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高红移长伽马射线暴及其宿主星系

Long Gamma-Ray Bursts and Their Host Galaxies at High Redshift

A. Lapi, N. Kawakatu, Z. Bosnjak, A. Celotti, A. Bressan, G. L. Granato, L. Danese

备注：
11 pages, 8 figures, uses mn2e.cls. Minor changes. In press on MNRAS
发表信息：
Mon. Not. Roy. Astron. Soc. 386 (2008) 608
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摘要：

最近的观测和理论证据表明，长伽马射线暴（GRB）可能与低金属丰度、快速旋转的大质量恒星有关。基于这一观点，我们研究了在临界金属丰度Z_crit Z_sun/10 - Z_sun/5以下的宇宙学恒星形成率（SFR），以估计高红移长GRB祖星的事件率。为了这个目的，我们利用一个星系形成的情况下已经成功地测试了丰富的观测数据（原）球状体，莱曼破裂星系，莱曼α发射器，亚毫米星系，类星体，和当地的早期型星系。我们发现，长伽玛暴的预测率约为300事件/年/sr，其中约30%发生在z>~6。相应地，GRB数计数与明亮的SWIFT数据很好地吻合，而不需要固有的光度演化。此外，上述框架使我们能够预测伽玛暴宿主星系的性质。大多数伽玛暴与低质量星系晕M_H<~10^11 M_sun有关，并有效地追踪了这些星系晕中小星系的形成。寄主很年轻，年龄小于5*10^7年，富含气体，但由于化学不成熟，吸收较差（A_V<~0.1）;这也意味着高的比SFR和相当极端的α增强。只有少数居住在M_H>~10^12 M_sun的大晕中的宿主具有较大的消光（A_V~0.7-1），SFR超过100 M_sun/yr，并且可以在亚毫米波段探测到。大部分寄主的紫外亮度在-20 <~M_1350<~-16之间，莱曼α光度在2*10^40 <~L_Lya<~2*10^42 erg/s之间。因此，伽玛射线暴的宿主正在追踪莱曼破裂星系和莱曼α发射体的光度函数的暗端。

英文摘要：

Motivated by the recent observational and theoretical evidence that long Gamma-Ray Bursts (GRBs) are likely associated with low metallicity, rapidly rotating massive stars, we examine the cosmological star formation rate (SFR) below a critical metallicity Z_crit Z_sun/10 - Z_sun/5, to estimate the event rate of high-redshift long GRB progenitors. To this purpose, we exploit a galaxy formation scenario already successfully tested on a wealth of observational data on (proto)spheroids, Lyman break galaxies, Lyman alpha emitters, submm galaxies, quasars, and local early-type galaxies. We find that the predicted rate of long GRBs amounts to about 300 events/yr/sr, of which about 30 per cent occur at z>~6. Correspondingly, the GRB number counts well agree with the bright SWIFT data, without the need for an intrinsic luminosity evolution. Moreover, the above framework enables us to predict properties of the GRB host galaxies. Most GRBs are associated with low mass galaxy halos M_H<~10^11 M_sun, and effectively trace the formation of small galaxies in such halos. The hosts are young, with age smaller than 5*10^7 yr, gas rich, but poorly extincted (A_V<~0.1) because of their chemical immaturity; this also implies high specific SFR and quite extreme alpha-enhancement. Only the minority of hosts residing in large halos with M_H>~10^12 M_sun have larger extinction (A_V~0.7-1), SFRs exceeding 100 M_sun/yr and can be detected at submm wavelengths. Most of the hosts have UV magnitudes in the range -20 <~M_1350<~ -16, and Lyman alpha luminosity in the range 2*10^40 <~L_Lya<~2*10^42 erg/s. GRB hosts are thus tracing the faint end of the luminosity function of Lyman break galaxies and Lyman alpha emitters.

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