In
the 16th March 2011 OOTD thread on a controversial galaxy, several interesting galaxy-quasar associations were mentioned by various zooites. All these involve galaxies with redshifts considerably smaller than the redshifts of the associated quasars (or QSOs) ...
...
except one!
The object in the crosshairs is
SDSS J204956.15-001204.5 (DR8 Explore link); the quasar - the bright blue blob to the top-left (NE) - is
SDSS J204956.61-001201.7 (also DR8 Explore link).
c_cld, who posted this interesting pair, provided a link to a recent paper (
preprint actually), by da Silva et al. entitled "
Shining Light on Merging Galaxies I: The Ongoing Merger of a Quasar with a 'Green Valley' Galaxy", which discusses this pair at considerable length. c_cld's comment? "no redshift discrepancy
"
The paper tells an interesting tale of astronomical sleuthing, of
luck serendipity, and provides a good snapshot of where astronomers are at, today, in terms of understanding what turns a quasar on,
How Green Was My Valley why some galaxies live in
a the green valley, and how Hanny's Voorwerp came to be.
The plot begins several decades ago, shortly after quasars were discovered to be high-redshift objects.
Very quickly it was realized that these distant, intrinsically very bright, point sources were almost custom-made for extra-galactic astronomers: they act like the light source in absorption spectroscopy.
The medium between the distant quasar and a spectroscope here on Earth (or just off it, aboard the Hubble Space Telescope, say) is illuminated by a beam of light whose photons have sufficient energy to cause any^ electrons in atoms or ions in between to 'jump' to a higher energy level, and leave their footprints in the spectrum we see, as absorption lines. No surprise, then, to learn that one of the Hubble Space Telescope's initial Key Projects - i.e. the major scientific research for which it was primarily built - was the "Quasar Absorption Line" Key Project (I wrote about this briefly
here; more detail
here).
Thousands of papers have been published on these quasar absorption lines. One subset is devoted to exploring the nature of the 'outer atmosphere' (if I may call it that) of galaxies ... the sight-line to the distant quasar passes near the bright part of a galaxy - as seen in an SDSS image, say - and so very likely through the extended halo of gas/plasma a few dozen to hundred kpc from it.
And that was what Jason Tumlinson (et al.) - one of the paper's authors - was (were) doing, looking for "
targets for a study of gas in the extended halos of galaxies via quasar sight lines". They came across this pair. At the time, the galaxy's redshift had not been estimated spectroscopically, and a photometric redshift (based on its SDSS colors) suggested that it was in the foreground (the quasar's SDSS spectroscopically determined redshift is 0.369; the galaxy's photometrically estimated one 0.28).
However, when they took a detailed spectrum, they found the quasar and galaxy have the same redshift (image is Figure 3
1 from da Silva et al.)!
Having discovered its true nature, the team seems to have switched the focus of their research, and have begun looking at this object as a way to test various hypotheses about how quasars are turned on (the ones in the sort of environment of the pair are thought to be fired up by galaxy mergers), why a galaxy lives in the green valley (a question which zookeeperKevin has studied,
and whose research da Silva et al. quoted), and what the nature of the 'tidal debris' is.
And this last point provides the link to Hanny's Voorwerp!
In Hanny's Voorwerp, a now dead (or merely sleeping?) quasar shone enough high-energy light (x-rays, UV) onto the gas that was likely pulled out of the parent galaxy by a past merger (now otherwise unseen) to make it shine brightly in 'forbidden' light (i.e. emission of 'nebular' lines, ones which correspond to atomic transitions whose probabilities are so low that they can only be seen in conditions that we'd call hard vacuums here on Earth; the two green [OIII] lines near 500 nm are perhaps the best known, and most prominent, of these).
In this pair (no one's given them a name yet, perhaps the Tumlinson duo?), the quasar is still active, and the tidal debris is beautifully lit up, as this 2D spectrum
2 clearly shows (if the Tumlinson duo were as close to us as Hanny's Voorwerp is, it would be a spectacular sight!):
I'll let NGC3314 make the final comment; he wrote,
in a post in the earlier OOTD thread:
That latest paper - on the QSO ionizing a tidal tail and gas in the companion, at z=0.37 - is a striking instance of something we see in the Voorwerpje sample (and in fact I just added that reference to the draft). The easiest way to put a lot of cold gas in a position to be ionized by an AGN that far out seems to be a galaxy encounter pulling out a gas-rich tidal stream. If we knew enough of these, they might give us a way to look at the angular pattern of emerging radiation, seeing which parts of the gas stream are and are not ionized.
^ well, strictly speaking, only those 'allowed' by their quantum states and the energy of the incident photons
1 caption:
One dimensional Keck/LRIS spectra of the galaxy and quasar comprising our interacting pair (the J2049−0012 system). Note the strong Balmer series absorption lines in the galaxy spectrum; these are suggestive of a starbust population. Also note that the quasar and galaxy are at nearly identical redshift. The quasar is at zq = 0.3691 ± 0.001 and the companion galaxy is offset by only 160 ± 20 km s−1. As a clarification, we note that this quasar velocity is not the systemic velocity of its host galaxy. We find the velocity difference between the two galaxies to be 30 ± 30 km/s. The strongest absorption and emisson lines are labelled by their ion. Fig. 10 shows an enlarged view of the post-starburst features along with models used to constrain the age of the stellar population.
2 Figure 4 in da Silva et al.; caption:
2D discovery spectrum of the J2049-0012 quasar/galaxy system centered near Hβ and [O III] emission made using the Keck/LRIS 600/7500 grating. The continuum light of the quasar (top) and galaxy (bottom) are the bright horizontal bands in the image. The bright quasar spectrum is centered at the 0 kpc (0") spatial position and saturates the image in this stretch. The image covers, from left to right, the H β, [O III]λ4959, and [O III]λ5007 transitions corresponding to z = 0.37. These emission lines are detected for the quasar, its host galaxy, and the companion galaxy as well as a 'bridge' of emission that connects the two objects. The emission lines are not detected beyond the spatial slice presented here.
