HEIC: Creative Destruction as Galaxies Collide (NGC 4485)

[bystander]

Hubble Observes Creative Destruction as Galaxies Collide
ESA Hubble Photo Release | 2019 May 16

Image Credit: NASA, ESA, Hubble -- Acknowledgment: T. Roberts (Durham University, UK), D. Calzetti (University of Massachusetts) and the LEGUS Team, R. Tully (University of Hawaii), and R. Chandar (University of Toledo)

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The NASA/ESA Hubble Space Telescope has taken a new look at the spectacular irregular galaxy NGC 4485, which has been warped and wound by its larger galactic neighbour. The gravity of the second galaxy has disrupted the ordered collection of stars, gas and dust, giving rise to an erratic region of newborn, hot, blue stars and chaotic clumps and streams of dust and gas.

The irregular galaxy NGC 4485 has been involved in a dramatic gravitational interplay with its larger galactic neighbour NGC 4490 — out of frame to the bottom right in this image. Found about 30 million light-years away in the constellation of Canes Venatici (The Hunting Dogs), the strange result of these interacting galaxies has resulted in an entry in the Atlas of Peculiar Galaxies: Arp 269.

Having already made their closest approach, NGC 4485 and NGC 4490 are now moving away from each other, vastly altered from their original states. Still engaged in a destructive yet creative dance, the gravitational force between them continues to warp each of them out of all recognition, while at the same time creating the conditions for huge regions of intense star formation.

This galactic tug-of-war has created a stream of material about 25 000 light-years long which connects the two galaxies. The stream is made up of bright knots and huge pockets of gassy regions, as well as enormous regions of star formation in which young, massive, blue stars are born. Short-lived, however, these stars quickly run out of fuel and end their lives in dramatic explosions. While such an event seems to be purely destructive, it also enriches the cosmic environment with heavier elements and delivers new material to form a new generation of stars.

Two very different regions are now apparent in NGC 4485; on the left are hints of the galaxy’s previous spiral structure, which was at one time undergoing “normal” galactic evolution. The right of the image reveals a portion of the galaxy ripped towards its larger neighbour, bursting with hot, blue stars and streams of dust and gas.

This image, captured by the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope, adds light through two new filters compared with an image released in 2014. The new data provide further insights into the complex and mysterious field of galaxy evolution.

Galaxy Blazes with New Stars Born from Close Encounter
NASA | GSFC | STScI | HubbleSite | 2019 May 16

[Ann]

Click to view full size image

NGC 4485. ESA/Hubble.
Acknowledgement: Kathy van Pelt.
https://www.spacetelescope.org/images/potw1419a/

Click to view full size image

NGC 4485. ESA/Hubble, NASA.
https://spacetelescope.org/images/heic1910a/

There exist two very different versions of the Hubble image of NGC 4485. At left, you can see a version where the main "body" of NGC 4485 is peppered with bright blue stars. The stars on the starbursting right side of the galaxy look a bit brighter, but not by much. I strongly recommend that you go to this page to see a larger version of the picture at left.

Now compare the picture of NGC 4485 at left with the picture of the same galaxy at right. I recommend that you go to this page, where you can enlarge the picture as you please. Note that the main body of the galaxy is relatively smooth, pinkish-beige in color, and slightly blurred or out of focus, as if we were viewing this part of the galaxy through a semi-transparent veil. Also this part of the galaxy is completely devoid of bright blue stars.

What gives? Well, this page says that NGC 4485 has been imaged through no less than six different filters, but in the picture at left, only two filters have been used: 435 nm (blue) and 606 nm (orange). The stars of the main "body" of NGC 4485 look quite bright through the 435 nm filter, and they look fainter through the 606 and 814 nm filters. As a result, they look rather bright and blue-white.

For the other picture, I would imagine that another set of filter images have been used. An ultraviolet exposure, either through a 275 nm or 336 nm filter, has probably been used for the blue channel. The green channel is likely once again 606 nm, and the red channel, if one has been used, is either 657 nm for H-alpha and N III, or 814 nm for infrared light.

Replacing the 435 nm filter for a 275 nm or 336 nm would explain why the body of NGC 4485 in one picture is peppered with bright blue stars in one picture, but looks so pale and wan in the other. If the main body of NGC 4485 is dominated by A- and F-type stars, these stars would look bright through a blue filter, but not through an ultraviolet one. But B- and O-type stars, which strongly dominate the starburst region, would look just a bit brighter than the A- and F-stars through a blue filter, but completely outshine them through an ultraviolet filter.

It is however totally weird that the main body of NGC 4485 appears completely devoid of red stars in both of the Hubble images, because red giants are everywhere. I almost have to wonder if the main body of NGC 4485 simply wasn't imaged though a long-wave filter. In fact, that might be the explanation here. A 606 nm filter would not do a tremendously good job at enhancing the kind of modest red giants that we expect to see in a galactic body dominated by stars like Vega or Sirius.

Click to view full size image

NGC 4485 through SDSS filters. Source:
http://skyserver.sdss.org/dr14/SkyServe ... pt=&query=

In any case, it appears true that NGC 4485 simply lacks a yellow center. In the picture at left, you can see a picture of NGC 4485 through SDSS filters. SDSS shows red H-alpha regions as green, but otherwise the SDSS colors are relatively similar to "normal" RGB colors. The fact that the main body of NGC 4485 is bluish in SDSS means that there are extremely few infrared-bright stars in there. (But note that the outer halo of NGC 4485 is definitely more yellowish than the center of NGC 4485 in the SDSS picture, meaning that the halo contains a greater proportion of cool red stars.)

Ann

[geckzilla]

A darkened picture of NGC 4485's swath of older stars for Ann.

[Ann]

Thanks!!! Geck, you have solved the mystery. Now the ubiquitous red giant stars can be seen in NGC 4485, and the main galactic body looks properly dotted again!

Ann