pressure to condense hydrogen, dust and other components that limits nucleation. In the case of a spiral galaxy, molecular clouds at the edge or even between spiral arms are the most likely locales of sufficient turbulence and pressure for star or even small cluster formation. However, if turbulence gets deep into the molecular clouds to produce at least one “starter star”, a nursery may result that can ultimately deliver many, many stars in clusters. This occurs via a process where one or more early stars, reaching fusion ignition, will create radiative pressure that can help nucleate more stars. The net result can be clusters of new stars limited only by the local hydrogen supply, any warming effects, and sufficient radiative pressure. Please stay tuned….
