Sky & Telescope - May 2019 - 58

MAY 2019 OBSERVING
Going Deep
N

lensed image forms a ring-shaped
structure that threads around the core.
And a more distant galaxy (4.5 billion
light-years) is gravitationally distorted
into an extended tangential arc. Based
on these features the mass of ESO 146-5
is up to 30 trillion solar masses - the
highest known for any galaxy out to 1.5
billion light-years.
The farthest cD galaxy I've managed
is 15.2-magnitude PGC 37477 (also
known as MCG +04-28-097), a short 2°
star-hop from 6.6-magnitude 1 Comae
Berenices. This giant X-ray-emitting
galaxy anchors the rich cluster Abell
1413. It displays a highly elliptical halo
that mimics the north-south elongation
of the cluster, an alignment feature seen
with other cD galaxies. I've only viewed
this galaxy in my 24-inch reﬂector, but
it should be within reach of an 18-inch.
In 1956 American astronomers
Milton Humason, Nicholas Mayall, and
Allan Sandage published a landmark
survey of 800 galactic redshifts that
provided a solid basis for the Hubble
constant (H0). The shifts in the spectral lines of PGC 37477 implied a high
recessional velocity of 42,844 km/s.
Using an average modern value for H0 of
70 km/s/Mpc, its light-travel time is a
whopping 1.8 billion years.
58

M AY 2 0 1 9 * S K Y & T E L E S C O P E

To reach further back in time let's
take a look at a few quasars, the most
luminous objects in the universe. The
optically brightest quasar, 3C 273, was
ﬁrst cataloged as a radio source in 1959.
A lunar occultation in 1962 allowed
Caltech astronomer Maarten Schmidt
to associate 3C 273 with a 13th-magnitude stellar object. In December that
year Schmidt obtained a spectrum using
the 200-inch Palomar telescope, but he
couldn't make any sense of the emission
line wavelengths. The "eureka" moment
came a month later when he suddenly
realized the hydrogen emission spectrum (Balmer series) was redshifted by
16%, implying a light-travel time of an
incredible (at the time) 2 billion years.
Quasar 3C 273 shines with a mean
visual magnitude of 12.8 and is visible
in a 6-inch scope, but over time it can
vary from 12.2 to 13.6. You'll need a
detailed ﬁnder chart to make a positive
identiﬁcation as a half-dozen Milky
Way stars from 12th to 13th magnitude lie within 10′. The quasar displays
a knotty, relativistic jet with optical,
radio, and X-ray emission. Although
the jet isn't considered within range of
amateur telescopes, I've glimpsed it at
813× as a thin spike through Jimi Lowrey's 48-inch telescope in west Texas.

CTA 102 was discovered in Pegasus in 1960 at Caltech's Owens Valley
Radio Observatory, and it's the rock
star of quasars. Russian astronomer
Nikolai Kardashev caused a stir in
1963 when he proposed CTA 102 was
a beacon from an advanced civilization based on the shape of its radio
spectrum. Follow-up observations,
which showed CTA 102 was a variable radio source with a period of 100
days, added fuel to this speculation. In
1965 Maarten Schmidt determined a
redshift of 1.037, yielding a light-travel
time of 8 billion years. But the media
excitement inspired The Byrds to muse
on extraterrestrial life in the 1967
recording "C.T.A.-102."
CTA 102 is classiﬁed as an optically
violent variable quasar. It contains
high-energy plasma jets launched
from a supermassive black hole - one
of which is pointed in our direction.
As we look down the throat of the jet,
changes in the black hole's accretion
disk can trigger dramatic ﬂares. CTA
102 has a normal quiescent brightness
of only 17th magnitude, but it staged
an epic show that began in November
2016. It was fascinating to follow the
gyrations in brightness over several
nights. The outburst peaked on Decem-

IC 1101: N ASA / SDSS; ESO 146 - 5: ESO

p Left: IC 1101 houses a supermassive black hole that weighs in at 40 to 100 billion solar masses. The diffuse halo that surrounds the galaxy extends
for some 4 million light-years, the largest known to date. Galaxies such as this cD elliptical are thought to grow to their gargantuan sizes through
mergers. Right: ESO 146-5 is a cD galaxy in the southern constellation Indus. (This is not accessible to North American observers - you'll have to
travel south to see this target.) This supergiant elliptical in the cluster Abell 3827 appears to be devouring at least four other galaxies. It also shows
features of the gravitational lensing of a background galaxy that shows up as the wispy blue structures in the image.

Sky & Telescope - May 2019

Table of Contents for the Digital Edition of Sky & Telescope - May 2019