Science & tech · 2022
Can distant galaxies selected by absorption have conditions like those in the Milky Way?
Yes: two of the four galaxies had CO spectral-line energy distributions consistent with the Milky Way’s inner disc, including one at redshift z ∼ 2. The other two, both at z > 2, showed more interstellar gas conditions.
Details
Galaxy surveys that select galaxies by their emission are biased toward the most luminous members of the galaxy population. Selection by absorption is a potentially unbiased technique with respect to luminosity, but the physical properties of absorption-selected galaxies are not well characterized.
This observational study investigated excitation conditions in the (ISM) of galaxies selected through damped Ly α (DLA) absorption. The researchers studied the CO spectral-line energy distribution (SLED) in four high-metallicity, absorption-selected galaxies with previously reported CO detections, at intermediate (z ∼ 0.7) and high (z ∼ 2) redshifts.
The findings provide further evidence for a wide variety of ISM conditions in these galaxies. Two of the four galaxies had CO SLEDs consistent with that of the Milky Way’s inner disc. One of these was at z ∼ 2 and had a CO SLED below that of main-sequence galaxies at similar redshifts.
The other two galaxies, at z > 2, showed more excited ISM conditions. One showed thermal excitation of the mid-J (J = 3, 4) levels, similar to that seen in two massive main-sequence galaxies at these redshifts.
Overall, the study finds that absorption selection traces a diverse population of galaxies.
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