矮不规则星系成星盘的外向内收缩
Outside-in Shrinking of the Star-forming Disk of Dwarf Irregular Galaxies
摘要：
我们研究了34个邻近矮不规则星系（dIrrs）的代表性样本的多波段表面亮度轮廓。我们的数据包括GALEX FUV/NUV、UBV、Hα和Spitzer 3.6 \mum图像。这些星系构成了小东西调查的大部分。通过模拟的光谱能量分布的方位角平均值与一个完整的图书馆的恒星形成（SF）的历史，我们推导出的恒星质量表面密度分布和SF率平均在三个不同的时间尺度：最近的0.1 Gyr，1 Gyr和哈勃时间。我们发现，对于\sim 80%（27个星系）的样本星系，径向轮廓（至少在外部）在较短的波长比那些在较长的波长有较短的磁盘尺度长度。这表明恒星形成盘一直在缩小。此外，恒星质量面密度的径向分布可以用分段指数分布来描述，而且\sim 80%的星系外盘的质量分布比内盘陡.与内部盘相比，外部盘的SF率径向急剧下降，这为下弯的恒星质量表面密度分布提供了一个自然的解释。在内部磁盘，我们的样本星系平均有恒定的比例最近SF率恒星质量与半径。然而，\sim 35%（12个星系，其中7个重子质量< 10^8 M \odot）的样本显示出负斜率在整个观察到的磁盘，这是相反的“由内而外”的磁盘增长的情况下提出的发光螺旋星系。SF的倾向，成为集中在低质量dIrrs的内部磁盘解释为由于其敏感性的环境影响和调节，通过恒星反馈。
英文摘要：
We have studied multi-band surface brightness profiles of a representative sample of 34 nearby dwarf irregular galaxies (dIrrs). Our data include GALEX FUV/NUV, UBV, Hα, and Spitzer 3.6 \mum images. These galaxies constitute the majority of the LITTLE THINGS survey. By modeling the azimuthal averages of the spectral energy distributions with a complete library of star formation (SF) histories, we derived the stellar mass surface density distributions and the SF rate averaged over three different timescales: the recent 0.1 Gyr, 1 Gyr and a Hubble time. We find that, for \sim 80% (27 galaxies) of our sample galaxies, radial profiles (at least in the outer part) at shorter wavelengths have shorter disk scale lengths than those at longer wavelengths. This indicates that the star-forming disk has been shrinking. In addition, the radial distributions of the stellar mass surface density are well described as piece-wise exponential profiles, and \sim 80% of the galaxies have steeper mass profiles in the outer disk than in the inner region. The steep radial decline of SF rate in the outer parts compared to that in the inner disks gives a natural explanation for the down-bending stellar mass surface density profiles. Within the inner disks, our sample galaxies on average have constant ratios of recent SF rate to stellar mass with radius. Nevertheless, \sim 35% (12 galaxies, among which 7 have baryonic mass < 10^8 M\odot) of the sample exhibit negative slopes across the observed disk, which is in contrast with the "inside-out" disk growth scenario suggested for luminous spiral galaxies. The tendency of SF to become concentrated toward the inner disks in low mass dIrrs is interpreted as a result of their susceptibility to environmental effects and regulation through stellar feedback.
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