Nearby Young Dwarf Galaxies: Primordial Gas and Lyα Emission
Nearby Young Dwarf Galaxies: Primordial Gas and Lyα Emission
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附近的年轻矮星系：原始气体和 Lyα 发射
DOI：
10.1086/304826
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发表时间：
1997
期刊：
影响因子：
--
通讯作者：
Y. Izotov
中科院分区：
文献类型：
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作者：
T. Thuan;Y. Izotov
利用哈勃空间望远镜紫外分光光度法对三个极度缺金属的蓝致密矮星系SBS 0335-052（Z ~ Z嫌/40）、Tol 65（Z ~ Z嫌/24）和T1214-277（Z ~ Z嫌/23）进行了观测。在前两个BCD中观察到宽阻尼Lyα吸收。对于SBS 0335-052，通过拟合Lyα吸收曲线得出的H I柱密度为N（H I）=（7.0 ± 0.5）× 1021 cm-2，这是迄今为止针对BCD得出的最高H I柱密度，比I Zw 18中的H I柱密度大约2倍。对于Tol 65，N（H I）=（2.5 ± 1.0）× 1021 cm-2，这也在BCD导出的H I柱密度的高范围内，仅比I Zw 18中的H I柱密度小约1.4倍。星际吸收线O I λ1302也在两个星系中被探测到。与高分辨率类星体光谱的比较表明，在SBS 0335-052中，O I λ1302线，沿着其他重元素星际吸收线，如Si II λ1304和S II λ1251，λ1254和λ1259，是不饱和的，这使我们能够推导出丰度。假设这些谱线起源于氢气体，我们得到氧、硅和硫的丰度分别比太阳值低37，000、4000和116倍。氧丰度比I Zw 18的中性气体中的氧丰度低整整37倍。然而，不同元素的这些高度不一致的亏损因子表明，吸收线不是在H I中产生的，而是在H II气体中产生的。采用这一假设，从紫外吸收线导出的丰度与从光发射线导出的丰度一致（Z ~ Z返/40）。通过对S Ⅱ λ1259、Si Ⅱ λ1260、O Ⅰ λ1302、Si Ⅱ λ1304等几个蓝移系统的探测，证实了重元素吸收线起源于H Ⅱ区的结论。和C II λ1335吸收线起源于速度高达~1500 km s的快速移动云-1，也通过存在的重元素吸收线与激发较低的水平。如果这个结论成立，那么SBS 0335-052中的H I云确实是原始的，没有受到重元素的污染。这将减轻假设广泛分布的第三星族恒星先前富集的必要性，并提出了这样的云如何能够在哈勃时间内持续而不会在当前时代之前制造恒星的问题。由于O I λ1302可能在Tol 65中饱和，我们只能得出氧丰度的下限，比太阳值低约6000倍。与前两个BCD的情况相反，在T1214-277的光谱中观察到一条强的Lyα线，这使其成为具有检测到的Lyα发射的最低金属量BCD。它的等效宽度为70埃，是恒星形成星系中最大的。Lyα发射相对于H II气体速度没有红移，因此T1214-277中Lyα光子的逃逸不是由H II气体的速度结构决定的，而是可能由其孔隙度决定的。SBS 0335-052和Tol 65中没有Lyα发射可能是由尘埃消光、在整个H I云区域上多次散射导致的Lyα光子重新分布以及云的几何形状共同造成的。SBS 0335-052的光谱显示出恒星Si IV λ1394和λ1403线，具有天鹅座P剖面，这表明存在许多热超巨星。在T1214-277的光谱中还可以看到一条强的恒星N V λ1240线，具有天鹅座P的轮廓，这表明存在着大量的大质量（M <$60 M）恒星。SBS 0335-052的恒星风终端速度非常低，只有~500 km s-1。它是较高的，~2000 km s-1的T1214-277，在低范围内的终端速度发现其他BCD。这两个BCD是已知的天鹅座P型星系中最缺乏金属的两个。这种剖面的存在提出了一个问题，即如何在SBS 0335-052中缺乏重元素的恒星大气中建立风。
We present Hubble Space Telescope UV spectrophotometry of three extremely metal-deficient blue compact dwarf (BCD) galaxies, SBS 0335-052 (Z ~ Z☉/40), Tol 65 (Z ~ Z☉/24), and T1214-277 (Z ~ Z☉/23). Broad damped Lyα absorption is seen in the first two BCDs. For SBS 0335-052, the H I column density derived by fitting the Lyα absorption profile is N(H I) = (7.0 ± 0.5) × 1021 cm-2, which is the highest H I column density derived thus far for a BCD and which is ~2 times larger than that in I Zw 18. As for Tol 65, N(H I) = (2.5 ± 1.0) × 1021 cm-2, which is also in the high range of H I column densities derived for BCDs and which is only ~1.4 times smaller than that in I Zw 18. The interstellar absorption line O I λ1302 has also been detected in both galaxies. Comparison with high-resolution quasar spectra implies that in SBS 0335-052, the O I λ1302 line, along with other heavy element interstellar absorption lines such as Si II λ1304 and S II λ1251, λ1254, and λ1259, are not saturated, which allows us to derive abundances. Assuming that these lines originate in the H I gas, we derive abundances of oxygen, silicon, and sulfur, respectively, as 37,000, 4000, and 116 times lower than the solar values. The oxygen abundance is a whole 37 times lower than in the neutral gas of I Zw 18. However, these highly discrepant deficiency factors of different elements suggest that the absorption lines are produced, not in the H I, but in the H II gas. Adopting that hypothesis, the derived abundances from the UV absorption lines are then consistent with that derived from the optical emission lines (Z ~ Z☉/40). The conclusion that the heavy element absorption lines originate in the H II region is supported by the detection of several systems of blueshifted S II λ1259, Si II λ1260, O I λ1302, Si II λ1304, and C II λ1335 absorption lines originating in fast-moving clouds with velocities up to ~1500 km s-1 and also by the presence of heavy element absorption lines with excited lower levels. If this conclusion holds, then the H I cloud in SBS 0335-052 is truly primordial, unpolluted by heavy elements. This would alleviate the need for postulating previous enrichment by widespread Population III stars and poses the problem of how such a cloud could last for a Hubble time without making stars before the present epoch. Because the O I λ1302 is likely to be saturated in Tol 65, we could derive only a lower limit for the oxygen abundance, ~6000 times lower than the solar value. Contrary to the situation in the two previous BCDs, a strong Lyα line is seen in the spectrum of T1214-277, which makes it the lowest metallicity BCD with detected Lyα emission. Its equivalent width of 70 Å is the largest found in star-forming galaxies. The Lyα emission is not redshifted with respect to the H II gas velocity, so that the escape of Lyα photons in T1214-277 is not dictated by the velocity structure of the H I gas but probably by its porosity. The absence of Lyα emission in SBS 0335-052 and Tol 65 is probably caused by a combination of dust extinction, redistribution of the Lyα photons by multiple scattering over the whole area of the H I cloud, and the geometry of the cloud. The spectrum of SBS 0335-052 shows stellar Si IV λ1394 and λ1403 lines with P Cygni profiles, which suggests the presence of numerous hot supergiant stars. A strong stellar N V λ1240 line with a P Cygni profile is also seen in the spectrum of T1214-277, which suggests that a large population of massive (M ≳ 60 M☉) stars is present. The stellar wind terminal velocity is very low in SBS 0335-052, being only ~500 km s-1. It is higher, ~2000 km s-1 in T1214-277, in the low range of terminal velocities found for other BCDs. These BCDs are the two most metal-deficient galaxies known with P Cygni profiles. The presence of such profiles raises the question of how to set up a wind in stellar atmospheres devoid of heavy elements as in SBS 0335-052.