High Redshift Galaxies And Extreme Strong Emission Line Galaxies

The epoch of redshift10is a cosmic milestone as it marks the beginning of reion-ization of the intergalactic medium. Nearly all the discoveries for this early Universe is made with the Hubble Space Telescope (HST), because of its unparalleled high sen-sitivity. However, even the HST is reaching its limit as the flux of z～10objects are detected only in the reddest bands. Along with z～10objects, the HST has also detected a group of interesting galaxies called Extreme Strong Emission Line Galaxies (EELGs) at redshift range1-2. In these galaxies, emission lines can be strong that their equivalent widths (EWs) greater than500A, thus affecting broad-band photometry and mimicking the properties of high-redshift objects. EELGs are believed to be of very young age, and their properties help us understand the formation of low-mass galaxies.In this thesis, we use the HST data from the two major HST projects, CLASH and Frontier Fields, to study the high-redshift galaxies and EELGs. In the first chapter, I will show the picture of galaxy-formations from the Big Bang and then introduce the recent progresses related to high-redshift galaxies and EELGs.In the second chapter, I will describe the basic theory of gravitational lensing and its application in the HST observations of cluster fields. Then I will present the results of the CLASH and the Frontier Fields. In these two projects, we have systematically selected galaxies above redshift7. In the Abell2744fields, we have detected a z-10galaxy with three counter images which well match the lensing model. With additional data from the fields of Abell2744and MACS J0416-2403, we have attempted to con-strain the luminosity functions at redshifts9and10.In the third chapter, I will give the details about our EELG studies in the CLASH fields. We have successfully selected52EELGs from the25cluster fields with two color-color methods. The primary lines in these EELGs are the [O Ⅲ]+Hβ, which are found to be with EWs larger than600A. There EELGs are detected rather stronger emission lines with EWs larger than3000A. The redshifts of these candidates are around1.03or1.67. But we also find three EELGs with photometric redshift around6. The analyses show the non-negligible effect of EELGs for the selection of high-redshift galaxies. We also show that the contaminations from EELGs are more significant in the ongoing Frontier Fields project.In the fourth chapter, I will present my contributions to the HST pipeline and the photometry with Spitzer/IRAC images. First, we achieve the HST pipeline called the APLUS. The APLUS runs with the original HST images, and provides the well com-bined scientific images and reliable catalogs. What is more, we introduce our solution to make the photometry with "confused" Spitzer/IRAC images. Although the sources in most Spitzer/IRAC images overlap with each other due to the poor space resolution, we can still well constrain their flux.I also present works related to the Active Galaxy Nuclei (AGNs) in the chapter five. With helps of the Suzaku X-ray observation, we found a spectacular Seyfert2galaxies, IRAS F01475-0740. This AGN shows relatively low neutral X-ray absorption, while the optical spectra show high optical depth from the lack of broad emission lines. The properties in these two bands are hardly reconciled based on the AGN unification model. We introduce a possible explanation that the material in our line of sight have unusual high dust-to-gas ratio, which is at least4times higher than the Galactic value.My future works are shown in the last chapter. First, we will study EELGs with other observations. With the larger area in CANDELS project, we will make the color selection of EELGs and use simulations to constrain the luminosity function of EELGs. The possible relation of EWs and luminosities can be studied. With the HST grism observations, we will be able to confirm the nature of both EELGs and the high-redshift galaxies in the HST fields. Ground based high resolution NIR observations can be used to study the physical parameters such as the viral masses and metallicities. With the whole Frontier Fields observations, more fainter EELGs with higher EWs can be detected. These galaxies could be at the initial stage of low-mass galaxies. Second, We will also keep searching the high-redshift galaxies in the Frontier Fields and could detect about10galaxies higher than redshift10. Finally, the QSO in the CLASH fields have been systematically selected, we will complete the follow-up spectral observations.