Matching Items (10)
Description
Green pea galaxies are a class of rare, compact starburst galaxies that have powerful optical emission line [OIII]$\lambda$5007. They are the best low-redshift analogs of high-redshift (z$>$2) Lyman-alpha emitting galaxies (LAEs). They provide unique opportunities to study physical conditions in high-redshift LAEs in great detail. In this dissertation, a few physical properties of green peas are investigated. The first study in the dissertation presents star formation rate (SFR) surface density, thermal pressure in HII regions, and a correlation between them for 17 green peas and 19 Lyman break analogs, which are nearby analogs of high-redshift Lyman break galaxies. This correlation is consistent with that found from the star-forming galaxies at z $\sim$ 2.5. In the second study, a new large sample of 835 green peas in the redshift range z = 0.011 -- 0.411 are assembled from Data Release 13 of the Sloan Digital Sky Survey (SDSS) with the equivalent width of the line [OIII]$\lambda$5007 $>$ 300\AA\ or the equivalent width of the line H$\beta$ $>$ 100\AA. The size of this new sample is ten times that of the original 80 star-forming green pea sample. With reliable T$_e$-based gas-phase metallicity measurements for the 835 green peas, a new empirical calibration of R23 (defined as ([OIII]$\lambda$$\lambda$4959,5007 + [OII]$\lambda$$\lambda$3726,3729)/H$\beta$) for strong line emitters is then derived. The double-value degeneracy of the metallicity is broken for galaxies with large ionization parameter (which manifests as log([OIII]$\lambda$$\lambda$4959,5007/[OII]$\lambda$$\lambda$3726,3729) $\geq$ 0.6). This calibration offers a good way to estimate metallicities for extreme emission-line galaxies and high-redshift LAEs. The third study presents stellar mass measurements and the stellar mass-metallicity relation of 828 green peas from the second study. The stellar mass covers 6 orders of magnitude in the range 10$^{5}$ -- 10$^{11}$ M$_{\odot}$, with a median value of 10$^{8.8}$ M$_{\odot}$. The stellar mass-metallicity relation of green peas is flatter and displays about 0.2 - 0.5 dex offset to lower metallicities in the range of stellar mass higher than 10$^{8}$ M$_{\odot}$ compared to the local SDSS star-forming galaxies. A significant dependence of the stellar mass-metallicity relation on star formation rate is not found in this work.
ContributorsJiang, Tianxing (Author) / Malhotra, Sangeeta (Thesis advisor) / Rhoads, James E (Committee member) / Scannapieco, Evan (Committee member) / Borthakur, Sanchayeeta (Committee member) / Jansen, Rolf A (Committee member) / Arizona State University (Publisher)
Created2018
Description
In the past three decades with the deployment of space-based from x-rays to infrared telescopes and operation of 8-10 m class ground based telescopes, a hand-full of regions of the sky have emerged that probe the distant universe over relatively wide fields with the aim of understanding the assembly of apparently faint galaxies. To explore this new frontier, observations were made with the Large Binocular Cameras (LBCs) on the Large Binocular Telescope (LBT) of a well-studied deep field, GOODS-North, which has been observed by a wide range of telescopes from the radio to x-ray. I present a study of the trade-off between depth and resolution using a large number of LBT/LBC U-band and R-band imaging observations in the GOODS-N field. Having acquired over 30 hours of data (315 images with 5-6 minute exposures) for U-band and 27 hours for R-band (828 images with 2 minute exposures), multiple mosaics were generated, starting with images taken under the best atmospheric conditions (FWHM <0.8"). For subsequent mosaics, data with coarser seeing values were added in until the final, deepest mosaic included all images with FWHM <1.8". For each mosaic, object catalogs were made to compare the optimal-resolution, yet shallower image to the low-resolution but deeper image. For the brightest galaxies within the GOODS-N field, structure and clumpy features within the galaxies are more prominent in the optimal-resolution image compared to the deeper mosaics. I conclude that for studies of brighter galaxies and features within them, the optimal-resolution image should be used. However, to fully explore and understand the faintest objects, the deeper imaging with lower resolution are also required. For the 220 and 360 brightest galaxies in the U-band and R-band images respectively, there is only a marginal difference between the optimal-resolution and lower-resolution light-profiles and their integrated total fluxes. This helps constrain how much flux can be missed in galaxy outskirts, which is important for studies of Extragalactic Background Light. Finally, I also comment on a collection of galaxies in the field with tidal tails and streams, diffuse plumes, and bridges.
ContributorsAshcraft, Teresa Ann (Author) / Windhorst, Rogier A (Thesis advisor) / Borthakur, Sanchayeeta (Committee member) / Jansen, Rolf A (Committee member) / Scowen, Paul (Committee member) / Groppi, Chris (Committee member) / Arizona State University (Publisher)
Created2018
Description
Galaxies in the universe are surrounded by a hot medium called the Circum-Galactic Medium (CGM). Present the CGM is gas that forms up clouds which travel within the CGM at speeds that approximately range between 100 km/s and 300 km/s. These gas clouds are very interesting because they play a crucial in the formation of stars within the galaxies and also in the overall evolution of galaxies. The clouds could in fact be thought of as mobile "gas stations" whose sole purpose is facilitate the ionization of elements and ultimately supply gas to galaxies. My thesis project is a follow-up study on CGM gas cloud observations that were made by Borthakur et. al. (2016). Using Cosmic Origins Spectrograph (COS) data from the Hubble Space Telescope (HST), Borthakur et. al. (2016) observed the presence of both Carbon IV (C IV) and Oxygen VI (O IV) but did not observe any Nitrogen V (N V) in the gas cloud when expected to be observable. Therefore, the ultimate goal of my research was to determine whether indeed CGM gas clouds have an actual shortage of the N V ion. My research involves the generation of cosmological simulations of a cold gas cloud that has a radius of 98 parsecs, relative velocity of 200 km/s, density range of 10-3 to -5 and a temperature in the range of ~104 to 5 K, and also a hot CGM that has density in the range of 10-4.5 to -6 particles/cm3 and temperature of approximately 106 K. Traces of N v are observed in my simulations.
ContributorsSaboi, Kezman (Author) / Scannapieco, Evan (Thesis director) / Borthakur, Sanchayeeta (Committee member) / Cottle, JNeil (Committee member) / School of Earth and Space Exploration (Contributor) / Barrett, The Honors College (Contributor)
Created2018-05
Description
The Balloon-borne Large Aperture Submillimeter Telescope - The Next Generation (BLAST-TNG) was designed to map the polarized emission from dust in star forming regions of our galaxy. The dust is thought to trace magnetic fields and thus inform us of the role that it plays in star formation. BLAST-TNG improves upon the previous generation of balloon-borne sub-mm polarimeters by increasing the number of detectors by over an order of magnitude. A novel detector technology which is naturally multiplexed, Kinetic Inductance Detectors have been developed as an elegant solution to the challenge of packing cryogenic focal plane arrays with detectors. To readout the multiplexed arrays, custom firmware and control software was developed for the ROACH2 FPGA based system. On January 6th 2020 the telescope was launched on a high-altitude balloon from Antarctica and flew for approximately 15 hours in the mid-stratosphere. During this time various calibration tasks occurred such as atmospheric skydips, the mapping of a sub-mm source, and the flashing of an internal calibration lamp. A mechanical failure shortened the flight so that only calibration scans were performed. In this dissertation I will present my analysis of the in-flight calibration data leading to measures of the overall telescope sensitivity and detector performance. The results of which prove kinetic inductance detectors as a viable candidate for future space based sub-mm telescopes. In parallel the fields of digital communications and radar signal processing have spawned the development of the Radio Frequency System On a Chip (RFSoC). This product by Xilinx incorporates a fabric of reconfigurable logic, ARM microprocessors, and high speed digitizers all into one chip. The system specs provide an improvement in every category of size, weight, power, and bandwidth.This is naturally the desired platform for the next generation of far-infrared telescopes which are pushing the limits of detector counts. I present the development of one of the first frequency multiplexed detector readouts on the RFSoC platform. Alternative firmware designs implemented on the RFSoC are also discussed. The firmware work presented will be used in part or in full for multiple current and upcoming far-infrared telescopes.
ContributorsSinclair, Adrian Kai (Author) / Mauskopf, Philip D (Thesis advisor) / Borthakur, Sanchayeeta (Committee member) / Groppi, Christopher (Committee member) / Jacobs, Daniel (Committee member) / Hubmayr, Johannes (Committee member) / Arizona State University (Publisher)
Created2021
Description
White Dwarf stars are the stellar remnants of low mass stars which have completed their evolution. Nearly all stars will become white dwarfs. The interior of a white dwarf encapsulates its evolution history: unraveling a white dwarf’s internal structure constrains the physical events which occurred to construct its composition. Variable, or pulsating, white dwarfs emit pulsations which are sensitive to their internal stratification. Just as seismology reveals Earth’s interior, asteroseismology can reveal stellar interiors. The standard approach to construe an observed white dwarf’s chemical makeup is to match observed pulsation properties to theoretical stellar models. Observed white dwarf pulsation data has reached 6-7 significant digits of precision. As such, it is important for computational modeling to consider systematic offsets from initial conditions and theoretical uncertainties that are within the detectable threshold. By analyzing the magnitude of pulsation differences among various uncertainties from white dwarf models, one can place constraints on important theoretical uncertainties. In this thesis, I explore impacts on white dwarf pulsations that result from accounting for various uncertainties in computational models. I start by showing the importance of 22Ne, and its impact on the pulsations in Helium atmosphere white dwarfs. Next, I discuss how certain trapped modes of white dwarfs may yield a signal for the 12C(α,γ)16O reaction rate probability distribution function. This reaction occurs during the Helium core burning phase in stellar evolution, and chiefly determines the Carbon and Oxygen abundance of white dwarfs. Following this work, I show how overshooting impacts the pulsation signatures of the 12C(α, γ)16O reaction rate. I then touch on the analytical work I’ve done regarding educational research in the HabWorlds course offered at Arizona State University (ASU). I then summarize my conclusions from these efforts.
ContributorsChidester, Morgan Taylor (Author) / Timmes, Francis X (Thesis advisor) / Young, Patrick (Committee member) / Li, Mingming (Committee member) / Borthakur, Sanchayeeta (Committee member) / Line, Michael (Committee member) / Arizona State University (Publisher)
Created2023
The Dependence of Star Formation Quenching and of Lyman Alpha Escape on Galaxy Structural Properties
Description
Galaxy structural properties such as size, morphology, and surface brightness bear the imprint of galaxies' evolutionary histories, and so are related with other properties such as stellar mass, star formation rate, and emergent spectra. In this dissertation, I present three studies exploring such relationships. In the first, I investigated the relationships between 4000 Å break (D4000) strength, colors, stellar masses, and morphology in a sample of 352 galaxies at intermediate redshifts based on photometric and spectroscopic data from the Hubble Space Telescope (HST). I explored several diagrams such as UVJ color space combined with the D4000 strengths and the structural parameters of sample galaxies. The analysis shows that the presence of a bulge component is a necessary but not sufficient requirement for star formation quenching at intermediate redshifts. In the second study, I investigated the central 250 pc UV star formation intensity (SFI, star formation rate per unit area) of a sample of 40 Green Pea (GP) galaxies and 15 local Lyman Break Galaxy Analogs (LBAs) to understand the Lyα escape mechanisms and the associations with the SFI in Lyα-emitters (LAEs). I utilized the Cosmic Origins Spectrograph near-ultraviolet (COS/NUV) images from the HST. I found that the Lyα equivalent width (EW(Lyα)) and the Lyα escape fraction are positively correlated with the ratio of SFI to galaxy stellar mass. These correlations suggest the importance of the central SFI in Lyα photon escape. In the third study, I investigated the UV photometric properties of a sample of 40 GPs and the possible associations with Lyα escape mechanisms. I measured the UV-continuum size and luminosity of the sample galaxies by employing the COS/NUV images. The circularized half-light radius of GPs shows compact sizes and it further shows the statistically significant anti-correlations with EW(Lyα) and the Lyα escape fraction. The size comparison of GPs to those of high-redshift LAEs shows that their sizes are similar, once spatial resolution effects are properly considered. These results show that a compact size is crucial for escape of Lyα photons, and that Lyα emitters show constant characteristic size independent of their redshift. Therefore, the results presented in this dissertation emphasize the importance of galaxy structural properties in star formation quenching and in Lyα escape.
ContributorsKim, Keunho (Author) / Malhotra, Sangeeta (Thesis advisor) / Butler, Nathaniel R (Thesis advisor) / Rhoads, James E (Committee member) / Borthakur, Sanchayeeta (Committee member) / Jansen, Rolf A (Committee member) / Arizona State University (Publisher)
Created2020
Description
Reionization is the phase transition of intergalactic atoms from being neutral to
becoming fully ionized. This process began ∼400 Myr after the Big Bang, when the first
stars and black holes began emitting ionizing radiation from stellar photospheres and
accretion disks. Reionization completed when all of the neutral matter between galaxies
became ionized ∼1 Gyr after the Big Bang, and the Universe became transparent as
it is today.
Characteristics of the galaxies that drove reionization are mostly unknown. The
physical mechanisms that create ionizing radiation inside these galaxies, and the
paths for this light to escape are even more unclear. To date, only a small fraction of
the numerous searches for this escaping light have been able to detect a faint signal
from distant galaxies, and no consensus on how Reionization was completed has been
established.
In this dissertation, I discuss the evolution of the atomic matter between galaxies
from its initially ionized state, to its current re-ionized state, potential sources of
re-ionizing energy, and the theoretical and observational status of the characteristics of
these sources. I also present new constraints on what fraction of the ionizing radiation
escapes from galaxies using Hubble Space Telescope UV imaging, theoretical models
of the stellar and accretion disk radiation, and models of the absorption of ionizing
radiation by the intergalactic medium.
becoming fully ionized. This process began ∼400 Myr after the Big Bang, when the first
stars and black holes began emitting ionizing radiation from stellar photospheres and
accretion disks. Reionization completed when all of the neutral matter between galaxies
became ionized ∼1 Gyr after the Big Bang, and the Universe became transparent as
it is today.
Characteristics of the galaxies that drove reionization are mostly unknown. The
physical mechanisms that create ionizing radiation inside these galaxies, and the
paths for this light to escape are even more unclear. To date, only a small fraction of
the numerous searches for this escaping light have been able to detect a faint signal
from distant galaxies, and no consensus on how Reionization was completed has been
established.
In this dissertation, I discuss the evolution of the atomic matter between galaxies
from its initially ionized state, to its current re-ionized state, potential sources of
re-ionizing energy, and the theoretical and observational status of the characteristics of
these sources. I also present new constraints on what fraction of the ionizing radiation
escapes from galaxies using Hubble Space Telescope UV imaging, theoretical models
of the stellar and accretion disk radiation, and models of the absorption of ionizing
radiation by the intergalactic medium.
ContributorsSmith, Brent Matthew (Author) / Windhorst, Rogier A. (Thesis advisor) / Bowman, Judd (Committee member) / Borthakur, Sanchayeeta (Committee member) / Butler, Nathaniel (Committee member) / Mauskopf, Phillip (Committee member) / Arizona State University (Publisher)
Created2019
Description
A key open problem within galaxy evolution is to understand the evolution of galaxies towards quiescence. This work investigates the suppression of star-formation through shocks and turbulence at low-redshift, and at higher-redshifts, this work investigates the use of features within quiescent galaxy spectra to redshift estimation, and passive evolution of aging stellar populations to understand their star-formation histories.
At low-$z$, this work focuses on the analysis of optical integral field spectroscopy data of a nearby ($z\sim0.0145$) unusual merging system, called the Taffy system because of radio emission that stretches between the two galaxies. This system, although a recent major-merger of gas-rich spirals, exhibits an atypically low star-formation rate and infrared luminosity. Strong evidence of shock heating as a mechanism for these atypical properties is presented. This result (in conjunction with many others) from the nearby Universe provides evidence for shocks and turbulence, perhaps due to mergers, as an effective feedback mechanism for the suppression of star-formation.
At intermediate and higher-$z$, this work focuses on the analysis of Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) G800L grism spectroscopy and photometry of galaxies with a discernible 4000\AA\ break. The usefulness of 4000\AA/Balmer breaks as redshift indicators by comparing photometric, grism, and spectrophotometric redshifts (SPZs) to ground-based spectroscopic redshifts, is quantified. A spectral energy distribution (SED) fitting pipeline that is optimized for combined HST grism and photometric data, developed for this project, is presented. This pipeline is a template-fitting based routine which accounts for correlated data between neighboring points within grism spectra via the covariance matrix formalism, and also accounts for galaxy morphology along the dispersion direction. Evidence is provided showing that SPZs typically improve the accuracy of photometric redshifts by $\sim$17--60\%. For future space-based observatories like the Nancy Grace Roman Space Telescope (formerly the Wide Field InfraRed Survey Telescope, i.e., WFIRST) and Euclid, this work predicts $\sim$700--4400 galaxies\,degree$^{-2}$, within $1.6 \lesssim z \lesssim 3.4$, for galaxies with 4000\AA\ breaks and continuum-based redshifts accurate to $\lesssim$2\%.
This work also investigates the star-formation histories of massive galaxies ($\mathrm{M_s \geq 10^{10.5}\, M_\odot}$). This is done through the analysis of the strength of the Magnesium absorption feature, Mgb, at $\sim$5175\AA. This analysis is carried out on stacks of HST ACS G800L grism data, stacked for galaxies binned on a color vs stellar mass plane.
At low-$z$, this work focuses on the analysis of optical integral field spectroscopy data of a nearby ($z\sim0.0145$) unusual merging system, called the Taffy system because of radio emission that stretches between the two galaxies. This system, although a recent major-merger of gas-rich spirals, exhibits an atypically low star-formation rate and infrared luminosity. Strong evidence of shock heating as a mechanism for these atypical properties is presented. This result (in conjunction with many others) from the nearby Universe provides evidence for shocks and turbulence, perhaps due to mergers, as an effective feedback mechanism for the suppression of star-formation.
At intermediate and higher-$z$, this work focuses on the analysis of Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) G800L grism spectroscopy and photometry of galaxies with a discernible 4000\AA\ break. The usefulness of 4000\AA/Balmer breaks as redshift indicators by comparing photometric, grism, and spectrophotometric redshifts (SPZs) to ground-based spectroscopic redshifts, is quantified. A spectral energy distribution (SED) fitting pipeline that is optimized for combined HST grism and photometric data, developed for this project, is presented. This pipeline is a template-fitting based routine which accounts for correlated data between neighboring points within grism spectra via the covariance matrix formalism, and also accounts for galaxy morphology along the dispersion direction. Evidence is provided showing that SPZs typically improve the accuracy of photometric redshifts by $\sim$17--60\%. For future space-based observatories like the Nancy Grace Roman Space Telescope (formerly the Wide Field InfraRed Survey Telescope, i.e., WFIRST) and Euclid, this work predicts $\sim$700--4400 galaxies\,degree$^{-2}$, within $1.6 \lesssim z \lesssim 3.4$, for galaxies with 4000\AA\ breaks and continuum-based redshifts accurate to $\lesssim$2\%.
This work also investigates the star-formation histories of massive galaxies ($\mathrm{M_s \geq 10^{10.5}\, M_\odot}$). This is done through the analysis of the strength of the Magnesium absorption feature, Mgb, at $\sim$5175\AA. This analysis is carried out on stacks of HST ACS G800L grism data, stacked for galaxies binned on a color vs stellar mass plane.
ContributorsJoshi, Bhavin (Author) / Windhorst, Rogier (Thesis advisor) / Jansen, Rolf (Committee member) / Appleton, Philip (Committee member) / Scannapieco, Evan (Committee member) / Borthakur, Sanchayeeta (Committee member) / Arizona State University (Publisher)
Created2020
Description
Learning how properties of galaxies such as star formation, galaxy interactions, chemical composition, and others evolve to produce the modern universe has long been a goal of extragalactic astronomy. In recent years, grism spectroscopy from the Hubble Space Telescope (HST) has provided a means to study these properties with spectroscopy while avoiding the limitations of ground-based observation. In this dissertation, I present several studies wherein I used HST G102 grism spectroscopy from the Faint Infrared Grism Survey (FIGS) to investigate these fundamental properties of galaxies and how they interact and evolve. In the first study, I combined the grism spectra with broadband photometry to produce a catalog of redshifts with improved accuracy, reducing the median redshift error from 3\% to 2\%. With this redshift catalog, I conducted a systematic search for galaxy overdensities in the FIGS fields, producing a list of 24 significant candidates. In the second study, I developed a method for identifying emission line galaxy (ELG) candidates from continuum-subtracted 1D spectra, and identified 71 ELGs in one FIGS field. In matching MUSE/VLT spectra, I measured the [OIII]$\lambda$4363 emission line for 14 FIGS ELGs, and used this to measure their $T_e$-based gas-phase metallicities. These ELGs show a low-metallicity offset on the Mass-Metallicity Relation, and I demonstrated that this offset can be explained by recent star formation. In the third study, I expanded the ELG search to all four FIGS fields, identifying 208 H$\alpha$, [OIII]$\lambda\lambda$4959,5007, and [OII]$\lambda\lambda$3727,3729 line emitters. I compiled a catalog of line fluxes, redshifts, and equivalent widths. I combined this catalog with the overdensity study to investigate a possible relationship between line luminosity, star formation, and an ELG's environment. In the fourth study, I usde 15 FIGS H$\alpha$ emitters and 49 ``green pea'' line emitters to compare H$\alpha$ and the far-UV continuum as tracers of star formation. I explored a correlation between the H$\alpha$-FUV ratio and the ratio of [OIII]$\lambda\lambda$4959,5007 to [OII]$\lambda\lambda$3727,3729 and its implications for star formation history.
ContributorsPharo, John (Author) / Malhotra, Sangeeta (Thesis advisor) / Young, Patrick (Thesis advisor) / Rhoads, James (Committee member) / Scannapieco, Evan (Committee member) / Borthakur, Sanchayeeta (Committee member) / Arizona State University (Publisher)
Created2019
Description
The interaction between galaxies and the surrounding gas plays a key role in galaxy formation and evolution. Feedback processes driven by star formation and active galactic nuclei facilitate the exchange of mass and energy between the galaxy and the circumgalactic medium through inflowing and outflowing gas. These outflows have a significant impact on the star formation rate and metallicity of the galaxy. Observations of outflows have provided evidence that these outflows are multi-phase in nature, identifying both low energy ions such as Mg II and C III and high energy ions such as O VI. The underlying physics maintaining the two phases as well as the ionization mechanism for these phases remains unclear. In order to better understand galactic outflows, hydrodynamic simulations are used to study the evolution of wind-cloud interactions. In this work, I carried out a suite of magnetohydrodynamic simulations to characterize the influence of magnetic fields on the evolution and lifetime of cold clouds. I found magnetic fields either provided little improvement to cloud stability over other influences such as radiative cooling or accelerated cloud disruption by pushing cloud material in the direction orthogonal to the wind and magnetic fields. To investigate the ionization mechanism of the material within outflows I first considered estimating the column densities of various ions within wind-cloud simulations with the post-processing tool Trident. Under the assumption of ionization equilibrium, the simulations did not reproduce the observed absorption profiles demonstrating the need for a more detailed treatment of the ionization processes. I then performed a new set of simulations with the non-equilibrium chemistry solver, MAIHEM. The column densities produced in the non-equilibrium model alter the evolution of the cloud and highlight the increased ionization along the boundary of the cloud.
ContributorsBlough-Swingen, J'Neil (Author) / Scannapieco, Evan (Thesis advisor) / Groppi, Christopher (Committee member) / Young, Patrick (Committee member) / Borthakur, Sanchayeeta (Committee member) / Mauskopf, Phillip (Committee member) / Arizona State University (Publisher)
Created2021