Thick disks in external galaxies.

Since thick disks in SO galaxies were originally detected in 1979,thick-disk compo- nents have been discovered in many disk galaxies,and there are more and more observational evidences that the thick disk in an external galaxy should be a component of the galaxy which is structurally,chemically and kinematically distinct from the thin disk of the galaxy.Generally speaking,for a galaxy the thick disk has its scale height significantly larger than the thin disk. Thick-disk stars are more metal-poor and older than stars in the thin disk and have both a slower rotation around the galaxy center and a larger velocity dispersion.It can be concluded from all these observational facts that thick disks are a kind of relic,which can be used to trace the formation and evolutional history of external galaxies. Edge-on disk galaxies have been extensively used by different authors to measure structural parameters and other observational properties of thin and thick disks.It is found from some recent studies that thick-disk scale heights are generally 2-6 times larger than thin-disk scale heights and the thick-disk scale lengths are slightly larger,which agree with what observed in the Milky Way Galaxy.The axial ratios of thick disks are normally in a range of 3:1-4:1,and those of thin disks can be over 8:1 in some galaxies. The ages of stars in thick disk are estimated to be 5-13 Ga,and their colors are significantly redder than thin-disk stars.Thick-disk stars are metal-poor with median metallicity of[Fe/H]～-1 in some low mass galaxies and show no evidence for the strong vertical matallicity gradients.It has been found that the contribution of the thick disk to the total luminosity shows an obvious trend with mass.For high-mass galaxies(V_c120 km·s~(-1)),thick disks can contribute 10% of the total luminosity of the galaxy and for lower mass systems this value can be up to 40%.The analysis of vertical color gradients for some galaxies suggests that the color of thick disks are fairly uniform.The stellar mass of low mass galaxies are dominated by thick disk stars,its fraction being up to 50% of the total stellar mass in some cases.Unfortunately,stellar populations above the galaxies' midplanes have been studied in fewer than 10 galaxies to date. The number of kinematic studies of thick disks in external galaxies is even more sparse than the stellar population studies.It is revealed from the two edge-on galaxies studied recently that one shows the thick disk rotates at 30%-40% of the rotation speed of the thin disk and the other shows clear evidence of counter-rotation. From the systematic differences between the properties in structure,chemical abundance and kinematics of thick and thin disks,they are likely to have distinct formation scenarios.It has been extensively recognized that the majority of thin-disk stars formed gradually from a rotating disk of high angular momentum gas.However,the formation of the thick disk is still poorly understood and is more complex.According to the properties of thick disks,their stars could form in one of the following ways:(1)they were kinematically heated from an earlier thinner disk,which means that disks form largely through smooth gas accretion,and the thick-disk stars in a galaxy form initially in a thin disk;(2)they were formed from gas with large scale height, which suggests that disks form primarily through smooth monolithic collapse and thick-disk stars precipitate out of the collapsing gas cloud,or the thick disk forms from a disk of gas that has been energetically heated,so that they form in the thick disk itself;(3)they were directly deposited at large scale heights during the accretion of smaller satellite galaxies,which means that these stars can form entirely from merging subunits outside the present-host galaxy.It is not very easy to distinguish among the above formation scenarios of thick disks for individual galaxies,and probably thick disks of different galaxies have different formation processes,or even more than one mechanism can play their own roles in formation processes of the thick disk around a disk galaxy.While the broad outlines of thick disk formation are in place,there are many details to be worked out.