Carbon and oxygen isotope ratios in starburst galaxies: New data from NGC 253 and Mrk 231 and their implications
Carbon and oxygen isotope ratios in starburst galaxies: New data from NGC 253 and Mrk 231 and their implications
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DOI：
10.1051/0004-6361/201322962
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发表时间：
2014-05-01
影响因子：
6.5
通讯作者：
Harada, N.
中科院分区：
文献类型：
--
作者：
Henkel, C.;Asiri, H.;Harada, N.
碳和氧同位素比率是核处理的极好指标，但到目前为止，很少有这样的数据被带到河外目标。因此，利用IRAM 30-m望远镜，对附近的星暴星系NGC253和典型的超亮度红外星系MRK231进行了CN和CO同位素测量。在NGC253的中心，CN和(CN)-C-13N=1->；0谱线表明，在考虑了主要物种的两个光谱成分的中等饱和效应(10%和25%)后，没有明显偏离预期的局部热力学平衡。考虑到影响误差预算的校准不确定因素，C-12/C-13比值变为40+/-10。这比银河系中央分子带的比值要大，这表明加工不良的气体向中央区域流入的比率更高。假设这个比值对于CO发射气体也成立，则得到O-16/O-18=145+/-36和O-16/O-17=1290+/-365，S-32/S-34的比值接近本地星际介质(2025年)的测量值。在均方根噪声为3和4mK(15和20mJy)的8.5公里S-1宽沟道中，在N=1-和2-gt；1跃迁的低频精细结构成分中没有发现振动激发CN的迹象。当(CO)-C-12-O-16和(CO)-C-12-O-18J=1-gt；0时，NGC253和MRK231之间的谱线强度比类似于100，而(CO)-C-13-O-16J=1-gt；0时，峰值强度比类似于250。这以及在MRK 231的J=1-gt；0和2-gt；1跃迁中类似的(CO)-C-13和(CO)-O-18谱线强度表明，C-12/C-13类似于100，O-16/O-18类似于100，这与进化较少的超铝合并Arp 220获得的值一致。此外，当考虑到其他(几乎没有可用的)河外数据时，C-12/C-13比率似乎在一个完整的数量级上变化，从超明亮高红移星系的>；100到更局部的此类星系类似于100，从没有经历大规模合并的较弱恒星爆发的类似40，到银河系中央分子带的25。由于C-12主要是在大质量恒星中合成的，而C-13主要是在较晚的时间被寿命更长的低质量恒星抛射出来的，这与我们的碳同位素比率随时间下降和金属丰度上升的结果定性一致。然而，需要强调的是，无论是注入加工不良的物质，引发核星暴，还是新形成的大质量恒星(特别是在顶重恒星初始质量函数的情况下)抛出的物质，都可以在有限的时间内提高碳同位素比率。
Carbon and oxygen isotope ratios are excellent measures of nuclear processing, but few such data have been taken toward extragalactic targets so far. Therefore, using the IRAM 30-m telescope, CN and CO isotopologues have been measured toward the nearby starburst galaxy NGC 253 and the prototypical ultraluminous infrared galaxy Mrk 231. Toward the center of NGC 253, the CN and (CN)-C-13 N = 1 -> 0 lines indicate no significant deviations from expected local thermodynamical equilibrium after accounting for moderate saturation effects (10 and 25%) in the two detected spectral components of the main species. Including calibration uncertainties, which dominate the error budget, the C-12/C-13 ratio becomes 40 +/- 10. This is larger than the ratio in the central molecular zone of the Galaxy, suggesting a higher infall rate of poorly processed gas toward the central region. Assuming that the ratio also holds for the CO emitting gas, this yields O-16/O-18 = 145 +/- 36 and O-16/O-17 = 1290 +/- 365 and a S-32/S-34 ratio close to the one measured for the local interstellar medium (2025). No indication of vibrationally excited CN is found in the lower frequency fine structure components of the N = 1 -> 0 and 2 -> 1 transitions at rms noise levels of 3 and 4 mK (15 and 20 mJy) in 8.5 km s-1 wide channels. Peak line intensity ratios between NGC 253 and Mrk 231 are similar to 100 for (CO)-C-12-O-16 and (CO)-C-12-O-18 J = 1 -> 0, while the ratio for (CO)-C-13-O-16 J = 1 -> 0 is similar to 250. This and similar (CO)-C-13 and (CO)-O-18 line intensities in the J = 1 -> 0 and 2 -> 1 transitions of Mrk 231 suggest C-12/C-13 similar to 100 and O-16/O-18 similar to 100, in agreement with values obtained for the less evolved ultraluminous merger Arp 220. Also, when accounting for other (scarcely available) extragalactic data, C-12/C-13 ratios appear to vary over a full order of magnitude, from >100 in ultraluminous high redshift galaxies to similar to 100 in more local such galaxies to similar to 40 in weaker starbursts that are not undergoing a large scale merger to 25 in the central molecular zone of the Milky Way. With C-12 being predominantly synthesized in massive stars, while C-13 is mostly ejected by longer lived lower mass stars at later times, this is qualitatively consistent with our results of decreasing carbon isotope ratios with time and rising metallicity. It is emphasized, however, that both infall of poorly processed material, initiating a nuclear starburst, and the ejecta from newly formed massive stars (in particular in the case of a top-heavy stellar initial mass function) can raise the carbon isotope ratio for a limited amount of time.