Uncovering the absorbed atomic Universe with the [OI]63um line

Abstract

We report the discovery of strongly absorbed [OI]63um in a sample of 12 DSFGs at 4.2<z<5.8 selected from the SPT survey. This is the first systematic survey of the [OI]63um fine-structure line at z>4. Using ALMA Bands 9 and 10, we obtain spatially and spectrally resolved observations that probe the interstellar medium on sub-kpc scales. Despite reaching sensitivities 10-100x deeper than most previous studies, we detect [OI]63um in emission in only 2 sources at low significance, with the remaining galaxies yielding stringent non-detections over the full velocity range covered by robust detections of other far-infrared lines, including [CII] and [NII]205um. We identify several compact (0.05-0.2") regions having [OI]63um absorption against the far-infrared dust continuum, some of which are possibly reaching below rest-frame CMB radiation level. We also detect narrow, spatially localised [OI]63um emission "escape channels" preferentially detected in regions with weak or absent dust continuum emission. We predict that similar absorption effects may appear in the [CII] line, particularly when concentrating on the regions with the densest foreground material along the line of sight. The [OI]63um line appears to be originate from a mix of compact, high optical depth [OI]63um emitting regions and sub-thermally excited, oxygen-rich molecular clouds dispersed throughout high-redshift starbursts that are capable of absorbing the ground-state line emission. Combined with a comparison to cosmological radiation hydrodynamical simulations, this supports the interpretation that regions with higher gas and dust column densities may lead to weakening an intrinsically strong [OI]63um line emission. We argue that the high [OI]63um optical depth is the dominant effect causing the strong absorption, limiting the diagnostic power of this line to trace regions of massive star formation in high-redshift DSFGs.

Figures (11)

• Figure 1: Overview of the results for SPT0106-64 and SPT0136-63 The central panel shows the Band 10 or 9 line+continuum image (at central frequency listed in Table \ref{['table:observations']}) in colour with the archival Band 7 continuum in white contours for reference. The beam sizes for both images are shown as red and white filled ellipses. The apertures used in Table \ref{['table:measurements']} are marked in red, and the extracted line+continuum spectra are shown in the left and right side panels. The top panel shows the continuum-subtracted [O i] 63 ${\mathrm{\mu m} }$ spectrum in yellow with the appropriate atmospheric transmission in both the signal and image band overplotted in blue and grey, respectively. The [C ii] 158 ${\rm \mu m}$ spectra from APEX or the ALMA 7m-array are overplotted in black, and where available, the [N ii] 205 ${\mathrm{\mu m} }$ spectrum is overplotted in green, with the scaling factors marked in the legend.
• Figure 2: Overview of SPT0155-62 and SPT0243-49 as in Fig. \ref{['figure:SPT0106OI-overview']}.
• Figure 3: Overview of SPT0418-47 and SPT0441-46 as in Fig. \ref{['figure:SPT0106OI-overview']}.
• Figure 4: Overview of SPT0459-58 and SPT0459-59 as in Fig. \ref{['figure:SPT0106OI-overview']}.
• Figure 5: Overview of SPT2132-58 and SPT2146-55 as in Fig. \ref{['figure:SPT0106OI-overview']}.