“Horsefeathers” – NGC 7770/71 Galaxy Group by Ken Crawford

Article written: 30 Jan , 2010
Updated: 24 Dec , 2015
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Away in Pegasus and 225 million light years from Earth, a barred spiral galaxy designated as NGC 7771 spans across 164,000 light years of space. It is a part of a small galaxy group consisting of lenticular galaxy NGC 7770 to the south and edge-on NGC 7771A to the west connected to magnificent spiral NGC 7769. But horsefeathers doesn’t mean this little clump of galactic partners is nonsense… Horsefeathers are what we need to make an intergalactic duster just to see them!

“This interacting group of galaxies reside behind an amazing amount of foreground dust. This high latitude dust reflects the light from our own galaxy.” says astrophotographer, Ken Crawford. “This dust makes color changes by scattering light, especially blue light making image processing a challenge. This dust is very faint and I enhanced it to make it easier to see its interesting structures.”

Not only does looking through a dust cloud make imaging galactic structure more difficult, but it also makes studying galactic structure more difficult, too. “We present a multiwavelength study of the interacting starburst galaxy NGC 7771, including new optical and ultraviolet spectra and a previously unpublished soft X-ray ROSAT image and spectrum. The FIR, radio, and X-ray fluxes suggest that a massive burst of star formation is currently in progress, but the small equivalent width of the Balmer emission lines, the weak UV flux, the low abundance of ionized oxygen, and the shape of the optical spectrum lead us to conclude that there are few O stars. This might normally suggest that star formation has ceased, but the barred gravitational potential and large gas reserves of the galaxy imply that this should not be so, and we therefore consider other explanations.” says Richard Davies, et al. “We argue that the observations cannot be due to effects of geometry, density-bounded nebulae, or dust within the nebulae, and conclude that a truncated IMF is required. The dwarf galaxy NGC 7770 appears to be in the initial stages of a merger with NGC 7771, and the resulting tidal perturbations may have induced the apparent two-armed spiral pattern, and driven a substantial fraction of the disk gas inwards. The presence of a bulge in NGC 7771 may be moderating the starburst so that, while still occurring on a large scale with a supernova rate of 0.8-1/yr, it is less violent and the IMF has a relatively low upper mass limit. We find that there is a cluster of stars obscuring part of the starburst region, and we offer an explanation of its origin.”

Through Ken’s mastery of imaging, we don’t need quite such sophisticated equipment to take a look at all the action going on with the NGC 7771 grouping. The primary galaxy’s well known starburst ring easily distinguishes itself and the tidal tails of its interacting companions come to light. Is it possible this giant molecular dust cloud could be contributing to the rapid formation of stars? Or… Is the region itself causing all the dust?

“Many statistical studies of interacting galaxies have shown that galaxy interactions can enhance star-forming activity. Luminous infrared galaxies are galaxies which emit the bulk of their energy in the far-infrared (FIR) and often show signs of interaction, such as tidal tails, multiple nuclei, or disturbed outer envelopes. LIRGs are considered to be extreme objects, where strong starbursts are induced by galaxy interactions, because in many such objects star formation can account for the infrared emission.” says T. Hattori (et al). “Theoretical work supports the idea that interactions play an important role in stimulating starbursts in galaxies. Numerical simulations of merging gas-rich spiral galaxies show that during the merging process, gas clouds lose their angular momentum and flow into the circumnuclear region of the host galaxies. The resulting high concentration of molecular gas can fuel star-formation activity in the circumnuclear region. This is consistent with compact nuclear starbursts and gas condensation in ultra-luminous infrared galaxies observed in mid-infrared. Therefore, a nuclear starburst triggered by gas inflow has generally been assumed to be the mechanism for producing enhanced star formation in interacting galaxies.”

Although it sounds good in theory, the reality check is the dust is between us and the galaxy grouping – like a thin fog seen at a great distance. Says Ken, “The interesting fact is this dust is illuminated by our own galaxy and reflected back to us. This dust scatters light, especially the blue light and this scattering is called light extinction. Light extinction plays havoc on color balance when it is this prominent and it makes processing a challenge.”

It’s a challenge we’re glad you took on… Because the results are amazing!

Many thanks to outstanding astrophotographer Ken Crawford for sharing his amazing work with us…


1 Response

  1. Lawrence B. Crowell says

    These are pretty pictures. This technique is of possible value in identifying what data should be filtered out in look out to intergalactic distances.

    LC

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