Metallicity and Kinematics of M31’s Outer Stellar Halo from a Keck Spectroscopic Survey
Metallicity and Kinematics of M31’s Outer Stellar Halo from a Keck Spectroscopic Survey
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DOI：
10.1086/341175
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发表时间：
2002-03
期刊：
影响因子：
--
通讯作者：
D. Reitzel;P. Guhathakurta
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文献类型：
--
作者：
D. Reitzel;P. Guhathakurta
我们目前的第一个光谱调查的结果，旨在研究金属丰度和运动学的个别红巨星分支恒星的外晕的仙女座螺旋星系（M31）。本研究基于Keck II 10 m望远镜和低分辨率成像摄谱仪的多狭缝光谱，在东南短轴上R = 19 kpc的区域中，对99个M31晕候选者中的Ca II近红外三重态进行了研究，亮度为20 < I < 22。这些光谱被用来将M31晕红巨星与前景银河系矮星、暗淡的致密背景星系和M31盘巨星分离开来。观测到的径向速度分布很好地拟合了前景银河系矮星和M31晕巨星的等量混合，前者来自标准银河系模型，速度为v <$0 km s-1，后者由宽度为σ <$150 km s-1的高斯表示，中心速度为v <$-300 km s-1。29颗M31红巨星的安全样本是根据径向速度（v < -220 km s-1）进行识别的，对于四颗中等速度恒星（-160 < v < -220 km s-1），则根据宽带B-I颜色进行识别。对于这个样本的对象，有粗略的协议之间的金属丰度获得独立的方式：两种不同的校准的钙II吸收线强度和光度估计的基础上拟合模型恒星等时线的对象的位置在（B-I，I）色星等图。M31晕巨星的[Fe/H]分布具有至少0.6 dex的均方根分布，并且跨越了丰度测量方法校准的102 dex范围。M31晕的平均/中值金属丰度约为[Fe/H] dex = -1.9至-1.1 dex（取决于金属丰度校准和样本选择的细节），可能更高：由于安全的M31样本的选择函数排除了超过80%的太阳/超太阳金属丰度范围内的巨星，因此我们的数据对高金属丰度分布的限制很差。可能的原因进行了探讨的平均值[Fe/H]发现在我们的光谱调查（金属丰度选择偏差校正）和稍高的平均值发现在早期的光度研究之间的明显差异。M31中的场晕红巨星似乎比银河系中的红巨星平均更富含金属。M31的晕[Fe/H]分布与M31球状星团、银河系球状星团和本星系群矮卫星星系相当。在这19千秒差距的外晕领域的数据是大致一致的情况下，该晕是从小恒星子系统的吸积。在M31的安全样本中有四颗恒星有特别强的Ca II线，表明太阳的金属丰度，它们的共同速度为100 - 340 km s-1，接近星系的系统速度，类似于短轴上M31盘巨星的预期。然而，对内盘亮度轮廓的外推，福尔斯远远不能解释这四颗恒星--内盘必须非常大（Rdisk = 80 kpc）和/或弯曲。更有可能的是，这四颗恒星代表了过去在光环中吸积事件的富含金属的碎片痕迹。
We present first results from a spectroscopic survey designed to examine the metallicity and kinematics of individual red giant branch stars in the outer halo of the Andromeda spiral galaxy (M31). This study is based on multislit spectroscopy with the Keck II 10 m telescope and Low Resolution Imaging Spectrograph of the Ca II near-infrared triplet in 99 M31 halo candidates in a field at R = 19 kpc on the southeast minor axis with brightnesses from 20 < I < 22. The spectra are used to isolate M31 halo red giants from foreground Milky Way dwarf stars, faint compact background galaxies, and M31 disk giants. The observed distribution of radial velocities is well fitted by an equal mix of foreground Milky Way dwarf stars, drawn from a standard Galactic model and with velocities v ≲ 0 km s-1, and M31 halo giants represented by a Gaussian of width σ ∼ 150 km s-1 centered on its systemic velocity of v ≈ -300 km s-1. A secure sample of 29 M31 red giant stars is identified on the basis of radial velocity (v < -220 km s-1) and, in the case of four intermediate-velocity stars (-160 < v < -220 km s-1), broadband B-I color. For this sample of objects, there is rough agreement between the metallicities derived in independent ways: two different calibrations of the Ca II absorption-line strength and a photometric estimate based on fitting model stellar isochrones to an object's location in a (B-I, I) color-magnitude diagram. The [Fe/H] distribution of M31 halo giants has an rms spread of at least 0.6 dex and spans the ≳2 dex range over which the abundance measurement methods are calibrated. The mean/median metallicity of the M31 halo is about ⟨[Fe/H]⟩ = -1.9 to -1.1 dex (depending on the details of metallicity calibration and sample selection) and possibly higher: the high-metallicity end of the distribution is poorly constrained by our data since the selection function for the secure M31 sample excludes over 80% of the giants in solar/supersolar metallicity range. Possible reasons are explored for the apparent discrepancy between the mean [Fe/H] found in our spectroscopic survey (corrected for metallicity selection bias) and the slightly higher mean values found in earlier photometric studies. Field halo red giants in M31 appear to be somewhat more metal-rich on average than their Milky Way counterparts. The M31 halo [Fe/H] distribution is comparable to that of M31 globular clusters, Galactic globular clusters, and Local Group dwarf satellite galaxies. The data in this 19 kpc outer halo field are broadly consistent with a scenario in which the halo is built from the accretion of small stellar subsystems. There are four stars in the secure M31 sample that have particularly strong Ca II lines, indicating solar metallicity, at a common velocity of ≈-340 km s-1 close to the galaxy's systemic velocity, similar to what might be expected for M31 disk giants on the minor axis. An extrapolation of the inner disk brightness profile, however, falls far short of accounting for these four stars—the disk would instead have to be very large (Rdisk ≳ 80 kpc) and/or warped. More likely, these four stars represent a metal-rich debris trail from a past accretion event in the halo.