Sky and Telescope - September 2018 - 34

Mystery Object

through. Using this method, they obtained a spectrum that
revealed emission lines, the analysis of which suggested that
they originated from a giant elliptical galaxy. Using these
lines, they calculated BL Lac's redshift and determined that it
corresponded to a distance of 1.1 billion light-years.
So BL Lac was indeed not only extragalactic but also at a
great distance from our galaxy!
There then followed a series of back and forths between
different observing groups using a variety of telescopes,
including the 120-inch Shane reﬂector at Lick Observatory,
that failed to reproduce the emission lines. A group using the
Kitt Peak 84-inch reﬂector instead detected the presence of
an elliptical galaxy. Subsequent observations by Gunn and
Oke, again with the 200-inch Hale telescope, conﬁrmed their
earlier analyses. Gradually the community began to accept
that BL Lac was indeed the core of a giant elliptical galaxy
more than one billion light-years away.
Within a short time, astronomers determined the redshifts of several other objects similar to BL Lac using Gunn
and Oke's masking technique. One extreme example was
PKS 0735+178, discovered with the Parkes Radio Telescope in
Australia. It was originally mistaken for a planetary nebula,
but astronomers subsequently measured its distance: 8 billion light-years! All these sources sharing the observational
properties of BL Lac collectively came to be known as "BL Lac
Objects," or simply "BL Lacs."

So what are BL Lacs? This is what astronomers presumed they
knew in the 1970s:
1. They are the cores of galaxies.
2. They lie at great distances from Earth.
3. At these great distances, they have to be the most luminous objects in the universe.
4. Their energy output varies frequently and irregularly.
5. Spectra of their cores display no lines, either in emission or
in absorption.
6. Radio and optical emission is polarized, with the direction
of the polarization varying on short timescales.
7. Energy pours out constantly.
So what type of object could explain all of this behavior?
Astronomers went through the list of possible suspects and
eliminated them one by one.
Stars could not be the source of this great outpouring of
energy. BL Lac's spectrum precluded any known or postulated star. In fact, there were good arguments against such a
star existing at all, given the known laws of physics. Normal
stellar fusion processes could not produce the types of energy
output seen from BL Lacs.
Astronomers also eliminated white dwarfs. Their energy
output was minuscule in comparison, even at galactic dis-

Quasar

Intensity

Blazar

A Proﬁle of the Unsub

Jet

Magnetic
field lines

BL Lac object

Wavelength

T
Turbule
Turbulenc
enc
nce

Wavelength

THE SPECTRUM REVEALS NOTHING Frustratingly to astronomers, BL Lac's spectrum contains
neither absorption nor emission lines with which to glean information on its true nature. It was
only when redshift measurements firmly placed the source at cosmological distances that astronomers grasped that BL Lac might be the shining core of a galaxy far away. The type of object
observed - for example, blazar or quasar - depends on line of sight, as shown by the arrows.

S E P TE M B E R 2 018 * SK Y & TELESCOPE

Quasar

Infalling material

Intensity

Wind

Accreting
central
object

Intensity

Accretion disk

Normal galaxy

Wavelength

Sky and Telescope - September 2018

Table of Contents for the Digital Edition of Sky and Telescope - September 2018