Pink Black Holes: Technical Details
[Mt Stromlo Observatory]
[Department of Physics]
[Astrophysical Theory Centre]
This page is intended for those with a technical background
who want more detail than was available in the press
These "pink black holes" are quasars. They were selected in a
radio survey: the Parkes Half Jansky survey of the southern sky
(Drinkwater et al. 1997, MNRAS 284, 85). This survey consists of
323 sources selected to have flat radio spectra. The optically
bright ones have long been studied: our innovation was to energetically
pursue the optically faint ones. We obtained accurate radio positions
with the VLA and ATCA, and identified all sources with deepish CCD and
near-IR array camera imaging. We have spectra for about 80% of the sample,
including all but the very faintest (B>23) 5% of the sources.
In 1994-5, we discovered that many of the optically faint Parkes
sources were extremely luminous in the near-IR (K-band: 2.1 microns).
Radio quiet quasars have essentially uniform B-K colours (around 2.5)
while these Parkes sources had B-K ranging from 2.5 out to 8! Their
colours were thus clearly much redder than normal quasars.
(Webster et al. 1995, Nature 375, 469).
Why the difference? Several rival theories were put forward:
Our spectra demonstrated that these sources were not galaxies (apart from
a few of the nearest ones, with radio spectra near the steep cut-off)
(Masci et al. 1998, MNRAS 301, 975). They have broad emission lines with
roughly normal line ratios and equivalent widths. We obtained
quasi-simultaneous BVRIJHK imaging of about half the sample, which
showed that most had spectra well fit by pure power-laws, with
indexes (F(nu) = nu^alpha) 0 > alpha > -2. A small sub-set were much
redder, and had spectra turning down in the blue and reddened line
ratios: these few are well fit by dust obscuration of blue quasars.
- Dust: these actually are typical blue quasars obscured by dust
somewhere along the line of sight.
- Synchrotron emission: we know that these compact flat-radio-spectrum
sources have relativistic jets. Perhaps the synchrotron emission from
these jets extends into the near-IR?
- Host galaxy contamination: perhaps they are not quasars but high
redshift radio galaxies, reddened by the redshifted 4000A break.
The puzzle is: what are these red power-law sources? They appear pink
to the human eye (I combined out BVR imaging weighted such that a sun-like
star would appear white). They show broad emission-lines such as MgII and
CIII] with roughly normal equivalent widths, so these are not classical
BL Lac objects (H-beta, on the other hand, has an equivalent width that
anticorrelates nicely with redness). We've measured the near-IR polarisation
for a few of them and they are quite strongly polarised out beyond a micron.
Many are reasonable X-ray sources as detected by ROSAT (Seibert et al.
1998, MNRAS 301, 261).
The problem is: how do you get synchrotron emission with a slope of -2?
This is close to the reddest slope you can get for any plausible electron
energy distribution. We can fit it with a cut-off in the electron energy
distribution, but this cut-off has to be at 1 micron +- 30% in well over
half of the quasars. This would argue for some fundamental physical
process locking the cut-off here: we cannot think of any (naievely the
cut-off should go as the square of the magnetic field). Even then, the
predicted spectrum shouldn't be a pure power-law, due to the Big Blue
Bump component coming in as shorter wavelengths (we need something blue
to photoionise the BLR). So: a puzzle! Suggestions welcome!
We have a couple of papers almost ready to submit about the data and
our modelling of it: watch this space!
Last updated 12th May 1999.