Sky and Telescope - October 2017 - 66

An Awesome Surprise

Arp's Atlas of Peculiar Galaxies
◗ American astronomer Halton Arp focused on both galaxy
formation and quasars during his career, using his research
in these areas to argue against the Big Bang theory. Although
his rejection of the Big Bang remains controversial, his Atlas
of Peculiar Galaxies, produced in part to support his ideas,
remains one of the best catalogs of interacting galaxies.
Arp began gathering the photos that would serve as
the basis for his atlas in 1962. Using earlier, similar lists
compiled by Swiss astronomer Fritz Zwicky and Russian
astronomer Boris Vorontsov-Vel'yaminov as a starting
point, Arp sifted through images captured with the 48-inch
Schmidt telescope and the 200-inch Hale telescope, both
at Palomar Observatory. He eventually selected 338 images
of galaxies with unusual, or "peculiar," shapes and sorted
them into four groups based on each galaxy's deviation
from a classic spiral form.

depths of ﬁeld available to amateur observers. The star 16 Psc
lies in our Milky Way galaxy, about 101 light-years distant.
That doesn't seem so far away.
But as noted above, the two galaxies of Arp 284 are about
100 million light-years away, while the quasar's light has
taken about 10 billion years to get here. These numbers are
much more difﬁcult to relate to, so looking at events on Earth
when these photons started their journey helps give them
some perspective.
World War I was raging 101 years ago. Albert Einstein's
theory of general relativity had been published for only a year
and was as yet unproven.
The Cretaceous Period was in full swing 100 million years
ago; the ﬁrst ﬂowering plants made their appearance. Dinosaurs were leaving their tracks on the muddy shorelines of
Earth's lakes and seas.

p WOW!!! A view of a quasar 2333+019 warrants three exclamation
marks in anybody's observing log. The author's sketch of Arp 284 shows
the 18th-magnitude quasar as viewed through Jimi Lowrey's 48-inch
telescope at 609× and 812×. South is up.

O C T O B E R 2 0 17 * S K Y & T E L E S C O P E

The Earth didn't exist 10 billion years ago, and its Sun
wouldn't form for another 5 billion years or so. Everything
that makes up the solar system today was part of some nowextinct star, the interstellar medium, or a mixture of both.
Okay, that's incredible.
Now consider that in the photons' frame of reference,
time and distance have both become something outside the
human experience. At light speed, time slows to a stop and
distance contracts to zero - something I didn't recall while at
the eyepiece. So in the photons' frame of reference, they were
emitted by the quasar at the same instant they were absorbed
by the retina in my left eye. No time and no distance.
This is equally true for the photons coming from Arp 284,
16 Psc, or any light source for that matter. Goodness.
As if this isn't mind-bending enough, the universe was
expanding during the 10-billion-year light-travel time. When
the photons I saw left their source, the quasar was only 5.8
billion light-years from Earth. Now, redshift tells the story of
the expanding space those photons have crossed: For a
z = 1.871 object, that space has enlarged by almost a factor of
three. I'm not sure if that's more or less crazy than a timeless,
distance-less photon traveling for 10 billion years.
All this is as unintuitive as it gets, and although it's an
accurate description of reality, I really don't understand how
it works. Nonetheless, I'll never get tired thinking about it,
especially when ageless photons entice me to try.
¢ Contributing Editor HOWARD BANICH's friends Jimi Lowrey
and Steve Gottlieb were every bit as excited as he was by these
observations. Howard can be reached at hbanich@gmail.com.

SK E TCH: HOWA RD BANICH; ATL AS OF PECULIAR G AL A XIES: H A LTON A RP/ N ASA / IPAC
E X TR AG A L ACTIC DATA BASE (NED) / CA LTECH / CA R NEGIE OBSERVATORIES

* The Atlas of Peculiar Galaxies is available online at
https://is.gd/ArpCatalog.

https://www.is.gd/ArpCatalog

Sky and Telescope - October 2017

Table of Contents for the Digital Edition of Sky and Telescope - October 2017