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We present a quantitative analysis of high-mass, low-z galaxies in order to investigate the 'downsizing' scenario of galaxy evolution. High-mass, low-z galaxies with ongoing star formation, antithetical to the 'downsizing' model, are identified using the 22[micrometer] data (W4 band) from the Wide-field Infrared Survey Explorer (WISE). A cluster and field sample is chosen to investigate any possible environmental effects. The cluster sample is based upon the GMBCG catalog, which contains 55,424 brightest cluster galaxies (BCGs) at 0:1 [approximately less than] z [approximately less than] 0:55 identified in the Sloan Digital Sky Survey (SDSS). We identify 389 W4-detected BCGs (W4BCGs) that have median SFRs of [approximately]50 M[dot in circle]/yr based upon their total IR luminosity (L[subscript IR]), which is attributed to dust-enshrouded star formation. BCGs with such high SFRs are found in "coolcore" clusters and the star formation is thought to be fueled by a cooling flow." Using Chandra X-ray data, it is shown that a subset of BCGs do reside within coolcores, but their mass deposition rates cannot account for the SFR. For comparison, a field sample is drawn from the Max-Planck Institute for Astrophysics - John Hopkins University (MPA-JHU) "value-added" SDSS DR7 catalog of spectrum measurements. A set of 1,244 high-mass, elliptical field galaxies within the same redshift range as the W4BCG catalog are identified for comparison. The median mass for the field sample is lower than the W4BCGs (Log(M/M[dot in circle])=10.9 and 11.2 respectively), as are their SFRs. However, the specific star formation rate (sSFR), the star formation rate per stellar mass, is comparable for both groups (Log(sSFR)[approximately]-9.7). This possibly reveals that there is no environmental dependence on the sSFR for these W4-detected galaxies. While a possible mechanism responsible for the SFR was identified for the W4BCGs, the process responsible for the star formation in the field sample requires further investigation.
We study the properties of star-forming galaxies at redshift z 2, an era in which a substantial fraction of the stellar mass in the universe formed. Using 114 near-IR spectra of the H-alpha and [N II] emission lines and model spectral energy distributions fit to rest-frame UV through IR photometry, we examine the galaxies' star formation properties, dynamical masses and velocity dispersions, spatially resolved kinematics, outflow properties, and metallicities as a function of stellar mass and age. While the stellar masses of the galaxies in our sample vary by a factor of 500, dynamical masses from H-alpha velocity dispersions and indirect estimates of gas masses imply that the variation of stellar mass is due as much to the evolution of the stellar population and the conversion of gas into stars as to intrinsic differences in the total masses of the galaxies. About 10% of the galaxies are apparently young starbursts with high gas fractions, caught just as they have begun to convert large amounts of gas into stars. Using the [N II]/H-alpha ratio of composite spectra to estimate the average oxygen abundance, we find a monotonic increase in metallicity with stellar mass. From the estimated gas fractions, we conclude that the observed mass-metallicity relation is primarily driven by the increase in metallicity as gas is converted to stars. The picture that emerges is of galaxies with a broad range in stellar population properties, from young galaxies with ages of a few tens of Myr, stellar masses M 10 DEGREES9 Msun, and metallicities Z 1/3 Zsun, to massive objects with M* 10 DEGREES11 Msun, Z Zsun, and ages as old as the universe allows. All, however, are rapidly star-forming, power galactic-scale outflows, and have masses in gas and stars of at least 10 DEGREES10 Msun, in keeping with their likely role as the progenitors of elliptical galaxies
This thesis presents a pioneering method for gleaning the maximum information from the deepest images of the far-infrared universe obtained with the Herschel satellite, reaching galaxies fainter by an order of magnitude than in previous studies. Using these high-quality measurements, the author first demonstrates that the vast majority of galaxy star formation did not take place in merger-driven starbursts over 90% of the history of the universe, which suggests that galaxy growth is instead dominated by a steady infall of matter. The author further demonstrates that massive galaxies suffer a gradual decline in their star formation activity, providing an alternative path for galaxies to stop star formation. One of the key unsolved questions in astrophysics is how galaxies acquired their mass in the course of cosmic time. In the standard theory, the merging of galaxies plays a major role in forming new stars. Then, old galaxies abruptly stop forming stars through an unknown process. Investigating this theory requires an unbiased measure of the star formation intensity of galaxies, which has been unavailable due to the dust obscuration of stellar light.
Star-formation is one of the key processes that shape the current state and evolution of galaxies. This volume provides a comprehensive presentation of the different methods used to measure the intensity of recent or on-going star-forming activity in galaxies, discussing their advantages and complications in detail. It includes a thorough overview of the theoretical underpinnings of star-formation rate indicators, including topics such as stellar evolution and stellar spectra, the stellar initial mass function, and the physical conditions in the interstellar medium. The authors bring together in one place detailed and comparative discussions of traditional and new star-formation rate indicators, star-formation rate measurements in different spatial scales, and comparisons of star-formation rate indicators probing different stellar populations, along with the corresponding theoretical background. This is a useful reference for students and researchers working in the field of extragalactic astrophysics and studying star-formation in local and higher-redshift galaxies.
The formation and evolution of galaxies is one of the most important topics in modern astrophysics. Secular evolution refers to the relatively slow dynamical evolution due to internal processes induced by a galaxy's spiral arms, bars, galactic winds, black holes and dark matter haloes. It plays an important role in the evolution of spiral galaxies with major consequences for galactic bulges, the transfer of angular momentum, and the distribution of a galaxy's constituent stars, gas and dust. This internal evolution is in turn the key to understanding and testing cosmological models of galaxy formation and evolution. Based on the twenty-third Winter School of the Canary Islands Institute of Astrophysics, this volume presents reviews from nine world-renowned experts on the observational and theoretical research into secular processes, and what these processes can tell us about the structure and formation of galaxies. The volume provides a firm grounding for graduate students and early career researchers working on galactic dynamics and galaxy evolution.
New observations of the period between the cosmic recombination and the end of reionization are posing intriguing questions about where the first generations of stars were formed, how the first galaxies were assembled, whether these galaxies have low redshift counterparts, and what role the early galaxies played in the reionization process. Combining the new observational data with theoretical models can shed new light on open issues regarding the star formation process, its role in the reionization of the Universe, and the metal enrichment in galaxies at those early epochs. This volume brings together leading experts in the field to discuss our current level of understanding and what may come in the near future as our observational as well as theoretical tools improve. The book confronts the theory of how the first stars, black holes, and galaxies formed with current and planned observations. This synthesis is very timely, just ahead of the establishment of major new facilities, such as the James Webb Space Telescope (JWST), a next-generation, millimeter/sub-millimeter observatory in the Atacama desert (ALMA), and ground-based Extremely Large Telescopes (ELT). Together, they will revolutionize the study of the most distant objects in the Universe. This volume is aimed at beginning graduate students but can also serve as a reference work for active researchers in the field. Apart from presenting the fundamental concepts involved, it also provides an introduction to the methods and techniques used. The book will also be useful to anyone with an astrophysical background who needs an effective starting point for learning about the first stars and galaxies.
Although low-mass metal-poor galaxies in the local universe have often been proposed as the 'primordial building blocks' in the hierarchical scenario of structure formation, several lines of evidence suggest that this may not be true. Moreover, it is not clear to what extent dwarf galaxies, because they are metal poor and because of their kinematics and structure, can tell us about how star formation proceeded in the early universe. This volume provides an overview and the most recent advances in this debate. IAU Symposium 255 presents the most up-to-date developments in six key areas, including: Population III and metal-free star formation; metal-enrichment, chemical evolution and feedback; explosive events in low-metallicity environments; dust and gas as seeds for metal-poor star formation; metal-poor initial mass functions, stellar evolution and star-formation histories; and low-metallicity star formation in the local universe. This overview is at a level suitable for research astronomers and graduate students.
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 book begins with a historical introduction, "Star Formation: The Early History", that presents new material of interest for students and historians of science. This is followed by two long articles on "Pre-Main-Sequence Evolution of Stars and Young Clusters" and "Observations of Young Stellar Objects". These articles on the fascinating problem of star formation from interstellar matter give a thorough overview of present-day theories and observations. The articles contain material so far unpublished in the astronomical literature. The book addresses graduate students and can be used as a textbook for advanced courses in stellar astrophysics.