Molecular Clouds Resolved at the Onset of Cosmic Noon

Abstract

We present the discovery of seven molecular clouds in the radio galaxy B2 0902+34 at redshift z=3.4. These clouds are detected as CO(0-1) absorption features against the bright radio continuum, and spectrally resolved using the Karl G. Jansky Very Large Array (VLA). The velocity dispersion of the individual absorption components ranges from 3-7 km/s, which is similar to values observed for molecular clouds in the Milky Way and nearby galaxies, and imply cloud radii of R~10$^{1-2}$ pc. The absorbing clouds are found in a region of high obscuration inside a 30 kpc wide stellar nebula, as revealed by rest-frame near-ultraviolet imaging performed with the Hubble Space Telescope (HST). The fact that we spectrally resolve molecular clouds at the onset of Cosmic Noon opens prospects for studying cloud chemistry and physics that drive the formation of stars in the Early Universe.

Figures (3)

• Figure 1: CO(0-1) absorption in B2 0902+34.Left: Contours of the 26 GHz radio continuum overlaid onto a total intensity image of the CO(0-1) absorption across the velocity range -80 to 55 km s$^{-1}$. Contour levels start at 0.5 mJy beam$^{-1}$ and increase by a factor of 2. The inset at the top-left shows the 1.65 GHz map from car95, which reveals additional large-scale emission that our 26 GHz data do not detect (credit: C. Carilli, A$\&$A, 298, 77, 1995, reproduced with permission $\copyright$ ESO). Right: CO(0-1) spectrum. The top panel shows a 2-dimensional spectrum along the radio axis, crossing both the southern and northern 26 GHz component. The molecular clouds seen against the southern component are resolved spectrally, but not spatially. The middle panel shows the 1-dimensional CO(0-1) spectrum taken against the peak of the southern 26 GHz radio component. The spectrum is Hanning smoothed (Sect. \ref{['sec:data']}) and has a root-mean-square (rms) noise of 0.15 mJy beam$^{-1}$ chan$^{-1}$ across the line-free channels. The red line represents a combined fit of seven Gaussian components (dashed lines). The blue dotted line is the low-resolution spectrum from emo24. The bottom panel shows the residuals after subtracting the model from the spectrum, which are consistent with noise.
• Figure 2: Near-UV starlight around radio galaxy B2 0902+34.Left: HST/WFC3$_{\rm F105W}$ image of the rest-frame near-UV starlight. The radio source is shown with the same red contours as in Fig. \ref{['fig:COspec']}. Right: Zoom-in on the stellar nebula, with a 5-pixel boxcar smoothing applied. The dashed yellow ellipse highlights the region with substantially lower stellar flux, likely due to high obscuration, seen at the location of the southern 26 GHz radio component. Orange contours show the stellar emission starting at 28$\%$ and increasing by 9$\%$ of the peak intensity of the nebula.
• Figure 3: Comparison to molecular clouds in the nearby Universe.Left: H$_{2}$ column density plotted against velocity dispersion for molecular clouds detected in low-$J$ CO($J$+1, $J$) emission in the Milky Way Galaxy spi22, M31 (lad24, L24; den25, D25), and nearby galaxies of the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) survey ros21. For the emission-line work, $N_{\rm H_2}$ is calculated from the cloud mass ($M$) and radius ($R$) as an average over a projected circular area, assuming a uniform volume density and spherical cloud geometry, following $N_{\rm H_2}$ = $M$/($\pi R^{2}$). All mass values are scaled to a mass-to-light ratio of $\alpha_{\rm CO}$ = 4.3 bol13, with the PHANGS result including an additional dependence on local metallicity ros21. The red circles show the CO(0-1) absorption components of B2 0902+34, with their size scaled to the integrated line flux (Table \ref{['tab:results']}). The dashed black line shows the correlation derived from Larson's scaling relations between $\sigma$, $R$, and $M$lar81. The arrow at the bottom left [right] corner shows how the emission [absorption] data decrease [increase] in value when $\alpha_{\rm CO}$ [$T_{\rm ex}$] changes from 4.3 to 1.0 [15 K to 50 K]. Right: Velocity dispersion plotted against the velocity offset from the systemic redshift for molecular absorption components in nearby galaxies, for which also the molecular emission is observed (from ros24, including data from ros20 and wik97). Black squares are absorption components found in galaxies with large-scale, edge-on (45$^{\circ}$$<$$i$$\le$ 90$^{\circ}$) molecular disks. Gray stars are absorption features in galaxies lacking an edge-on disk, and likely represent a combination of circum-nuclear disks (CND) and infalling gas ros24. Red circles show the absorption in B2 0902+34.