Stellar population parameters derived from spectral line-strengths provide a powerful probe of galaxy properties and formation histories. We implement the machinery for extracting single-stellar-population-equivalent stellar population parameters from synthetic spectra generated by a hierarchical galaxy formation model. We find that the SSP-equivalent age is related to the light-weighted age in a complicated fashion that reflects the influence of recently-formed stars and is poorly correlated with the mass-weighted age. The tendency for SSP-equivalent ages to be biased young means that archaeological downsizing overstates the mass-weighted downsizing in age with mass. We find that the SSP-equivalent metallicity closely tracks the mass- and light-weighted metallicities, so that observed mass--metallicity relations for old galaxies closely reflect the underlying trends. We construct mock catalogues of early-type galaxies in a Coma cluster-sized halo and compare them directly to observations of early-type galaxies in the Coma cluster. The similarity of the SSP-equivalent ages in the observational samples and the mock catalogues gives us confidence that the star-formation quenching implemented in the hierarchical galaxy formation model produces roughly the correct amount of recent star formation. The SSP-equivalent metallicities are however too low and have the wrong slope as a function of velocity dispersion, and the SSP-equivalent ages of the model galaxies may have an incorrect slope as a function of velocity dispersion. (Abridged)
Probing the cool ISM in galaxies via 21cm HI absorption
最近有针对性的研究在射电星系的相关HI吸收开始映射出的位置，和潜在的宇宙学演化，在活动星系核（AGN）的宿主星系的冷气体。观测到的21 cm吸收剖面通常显示出两种不同的谱线成分：来自星系扩展盘中的冷气的窄而深的谱线，以及来自活动星系核附近冷气的宽而浅的谱线（例如Morganti et al. 2011）。在这里，我们介绍了在本地宇宙中最年轻和最近触发的射电AGN中有针对性地搜索相关HI吸收的结果（Allison等人，2012 b）。到目前为止，通过使用最近委托的澳大利亚望远镜紧凑阵列宽带后端（CABB; Wilson et al. 2011），我们已经检测到两个新的吸收器和一个以前已知的系统。虽然其中两个显示了一个宽的，浅的分量和一个窄的，深的分量（见图1），其中一个新的检测只有一个宽的，浅的分量。有趣的是，前两个探测到的宿主星系被归类为富含气体的螺旋星系，而后者则是早期类型的星系。这些探测是使用基于贝叶斯推断的谱线查找方法获得的，该方法是为未来的大规模吸收调查开发的（Allison等人，2012年a）。
We present statistics of HeII Lya transmission spikes and large-scale absorption troughs using archival high-resolution ($R=λ/Δλ\simeq 12,500$-$18,000$) far-UV spectra of eight HeII-transparent quasars obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. The sample covers the redshift range 2.5<z<3.8, thereby probing the rapidly evolving HeII absorption at the end of the HeII reionization epoch. The measured lengths of the troughs decrease dramatically from L>100cMpc at z>3 to L~30cMpc at z~2.7, signaling a significant progression of HeII reionization at these redshifts…
Probing the metallicity and ionization state of the circumgalactic medium at z~6 and beyond with OI absorption
低电离金属吸收由于OI已被确定为一个重要的探针的物理状态的星际/环星系介质在尾端的再电离。我们在此使用高分辨率流体动力学模拟来解释由Becker等人（2011）观察到的z~6处OI吸收剂的发生率。我们推断弱OI吸收体（EW > 0.1A）在亚DLAs范围内具有典型的HI柱密度，密度为平均重子密度的80倍，金属丰度约为太阳的1/500。这与在z~3处类似密度下推断的金属丰度相似，表明低质量星系周围环星系介质的金属富集在z~6时已经有了相当大的进展。红移范围5<z 5内OI吸收发生率的明显快速演变<6 mirrors that of self-shielded Lyman-Limit systems at lower redshift and is mainly due to the rapid decrease of the meta-galactic photo-ionization rate at z>。我们预测，随着IGM变得更加中性，OI吸收体的发生率将随着红移的增加而继续迅速上升。如果金属的分布延伸到较低密度的区域，OI吸收剂将允许探测宇宙网络中越来越中性的附属结构的金属富集。
We study large-scale UV background inhomogeneity in three-dimensions associated with the observed quasar populations at high redshift. We do this by measuring metal absorption through quasar absorption spectrum stacking as a function distance to closest quasar in SDSS-IV/eBOSS on 10s of comoving megaparsec scales. We study both intergalactic medium absorbers and mixed circumgalactic medium absorbers and probe absorption in O VI, C IV, and Si IV, and Si III. Overall stronger high ionization species absorption is seen closer to quasars at $2.4<z<3.1$. O VI absorption shows a particularly strong …
Known as the "Missing Baryon Problem", about one-third of baryons in the local universe remain unaccounted for. The missing baryons are thought to reside in the warm-hot intergalactic medium (WHIM) of the cosmic web filaments, which are challenging to detect. Recent Chandra X-ray observations used a novel stacking analysis and detected an OVII absorption line toward the sightline of a luminous quasar, hinting that the missing baryons may reside in the WHIM. To explore how the properties of the OVII absorption line depend on feedback physics, we compare the observational results with prediction…
One of the major science goals of the Square Kilometre Array (SKA) is to understand the role played by atomic hydrogen (HI) gas in the evolution of galaxies throughout cosmic time. The hyperfine transition line of the hydrogen atom at 21-cm is one of the best tools to detect and study the properties of HI gas associated with galaxies. In this article, we review our current understanding of HI gas and its relationship with galaxies through observations of the 21-cm line both in emission and absorption. In addition, we provide an overview of the HI science that will be possible with SKA and its …
Enhanced ionizing radiation in close proximity to redshift $z\gtrsim 6$ quasars creates short windows of intergalactic Ly$α$ transmission blueward of the quasar Ly$α$ emission lines. The majority of these Ly$α$ near-zones are consistent with quasars that have optically/UV bright lifetimes of $t_{\rm Q}\sim 10^{5}-10^{7}\rm\,yr$. However, lifetimes as short as $t_{\rm Q}\lesssim 10^{4}\rm\,yr$ appear to be required by the smallest Ly$α$ near-zones. These short lifetimes present an apparent challenge for the growth of $\sim 10^{9}\rm\,M_{\odot}$ black holes at $z\gtrsim 6$. Accretion over longer…
We present an analysis combining the simultaneous measurement of intergalactic absorption by hydrogen (H I), helium (He II) and oxygen (O VI) in UV and optical quasar spectra. The combination of the H I and He II Lyman-alpha forests through $η$ (the ratio of column densities of singly ionized helium to neutral hydrogen) is thought to be sensitive to large-scale inhomogeneities in the extragalactic UV background. We test this assertion by measuring associated five-times-ionized oxygen (O VI) absorption, which is also sensitive to the UV background. We apply the pixel optical depth technique to …
