An Exploration Of Inter Stellar Gas And Its Role In Galaxy Assembly Over Cosmic Time
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An Exploration Of Inter Stellar Gas And Its Role In Galaxy Assembly Over Cosmic Time
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Author : Riccardo Pavesi
language : en
Publisher:
Release Date : 2019
An Exploration Of Inter Stellar Gas And Its Role In Galaxy Assembly Over Cosmic Time written by Riccardo Pavesi and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2019 with categories.
The key physical processes driving galaxy formation and evolution are controlled by gas and, in particular, the process of star formation from cold, dense gas is not well understood since it depends upon the gas cooling ability, its dynamical state and complex feedback processes. Galaxies were observed to form stars much more rapidly in the past (~10-11 billion years ago), which may be due to larger gas reservoirs or more efficient star formation processes. While previous studies have identified large molecular gas reservoirs in a few pre-selected star-forming galaxies, an unbiased survey for molecular gas is necessary to provide robust statistical constraints to the gas content of galaxies at the peak epoch of cosmic star formation. Taking advantage of the improved frequency coverage, sensitivity and bandwidth of the upgraded Very Large Array we have carried out the first unbiased survey by performing a deep-field blind search for CO(1-0) line emission at z~2-3 and CO(2-1) line emission at z~5-7, targeting CO(1-0) which is the most commonly used tracer of the cold, dense molecular gas which fuels star formation. Having detected the first CO(1-0)-selected galaxies at high redshift, we have used their luminosity and abundance to provide robust statistical constraints to the CO luminosity function at z~2-3, finding conclusive evidence for a much higher gas mass content relative to galaxies in the local Universe. This finding suggested that evolution in the mechanisms of star formation may not be the dominant contribution to the high observed star formation rates, but rather large amounts of available cold gas. In order to explore how this finding may apply to even higher redshift, we have also achieved the first detection of CO emission in "normal" galaxies at z>5 (in the first billion years of cosmic time) together with far-infrared fine structure line tracers of the atomic and ionized gas using the sensitive Atacama Large(sub-)Millimeter Array. We found that early galaxies appear to be extremely gas rich, relative to their stellar content, and to display comparable star formation efficiency to typical lower redshift "normal" galaxies. However, the interstellar medium in a fraction of such galaxies also appears to be strongly affected by lower metallicity, affecting the phase structure of the interstellar medium, and the usefulness of CO as a tracer of molecular gas.
The Assembly Of Galaxies Over Cosmic Time
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Author : Yicheng Guo
language : en
Publisher:
Release Date : 2012
The Assembly Of Galaxies Over Cosmic Time written by Yicheng Guo and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2012 with Cosmology categories.
To understand how galaxies were assembled across the cosmic time remains one of the most outstanding questions in astronomy. The core of this question is how today's Hubble Sequence, namely the differentiation of galaxy morphology and its correlation to galaxy physical properties, is formed. In this thesis, we investigate the origin of the Hubble Sequence through galaxies at z~2, an epoch when the cosmic star formation activity reaches its peak and the properties of galaxies undergo dramatic transitions. Galaxies at z~2 have two important features that are distinct from nearby galaxies: much higher frequency of clumpy morphology in star-forming systems, and much compacter size. To understand the nature of the two features requires investigations on the sub-structure of galaxies in a multi-wavelength way. In this thesis, we study samples of galaxies that are selected from GOODS and HUDF, where ultra-deep and high-resolution optical and near-infrared images allow us to study the stellar populations of the sub-structures of galaxies at the rest-frame optical bands for the first time, to answer two questions: (1) the nature of kiloparsec-scale clumps in star-forming galaxies at z$\sim$2 and (2) the existence of color gradient and stellar population gradient in passively evolving galaxies at z~2, which may provide clues to the mechanisms of dramatic size evolution of this type of galaxies. We further design a set of color selection criteria to search for dusty star-forming galaxies and passively evolving galaxies at z~3 to explore the question: when today's Hubble Sequence has begun to appear.
The Role Of Gas In Galaxy Evolution
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Author : John Caleb Barentine
language : en
Publisher:
Release Date : 2013
The Role Of Gas In Galaxy Evolution written by John Caleb Barentine and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2013 with categories.
The story of a typical spiral galaxy like the Milky Way is a tale of the transformation of metal-poor hydrogen gas to heavier elements through nuclear burning in stars. This gas is thought to arrive in early times during the assembly phase of a galaxy and at late times through a combination of hot and cold "flows" representing external evolutionary processes that continue to the present. Through a somewhat still unclear mechanism, the atomic hydrogen is converted to molecules that collect into clouds, cool, condense, and form stars. At the end of these stars' lives, much of their constituent gas is returned to the galaxy to participate in subsequent generations of star formation. In earlier times in the history of the universe, frequent and large galaxy mergers brought additional gas to further fuel this process. However, major merger activity began an ongoing decline several Gyr ago and star formation is now diminishing; the universe is in transitioning to an era in which the structural evolution of disk galaxies is dominated by slow, internal ("secular") processes. In this evolutionary regime, stars and the gas from which they are formed participate in resonant gravitational interactions within disks to build ephemeral structures such as bars, rings, and small scale-height central bulges. This regime is expected to last far into the future in a galaxy like the Milky Way, punctuated by the periodic accretion of dwarf satellite galaxies but lacking in the "major" mergers that kinematically scramble disks into ellipticals. This thesis examines details of the story of gas from infall to structure-building in three major parts. The High- and Intermediate-Velocity Clouds (HVCs/IVCs) are clouds of H [Iota] gas at velocities incompatible with simple models of differential Galactic rotation. Proposed ideas explaining their observed properties and origins include (1) the infall of low-metallicity material from the Halo, possibly as cold flows along filaments of a putative "Cosmic Web"; (2) gas removed from dwarf satellite galaxies orbiting the Milky Way via some combination of ram pressure stripping and tidal disruption; and (3) the supply and return feeds of a "Galactic Fountain" cycling gas between the Disk and Halo. Numerical values of their observed properties depend strongly on the Clouds' distances. In Chapter 2, we summarize results of an ongoing effort to obtain meaningful distances to a selection of HVCs and IVCs using the absorption-line bracketing method. We find the Clouds are not at cosmological distances, and with the exception of the Magellanic Stream, they are generally situated within a few kiloparsecs of the Disk. The strongest discriminator of the above origin scenarios are the heavy element abundances of the Clouds, but to date few reliable Cloud metal- licities have been published. We used archival UV spectroscopy, supplemented by new observations with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope and H [Iota] 21 cm emission spectroscopy from a variety of sources to compute elemental abundances relative to hydrogen for 39 HVC/IVC components along 15 lines of sight. Many of these are previously unpublished. We find support for all three origin scenarios enumerated above while more than doubling the number of robust measurements of HVCs/IVCs in existence. The results of this work are detailed in Chapter 3. In Chapter 4 we present the results of a spectroscopic study of the high-mass protostellar object NGC 7538 IRS 9 made with the Texas Echelon Cross Echelle Spectrograph (TEXES), a sensitive, high spectral resolution, mid-infrared grating spectrometer and compare our observations to published data on the nearby object NGC 7538 IRS 1. Forty-six individual lines in vibrational modes of the molecules C2H2, CH4, HCN, NH3 and CO were detected, including two isotopologues (13CO, 12C18O) and one combination mode ([nu]4+[nu]5 C2H2). Fitting synthetic spectra to the data yielded the Doppler shift, excitation temperature, Doppler b parameter, column density and covering factor for each molecule observed; we also computed column density upper limits for lines and species not detected, such as HNCO and OCS. We find differences among spectra of the two objects likely attributable to their differing radiation and thermal environments. Temperatures and column densities for the two objects are generally consistent, while the larger line widths toward IRS 9 result in less saturated lines than those toward IRS 1. Finally, we compute an upper limit on the size of the continuum-emitting region (~2000 AU) and use this constraint and our spectroscopy results to construct a schematic model of IRS 9. In Chapters 5 and 6, we describe studies of the bright, nearby, edge-on spiral galaxies NGC 4565 and NGC 5746, both previously classified as type Sb spirals with measured bulge-to-total luminosity ratios B/T [approximately equal to] 0.4. These ratios indicate merger-built, "classical" bulges but in reality represent the photometric signatures of bars seen end-on. We performed 1-D photometric decompositions of archival Hubble Space Telescope, Spitzer Space Telescope, and Sloan Digital Sky Survey images spanning a range of wavelengths from the optical to near-infrared that penetrate the thick midplane dust in each galaxy. In both, we find high surface brightness, central stellar components that are clearly distinct from the boxy bar and from the disk; we interpret these structures as small scale height "pseudobulges" built from disk material via internal, resonant gravitational interactions among disk material -- not classical bulges. The brightness profiles of the innermost component of each galaxy is well fitted by a Sersic function with major/minor axis Sersic indices of n = 1.55±0.07 and 1.33±0.12 for NGC 4565 and n = 0.99±0.08 and 1.17 ± 0.24 for NGC 5746. The true "bulge-to-total" ratios of these galaxies are considerably smaller than once believed: 0.061+0.009 and 0.136 ± 0.019, -0.008, respectively. Therefore, more galaxies than we thought contain little or no evidence of a merger-built classical bulge. We argue further that a classical bulge cannot hide behind the dust lane of either galaxy and that other structures built exclusively through secular evolution processes such as inner rings, both revealed through the infrared imagery, argue strongly against any merger violence in the recent past history of these objects. From a formation point of view, NGC 4565 and NGC 5746 are giant, pure-disk galaxies, and we do not understand how such galaxies form in a [Lamda]CDM universe. This presents a challenge to our picture of galaxy formation by hierarchical clustering because it is difficult to grow galaxies as large as these without making big, classical bulges. We summarize the work presented in this thesis in Chapter 7 and conclude with speculations about the future direction of research in this field.
Galaxies Through Cosmic Time
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Author : Amber Bauermeister
language : en
Publisher:
Release Date : 2012
Galaxies Through Cosmic Time written by Amber Bauermeister and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2012 with categories.
In the past decade, molecular gas observations have begun probing the high redshift universe in a systematic way using increasingly powerful millimeter instruments. This work has significantly advanced our understanding of the history of gas consumption by star formation in galaxies, revealing the high redshift universe to be similar in many ways to what we know locally. Specifically, molecular gas studies suggest that at both high and low redshift, the molecular gas reservoir in galaxies is insufficient to support on-going star formation. This is the molecular gas depletion problem, and motivates the research presented in this dissertation. I first investigate the molecular gas depletion problem on cosmic scales. Using the observed cosmic densities of the star formation rate, atomic gas and molecular gas, combined with measurements of the molecular gas depletion time in local galaxies, I derive the history of gas consumption by star formation from z = 0 to z ~ 4. I show that models in which the molecular gas is not replenished, or is only replenished by atomic gas, are not consistent with observational constraints. I find that star formation on cosmic timescales must be fueled by intergalactic ionized gas at an average rate that roughly traces the star formation rate density of the universe. Further, I predict the volume averaged density of molecular gas to increase by a factor of 1.5 - 10 to z ~ 1.5 over the currently measured value, which implies that galaxies at high redshift must, on average, be more molecular gas-rich than they are at the present epoch, consistent with observations. Next I focus on the observational constraints on the molecular gas content of galaxies from z ~ 1 - 2 to today. Recent observations suggest z ~ 1 - 2 galaxies harbor molecular gas reservoirs an order of magnitude larger than their local counterparts, implying significant evolution of the molecular gas content of galaxies over the past 8 billion years. However, this period of time has been relatively un-observed in molecular gas. To fill in this observational gap, I carry out the Evolution of molecular Gas in Normal Galaxies (EGNoG) survey, a study of molecular gas in 31 star-forming galaxies from z = 0.05 to z = 0.5. With observations of the CO(1-0) and CO(3-2) rotational lines using the Combined Array for Research in Millimeter-wave Astronomy (CARMA), the EGNoG survey accomplishes two goals: tracing the evolution of the molecular gas content of galaxies at intermediate redshifts and constraining the excitation of the molecular gas in these galaxies. With 24 detections out of 31 observed galaxies, I calculate an average molecular gas fraction of 7 - 20% at z ~ 0.05 - 0.5, which is in line with observations at high and low redshift and agrees well with the evolution predicted by a simple empirical prescription for gas consumption by star formation in galaxies from z ~ 1 - 2 to today. The EGNoG observations of four galaxies at z = 0.3 (the gas excitation subsample) yield robust detections of both lines in three galaxies (and an upper limit on the fourth). I find an average line ratio, r31 = L'(CO(3-2)) / L'(CO(1-0)), of 0.46 ± 0.07 (with systematic errors less than 40%), which implies sub-thermal excitation of the CO(3-2) line. As the EGNoG galaxies are representative of the main sequence of star-forming galaxies, I extend this result to include main sequence galaxies at high redshift. To support the observations carried out at CARMA as part of the EGNoG survey, I give two appendices. The first details the data reduction and flux measurement for the EGNoG survey, including a description of the use of polarized calibrators to calibrate data from single, linearly polarized feeds. In the second appendix, I describe the absolute flux calibration of CARMA data and the automated monitoring system I helped put in place in order to maintain a historical record of the flux of common calibrators. Finally, I return to the gas depletion problem in the local universe. I carry out a pilot study of atomic (HI) gas in groups of galaxies in order to investigate the role of tidal interactions in transporting atomic gas from the outskirts of galaxy disks to the central regions so that it may replenish the molecular gas and fuel ongoing star formation. I image three groups of galaxies in the 21 cm line of HI with the Allen Telescope Array (ATA), detecting many galaxies not previously observed in HI as well as four previously undetected clouds of HI between galaxies that account for up to 3% of the HI reservoir of the groups. To investigate the potential role of this gas in the ongoing star formation in the group, I compare the mass of the detected HI gas in and between galaxies in the group to the estimated star formation rates of the group members.
The Influence Of Cosmic Ray Transport On The Stability Of Interstellar Gas
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Author : Evan Mitchell Heintz
language : en
Publisher:
Release Date : 2022
The Influence Of Cosmic Ray Transport On The Stability Of Interstellar Gas written by Evan Mitchell Heintz and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2022 with categories.
Cosmic rays play a major role in the stability and evolution of galaxies. While they have a fairly small density inside galaxies, their energy density is comparable to the thermal gas and magnetic fields. We now believe that cosmic rays play a major role in galactic and interstellar dynamics, including helping to drive galactic outflows, modify the galaxy's interstellar chemistry, and form large-scale structures. To understand the process of how cosmic rays do this, their microscale interactions with the gas and magnetic fields must be accurate so that their macroscale effects can reproduce observations. Our work in this thesis specifically focuses on how different models of cosmic ray transport change these interactions and their effects. Through the combined approach of analytical solutions and numerical simulations, this thesis aims to better understand how cosmic ray transport affects the stability of the interstellar medium and drive galactic outflows. We begin in Chapter 2 with a linear stability analysis of the Parker instability, a Rayleigh-Taylor like instability with the thermal gas supported against gravity by magnetic fields and cosmic rays. We model three different cosmic ray transport models and find that the model where cosmic rays stream relative to the thermal gas most greatly enhances the instability due to the heating of the thermal gas by cosmic rays scattering off of magnetic fluctuations. We continue with the Parker instability in Chapter 3 where we add radiative cooling to the system and then run numerical simulations with a smooth gravitational potential in 2D and 3D to better understand the nonlinear evolution of the instability in a more realistic environment. When radiative cooling is added, we find it enhances the instability when cosmic rays are locked to the thermal gas while it dampens the instability when cosmic ray streaming is the primary mode of transport. In our MHD simulations, we find that both cosmic ray diffusion and streaming enhance the growth of the instability due to the motion of cosmic rays out of the compressive pockets of gas in the valleys of the magnetic field. While the instability growth seems similar, however, the two transport models result in quite different phase structures of the gas, especially at the top of the Parker loops where streaming cosmic rays heat the gas. We then explore the idea of a cosmic ray Eddington limit in Chapter 4. This theory supposes that cosmic rays, through their own pressure gradient, may be able to overcome hydrostatic equilibrium and launch an outflow if star formation is vigorous enough in that galaxy. For five different galaxies and many different transport models, we find that a cosmic ray Eddington limit does exist. However, the Eddington limit often requires gas densities and/or star formation rates that are far different from typical values for galaxies. Therefore, we conclude that it is unlikely that cosmic rays themselves can reach this Eddington limit and drive a galactic wind. We finally conclude in Chapter 5 with a summary of our results and a short discussion on the future research that could be done based around our conclusions.
Interstellar Grains
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Author : Chandra Wickramasinghe
language : en
Publisher:
Release Date : 1967
Interstellar Grains written by Chandra Wickramasinghe and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 1967 with Cosmic dust categories.
The Distribution And Motion Of Interstellar Matter In Galaxies
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Author : Lodewijk Woltjer
language : en
Publisher:
Release Date : 1962
The Distribution And Motion Of Interstellar Matter In Galaxies written by Lodewijk Woltjer and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 1962 with Galaxies categories.
Galaxy Formation And Evolution
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Author : Houjun Mo
language : en
Publisher: Cambridge University Press
Release Date : 2010-05-20
Galaxy Formation And Evolution written by Houjun Mo and has been published by Cambridge University Press this book supported file pdf, txt, epub, kindle and other format this book has been release on 2010-05-20 with Science categories.
A coherent introduction for researchers in astronomy, particle physics, and cosmology on the formation and evolution of galaxies.
Origin Of Matter And Evolution Of Galaxies
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Author : Shigeru Kubono
language : en
Publisher: World Scientific
Release Date : 1997-05-07
Origin Of Matter And Evolution Of Galaxies written by Shigeru Kubono and has been published by World Scientific this book supported file pdf, txt, epub, kindle and other format this book has been release on 1997-05-07 with categories.
This book focuses on nucleosynthesis and chemical evolution of the universe. The discussion on the universe, using a common language of atomic elements and nucleosynthesis, is presented by leading figures from a wide variety of fields — astronomy, astrophysics, cosmology, nuclear physics and particle physics. One of the highlights is the paper on MACHO's by C Alcock, which was the first to be released to the world. Perspectives of the fields are also presented, such as the SUBARU project and the Radioactive Nuclear Beam Project at INS, University of Tokyo.
Gas Distribution Around Galaxies In Cosmological Simulations
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Author : Mitali Damle
language : en
Publisher:
Release Date : 2023
Gas Distribution Around Galaxies In Cosmological Simulations written by Mitali Damle and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2023 with categories.
The evolution of a galaxy is pivotally governed by its pattern of star formation over a given period of time. The star formation rate at any given time is strongly dependent on the amount of cold gas available in the galaxy. Accretion of pristine gas from the Intergalactic medium (IGM) is thought to be one of the primary sources for star-forming gas. This gas first passes through the virial regions of the galaxy before reaching the Interstellar medium (ISM), the hub of star formation. On the other hand, owing to the evolutionary course of young and massive stars, energetic winds are ejected from the ISM to the virial regions of the galaxy. A bunch of interlinked, complex astrophysical processes, arising from the concurrent presence of both infalling as well as outbound gas, play out over a range of timescales in the halo region or the Circumgalactic medium (CGM) of a galaxy. It would not be incorrect to say that the CGM has a stronghold over the gas reserves of a galaxy and thus, plays a backhand, yet, rather pivotal role in shaping ...