Lyα IMAGING OF A PROTO--CLUSTER REGION AT (z) = 3.09

CHARLES C. STHIDEL; KURT L. ADELBERGER; ALICE E. SHAPLEY

摘要

We present very deep narrowband observations of a volume containing a significant overdensity of galaxies previously discovered in our survey for continuum--selected Lyman-break galaxies (LBGs) at redshifts 2.7 ≈＜ z ≈ 3.4. The new observations are used in conjunction with our spectroscopic results on LBGs to compare the effectiveness of continuum and emission-line searches for star-forming galaxies at high redshift, and to attempt to extend the search for members of the structure at (z) = 3.09 to much fainter continuum luminosities. The 8'7 × 8'9 field contains a very high surface density of emission-line candidates, approximately 6 times higher than in published blind narrowband searches to comparable depth and at similar redshift. This level of density enhancement for the (z) = 3.09 structure is consistent with that inferred from the analysis of the spectroscopic Lyman-break galaxy sample in the same region (6.0 ± 1.2), but extends to continuum luminosities up to 2 mag fainter. We find that only ~20/100-25/100 of all galaxies at a given UV continuum luminosity would be flagged as narrowband-excess objects subject to the typical limits W_λ ＞ 80 A in the observed frame. The remainder have lines that are too weak (<20 A rest equivalent width) to make selection by narrowband excess effective. There is no evidence for a significantly higher fraction of large Lyα line equivalent widths at faint continuum luminosities. We have also discovered two extremely bright, large, and diffuse Lyα-emitting “blobs” that are apparently associ- ated with, but not centered on, previously known Lyman-break galaxies at the redshift of the (z) = 3.09 structure. These nebulae have physical extents ≈＞ 100 h~-1 kpc and Lyα line fluxes of ～10~-15 ergs s~-1 cm~-2, both factors of ～20-40 times larger than the typical line emitters at the same redshifts in the field. In many respects, these blobs resemble the giant Lya nebulae associated with high-redshift radio galaxies, but have < 1/100 of the associated radio continuum flux and no obvious source of UV photons bright enough to excite the nebular emission. While the nature of these blobs remains unclear, it is pos- sible that they are excited by continuum sources that are heavily obscured along our line of sight, or that they are associated with cooling-flow--like phenomena. The effectiveness of narrowband imaging for isolating large, albeit incomplete, samples of high-redshift galaxies over a broad range of continuum luminosity makes the technique particularly well-suited to “mapping” known or suspected structures at high redshift. By combining the 24 spectroscopic members of the z = 3.09　“spike” with the narrowband candidates, we are able to produce a sample of 162 objects in a single relatively small field that are either known or likely members of this large structure. A smoothed surface density map shows three regions exceeding a local overdensity of δρ/ρ = 1 on 1 h~-1 Mpc (comoving) scales, interestingly, one is centered on a z = 3.083 QSO, and another on one of the giant Lyα nebulae. There is thus circumstantial evidence that the Lyα nebulae may, like QSOs, be linked to the sites of the largest density enhancements at high redshifts.

机译：我们提出了一个非常深的窄带观测值，该观测值包含先前在我们的调查中针对连续谱选择的莱曼断裂星系（LBG）在红移2.7≈＜z≈3.4时发现的显着密度过大的星系。这些新的观测结果与我们在LBG上的光谱结果结合使用，比较了在高红移条件下连续搜索和发射线搜索形成恒星的星系的有效性，并尝试扩大对（z）=的结构成员的搜索3.09到更微弱的连续光度。 8'7×8'9场包含非常高的发射谱线候选表面密度，比已发布的盲窄带搜索（可比较的深度和相似的红移）高大约6倍。（z）= 3.09结构的密度增强水平与对同一区域（6.0±1.2）的光谱莱曼破裂星系样品的分析所推断的密度相一致，但扩展到了2 mag微弱的连续光度。我们发现，在给定的UV连续性光度下，仅约20 / 100-25 / 100的所有星系会被标记为窄带多余的物体，但在观察到的帧中它们的典型限制为W_λ＞ 80A。其余的线太弱（<20 A静止等效宽度），不足以使窄带多余的选择有效。没有证据表明在连续微弱的发光度下，较大的Lyα线等效宽度比例要高得多。我们还发现了两个非常明亮，大而弥漫的发射Lyα的“斑点”，它们与（z）= 3.09结构的红移显然与先前已知的Lyman破裂星系相关，但并未居中。这些星云的物理范围约为≈＞ 100 h〜-1 kpc，Lyα线通量约为〜10〜-15 ergs s〜-1 cm〜-2，两者都是同一条件下典型线状发射器的约20-40倍。在该领域的红移。在许多方面，这些斑点类似于与高红移射电星系相关的巨型Lya星云，但具有小于相关射电连续性通量的1/100，并且没有明显的紫外线光子源足够亮以激发星云发射。虽然这些斑点的性质尚不清楚，但有可能是它们被连续的光源激发，这些光源在我们的视线范围内被严重遮盖，或者与类似冷却流的现象有关。窄带成像用于在较大的连续光度范围内隔离较大的，尽管不完整的高红移星系样本的有效性，使得该技术特别适合于在高红移条件下“映射”已知或可疑的结构。通过将z = 3.09的“尖峰”的24个光谱成员与窄带候选者结合，我们能够在一个相对较小的场中生成162个对象的样本，这些对象是该大型结构的已知成员或可能成员。平滑的表面密度图显示三个区域在1 h〜-1 Mpc尺度上超过δρ/ρ= 1的局部过密度，有趣的是，一个区域集中在az = 3.083 QSO上，另一个区域集中在一个巨型Lyα星云上。因此，有间接证据表明，Lyα星云可能像QSOs一样，与高红移时密度增加最大的部位有关。

Lyα IMAGING OF A PROTO--CLUSTER REGION AT (z) = 3.09

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