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Cougar
2015-Apr-03, 02:05 AM
Hubblesite's got some great images (http://hubblesite.org/newscenter/archive/releases/2015/13/) of these strange "objects" akin to Galaxy Zooite Hanny's voorweep. I gather these show early galaxies that have interacted, which sent a lot of material falling into the larger galaxy's supermassive black hole, turning it on as a quasar (if it wasn't one already). Then the quasar's beams ionize sections of the disrupted galaxies that didn't fall into the quasar, and that's what continues to glow, even after the quasar shuts off? Is that what's going on here? :surprised:

EigenState
2015-Apr-03, 02:43 AM
Greetings,

ncg3314 should be along any time to tell you everything known about the voorwerps.

Best regards,
ES

ngc3314
2015-Apr-03, 03:19 AM
That's indeed pretty much the sequence we infer. The timescales are so different that my suspicion is that the active nuclei "flicker" up and down for millions of years, and these are the ones we happen to catch in a very low state lasting maybe a couple of hundred thousand years (because we see about twice as many of the giant clouds lit up around nuclei that we see "on" as "too dim", and the time spans we can trace by light-travel time to the clouds range up to about 150,000 years). The interaction probably play a role in feeding more material inward to be accreted by the central black holes, but also play a key role in making the history of its radiation observable - they are a good way to throw out streamers of gas far beyond the usual galaxy disks, and out of the galaxy plane as well, where it can easily intercept emerging ionizing radiation. No interaction, no gas tails, nothing to see here, move along.

These are pretty local as galaxies go, essentially in the present-day Universe. In fact, work by Joss Bland-Hawthorn and colleagues suggests that the Milky Way nucleus did something in this vein a million years ago, leaving remnant ionization levels in some of the gas stripped from the Magellanic Clouds requiring such a source to explain. (I just realized that putting these ideas together, observers in Andromeda "now" might see the Milky Way as a Seyfert galaxy). This notion of quasars (and lower-power active nuclei) being episodic on rather short times would also explain something known as the transverse proximity effect, which shows that quasars in the distant Universe have not ionized the gas seen near to them in space as much as we expect from their observed luminosity.

While this project was in various phases, other groups have found analogous systems at somewhat greater distances (redshifts z=0.2-0.4 rather than 0.02-0.08), known as Green Beans for their colors in the Sloan survey and sizes, and in the very nearby well-studied merger NGC 7252 (whose nucleus has faded so much that we cannot detect its nonstellar activity at all).

Coauthor Peter Maksym and I talked about these issues and more in the Hubble Hangout webcast from today (April 2).

(Pedantic multilingual spelling thing: Voorwerp/voorwerpjes)

Margarita
2015-Apr-03, 08:39 AM
The Hubble Hangout ngc3314 mentions is here (https://m.youtube.com/watch?v=AHskCVTHnh8)

Cougar
2015-Apr-03, 11:17 AM
Thanks, ngc3314. I thought I recognized your name in the writeup. Fantastic work. I'm particularly intrigued by the image of UGC7342 and what appears to be a small satellite galaxy at the lower left. Is that incidental, or the core remains of the erstwhile smaller galaxy that interacted?

ngc3314
2015-Apr-03, 11:58 AM
That galaxy is a photobomb - redshifts show it to be far in the background, at about 1.5 times the distance of C 7342. It is itself an actively star-forming system whose far-IR emission confused the luminosity of UGC 7342 until WISE mid-infrared data sorted them out. This means that the merger in UGC 7342 is more advanced than it first seems (which makes sense for having the churned-up dust lanes and extensive tidal tails around it).

George
2015-Apr-03, 03:37 PM
Are they all predominantly green for a reason?

EigenState
2015-Apr-03, 04:25 PM
Greetings,


While this project was in various phases, other groups have found analogous systems at somewhat greater distances (redshifts z=0.2-0.4 rather than 0.02-0.08), known as Green Beans for their colors in the Sloan survey and sizes, and in the very nearby well-studied merger NGC 7252 (whose nucleus has faded so much that we cannot detect its nonstellar activity at all).

Beans or Peas?

Best regards,
ES

EigenState
2015-Apr-03, 04:42 PM
Greetings,

Peer-reviewed papers on Hanny's Voorwerp from GalaxyZoo:

Chris Lintott, et. al.,Galaxy Zoo : 'Hanny's Voorwerp', a quasar light echo? (http://arxiv.org/abs/0906.5304), Monthly Notices of the Royal Astronomical Society, 399, 129 (2009).

G. I. G. Jozsa, et. al., Revealing Hanny's Voorwerp: radio observations of IC 2497 (http://arxiv.org/abs/0905.1851), Astronomy & Astrophysics, Volume 500 (Issue 2), pages L33L36 (2009).

Best regards,
ES

ngc3314
2015-Apr-03, 04:45 PM
Beans - because while they can be found with the same color selection as Green Peas, they are much bigger (beans... peas...) - one description is here (http://www.gemini.edu/node/11904). I hope to draw the line at further green vegetables.

These things are all described as green because their strongest optical emission lines (by far) are the [O III] doublet at 496/501 nm (emitted frame), redshifted by as much as 8% in our most distant case. For the peas it's a little less direct - they look green in the SDSS images because the display mages map irg filters to RGB, and they are at typical redshifts which put the [O III] lines in the r filter. Likewise, Green Beans are at z=0.2-0.4 and have [O III] in the red filter.

Some of our original spectra are shown in this paper (http://adsabs.harvard.edu/abs/2012MNRAS.420..878K), and they all look a lot like that of Hanny's Voorwerp:

http://astronomy.ua.edu/keel/research/WHTHVcomposite.gif

The He II and [Ne V] lines immediately distinguish this from gas photoionized by hot stars; they don't emit enough far-UV to make He II on these galactic scales, much less [Ne V] which requires 95 eV to put in the appropriate ionization state.

(Note to Eigenstate - these objects have such weak astrophysical continuum emission that an important contributor in the UV is the two-photon decay continuum of metastable hydrogen atoms. This so rarely matters on galaxy scales that I never expected to use that section of my class notes; sure glad I kept that binder!).

EigenState
2015-Apr-03, 05:02 PM
Greetings,


These things are all described as green because their strongest optical emission lines (by far) are the [O III] doublet at 496/501 nm (emitted frame), redshifted by as much as 8% in our most distant case.

The [O III] transitions are all 3P - 1D which are electric dipole forbidden but either magnetic dipole (M1) or electric quadrupole (E2) allowed.


(Note to Eigenstate - these objects have such weak astrophysical continuum emission that an important contributor in the UV is the two-photon decay continuum of metastable hydrogen atoms. This so rarely matters on galaxy scales that I never expected to use that section of my class notes; sure glad I kept that binder!).

Most interesting! Thank you for sharing that information, and I will make a note to retain my notes. ;)

Best regards,
ES

George
2015-Apr-03, 08:37 PM
Ionized oxygen they be. Thanks.

I would suggest green jelly beans as a compromise, unless Veggie Tales has a song already on these things.

George
2015-Apr-04, 08:42 PM
Guess what's on APOD today? Yep!!!