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arXiv 1408.4451astro-ph.GAastro-ph.HE
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矮星系中心的黑洞

Black Holes at the Centers of Nearby Dwarf Galaxies

Edward C. Moran, Karlen Shahinyan, Hannah R. Sugarman, Darik O. Velez, Michael Eracleous

备注：
41 pages, 10 figures, accepted for publication in The Astronomical Journal
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摘要：

利用斯隆数字巡天（SDSS）数据Release 7的有限距离部分，我们已经在附近（d < 80 Mpc）低质量，低光度矮星系中确定了28个活动星系核（AGN）。在星系中心的吸积物体被认为是中等质量的黑洞（IMBHs），其M_BH <1 e6 M_sun。活动星系核是在对光谱中的连续谱进行仔细建模后，利用几种光学发射线诊断方法选出的。我们将调查范围限制在光谱特征与塞弗特核相似的物体上，不包括光谱模糊的发射线星系，这些星系可能是由恒星过程提供动力的。因此，我们样本中的宿主星系是非常局部的宇宙中质量最小的天体，而这个局部的宇宙肯定包含中心黑洞。考虑到我们对SDSS中最近天体的关注，我们的调查对低光度发射比以前在低质量星系中对AGN的光学搜索更敏感。在我们的样品中，Seyfert核的[O III]~（5007）亮度的中值为L_5007 = 2e5L_sun，并向下延伸到1e4L_sun。利用已发表的宽线IMBH候选者的数据，我们导出了log（L_bol/L_5007）= 3.0 +/- 0.3的[O III]热辐射校正，这是显着低于高光度AGN获得的值。对我们的样本应用这种校正，我们得到了主要落在10^3 M_sun- 10^4 M_sun范围内的最小黑洞质量估计，这大致是不同黑洞种子形成情景的预测质量函数重叠最多的地方。在恒星质量范围内，包括我们的样本中的AGN宿主星系的大部分，我们得出了几个百分点的AGN分数的下限，表明矮星系中的活动核并不像以前认为的那样罕见。

英文摘要：

Using a distance-limited portion of the Sloan Digital Sky Survey (SDSS) Data Release 7, we have identified 28 active galactic nuclei (AGNs) in nearby (d < 80 Mpc) low-mass, low-luminosity dwarf galaxies. The accreting objects at the galaxy centers are expected to be intermediate-mass black holes (IMBHs) with M_BH < 1e6 M_sun. The AGNs were selected using several optical emission-line diagnostics after careful modeling of the continuum present in the spectra. We have limited our survey to objects with spectral characteristics similar to those of Seyfert nuclei, excluding emission-line galaxies with ambiguous spectra that could be powered by stellar processes. The host galaxies in our sample are thus the least massive objects in the very local universe certain to contain central black holes. Given our focus on the nearest objects included in the SDSS, our survey is more sensitive to low-luminosity emission than previous optical searches for AGNs in low-mass galaxies. The [O III] lambda5007 luminosities of the Seyfert nuclei in our sample have a median value of L_5007 = 2e5 L_sun and extend down to 1e4 L_sun. Using published data for broad-line IMBH candidates, we have derived an [O III] bolometric correction of log (L_bol/L_5007) = 3.0 +/- 0.3, which is significantly lower than values obtained for high-luminosity AGNs. Applying this correction to our sample, we obtain minimum black-hole mass estimates that fall mainly in the 10^3 M_sun -- 10^4 M_sun range, which is roughly where the predicted mass functions for different black-hole seed formation scenarios overlap the most. In the stellar mass range that includes the bulk of the AGN host galaxies in our sample, we derive a lower limit on the AGN fraction of a few percent, indicating that active nuclei in dwarf galaxies are not as rare as previously thought.

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