Selection And Physical Properties Of High-redshift Galaxies

Extremely Red Objects (EROs) and BzKs continue to attract considerable interest, on the one hand, the research in the literature suggests that they may be the direct progenitors of present-day massive E/SO galaxies. On the other hand, they can provide crucial constraints on the current galaxy formation and evolution models. Therefore, the key question is to measure the relative fraction of OGs (old galaxies) and DGs (young, dusty starburst galaxies) in the sample of EROs. Many groups are currently investigating the fractions of these two ERO populations using a variety of observational approaches, but the fraction of OGs and DGs from different surveys is different. In the meantime, a number of observations suggest that the era of z-2is also important in galaxy formation and evolution for various reasons:the cosmic star formation rate density (SFRD) begins to drop at z～2from a flat plateau at higher redshifts; the morphological type mix of field galaxies changes remarkably at z～2; the number density of QSOs has a peak at z～2;～50%-70%of the stellar mass assembly of galaxies took place in the redshift range1<z<3. In this thesis, our main works are as follows:(1) Classification of z～1EROs in the UDF and COSMOS field.(2) A study on physical properties of passive and star-forming Galaxies at z-2in the AEGIS Field.(3) Mid-infrared spectroscopy and multi-wavelength study of galaxies at z-2in the AEGIS Field.We present a quantitative study of the classification of EROs in the UDF and COSMOS field. Our sample includes5264(COSMOS, KVega<19.2) and24EROs (UDF, Kvega<22.0) with (i-K)AB≥2.45. Using the spectral energy dis-tribution (SED) fitting method,[3.6]-[8.0] color, and nonparametric measures of galaxy morphology, we classify EROs into two classes:DGs and OGs. We found that the fraction of OGs and DGs in our sample (COSMOS) is similar, about52% of them are DGs, and48%of EROs in our sample are OGs. For24EROs in the UDF,16are classified as DGs, while8are classified as OGs. To reduce the re-dundancy of these three different classification methods, we performed a principal component analysis on the measurements of EROs, and find that nonparametric measures and SEDs are efficient in segregating DGs and OGs. We investigate the dependence of the fraction of EROs on their observational properties, and the results suggest that DGs become increasingly important at fainter magnitudes, redder colors, and higher redshifts. Moreover, we find that the clustering of EROs is much stronger than that of full K-limited samples of galaxies; the clustering amplitude of OGs is a factor of-2larger than that of DGs.We pick out1609star-forming galaxies (sgzKs:gzK=(z-K)AB-1.4(g-z)AB≥0.2) and422passively evolving galaxies (pgzKs:gzK<0.2and (z-K)AB≥2.7) at z～2in the AEGIS field (KAB<22.0). The number counts of pgzKs in our sample turn over at KAB～21.0, and both the number of faint and bright objects (including sgzKs and pgzKs) exceed the predictions of a recent semi-analytic model of galaxy formation, a more successful model is need to explain this diversity. We also find that the star formation rate (SFR) and specific SFR (sSFR) of sgzKs increases with redshift at all masses, implying that star-forming galaxies were much more active on average in the past. Moreover, the sSFR of massive galaxies is lower at all redshifts, suggesting that star formation contributes more to the mass growth of low-mass galaxies than to high-mass galaxies. From HST WFC3/F160W imaging data, we find that gzKs not only have diffuse structures, but also have single-object morphologies, implying that there are morphological variety and different formation process for these galaxies at z～2. On the other hand, we also find～-10%of828gzKs can be classified as AGNs.Finally, we present Spitzer/IRS spectra of a sample of14ULIRGs with0.2<F24μm<0.6mJy in the AEGIS field. The spectroscopic redshifts of this sample are in a very narrow range around z～1.95±0.19, and their IRS spec-tra have polycyclic aromatic hydrocarbon (PAH) features at7.7,8.6, and11.3μm. The ranges of stellar mass (M*) and SFR of these ULIRGs correspond to M*1011M⊙and410M⊙yr-1<SFR<1022M⊙yr-1, respectively. Their rest-frame optical morphologies are very diversified including string-like, extend-ed/diffused, and even early type spiral morphologies, implying that there are different formation process for these galaxies. We also search for active galac-tic nucleus (AGN) signature in our sample using X-ray, radio, and mid-infrared (MIR) emission. EGS22, EGS25, EGS27, and EGS34are detected in the X-ray imaging. The X-ray luminosities for EGS22and EGS34can be accounted for by their intensive star formation. EGS25and EGS27have higher L2-10keV, indicat-ing that they harbor AGNs. About14-29%of the sample show signatures of AGNs in X-ray, MIR or radio.