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arXiv 0909.4296astro-ph.CO
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SDSS低表面亮度盘星系的红晕

The red halos of SDSS low surface brightness disk galaxies

Nils Bergvall, Erik Zackrisson, Brady Caldwell

摘要:

星系微弱的恒星晕包含了关于最古老恒星和星系形成过程的关键信息。先前对盘状星系堆叠SDSS图像的研究揭示了一个具有异常红色r-i颜色的晕,似乎与我们目前对恒星晕的理解不一致。在这里,我们调查了低表面亮度盘星系的微弱信封的统计特性,以寻找进一步的支持红色过剩。从SDSS数据Release 5中选择了1510个边缘低表面亮度星系,重新缩放到相同的表观大小,对齐并堆叠。这一过程使我们能够达到mu_g ~ 31 mag arcsec^-2的表面亮度。在仔细评估仪器光散射效应后,我们得出了g,r和i的中值和平均径向表面亮度和颜色分布。然后根据g-r颜色将样本分为3个子样本。所有三个样品在厚盘/晕区域中表现出r-i的红色过量。整个样品的晕色g-r = 0.60+-0.15和r-i = 0.80+-0.15,与任何正常类型的恒星星族的颜色都不相容。事实上,没有类似的颜色异常是看到在可比的表面亮度水平沿磁盘排除了天空减法残差作为极端颜色的来源。一些可能的解释,这些异常的红色晕进行了讨论。我们发现,两种不同的情况-尘埃灭绝的河外背景光和一个恒星星族与一个非常底部重的初始质量函数-似乎是大致符合我们的观察和类似的红色过剩的其他类型的星系晕报告。

英文摘要:

The faint stellar halos of galaxies contain key information about the oldest stars and the process of galaxy formation. A previous study of stacked SDSS images of disk galaxies has revealed a halo with an abnormally red r-i colour, seemingly inconsistent with our current understanding of stellar halos. Here, we investigate the statistical properties of the faint envelopes of low surface brightness disk galaxies to look for further support for a red excess. 1510 edge-on low surface brightness galaxies were selected from the SDSS Data Release 5, rescaled to the same apparent size, aligned and stacked. This procedure allows us to reach a surface brightness of mu_g ~ 31 mag arcsec^-2. After a careful assessment of instrumental light scattering effects, we derive median and average radial surface brightness and colour profiles in g,r and i. The sample is then divided into 3 subsamples according to g-r colour. All three samples exhibit a red colour excess in r-i in the thick disk/halo region. The halo colours of the full sample, g-r = 0.60+-0.15 and r-i = 0.80+-0.15, are found to be incompatible with the colours of any normal type of stellar population. The fact that no similar colour anomaly is seen at comparable surface brightness levels along the disk rules out a sky subtraction residual as the source of the extreme colours. A number of possible explanations for these abnormally red halos are discussed. We find that two different scenarios -- dust extinction of extragalactic background light and a stellar population with a very bottom-heavy initial mass function -- appear to be broadly consistent with our observations and with similar red excesses reported in the halos of other types of galaxies.

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