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- Member of: ASU Electronic Theses and Dissertations
Description
Galaxies with strong Lyman-alpha (Lya) emission line (also called Lya galaxies or emitters) offer an unique probe of the epoch of reionization - one of the important phases when most of the neutral hydrogen in the universe was ionized. In addition, Lya galaxies at high redshifts are a powerful tool to study low-mass galaxy formation. Since current observations suggest that the reionization is complete by redshift z~ 6, it is therefore necessary to discover galaxies at z > 6, to use their luminosity function (LF) as a probe of reionization. I found five z = 7.7 candidate Lya galaxies with line fluxes > 7x10-18 erg/s/cm/2 , from three different deep near-infrared (IR) narrowband (NB) imaging surveys in a volume > 4x104Mpc3. From the spectroscopic followup of four candidate galaxies, and with the current spectroscopic sensitivity, the detection of only the brightest candidate galaxy can be ruled out at 5 sigma level. Moreover, these observations successfully demonstrate that the sensitivity necessary for both, the NB imaging as well as the spectroscopic followup of z~ 8 Lya galaxies can be reached with the current instrumentation. While future, more sensitive spectroscopic observations are necessary, the observed Lya LF at z = 7.7 is consistent with z = 6.6 LF, suggesting that the intergalactic medium (IGM) is relatively ionized even at z = 7.7, with neutral fraction xHI≤ 30%. On the theoretical front, while several models of Lya emitters have been developed, the physical nature of Lya emitters is not yet completely known. Moreover, multi-parameter models and their complexities necessitates a simpler model. I have developed a simple, single-parameter model to populate dark mater halos with Lya emitters. The central tenet of this model, different from many of the earlier models, is that the star-formation rate (SFR), and hence the Lya luminosity, is proportional to the mass accretion rate rather than the total halo mass. This simple model is successful in reproducing many observable including LFs, stellar masses, SFRs, and clustering of Lya emitters from z~ 3 to z~ 7. Finally, using this model, I find that the mass accretion, and hence the star-formation in > 30% of Lya emitters at z~ 3 occur through major mergers, and this fraction increases to ~ 50% at z~7.
ContributorsShet Tilvi, Vithal (Author) / Malhotra, Sangeeta (Thesis advisor) / Rhoads, James (Committee member) / Scannapieco, Evan (Committee member) / Young, Patrick (Committee member) / Jansen, Rolf (Committee member) / Arizona State University (Publisher)
Created2011
Description
The first part of this dissertation presents the implementation of Bayesian statistics with galaxy surface luminosity (SL) prior probabilities to improve the ac- curacy of photometric redshifts. The addition of the SL prior probability helps break the degeneracy of spectro-photometric redshifts (SPZs) between low redshift 4000 A break galaxies and high redshift Lyman break galaxies which are mostly catas- trophic outliers. For a sample of 1138 galaxies with spectroscopic redshifts in the GOODS North and South fields at z < 1.6, the application of the surface luminosity prior reduces the fraction of galaxies with redshift deviation sigma(z) > 0.2 from 15.0% to 10.4%. The second part of this dissertation presents the study of the chemical evolution of the star-forming galaxies. The Hubble Space Telescope Probing Evolution and Reionization Spectroscopically (PEARS) grism Survey effectively selects emission line galaxies (ELGs) to mAB ~ 27. Follow-up Magellan LDSS3+IMACS spectroscopy of the HST/ACS PEARS ELGs confirms an accuracy of sigma_z = 0.006 for the HST/ACS PEARS grism redshifts. The luminosity-metallicity (L-Z) relation and the mass-metallicity (M-Z) relation of the PEARS ELGs at z ~ 0.6 are offset by ~ - 0.8 dex in metallicity for a given rest-frame B absolute magnitude and stellar mass relative to the local relations from SDSS galaxies. The offsets in both relations are ~ - 0.4 dex larger than that given by other samples at same redshifts, which are demonstrated to be due to the selection of different physical properties of the PEARS ELGs: low metallicities, very blue colors, small sizes, compact disturbed morphologies, high SSFR > 10^-9 yr^-1 , and high gas fraction. The downsizing effect, the tidal interacting induced inflow of metal-poor gas, and the SNe driven galactic winds outflows, may account for the significant offset of the PEARS galaxies in the L-Z and the M-Z relations relative to the local relations. The detection of the emission lines of ELGs down to m ~ 26 mag in the HST/ACS PEARS + HST/WCF3 ERS NIR composit grism spectra enables to extend the study of the evolution of the L-Z and M-Z relations to 0.6 < z < 2.4.
ContributorsXia, Lifang (Author) / Malhotra, Sangeeta (Thesis advisor) / Rhoads, James (Committee member) / Scannapieco, Evan (Committee member) / Jansen, Rolf (Committee member) / Young, Patrick (Committee member) / Arizona State University (Publisher)
Created2012
Description
Lyman-alpha (Lyα) galaxies (LAEs) and Lyα blobs (LABs) are objects identified and studied due to their bright Lyα emission lines. This bright emission allows LAEs and LABs to be studied in the distant universe, providing a glimpse into the physical processes occuring in the early universe. This dissertation presents three complementary studies of LAEs and LABs at z ~ 3.1. The two main foci of this work are (1) to understand the gas kinematics in both classes of objects and (2) to improve spectral energy distribution (SED) fitting processes to better determine the physical characteristics of LAEs. Gas kinematics in this dissertation means looking for signatures of large-scale winds. This is an exciting astrophysical endeavor, because the results can provide insight into how Lyα photons escape distant galaxies and traverse the IGM, and the results have implications for how the epoch of reionization can be studied with the Lyα line and because winds can be a signature of powerful star formation events. In the first two studies we find signatures of winds in three LAEs by measuring the velocity offset between the redshifts of [OIII] and Lyα in these galaxies. The first two LAEs presented here represent the first ever measurements of [OIII] in Lyα-selected field galaxies. The third study reports no velocity offset between [OIII] and Lyα when the methodology is transferred to a z ~ 3.1 LAB. This lack of velocity offset is an interesting result, however, as powerful outflows and star formation events, which should impart a velocity offset, have been hypothesized as power sources for LABs. In addition to understanding the kinematics of these objects, we introduce a new parameter into the SED fitting process typically used to characterize LAEs. This new parameter enables better determination of characteristics like the age, mass, metallicity, dust content and star formation history of the galaxies in our sample. These characteristics provide a snapshot of galaxies in the universe ~ 11 billion years ago and also provide insight into how these characteristics compare to galaxies at other epochs.
ContributorsMcLinden, Emily (Author) / Rhoads, James (Thesis advisor) / Malhotra, Sangeeta (Committee member) / Timmes, Frank (Committee member) / Scowen, Paul (Committee member) / Young, Patrick (Committee member) / Arizona State University (Publisher)
Created2012
Description
New measurements of the Hα luminosity function (LF) and star formation rate
(SFR) volume density are presented for galaxies at z∼0.62 in the COSMOS field.
These results are part of the Deep And Wide Narrowband Survey (DAWN), a unique
infrared imaging program with large areal coverage (∼1.1 deg 2 over 5 fields) and
sensitivity (9.9 × 10 −18 erg/cm 2 /s at 5σ).
The present sample, based on a single DAWN field, contains 116 Hα emission-
line candidates at z∼0.62, 25% of which have spectroscopic confirmations. These
candidates have been selected through comparison of narrow and broad-band images
in the infrared and through matching with existing catalogs in the COSMOS field.
The dust-corrected LF is well described by a Schechter function with L* = 10 42.64±0.92
erg s −1 , Φ* = 10 −3.32±0.93 Mpc −3 (L* Φ* = 10 39.40±0.15 ), and α = −1.75 ± 0.09. From
this LF, a SFR density of ρ SF R =10 −1.37±0.08 M○ yr −1 Mpc −3 was calculated. An
additional cosmic variance uncertainty of ∼ 20% is also expected. Both the faint
end slope and luminosity density that are derived are consistent with prior results at
similar redshifts, with reduced uncertainties.
An analysis of these Hα emitters’ sizes is also presented, showing a direct corre-
lation between the galaxies’ sizes and their Hα emission.
(SFR) volume density are presented for galaxies at z∼0.62 in the COSMOS field.
These results are part of the Deep And Wide Narrowband Survey (DAWN), a unique
infrared imaging program with large areal coverage (∼1.1 deg 2 over 5 fields) and
sensitivity (9.9 × 10 −18 erg/cm 2 /s at 5σ).
The present sample, based on a single DAWN field, contains 116 Hα emission-
line candidates at z∼0.62, 25% of which have spectroscopic confirmations. These
candidates have been selected through comparison of narrow and broad-band images
in the infrared and through matching with existing catalogs in the COSMOS field.
The dust-corrected LF is well described by a Schechter function with L* = 10 42.64±0.92
erg s −1 , Φ* = 10 −3.32±0.93 Mpc −3 (L* Φ* = 10 39.40±0.15 ), and α = −1.75 ± 0.09. From
this LF, a SFR density of ρ SF R =10 −1.37±0.08 M○ yr −1 Mpc −3 was calculated. An
additional cosmic variance uncertainty of ∼ 20% is also expected. Both the faint
end slope and luminosity density that are derived are consistent with prior results at
similar redshifts, with reduced uncertainties.
An analysis of these Hα emitters’ sizes is also presented, showing a direct corre-
lation between the galaxies’ sizes and their Hα emission.
ContributorsGonzalez, Alicia (Author) / Rhoads, James E (Thesis advisor) / Malhotra, Sangeeta (Thesis advisor) / Butler, Nathaniel (Committee member) / Jansen, Rolf (Committee member) / Arizona State University (Publisher)
Created2017