极亮红外星系(ULIRGs)是指红外(IR,8~1 000 μm)光度LIR>1012 L☉的一类星系.研究表明,红移z≈2处极亮红外星系是大质量(M*>1011 M☉)、富尘埃和强恒星形成(大于100 M☉·a-1)的特殊星系.极亮红外星系可分成活动星系核起主导作用的源和恒星形成占主导的星系.恒星形成主导的源,中红外光谱有明显的多环芳香烃辐射;而活动星系核主导的星系,光谱呈现出幂律形式并有很强的硅线吸收.极亮红外星系的静止光学波段形态存在多样化,既有并合结构特征,又有椭圆形态.这类星系很可能是近邻大质量宁静星系的前身星系.介绍了红移z=2附近极亮红外星系的各种物理性质的研究进展,如形态和结构、光谱特征、成团性、尘埃分布和形成机制等,以及阐述了该领域未来的研究方向.
UltraLuminous InfraRed Galaxies (ULIRGs) are galaxies defined to have extremely high infrared (IR) luminosities (LIR > 1012 L☉).Observations have shown that high-redshift ULIRGs are massive galaxies (M* > 1011 M☉),with extremely high ratio of infrared to optical flux density (F24 μm/FR > 1 000) and intensive star formation (100 ~ 1 000 M☉ · a-1).These objects are relatively rare at z ≈ 0,but their space density rapidly increase with look-back time and apparently peaks around z =2 ~ 3.Upon it's discovery,ULIRGs were suggested to be a feasible evolutionary phase towards the formation of local massive early-type galaxies.Their rest-frame optical morphologies show that high-redshift ULIRGs are a mixture of mergers or interacting systems,irregular galaxies,disks,and ellipticals.The morphological diversities of ULIRGs suggest that there are different formation processes for these galaxies.Merger processes between galaxies and disk instabilities play an important role in the formation and evolution of ULIRGs at high redshift.Mid-infrared (MIR) spectra of ULIRGs are observed with the Spitzer/IRS (InfraRed Spectrograph) instrument.For ULIRGs at z ≈ 2,major spectral features including the PAH (Polycyclic Aromatic Hydrocarbon) emission features at 7.7,8.6,and 11.3 μm and silicate absorption from 8 to 13 μm (peaking at 9.7 μm) fall in the observable wavelength range.ULIRGs can be divided into two types according to their MIR spectra.Objects with strong power-law continua are powered mainly by active galactic nuclei (AGNs),while those with a strong PAH emission are powered by intensive star formation.The clustering signal of ULIRGs roughly corresponds to a correlation length of r0 =7.40 h-1.Mpc for the full F24 μm > 0.3 mJy sample.The clustering strength increases with luminosity,up to r0 =12.97 h-1.Mpc for F24 μm > 0.6 mJy ULIRGs.Moreover,observations demonstrate that a median dust temperature is about 40 K for ULIRGs at z ≈ 2.