APOD: Voorwerpjes in Space (2015 Apr 04)

[APOD Robot]

Voorwerpjes in Space

Explanation: Mysterious Hanny's Voorwerp, Dutch for "Hanny's Object", is really enormous, about the size of the Milky Way Galaxy and glowing strongly in the greenish light produced by ionized oxygen atoms. It is thought to be a tidal tail of material left by an ancient galaxy merger, illuminated and ionized by the outburst of a quasar inhabiting the center of distant spiral galaxy IC 2497. Its exciting 2007 discovery by Dutch schoolteacher Hanny van Arkel while participating online in the Galaxy Zoo project has since inspired a search and discovery of eight more eerie green cosmic features. Imaged in these panels by the Hubble Space Telescope, all eight appear near galaxies with energetic cores. Far outside their associated galaxies, these objects are also likely echoes of quasar activity, illuminated only as light from a core quasar outburst reaches them and ultimately fading tens of thousands of years after the quasar outburst itself has faded away. Of course a galaxy merger like the impending merger of our own Milky Way and the Andromeda Galaxy, could also trigger the birth of a quasar that would illuminate our distant future version of Hanny's Voorwerp.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

[daddyo]

A little bit scary what a quasar could do to a whole galaxy

[bystander]

http://asterisk.apod.com/viewtopic.php?t=34610

[Ann]

http://asterisk.apod.com/viewtopic.php?t=34610

First of all, thank you for posting all these fascinating breaking science news, bystander! You were away for a while a year ago or so and the Breaking Science News forum really withered away. Thank you again for your hard work!

I find these "voorwerpjes" really fascinating, and the mechanism behind their formation even more so. But as usual, wouldn't you know it, I wonder about the colors and what they mean. This list of filters behind the images makes me think that the green color has been arbitrarily chosen, or rather, arbitrarily mapped. Of course, I realize that because of the cosmic redshift induced by the expansion of the universe, intrinsically green distant nebulas just can't look green when their light reaches us. So what I'm wondering about is if these nebulas really are intrinsically green. In other words, do they glow so brightly from green OIII light? Or are they in fact dominated by red Ha emission, like most emission nebulas?

The original "voorwerp" was discovered in an SDSS image. In SDSS images, red Ha nebulas are usually mapped as green. I hope I'm allowed to hotlink this SDSS image of NGC 3310, a relatively nearby galaxy dominated by hot stars and brilliant emission nebulas. In the SDSS image, the regions dominated by hot stars all look blue, but the red Ha nebulas look green.

That's why I keep wondering if these green voorwerpjes really are intrinsically OIII-brilliant and green, or if they are mostly red like most emission nebulas.

Ann

[Boomer12k]

So...um...is ONLY IC 2497 illuminating all of these shown??? That is kind of what it sounds like.

:---[===] *

[inertnet]

The word "voorwerpjes" means "little objects", which is a little odd for such large objects. The proper plural of "voorwerp" is "voorwerpen".

[Jim Shea]

"the impending merger of our own Milky Way and the Andromeda Galaxy" I've seen this claim frequently, but I don't see how we can know that these two galaxies will, in fact, collide. Of course, we know that Andromeda has a considerable blue shift, which means it has a component of radial motion toward us. What we don't know and can't measure is its component of proper motion because it's so far away. That component, it seems to me, could be great enough to cause Andromeda to pass to one side or the other of the Milky Way. I am not an astronomer so perhaps I am missing something. Am I ???
Jim Shea

[geckzilla]

"the impending merger of our own Milky Way and the Andromeda Galaxy" I've seen this claim frequently, but I don't see how we can know that these two galaxies will, in fact, collide. Of course, we know that Andromeda has a considerable blue shift, which means it has a component of radial motion toward us. What we don't know and can't measure is its component of proper motion because it's so far away. That component, it seems to me, could be great enough to cause Andromeda to pass to one side or the other of the Milky Way. I am not an astronomer so perhaps I am missing something. Am I ???
Jim Shea

Because galaxies are not solid objects but rather elastic and can become very stretchy when they interact it's difficult for them to pass by one another and not appear to touch, at least from an outsider's perspective. Even if Andromeda just happens to be moving towards Milky Way instead of the attraction being primarily gravitational then there is a good chance pieces of the two galaxies will extend to meet one another anyway.

[Chris Peterson]

"the impending merger of our own Milky Way and the Andromeda Galaxy" I've seen this claim frequently, but I don't see how we can know that these two galaxies will, in fact, collide. Of course, we know that Andromeda has a considerable blue shift, which means it has a component of radial motion toward us. What we don't know and can't measure is its component of proper motion because it's so far away. That component, it seems to me, could be great enough to cause Andromeda to pass to one side or the other of the Milky Way. I am not an astronomer so perhaps I am missing something. Am I ???
Jim Shea

By looking at the radial motion of all the galaxies in the Local Group, it is possible to put together plausible simulations of the system that allow the orbital elements of Andromeda to be determined with a fair degree of confidence. Good enough, at least, to be pretty certain that our galaxies will at least partially pass through each other in a few billion years.

[Case]

The original "voorwerp" was discovered in an SDSS image. In SDSS images, red Ha nebulas are usually mapped as green. I hope I'm allowed to hotlink this SDSS image of NGC 3310, a relatively nearby galaxy dominated by hot stars and brilliant emission nebulas. In the SDSS image, the regions dominated by hot stars all look blue, but the red Ha nebulas look green.
That's why I keep wondering if these green voorwerpjes really are intrinsically OIII-brilliant and green, or if they are mostly red like most emission nebulas.

In the original image in which Hanny found the Voorwerp, it was blue.

SDSS surveyed with five filters: ultraviolet, green, red, near infrared and far infrared. The SDSS uses the green wavelength picture for the blue band, the red wavelength picture for the green band and the infrared wavelength picture for the red band. These assignments were chosen because they are the middle three filters.

Thus the blue in SDSS images would be green in visible wavelengths. The sharp Hubble image does show it as (mostly) green.

[NGC3314]

Wow, APOD got this really fast!

Indeed, the clouds are pretty green. The [O III] lines are by far the strongest in the optical range, outshining even H-alpha by 3x or more. Their spectra (not coincidentally) look almost identical to that of Hanny's Voorwerp:



We started calling these "voorwerpjes" informally because they were smaller than Hanny's Voorwerp (although later it turned out that UGC 7342 and NGC 5972 have ionized gas that may go even farther out; we can't be sure at this point because the angle between the AGN-cloud and our line of sight is poorly known). NGC 5972 has filaments that are outside the Hubble field of view. The Galaxy Zoo volunteers turned up 19 more of these after Hanny's Voorwerp; what's special about these 8 is that their nuclei are to dim to account for the ionization levels in the clouds, leading us to hypothesize that they have recently faded (maybe as part of a sort-of-cyclic episodic behavior), over the timescale it takes light to go from the nucleus to the clouds.

Once the survey results finding these had appeared, other groups started finding analogous objects - the "Green Beans" at higher redshift, and a cloud in the nearby merger NGC 7252 (which people had been studying closely for decades). In NGC 7252, the active nucleus had faded so much that (AFAIK) existing data do not detect it directly at any wavelength.

There will be more to come - analysis to reconstruct the history of the active nuclei is in the pipeline, and X-ray data are building up to check whether it is possible that much of the AGN radiation is being lost to changes in obscuration very close to the black holes. We are building up a set of integral-field spectroscopic data both around the nuclei and more coarsely sampled in the extended clouds to distinguish gas outflows from tidal tails simply orbiting in the galaxies' gravity.