The Caldwell Objects

cosmologist Halton Arp (Max-Planck Institute for
Astrophysics) thinks so. In a 1998 Astrophysics and
Space Science paper, Arp notes that the Rosat
spacecraft also has found a striking excess of Xray
sources — the brightest of which are quasars
— around NGC 1097's optical jets. If, as Arp
suspects, the fainter X-ray sources also are
predominantly quasars, "at least 10 and possibly
considerably more X-ray quasars appear to be
associated with NGC 1097." Most cosmologists
believe the quasars are the active nuclei of
galaxies that are vastly more distant than NGC
1097, but Arp has posited that quasars are ejected
from the nuclei of active galaxies.
NGC 1097 is a large galaxy, with a diameter
of 123,000 light-years and a mass of 200 billion
Suns, making it a good match for our own Milky
Way. It is receding from our solar system at a
speed of 1,275 km per second. We see NGC
1097's barred form inclined 33° from edge on.
High-resolution images of NGC 1097's nuclear
region reveal a ring of star formation with a
slightly higher metal abundance than our Sun's.
If we accept a distance of 47 million light-years
to NGC 1097, the starburst ring measures 6,500
light-years across. Outside the ring, ΗII regions
are enhanced where the galaxy's bar and inner
spiral arms intersect. No signs of star formation
appear inside the ring, though Hubble Space
Telescope (HST) images reveal narrow dust lanes
intricately winding between the ring and the
nucleus. Actually, the ring is incomplete; HST
resolved it into a two-armed spiral of ΗII regions
and clumps of bright stars that appear on the
leading edges of strong dust lanes. Such twoarmed
structure may be generic to nuclear rings
in barred spiral galaxies. This notion is bolstered
by computer simulations that show how gas
flows along a galaxy's bar to feed star formation
in a ring around the nucleus. In the case of NGC
1097,
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the burst of star-forming activity along the ring
(100 million solar masses' worth) may have
occurred nearly instantaneously some 6 to 7
million years ago (in the galaxy's frame of reference,
that is; recall that its light takes 47 million
years to reach us).
William Herschel did not see the ring when
he discovered the "nebula" in 1790, though I
wonder if his son, John, did, albeit indirectly,
since he saw a pretty large, "round" nucleus. The
Rev. T. W Webb . called the object simply "very
bright and large." The 9.8' x 6.1' galaxy shines at
magnitude 9.2, but its surface brightness is rather
paltry. I could not see the galaxy with 7x50
binoculars, though it has been spotted in 11 x80s
with averted vision. The best way to locate NGC
1097 is to star-hop from 3rd-magnitude Acamar,
Theta (θ) Eridani, the original end of the River
Eridanus. First go 5° north-northwest to the
sharp, 1°-wide triangle of Eta1,2,3 (η1,2,3 ) Fornacis.
These stars shine at magnitudes between 5.5 and
6.5. Note that Eta2 and Eta3 Fornacis form a nearisosceles
triangle with Theta and Iota (ι) Eridani;
if you cannot see Eta2 and Eta3 Fornacis with your
eyes, you can at least point your finder to their
location, using Theta and Iota Eridani as guides.
With binoculars you can confirm that you've
correctly located the field. Continue from Eta1,2,3 Fornacis about 3½° north to magnitudes.5 Beta
(β) Fornacis. The galaxy is just another 2¼° northnorthwest
of Beta Fornacis.
With a glance at 23x in the 4-inch NGC 1097
is not impressive at first, appearing only as a faint
glow with a starlike core just 15' north of a tiny
triangle of binocular stars about 5' across. By the
way, if you move your telescope 1½° to the west,
you'll see a beautiful dipper asterism of 6th- to
8th-magnitude suns. With north "up," the dipper
has its "bowl" on the "table." It also looks a bit like
the Pleiades. If you place the galaxy on the
eastern edge of a
The Caldwell Objects 267