EXTREMELY INEFFICIENT STAR FORMATION IN THE OUTER DISKS OF NEARBY GALAXIES
EXTREMELY INEFFICIENT STAR FORMATION IN THE OUTER DISKS OF NEARBY GALAXIES
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DOI：
10.1088/0004-6256/140/5/1194
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发表时间：
2010-07
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通讯作者：
F. Bigiel;A. Leroy;F. Walter;L. Blitz;E. Brinks;W. D. Blok;B. M. U. Berkeley;Mpia Heidelberg;Nrao;U. Hertfordshire;U. C. Town;C. Observatories
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作者：
F. Bigiel;A. Leroy;F. Walter;L. Blitz;E. Brinks;W. D. Blok;B. M. U. Berkeley;Mpia Heidelberg;Nrao;U. Hertfordshire;U. C. Town;C. Observatories
利用联合收割机的H i近星系巡天和GALEX近星系巡天观测数据，研究了17个旋涡星系和5个矮星系在光学半径（r25）外的氢原子（H i）和远紫外（FUV）辐射之间的关系.在这个制度中，H i很可能代表大部分的星际介质（ISM）和FUV发射跟踪最近的星星的形成，由于消光的偏差很小，所以这两个量密切跟踪气体和星星形成率（SFR）之间的潜在关系。方位平均的H i和FUV强度都随着半径的增加而下降，在这种情况下，FUV轮廓的标度长度通常是H i轮廓的一半。尽管轮廓不匹配，但局部FUV和H i强度之间存在显著的空间相关性（在15“分辨率下）;在r25附近，这种相关性相当强，实际上比r25内的任何地方都强（其中H i不是ISM大部分的良好示踪剂），并且显示出向更大半径的下降。的星星形成效率（SFE）-定义为FUV/H i的比率，因此气体耗尽时间的倒数-减少与银河中心半径在整个外磁盘，虽然比整个光盘浅得多。平均而言，我们发现气体耗尽时间远高于哈勃时间（1011年）。我们观察到一个明确的关系FUV/H i和H i柱的外磁盘，与SFE增加H i柱。尽管观察到FUV/H i的系统性变化，但我们没有发现阶跃函数型星星形成阈值的明确证据，尽管我们强调期望它们可能不现实。当与r25内部的结果相比时，我们发现外盘星星的形成在几个方面是不同的：它是非常低效的（与自由落体或轨道时标相比，许多哈勃时间的耗尽时间也很长），柱密度和SFR低于光盘内部的任何地方。看来，H i柱是一个关键的环境因素，也许是关键因素，在设置SFR在外星系盘。
We combine data from The H i Nearby Galaxy Survey and the GALEX Nearby Galaxy Survey to study the relationship between atomic hydrogen (H i) and far-ultraviolet (FUV) emission outside the optical radius (r25) in 17 spiral and 5 dwarf galaxies. In this regime, H i is likely to represent most of the interstellar medium (ISM) and FUV emission to trace recent star formation with little bias due to extinction, so that the two quantities closely trace the underlying relationship between gas and star formation rate (SFR). The azimuthally averaged H i and FUV intensities both decline with increasing radius in this regime, with the scale length of the FUV profile typically half that of the H i profile. Despite the mismatch in profiles, there is a significant spatial correlation (at 15″ resolution) between local FUV and H i intensities; near r25 this correlation is quite strong, in fact stronger than anywhere inside r25 (where H i is not a good tracer for the bulk of the ISM), and shows a decline toward larger radii. The star formation efficiency (SFE)—defined as the ratio of FUV/H i and thus the inverse of the gas depletion time—decreases with galactocentric radius across the outer disks, though much shallower than across the optical disks. On average, we find the gas depletion times to be well above a Hubble time (∼1011 yr). We observe a clear relationship between FUV/H i and H i column in the outer disks, with the SFE increasing with increasing H i column. Despite observing systematic variations in FUV/H i, we find no clear evidence for step-function-type star formation thresholds, though we emphasize that it may not be realistic to expect them. When compared with results from inside r25, we find outer disk star formation to be distinct in several ways: it is extremely inefficient (depletion times of many Hubble times which are also long compared to either the free fall or orbital timescale) with column densities and SFRs lower than found anywhere inside the optical disks. It appears that the H i column is one of the key environmental factors—perhaps the key factor—in setting the SFR in outer galaxy disks.