利用地面巡天与光谱后续观测统计 $z<4$ 的小红点
Counting Little Red Dots at $z<4$ with Ground-based Surveys and Spectroscopic Follow-up
AI总结：
本文利用地面超深巡天在 $z<4$ 范围内搜寻小红点，发现其数密度随红移下降而显著降低，并通过光谱后续观测验证了样本选择方法，为约束低红移 LRD 数密度提供依据。
摘要：
小红点（LRDs）是 JWST 在 $z>4$ 处发现的一类红色致密天体。在 $4<z<8$ 范围内，它们的数量密度大约是紫外选类星体的 100 倍。然而，由于 JWST 的天区覆盖范围较小且蓝波段覆盖有限，其在 $z<4$ 处的数密度仍不确定。我们展示了在 $2\lesssim z\lesssim4$ 范围内对 LRDs 的地面搜寻工作，结合了超深 Hyper Suprime-Cam 测光数据和多种（近）红外巡天数据，总覆盖面积约 $\sim3.1\\,\mathrm{deg^{2}}$。我们发现，对于 $M_{5500}<-22.5$ 的 LRDs，其数密度从 $z>4$ 时的 $\sim10^{-4.5}\\,\mathrm{cMpc^{-3}}$ 下降至 $2.7<z<3.7$ 时的 $\sim10^{-5.3}\\,\mathrm{cMpc^{-3}}$，并在 $1.7<z<2.7$ 时进一步降至 $\sim10^{-5.7}\\,\mathrm{cMpc^{-3}}$。我们还展示了首个后续观测候选体 DEEP23-z2LRD1 的 Magellan/FIRE 光谱，其光谱红移为 $z_\mathrm{spec}=2.26$，以此作为样本选择的概念验证。与高红移 LRDs 类似，DEEP23-z2LRD1 的光谱展现出宽 H$α$ 发射线，半高全宽 $\mathrm{FWHM}\approx2400\\,\mathrm{km\\,s^{-1}}$，并伴有近乎对称的窄 H$α$ 吸收。此外，DEEP23-z2LRD1 具有极窄的 [OIII] 线，$\mathrm{FWHM}\approx140\\,\mathrm{km\\,s^{-1}}$，表明在一个低质量宿主星系中存在吸积黑洞。受限于地面巡天的角分辨率，我们强调需要光谱后续观测来确定该样本的污染比例，并精确测定 $z<4$ 处 LRD 的数密度。
英文摘要：
Little red dots (LRDs) are a population of red, compact objects discovered by JWST at $z>4$. At $4<z<8$, they are roughly 100 times more abundant than UV-selected quasars. However, their number density is uncertain at $z<4$ due to the small sky coverage and limited blue wavelength coverage of JWST. We present our ground-based search for LRDs at $2\lesssim z\lesssim4$, combining ultra-deep Hyper Suprime-Cam photometry and various (near-)infrared surveys within a total area of $\sim3.1\,\mathrm{deg^{2}}$. We find that for LRDs with $M_{5500}<-22.5$, their number density declines from $\sim10^{-4.5}\,\mathrm{cMpc^{-3}}$ at $z>4$ to $\sim10^{-5.3}\,\mathrm{cMpc^{-3}}$ at $2.7<z<3.7$ and $\sim10^{-5.7}\,\mathrm{cMpc^{-3}}$ at $1.7<z<2.7$. We also present the Magellan/FIRE spectrum of our first followed-up candidate, DEEP23-z2LRD1 at $z_\mathrm{spec}=2.26$, as a proof of concept for our sample selection. Similar to high-redshift LRDs, the spectrum of DEEP23-z2LRD1 exhibits broad H$α$ emission with $\mathrm{FWHM}\approx2400\,\mathrm{km\,s^{-1}}$ and with nearly symmetric narrow H$α$ absorption. Additionally, DEEP23-z2LRD1 has extremely narrow [OIII] lines with $\mathrm{FWHM}\approx140\,\mathrm{km\,s^{-1}}$, suggesting the presence of an accreting black hole in a low-mass host galaxy. Limited by the angular resolution of ground-based surveys, we emphasize that spectroscopic follow-ups are required to characterize the contamination fraction of this sample and pin down LRD number density at $z<4$.
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