Absorption of Millimeter-band CO and CN in the Early Universe: Molecular Clouds in Radio Galaxy B2 0902+34 at Redshift 3.4. (arXiv:2401.03577v1 [astro-ph.GA])

Absorption of Millimeter-band CO and CN in the Early Universe: Molecular Clouds in Radio Galaxy B2 0902+34 at Redshift 3.4. (arXiv:2401.03577v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Emonts_B/0/1/0/all/0/1">Bjorn Emonts</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Curran_S/0/1/0/all/0/1">Steve Curran</a> (2), <a href="http://arxiv.org/find/astro-ph/1/au:+Miley_G/0/1/0/all/0/1">George Miley</a> (3), <a href="http://arxiv.org/find/astro-ph/1/au:+Lehnert_M/0/1/0/all/0/1">Matthew Lehnert</a> (4), <a href="http://arxiv.org/find/astro-ph/1/au:+Carilli_C/0/1/0/all/0/1">Chris Carilli</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Yoon_I/0/1/0/all/0/1">Ilsang Yoon</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Morganti_R/0/1/0/all/0/1">Raffaella Morganti</a> (5,6), <a href="http://arxiv.org/find/astro-ph/1/au:+Weeren_R/0/1/0/all/0/1">Reinout van Weeren</a> (3), <a href="http://arxiv.org/find/astro-ph/1/au:+Villar_Martin_M/0/1/0/all/0/1">Montserrat Villar-Martin</a> (7), <a href="http://arxiv.org/find/astro-ph/1/au:+Guillard_P/0/1/0/all/0/1">Pierre Guillard</a> (8), <a href="http://arxiv.org/find/astro-ph/1/au:+Cordun_C/0/1/0/all/0/1">Cristina Cordun</a> (5,6), <a href="http://arxiv.org/find/astro-ph/1/au:+Oosterloo_T/0/1/0/all/0/1">Tom Oosterloo</a> (5,6) ((1) NRAO, (2) Victoria Univ. Wellington, (3) Leiden Univ., (4) CRAL/Univ. Lyon, (5) ASTRON, (6) Univ. Groningen, (7) CAB/CSIC-INTA, (8) IAP)

Using the Karl G. Jansky Very Large Array (VLA), we have detected absorption
lines due to carbon-monoxide, CO(J=0-1), and the cyano radical, CN(N=0-1),
associated with radio galaxy B2 0902+34 at redshift z=3.4. The detection of
millimeter-band absorption observed 1.5 Gyr after the Big Bang facilitates
studying molecular clouds down to gas masses inaccessible to emission-line
observations. The CO absorption in B2 0902+34 has a peak optical depth of
$tau$ $ge$ 8.6% and consists of two components, one of which has the same
redshift as previously detected 21-cm absorption of neutral hydrogen (HI) gas.
Each CO component traces an integrated H$_2$ column density of N(H2) $ge$
3×10$^{20}$ cm$^{-2}$. CN absorption is detected for both CO components, as
well as for a blueshifted component not detected in CO, with CO/CN line ratios
ranging from $le$0.4 to 2.4. We discuss the scenario that the absorption
components originate from collections of small and dense molecular clouds that
are embedded in a region with more diffuse gas and high turbulence, possibly
within the influence of the central Active Galactic Nucleus or starburst
region. The degree of reddening in B2 0902+34, with a rest-frame color B-K ~
4.2, is lower than the very red colors (B-K > 6) found among other known
redshifted CO absorption systems at z<1. Nevertheless, when including also the
many non-detections from the literature, a potential correlation between the
absorption-line strength and B-K color is evident, giving weight to the
argument that the red colors of CO absorbers are due to a high dust content.

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