YSM Issue 91.1

astronomy
FEATURE
IMAGE COURTESY OF WIKIMEDIA COMMONS
The James Webb Space Telescope is a space telescope developed in
coordination with NASA.
if the atom is moving away from us, this wavelength will get longer and
redder; if it is moving towards us, the light gets shorter and bluer. This
is known as the Doppler effect; most of us experience the aural version
of it every time an ambulance drives past, its siren getting shriller as it
approaches and then dropping as it drives away.
Since the Universe is expanding away from us, light coming from
its most distant sources is red-shifted. Smit explains that the brightest
emission in the most distant galaxies, which has optical wavelengths in
a lab, becomes redshifted into the mid-infrared. In her PhD thesis at
the Leiden observatory in the Netherlands, Smit used the infrared space
telescope, Spitzer, to measure redshifts precisely for a very large sample
of galaxies. The CII line, with a wavelength of 157.7 micrometers, is
already in the infrared, but it gets further redshifted into longer radio
waves. This is exactly what ALMA can detect.
“Getting time on ALMA is hard,” said Pascal Oesch, former postdoctoral
fellow at Yale and now assistant professor at the University of
Geneva, a co-author on the paper. The entire array of telescopes must be
reconfigured every time a researcher wants to look at a different range
of wavelengths. “You really have to know the redshifts and locations of
the targets, and Renske constrained those tightly with her Spitzer observations,”
Oesch said.
Now picture a disk of swirling gas, moving clockwise. Rotate the disk
so you’re viewing it from the side. Gas to the right half of the disk will
appear to be moving towards you, and on the left it’ll be moving away
from you. If there were carbon atoms emitting photons at exactly 158
micrometers everywhere in the disk, you’d think the light from the right
side of the disk actually had a wavelength shorter and bluer than that,
and that from the left larger and redder. Now think of two coins sitting
on this disk, at different distances from the center. As the disk rotates, the
coin farther from the center moves a longer total distance than the one
closer in. In other words, the velocity of the disk is greater at larger radii.
So the 157.7 micrometer line is redshifted increasingly more the further
left you look from the center, and blueshifted more the further you look
right. The line gets broadened into a bell shaped curve, the width of
which tells you how fast the gas in the disk is rotating.
Current simulations of galaxy formation in the early Universe show
these galaxies colliding with their neighbors often in what are called mergers.
These mergers disrupt the formation of any disks, and tend to leave
IMAGE COURTESY OF WIKIMEDIA COMMONS
The Atacama Large Millimeter Array (ALMA) is a collection of radio
telescopes in the Chile, five kilometers (sixteen thousand feet) above sea
level. Since light from a single source in the sky lands at slightly different
points on each telescope, the images can be combined using a technique
called interferometry to get extremely high resolution. Together, the
telescopes can see fainter objects than any one of them could on its own.
Source: The European Space Organization.
behind gas clumps with lots of star formation in the center. “We expected
the carbon emission to be pretty concentrated,” Smit said. Therefore, she
only expected to see lines from the center of the rotating disk. Instead, the
line-emission was extended enough that she could measure the velocity
variations across the galaxy. “The fact that we could actually see the rotation
meant that CII emission was more spread out,” Smit said.
That was not the only surprise. The CII line emission is only generated
if the carbon undergoes a lot of collisions, usually with electrons coming
from dust. “We see the carbon line but we don’t see any dust, and that is
a puzzle we haven’t solved yet,” Smit said.
Oesch doesn’t think it’s that surprising to find so little dust in these galaxies:
dust is released during a relatively late stage in a galaxies’ evolution,
and since they are still so young, the stars simply may not have reached
this phase of their lives. Further, he says, they may not have found
dust because they were looking for a very specific kind. “You have to
make assumptions of the temperature of the dust to predict how much
emission you would see,” said Oesch. It is just another example of how
carefully astronomers have to design their experiments to encompass all
of the components of a galaxy that they’re interested in.
Astronomers are also very careful about making inferences from
small samples. Smit is excited about the upcoming James Webb Space
Telescope, which will detect hundreds or even thousands of galaxies
at these high redshifts. James Webb will have an IFU, or Integral Field
Unit, a device that takes a spectrum on every pixel of the camera.
“We’ll at least be able to get low resolution but large samples,” she said.
Smit already has time on ALMA to look at six more galaxies, which
will help us build a picture of how common galaxy disks really are in the
early Universe. She is also preparing to observe one of these galaxies at
a much higher resolution. “We’ll be able to see how organized the disk
is, or if its messier than we thought. It’ll tell us about the physics of at
least one disk in much more detail,” Oesch said. She emphasizes that
this really a new frontier of observation. “Maybe it’s not a single disk—
maybe it’s multiple clumps merging! We really haven’t been able to do
any of this before.”
www.yalescientific.org
March 2018
Yale Scientific Magazine
31