Sky & Telescope - USA (2020-01)

The Infrared Sky

24 JANUARY 2020 • SKY & TELESCOPE

The Farthest Galaxy

Spitzer Can See

Spitzer’s redshift limit is currently set by the heroic

observation by Pascal Oesch (now University of

Geneva) and his colleagues of a galaxy at a redshift

11.1. The detection required about 70 hours of

Spitzer observations at 4.5 microns. We see this

galaxy at a time when the universe was only 3%

of its current age.

Since the galaxy is observed at a lookback time of 13.2 billion

years, the result suggests that this galaxy’s star-formation

episode occurred about 300 million years after the Big Bang.

These observations enable Spitzer to measure the growth

of galaxies in two ways: by measuring how much mass is

in galaxies at a given time, and by measuring how fast the

galaxies are growing by forming stars. Comparing what we’d

expect the stellar masses to be, based on the starbirth rates,

with what we actually observe yields a gratifying confl u-

ence across more than 12 billion years of cosmic history. The

strong agreement demonstrates that with Spitzer, Hubble, and

large ground-based telescopes, we are indeed developing an

accurate picture of the growth and evolution of galaxies in

the universe.

Infrared Leaps

This wide-ranging scope of discovery is now coming to an

end. Faced with a limited pool of funds, NASA has chosen to

retire Spitzer because the high operating cost inherent to its

mission design made it less attractive than other operating

missions that were competing for the same funds.

Because Spitzer was such a leap in capability compared to

what had come before, it was able to lead the way in astro-

physical exploration over the last decade and a half. This

is the constant lesson of advances in technology that have

driven astrophysics since the end of World War II. We saw

this with the early infrared missions, with space observato-

ries, with the twin Keck telescopes, and with the Very Large

Telescope quartet in Chile, as well as with myriad other

instruments, all of which, in one way or another, have probed

the mysteries of the infrared universe. Doubtless we will

continue to see it with future ground- and space-based tele-

scopes, including the next major infrared facility, the James

Webb Space Telescope, set to launch in 2021.

Even as Spitzer sends its fi nal data back to Earth at the end

of January 2020, commands will be sent to place the space-

pDISTANT GALAXIES Every circle in this composite visible and infrared image marks a galaxy with a redshift of more than 7, corresponding to a

lookback time of nearly 13 billion years. The inset is a Spitzer image of one of the galaxies. The main image is of part of the sky near the Draco-Ursa

Major boundary, and most of the objects in the image are galaxies.

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