With the excitement about
new observations of Hanny's Voorwerp, this might be a good time to point out that Zoo participants are finding new examples of a related (albeit less spectacular) class of objects. (That was evidently my roundabout way of saying "hey, everyone, you can look for these too!").
What do these four galaxies have in common?
NGC 5972
588017991773520114
587736809916399664
Mkn 266First off, they all have active galactic nuclei (AGN) as revealed by the spectra. Second, and more unusual, the SDSS images all show oddly colored filaments or loops around the nucleus. (Kevin named that second one the Teacup because of the loop). Colors this pure don't come from starlight - and indeed, wherever we have the additional information to say, we find that these are ionized gas illuminated by the nucleus. (This should sound familiar to member of the Voorwerp Van Club). Seeing these giant emission-line clouds tells us about the environment of the AGN - how much gas is out there, what its density and ionization state are... It also tells us about the AGN, since the gas views it from various directions that we don't. Some AGN are partially surrounded by cods of absorbing material so that our view isn't necessarily representative.
What else do we know about these galaxies? NGC 5972 attracted some interest as possibly being a spiral with a giant double-lobed radio source, which would be quite rare. In a study
published in 1995, Phillippe Veron and Mira Veron-Cetty showed that the "spiral arms" are filaments of gas ionized by the central AGN, whose motions are mostly smooth rotation about the core.
Colleagues at Georgia State University kindly measured a spectrum for part of the handle of Kevin's Teacup, using the 1.8m Perkins telescope at Lowell Observatory. Same story - the gas is ionized by the copious UV and X-radiation of an AGN. The loop shape is interesting, spanning something like 15,000 light-years. Could this be a bubble driven by some kind of hot outflow from the core? Scribble, consult Chandra instrument guide...
Mkn 266 (AKA NGC 5256) is a picturesque merger with embedded AGN. Its emission-line features have received attention all the way back to 1988, with interpretations including gas shocked by collisions during the merger.
I'd be willing to bet that there are more of these hiding in the SDSS data. In the usual color mapping, strong [O III] emission shows up as blue and H-alpha as green at low redshifts. H-alpha shifts to the red (
i) band at greater redshifts. The Teacup shows both [O III] and H-alpha, and at its redshift these add to purple.
Find another one? Feel free to chime in and share it here! Sharing common features but being much smaller, maybe we could call them voorwerpjes.
