银河系恒星潮汐流的探测进展

2014-05-13 01:24赵君亮
天文研究与技术 2014年4期
关键词:人马星团球状

赵君亮

(1.中国科学院上海天文台,上海 200030;2.上海师范大学,上海 200233)

银河系恒星潮汐流的探测进展

赵君亮1,2

(1.中国科学院上海天文台,上海 200030;2.上海师范大学,上海 200233)

银晕外区存在众多星流,它们或源自银河系的矮伴星系,或源自晕族球状星团,常分别称为矮星系星流和球状星团潮汐尾。星流可以利用各类示踪星,并通过不同的途径加以探测,对若干代表性矮星系星流和球状星团潮汐尾的探测进展做了简要的介绍。

银河系;星流;人马星流;球状星团;潮汐尾;暗晕

CN53-1189/P ISSN1672-7673

“星流(stellar stream)”,是指具有共性运动学和物理学特征的一群恒星,因其在位置空间中呈现长条形水流状分布得名,与20世纪初Kapteyn[1-2]提出的“二星流假设”中的“星流”有着不同的物理含义。

关于星流的有效实测研究始于1994年文[3]的工作,他们在人马座方向银河系中心以远处探测到一批有共性运动特征的恒星,从而发现了距银河系最近的伴星系——人马矮椭球星系。该星系的结构表现为朝银道面方向拉长,并正处于瓦解过程之中,最终将会与银河系并合[3-5]。随后,人们对人马矮星系进行了多方面的探测研究[6-18]。

继人马星流之后,银河系周边又陆续发现了众多星流,与矮伴星系有关的如室女星流[19-23]、麒麟星流[19,24-26]、鲸鱼星流[15,27]等;与晕族球状星团有关的一些星流[28-32],特别是星团Palomar 5(Pal 5)的星流[19,33-37],以及若干尚未确认其本源的星流[38-44]。

星流是因矮伴星系或晕族球状星团在银河系潮汐力场的长期作用下生成的,在这一过程中伴星系或星团不断丢失质量,并最终会导致它们瓦解——已发现有些星流的母星系(或母星团)因瓦解而难觅踪影[45-46]。另一方面,因星流物质向银河系内落,甚至最终与之并合,银河系的结构会随之发生变化。所以,有关星流的观测研究,对探索银河系的结构和演化有重要的意义[8,14,44]。

由银河系伴星系生成的星流,常以其主要部分所在的星座名命名,如室女星流[20]、麒麟星流[24]、鲸鱼星流[27]等;个别以母星系的名称命名,如人马星流[47]。与星团有关的星流,因数目众多、尺度又相对较小,通常并不赋以特定的名称[28]。

星流常表现为由诸多小块恒星集聚区构成的条状结构,故又可称为恒星碎片流[48]。星流是在银河系潮汐力场作用下生成的,有人便称之为潮汐流[49],其中尺度较小的亦称潮汐尾(tidal tail)[33],如球状星团的此类结构常称为潮汐尾。上述多种名称反映的乃是天文学上的一类重要观测现象——在银河系引力场作用下,从伴星系或球状星团中拖曳出来,由众多恒星构成的、长短不一的条形结构。

与银河系的总体结构相比,星流可归于银晕周边尺度不等的次结构,或者说密度超区——相空间中星流成员星的数密度要高于周围同类场星的数密度。不过,此类密度超区并非仅限于条状的星流。如位于武仙座和天鹰座方向的密度超区的投影尺度为~15 kpc×20 kpc,称为武仙-天鹰恒星云[50];位于大犬座的环状星流冠名为大犬恒星环[25,51]。

本文仅限于简要介绍与星流有关的探测结果。为明确起见,除个别小节(§3.3)外也不涉及如麦哲伦流那样的气体流。

图1 银河系周边不同尺度星流结构的示意图Fig.1 A sketch diagram showing the 3D distributions of different-scale stellar tidal streams around the Galaxy

1 探测途径

星流的成员星源自同一个伴星系或球状星团(即星流本源),且已经历了长时间的演化,它们在参数(如位置、速度、星等、颜色、金属度)空间中会呈现与场星不同的共性特征。一旦从观测上发现某种可能的次结构,接下来就要在相应的参数空间中把它与某种银河系模型的恒星预期分布进行比较,以证实该观测次结构在理论上的合理性,同时亦可给相关理论模型以观测约束[52-54]。

星流可以通过以下几种途径(或者它们的综合利用)加以探测[54]:

(1)位置空间(距离或球面位置,或两者兼有之)中的恒星密度分布[55];

(2)恒星的CM图[3,24];

(3)速度(视向速度和或自行)空间中恒星的共性运动学特征[14,25];

(4)角动量-能量相空间图上恒星的成团性[56-57]。

观测者必需借助能反映银河系物质分布的所谓“示踪天体”,间接地探究银河系的结构成分。示踪天体可以是恒星,也可以是气体,如探测银河系棒结构就可以利用中性氢气体。对于星流探测,示踪星应满足以下条件:(1)能起到标距天体的作用,能方便地确定其光度距离;(2)应有较高的光度,在很远处也能取得所需要的观测资料;(3)在星流结构中应普遍存在,能保证统计分析结论的可靠性;(4)不难在众多恒星中加以识别。

目前较为广泛用于星流探测的示踪星有:

(1)天琴RR(RRL)型变星。这是星流研究中最常用的示踪星之一,有关研究相当多[10,19,55,58-60]。

(2)红团簇(RC)星[55,61]。

(3)蓝水平支(BHB)星[13,62-65]。

(4)主序折向点(MSTO)星,但它们并非是十分理想的标距天体,故常与其他类别的示踪星综合利用[38,55]。

(5)随着SDSS巡天资料的面世,M型巨星被单独或综合用于银河系星流研究[3,8,15,64]。

此外,还有其他一些类别的天体被用作探测星流的示踪星,如蓝离散(BS)星[66]、K型巨星[3,12,44]、红巨星支(RGB)星[18]、碳星[67]、球状星团[68-69]等,不过研究相对较少。另外,在同一项研究中会用到2种甚至2种以上的示踪星[3,39,66]。

大尺度数字巡天(如SDSS、2MASS、Pan-STARRS 1[55]等)计划的成功实施,大大提高了人们探测银河系次结构的能力,尤其是SDSS资料被广泛用于星流探测,并得到了一系列新的成果,包括已知星流之细结构[11,64],发现新的星流[11]和恒星密度超区[24]。既有矮伴星系产生的较大尺度星流[24-25,47,70],也有晕族球状星团恒星形成的潮汐尾[31,71],或者本源尚不确准的小尺度星流[11,72]。

2 矮星系星流

目前,在诸多矮星系星流中,研究工作做得最多、认识最为清楚且结论最为一致的当推射手星流。

2.1 人马星流

人马矮椭球星系为小星系在大星系潮汐力场作用下瓦解,并最终融入大星系的过程提供了一个极好的范本。最初,目标天区大多限于人马矮星系本体10°~15°范围内[5,73],随后被探测到的星流尺度渐而扩大[74-75],如2001年发现了距人马矮星系本体60°处的潮汐碎片[76],2003年发现了日心距r=90 kpc处的人马潮汐碎片[62]。事实上人马矮星系的潮汐碎片流可能包围了整个银河系,而且碎片流大致分布在该伴星系的运动轨道附近[74]。21世纪初,利用SDSS资料提供的多种示踪星所做的一系列研究,都证实了上述基本结论[24,47,58,76-79]。

2003年文[8]作者以2MASS提供的M型巨星作为示踪星,发现从人马矮星系本体向外存在两条明显的潮汐流,分别称为前导星流和后随星流,它们在天空中大致呈现一个360°的大圆。翌年,通过视向速度测定,发现前导星流成员星的动力学年龄比后随星流恒星较为年老[80]。2006年证实沿着人马星流呈现明显的年龄/金属度梯度——早期剥离的恒星更多的是一些老年贫金属星[81],由此推知母星系应该存在径向年龄/金属度梯度。2007年发现,人马前导星流在太阳圈以远处穿过银道面,星流碎片之最小银心距r>15 kpc[22]。

2006年文[11]作者利用SDSS(DR5)多色测光资料,确认了沿着人马星流存在明显的距离梯度,并探测到人马前导星流从北银极附近位置起呈现分叉结构,分别冠名为分支A和分支B。最近,文[15]作者发现南银半球人马后随星流呈现与北部分叉相似的分叉结构,两条分支相距~10°,有类似的距离梯度但星族成份有所不同。

近期,关于人马星流的工作可谓方兴未艾,如人马星流的三维运动学研究[14-15],以及星流分叉结构的研究(图2)等[55]。

2.2 鲸鱼星流(Cetus stream)

2009年文[12]作者注意到,在南银冠人马后随星流附近,有一群具有共性运动特征的低金属度BHB星,运动速度亦接近人马星流。同年,文[27]作者通过对SDSS(DR7)资料的分析指出,这很可能说明那儿存在一条新的星流——鲸鱼星流,因轨道路径近乎极向又称鲸鱼极向星流(CPS)。鲸鱼极向星流与人马星流在位置上是交叉的,但星族和运动学特征则不同。上述基本结论后来为文[15]作者的研究所证实。

关于鲸鱼极向星流的前身天体有两种可能:(1)鉴于鲸鱼极向星流恒星的银纬跨度至少有15° (~10 kpc),前身天体很可能是一个矮星系,不过在其轨道附近并未探测到任何矮星系;(2)球状星团NGC5824的位置距鲸鱼极向星流轨道不到3°,两者的日心距、视向速度和星族特征相近,它也许就是鲸鱼极向星流的母天体,而这个大质量球状星团则可能曾经是一个矮星系的核[12,82]。

图2 射手星流在银道直角坐标系(X,Z)平面(左)和(Y,Z)平面(右)上的投影位置,前者大致位于星流轨道面附近,后者则与轨道面正交。中央水平线代表银盘所处位置,叉号代表太阳。图中的圆圈和三角分别表示亮支和暗支上RC星的观测位置,小黑点取自模拟数据[55]Fig.2 The projections of the Sgr.stream onto the Galactic X-Z plane and Y-Z plane,respectively.The X-Z plane is about the orbital plane of the stream,and the Y-Z plane is about perpendicular to the orbit.The Galactic disk is represented by the horizontal line,and the location of the Sun is marked with a cross.Circles and triangles indicate the positions of the RC stars in the fields of the bright and faint arms.The small solid dots represent the simulated data[55]

2.3 独孤星流(Orphan stream)

独孤星流是由文[11]和文[83]作者于2006年独立发现的,因当时尚未有明确的前身母天体而得其名。之后,人们随即对它进行了多方面的探究[39-44]。

2007年文[39]作者的研究表明,SDSS(DR5)资料所显示的独孤星流的角尺度约为50°。该星流存在距离梯度:天赤道附近星流的日心距约为20 kpc,并以~40 km s-1的速度朝向地球运动;在高赤纬区星流日心距最大可达~32 kpc,以~100 km s-1的速度远离地球而去。

2010年文[43]作者利用SDSS和SEGUE资料详细地讨论了独孤星流的轨道,发现该星流位于一条顺行轨道上,远银心距90 kpc,近银心距16.4 kpc,轨道偏心率e=0.7。最近,文[44]作者由K型巨星样本得出,该星流的角宽度只有2°左右,约只及人马星流的1/5。

关于独孤星流的前身天体,迄今尚未得出一致性的结论。2007年文[39]作者率先提出,独孤星流与新发现的矮椭球星系大熊II可能有物理上的联系,主要理由是两者的距离颇为接近。大熊II由文[84]作者于2006年发现并命名,其日心距约为30 kpc,且正处于瓦解之中。同年,文[40]作者的理论工作支持了上述观点,并进而提出了较为连贯的演化图像——很久以前,某个伴星系与银河系相并合,独孤星流、大熊II以及若干年轻晕族球状星团,都是这一并合过程中,因该伴星系被撕裂而生成的碎片。尽管如此,不同的观点依然存在[42-44]。

除上述星流外,银晕外区还存在其他一些矮星系星流。如室女星流[19-23,71]、麒麟星流[71,85-88]、大犬恒星环[25,51]、反银心星流(ACS)[89-90]、Styx星流[48]等等。

3 球状星团与星流

球状星团在星流探测中可能具有双重身份:其一,在银河系引力场作用下,部分星团成员星被剥离而生成潮汐尾,星团是潮汐尾的母天体;其二,球状星团自身可能来自近邻矮星系,它们属于矮星系星流的成员,矮星系是星团的母天体。

3.1 球状星团潮汐尾

20世纪90年代中期,人们发现许多星团的两侧存在潮汐碎片,表现为在星团的最外缘部分,实测密度轮廓与King模型不符,即超出了模型潮汐半径的范围[28-29,91]。碎片来自瓦解中的球状星团,与星团有着类似的运动轨道,它们会沿着轨道路径从两个方向伸出,形成星团的前导尾和后随尾[33]。

数字巡天使探测工作进一步深入[33,35,71]。在众多球状星团中,以星团Pal 5潮汐尾的工作做得最多,也最为细致[19,33-37,71],而其他球状星团则显得较为零碎[31-32,36,38,48,92-97]。

3.1.1 球状星团Pal 5潮汐尾

Pal 5是一个结构相对松散的远距离球状星团,日心距r=23.2 kpc,质量M=(0.8-1.3)×104M☉,核半径约20 pc(角半径2.′9)。Pal 5可归于光度最低的球状星团之列,总光度仅约为MV=-5.0 mag,质量只及银河系球状星团中位质量的1/30[33-34]。

2001年,文[33]作者率先利用SDSS的5色深度测光资料对Pal 5结构进行了探究。他们发现从星团本体的南、北两侧、距团中心约0.2° (投影线距离为80 pc)处出发,分别朝东北和西南方向(也就是星团的轨道路径方向)伸出2条近乎对称的潮汐尾,其银纬大体上保持不变,整个潮汐尾结构的张角约为2.6°(图3)。

上述基本结论为多项后续工作所证实或修正。如2003年给出潮汐尾的总长度超过10°(线尺度40 kpc),其中前导尾长3.5°,后随尾长6.5°,远大于2年前得到的长度2.6°[35]。3年后,利用SDSS(DR4)资料得出,Pal 5后随尾的长度接近19°,潮汐尾结构总长度可达22.5°[36]。显然,潮汐尾长度的增大与观测天区的范围有关——随着目标天区的扩大,发现了距团中心越来越远的潮汐尾成员。另一方面,恒星从星团中剥离是一种间歇性、而不是连续性过程:星团通过近银心点附近时会有较多恒星剥离出来,而在远银心点附近则少有、甚至没有恒星被剥离,早期的观测天区较小,远处的潮汐碎片就发现不了。

图3 球状星团Pal 5的面密度轮廓,2条潮汐尾清晰可见[33]。横坐标为赤经,纵坐标是赤纬,其单位均为“°”Fig.3 Contours of the surface-density profile of the globular cluster Pal 5.Two tidal tails of the Pal 5 are obvious in this contour plot[33].The horizontal axis is for Right Ascensions in degrees,and the vertical axis is for Declinations in degrees

文[33]作者2001年的工作表明,在19.5≤i≤22.0星等范围内,Pal 5潮汐尾中的恒星约占目前该星团总星数的32%,可见Pal 5已经历了较为显著的质量损失。2003年发现,潮汐尾中原为母星团成员的恒星总数是目前星团成员星数的1.2倍——潮汐尾质量已超过了星团自身的质量[35]。据此他们认为质量损失主要是在最近20亿年内发生的,并估算了平均质量损失率,同时指出该星团的原始质量可能是它现有质量的6~10倍。

3.1.2 多星团样本的探测研究

1995年,文[20]作者讨论了NGC288等12个球状星团的外部结构,发现对于大部分样本星团,面密度轮廓都存在因外部潮汐力生成的侧翼结构。不久,文[91]作者计算了M5等7个球状星团的潮汐半径,发现其中5个星团呈现潮汐尾结构。

2000年文[30]作者证实,星团潮汐尾的存在乃是一种普遍现象。他们的观测样本中计有20个球状星团,所有星团表现出有潮汐尾结构,其投影方向偏于朝向银心。在这些星团中,有7个团的潮汐尾取向与潮汐场梯度相一致,说明它们起因于银河系激波;有9个团的潮汐尾结构可能反映了星团的轨道路径,且有着不同程度的质量损失。

3.2 作为矮星系星流成员的球状星团

银河系内约有150个球状星团,它们大多形成于约120亿年前。但晕族球状星团有很宽的年龄谱,最年轻的(如Pal 1)甚至比厚盘球状星团(如杜鹃47)的年龄小得多。对这种异常情况的解释是大星系在形成过程中吸积了多个小星系,而被吸积的矮星系各有其恒星和星团的形成史。因此,需要找到能支持这一观点的观测证据。

早在1992年文[92]作者就已指出,年轻球状星团Ruprecht 106很可能是因银河系潮汐力作用从麦哲伦云俘获而来,另一个年轻星团Pal 12也许有着同样的起源,后者是一个很远的球状星团,日心距约19 kpc。但是,2000年,文[93]作者的研究表明,Pal 12更可能是人马星流的成员,而非源自麦哲伦云,并估计出Pal 12从矮星系经潮汐剥离而被俘获的时间大约发生在17亿年之前。

2002年文[94]作者的研究支持了Dinescu等人的观点,他们发现Pal 12的球面位置距人马矮星系主体约40°,且很靠近南方人马星流。他们还在Pal 12附近发现了一个同样属于人马星流成员的低面亮度(LSB)恒星系统,并在总结前人工作[3,24,98]的基础上指出,人马矮星系在瓦解过程中至少为银晕注入了5个球状星团,其中M54和Ter 8两个星团的年龄与银河系最年老球状星团相仿,第3个星团Arp 2较之略微年轻些,还有2个星团(Ter 7和Pal 12)的年龄则要比上述3个星团年轻几十亿年,属于银晕中最年轻、金属丰度最高的球状星团。除球状星团外,人马矮星系在瓦解过程中还为银晕注入了许多不同年龄和不同金属度的恒星——这些正是文[99]银河系演化模型的预期结果。

5年后,文[95]作者发现了源自人马矮星系的第6个银晕球状星团——Whiting 1,年龄仅为6.5 Ga,较上述5个星团更为年轻,日心距r=29.4 kpc,金属度为[Fe/H]=-0.65。Whiting 1经历了来自银河系的潮汐剥离过程,它的天球位置、日心距和视向速度(-130.6 km s-1)与人马后随星流的观测特征相一致。Whiting 1母星系的确认,说明至少在60亿年前人马矮星系中还能形成星团,有力地支持了年轻晕族球状星团来自被银河系吸积的矮星系的观点。

顺便指出,有些结构相对松散的高光度晕族球状星团,可能就是很久以前因经历潮汐剥离作用而剩下的矮星系核,或者说是被吸积矮星系的遗迹,它们的母星系早已不复存在,如NGC2419即属此列——NGC2419是一个现已瓦解的矮星系的核,室女星流亦属该星系的一部分[100-101]。

最早提出“星流”概念的也许可追溯到1894年Holmes的工作[102]。不过,关于银河系星流仅有约20年的有效观测研究史,而期间所取得成果之丰富却远非拙文所能充分述及。随着观测资料的累积,以及一些近距河外星系中矮星系星流和星团潮汐尾的发现[103-105],相关实测结果必将受到人们更为广泛的关注。

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Progress of Detections of Stellar Tidal Streams around the Galaxy

Zhao Junliang1,2
(1.Shanghai Astronomical Observatory,Chinese Academy of Sciences,Shanghai 200030,China,Email:jlzhao@shao.ac.cn;2.Shanghai Normal University,Shanghai 200233,China)

Many stellar tidal streams in the outer halo of the Galaxy have been found since 1994.Some streams were formed from dwarf satellites of the Galaxy and others were from globular clusters.The streams of these two types of origin are usually called dwarf streams and tidal tails of clusters,respectively.Observational and theoretical studies of stellar tidal streams have been steadily accumulated throughout the past 20 years or so.Stellar tidal streams can be identified by using different luminous tracers,such as RR Layaes,red clump stars,blue horizontal-branch stars,main-sequence turnoff stars,M giant stars,and red giant branch stars. These stars are observable even if they are rather far away from the Sun and can be used to determine the heliocentric distances of streams.Various approaches can be used to analyze parameter distributions of these tracers to detect stellar tidal streams.The parameter distributions include distributions of positions,velocities,and Color-Magnitude values.Among the many streams discovered the Sagittarius(Sgr)stream is most spectacular.Since its discovery the Sgr dwarf spheroid galaxy(dSph)has been considered the illustrative example of the assimilation of a small galaxy into a larger one through tidal destruction.In 2003 two tidal substreams of the Sgr stream were discovered;one of them is leading the main body of the dSph galaxy and the other is trailing.A few years later,a bifurcation of the leading sub-stream(into branches A and B)starting at about the North Galactic Pole was identified.Recently,a similar bifurcation of the trailing sub-stream in the Southern Galactic Hemisphere was also identified.Since mid-1990s it has been known that most of the Galactic globular clusters show leading or trailing tidal tails out of their main bodies.Among these,tidal tails of the Palomar 5(Pal 5)are the most closely studied.It has been known over a decade that the Pal 5 has two tails symmetrically stretching out from the main body with a total angular extension of at least 2.6°.Subsequent observations show that the extension of the tails can be even more than known previously,reaching up to 22.5° according to 2006 data.Studies of stellar tidal streams are important for several reasons.First,they yield details about tidal destruction and galaxy merging.Second,they may reveal interesting aspects of galaxy evolution in the presence of strong tidal forces.Third,tidal streams can be used to effectively probe the gravitational potential of the Galaxy(including the dark halo).On the last aspect,however,observations and models of the disruption of the Sgr stream have yielded ambiguous results,which are affected by different choices of data,analysis approaches,and the shape of the Galactic halo(which has been modeled to be oblate,protracted,spherical,or even triaxial).

The Galaxy;Stellar tidal stream;Sagittarius stream;Globular cluster;Tidal tail;Dark halo

P156

A

1672-7673(2014)04-0323-12

2014-02-21;

2014-04-16

赵君亮,男,硕士.研究方向:星团与银河系结构.Email:jlzhao@shao.ac.cn

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词语逗趣
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