We present a pilot search of CO emission in three H2-absorbing, long-duration gamma-ray burst (GRB) host galaxies at z ∼ 2–3. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to target the CO(3 − 2) emission line and report non-detections for all three hosts. These are used to place limits on the host molecular gas masses, assuming a metallicity-dependent CO-to-H2 conversion factor (αCO). We find, $M_{\rm mol} \lt 3.5\times 10^{10}\, M_{\odot }$ (GRB 080607), $M_{\rm mol} \lt 4.7\times 10^{11}\, M_{\odot }$ (GRB 120815A), and $M_{\rm mol} \lt 8.9\times 10^{11}\, M_{\odot }$ (GRB 181020A). The high limits on the molecular gas mass for the latter two cases are a consequence of their low stellar masses M⋆ ($M_\star \lesssim 10^{8}\, M_{\odot }$) and low gas-phase metallicities ($Z\sim 0.03\, Z_{\odot }$). The limit on the Mmol/M⋆ ratio derived for GRB 080607, however, is consistent with the average population of star-forming galaxies at similar redshifts and stellar masses. We discuss the broader implications for a metallicity-dependent CO-to-H2 conversion factor and demonstrate that the canonical Galactic αCO will severely underestimate the actual molecular gas mass for all galaxies at z > 1 with $M_\star \lt 10^{10}\, M_\odot$. To better quantify this we develop a simple approach to estimate the relevant αCO factor based only on the redshift and stellar mass of individual galaxies. The elevated conversion factors will make these galaxies appear CO-‘dark’ and difficult to detect in emission, as is the case for the majority of GRB hosts. GRB spectroscopy thus offers a complementary approach to identify low-metallicity, star-forming galaxies with abundant molecular gas reservoirs at high redshifts that are otherwise missed by current ALMA surveys.

中文翻译：

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GRB宿主星系具有强H2吸收：宇宙恒星形成高峰期的CO-暗分子气体

我们在 z ∼ 2-3 处对三个吸收 H2 的长持续时间伽马射线爆发 (GRB) 宿主星系进行了 CO 排放的试点搜索。我们使用阿塔卡马大毫米/亚毫米阵列 (ALMA) 来瞄准 CO(3-2) 发射线并报告所有三个主机的未检测到。这些用于限制主体分子气体质量，假设金属量依赖的 CO-to-H2 转换因子 (αCO)。我们发现，$M_{\rm mol} \lt 3.5\times 10^{10}\, M_{\odot }$ (GRB 080607), $M_{\rm mol} \lt 4.7\times 10^{11} \, M_{\odot }$ (GRB 120815A), 和 $M_{\rm mol} \lt 8.9\times 10^{11}\, M_{\odot }$ (GRB 181020A)。后两种情况的气体分子质量上限是由于它们的低恒星质量 M⋆ ($M_\star \lesssim 10^{8}\, M_{\odot }$) 和低气相金属丰度($Z\sim 0.03\, Z_{\odot }$)。然而，从 GRB 080607 推导出的 Mmol/M⋆ 比率的限制与具有相似红移和恒星质量的恒星形成星系的平均数量一致。我们讨论了金属量依赖的 CO 到 H2 转换因子的更广泛含义，并证明规范的银河系 αCO 将严重低估所有星系在 z > 处的实际分子气体质量。1 与 $M_\star \lt 10^{10}\, M_\odot$。为了更好地量化这一点，我们开发了一种简单的方法来仅基于单个星系的红移和恒星质量来估计相关的 αCO 因子。升高的转换因子将使这些星系看起来像 CO-'dark' 并且难以在发射中检测到，就像大多数 GRB 宿主的情况一样。因此，GRB 光谱提供了一种补充方法来识别低金属性，