Author
Debra Meloy Elmegreen
Other affiliations: National Science Foundation, Association of Universities for Research in Astronomy
Bio: Debra Meloy Elmegreen is an academic researcher from Vassar College. The author has contributed to research in topics: Galaxy & Star formation. The author has an hindex of 64, co-authored 186 publications receiving 11204 citations. Previous affiliations of Debra Meloy Elmegreen include National Science Foundation & Association of Universities for Research in Astronomy.
Topics: Galaxy, Star formation, Spiral galaxy, Elliptical galaxy, Bulge
Papers published on a yearly basis
Papers
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TL;DR: In this article, it was shown that clump formation, migration, disruption, and interaction with the disk cause these systems to evolve from initially uniform disks into regular spiral galaxies with an exponential or double-exponential disk profile and a central bulge.
Abstract: Many galaxies at high redshift have peculiar morphologies dominated by 108-109 M☉ kpc-sized clumps. Using numerical simulations, we show that these "clump clusters" can result from fragmentation in gravitationally unstable primordial disks. They appear as "chain galaxies" when observed edge-on. In less than 1 Gyr, clump formation, migration, disruption, and interaction with the disk cause these systems to evolve from initially uniform disks into regular spiral galaxies with an exponential or double-exponential disk profile and a central bulge. The inner exponential is the initial disk size, and the outer exponential is from material flung out by spiral arms and clump torques. A nuclear black hole may form at the same time as the bulge from smaller black holes that grow inside the dense cores of each clump. The properties and lifetimes of the clumps in our models are consistent with observations of the clumps in high-redshift galaxies, and the stellar motions in our models are consistent with the observed velocity dispersions and lack of organized rotation in chain galaxies. We suggest that violently unstable disks are the first step in spiral galaxy formation. The associated starburst activity gives a short timescale for the initial stellar disk to form.
498 citations
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TL;DR: The photometric redshift distributions, spectral types, Sersic indices, and sizes of all resolved galaxies in the Hubble Space Telescope Ultra Deep Field (UDF) are studied in this article to understand the environment and nature of star formation in the early universe.
Abstract: The photometric redshift distributions, spectral types, Sersic indices, and sizes of all resolved galaxies in the Hubble Space Telescope Ultra Deep Field (UDF) are studied in order to understand the environment and nature of star formation in the early universe. Clumpy disk galaxies that are bright at short wavelengths (rest-frame λ < 5000 A) dominate the UDF out to z ~ 5.5. Their uniformity in V/Vmax and comoving volume density suggest that they go even further, spanning a total time more than an order of magnitude larger than their instantaneous star formation times. They precede as well as accompany the formation epoch of distant red galaxies and extreme red objects. Those preceding could be the premerger objects that combined to make red spheroidal types at z ~ 2-3. Clumpy disks that do not undergo mergers are likely to evolve into spirals. The morphology of clumpy disks, the size and separation of the clumps, and the prevalence of this type of structure in the early universe suggests that most star formation occurs by self-gravitational collapse of disk gas.
341 citations
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TL;DR: In this article, color-color diagrams for the clump and interclump emission in 10 clump-cluster galaxies of the Hubble Ultra Deep Field (UDF) are made from B,V, i, and z images and compared with models to determine redshifts, star formation histories, and galaxy masses.
Abstract: Color-color diagrams for the clump and interclump emission in 10 clump-cluster galaxies of the Hubble Ultra Deep Field (UDF) are made from B,V, i, and z images and compared with models to determine redshifts, star formation histories, and galaxy masses. These galaxies are members of a class dominated by 5-10 giant clumps, with no exponential disk or bulge. The redshifts are found to be in the range from 1.6 to 3. The clump emission is typically 40% of the total galaxy emission, and the luminous clump mass is 19% of the total galaxy mass. The clump colors suggest declining star formation over the last ~0.3 Gyr, while the interclump emission is redder than the clumps, corresponding to a greater age. The clump luminous masses are typically 6 × 108 M☉, and their diameters average 1.8 kpc, making their average density ~0.2 M☉ pc-3. Including the interclump populations, assumed to begin forming at z = 6, the total galaxy luminous masses average 6.5 × 1010 M☉ and their diameters average 19 kpc to the 2 σ noise level. The expected galaxy rotation speeds average ~150 km s-1 if they are uniformly rotating disks. The ages of the clumps are longer than their internal dynamical times by a factor of ~8, so they are stable star clusters, but the clump densities are only ~10 times the limiting tidal densities, so they could be deformed by tidal forces. This is consistent with the observation that some clumps have tails. The clumps could form by gravitational instabilities in accreting disk gas and then disperse on a ~1 Gyr timescale, building up the interclump disk emission, or they could be captured as gas-rich dwarf galaxies, flaring up with star formation at first and then dispersing. Support for this second possibility comes from the high abundance of nearly identical clumps in the UDF, smaller than 6 pixels, whose distributions on color-magnitude and color-color plots are the same as the galaxy clumps studied here. The distribution of axial ratios for the combined population of chain and clump-cluster galaxies in the UDF is compared with models and shown to be consistent with a thick-disk geometry. If these galaxies evolve into today's disk galaxies, then we are observing a stage in which accretion and star formation are extremely clumpy and the resulting high velocity dispersions form thick disks. Several clump-clusters have disk densities that are much larger than in local disks, however, suggesting an alternate model in which they do not survive until today, but get converted into ellipticals by collisions.
277 citations
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TL;DR: In this article, the authors presented the first merger simulations with high fractions of cold, turbulent, and clumpy gas, and discussed the major new features of these models compared to models where the gas is artificially stabilized and warmed.
Abstract: Disk galaxies at high redshift (z ~ 2) are characterized by high fractions of cold gas, strong turbulence, and giant star-forming clumps. Major mergers of disk galaxies at high redshift should then generally involve such turbulent clumpy disks. Merger simulations, however, model the interstellar medium as a stable, homogeneous, and thermally pressurized medium. We present the first merger simulations with high fractions of cold, turbulent, and clumpy gas. We discuss the major new features of these models compared to models where the gas is artificially stabilized and warmed. Gas turbulence, which is already strong in high-redshift disks, is further enhanced in mergers. Some phases are dispersion dominated, with most of the gas kinetic energy in the form of velocity dispersion and very chaotic velocity fields, unlike merger models using a thermally stabilized gas. These mergers can reach very high star formation rates, and have multi-component gas spectra consistent with SubMillimeter Galaxies. Major mergers with high fractions of cold turbulent gas are also characterized by highly dissipative gas collapse to the center of mass, with the stellar component following in a global contraction. The final galaxies are early type with relatively small radii and high Sersic indices, like high-redshift compact spheroids. The mass fraction in a disk component that survives or re-forms after a merger is severely reduced compared to models with stabilized gas, and the formation of a massive disk component would require significant accretion of external baryons afterwards. Mergers thus appear to destroy extended disks even when the gas fraction is high, and this lends further support to smooth infall as the main formation mechanism for massive disk galaxies.
276 citations
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University of Alabama1, National Radio Astronomy Observatory2, Aix-Marseille University3, University of La Laguna4, Spanish National Research Council5, University of Turku6, University of Oulu7, Vassar College8, Carnegie Institution for Science9, Peking University10, University of Arizona11, University of Hawaii12, University of Lyon13, European Southern Observatory14, Seoul National University15, Space Telescope Science Institute16, Complutense University of Madrid17, Florida Institute of Technology18, Leiden University19
TL;DR: In this paper, the authors presented an atlas and classifications of S(4)G galaxies in the Comprehensive de Vaucouleurs revised Hubble-Sandage (CVRHS) system.
Abstract: The Spitzer Survey of Stellar Structure in Galaxies (S(4)G) is the largest available database of deep, homogeneous middle-infrared (mid-IR) images of galaxies of all types. The survey, which includes 2352 nearby galaxies, reveals galaxy morphology only minimally affected by interstellar extinction. This paper presents an atlas and classifications of S(4)G galaxies in the Comprehensive de Vaucouleurs revised Hubble-Sandage (CVRHS) system. The CVRHS system follows the precepts of classical de Vaucouleurs morphology, modified to include recognition of other features such as inner, outer, and nuclear lenses, nuclear rings, bars, and disks, spheroidal galaxies, X patterns and box/peanut structures, OLR subclass outer rings and pseudorings, bar ansae and barlenses, parallel sequence latetypes, thick disks, and embedded disks in 3D early-type systems. We show that our CVRHS classifications are internally consistent, and that nearly half of the S(4)G sample consists of extreme late-type systems (mostly bulgeless, pure disk galaxies) in the range Scd-Im. The most common family classification for mid-IR types S0/a to Sc is SA while that for types Scd to Sm is SB. The bars in these two type domains are very different in mid-IR structure and morphology. This paper examines the bar, ring, and type classification fractions in the sample, and also includes several montages of images highlighting the various kinds of "stellar structures" seen in mid-IR galaxy morphology.
272 citations
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TL;DR: In this article, the authors focus on the broad patterns in the star formation properties of galaxies along the Hubble sequence and their implications for understanding galaxy evolution and the physical processes that drive the evolution.
Abstract: Observations of star formation rates (SFRs) in galaxies provide vital clues to the physical nature of the Hubble sequence and are key probes of the evolutionary histories of galaxies. The focus of this review is on the broad patterns in the star formation properties of galaxies along the Hubble sequence and their implications for understanding galaxy evolution and the physical processes that drive the evolution. Star formation in the disks and nuclear regions of galaxies are reviewed separately, then discussed within a common interpretive framework. The diagnostic methods used to measure SFRs are also reviewed, and a self-consistent set of SFR calibrations is presented as an aid to workers in the field. One of the most recognizable features of galaxies along the Hubble sequence is the wide range in young stellar content and star formation activity. This variation in stellar content is part of the basis of the Hubble classification itself (Hubble 1926), and understanding its physical nature and origins is fundamental to understanding galaxy evolution in its broader context. This review deals with the global star formation properties of galaxies, the systematics of those properties along the Hubble sequence, and their implications for galactic evolution. I interpret “Hubble sequence” in this context very loosely, to encompass not only morphological type but other properties such as gas content, mass, bar structure, and dynamical environment, which can strongly influence the largescale star formation rate (SFR).
6,640 citations
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TL;DR: The Virgo Consortium's EAGLE project as discussed by the authors is a suite of hydrodynamical simulations that follow the formation of galaxies and black holes in representative volumes, where thermal energy is injected into the gas, allowing winds to develop without predetermined speed or mass loading factors.
Abstract: We introduce the Virgo Consortium's EAGLE project, a suite of hydrodynamical simulations that follow the formation of galaxies and black holes in representative volumes. We discuss the limitations of such simulations in light of their finite resolution and poorly constrained subgrid physics, and how these affect their predictive power. One major improvement is our treatment of feedback from massive stars and AGN in which thermal energy is injected into the gas without the need to turn off cooling or hydrodynamical forces, allowing winds to develop without predetermined speed or mass loading factors. Because the feedback efficiencies cannot be predicted from first principles, we calibrate them to the z~0 galaxy stellar mass function and the amplitude of the galaxy-central black hole mass relation, also taking galaxy sizes into account. The observed galaxy mass function is reproduced to ≲0.2 dex over the full mass range, 108
2,828 citations
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TL;DR: In this paper, supermassive black holes (BHs) have been found in 85 galaxies by dynamical modeling of spatially resolved kinematics, and it has been shown that BHs and bulges coevolve by regulating each other's growth.
Abstract: Supermassive black holes (BHs) have been found in 85 galaxies by dynamical modeling of spatially resolved kinematics. The Hubble Space Telescope revolutionized BH research by advancing the subject from its proof-of-concept phase into quantitative studies of BH demographics. Most influential was the discovery of a tight correlation between BH mass and the velocity dispersion σ of the bulge component of the host galaxy. Together with similar correlations with bulge luminosity and mass, this led to the widespread belief that BHs and bulges coevolve by regulating each other's growth. Conclusions based on one set of correlations from in brightest cluster ellipticals to in the smallest galaxies dominated BH work for more than a decade. New results are now replacing this simple story with a richer and more plausible picture in which BHs correlate differently with different galaxy components. A reasonable aim is to use this progress to refine our understanding of BH-galaxy coevolution. BHs with masses of 105−106M...
2,804 citations
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TL;DR: In this paper, an overall theoretical framework and the observations that motivate it are outlined, outlining the key dynamical processes involved in star formation, including turbulence, magnetic fields, and self-gravity.
Abstract: We review current understanding of star formation, outlining an overall theoretical framework and the observations that motivate it. A conception of star formation has emerged in which turbulence plays a dual role, both creating overdensities to initiate gravitational contraction or collapse, and countering the effects of gravity in these overdense regions. The key dynamical processes involved in star formation—turbulence, magnetic fields, and self-gravity— are highly nonlinear and multidimensional. Physical arguments are used to identify and explain the features and scalings involved in star formation, and results from numerical simulations are used to quantify these effects. We divide star formation into large-scale and small-scale regimes and review each in turn. Large scales range from galaxies to giant molecular clouds (GMCs) and their substructures. Important problems include how GMCs form and evolve, what determines the star formation rate (SFR), and what determines the initial mass function (IMF). Small scales range from dense cores to the protostellar systems they beget. We discuss formation of both low- and high-mass stars, including ongoing accretion. The development of winds and outflows is increasingly well understood, as are the mechanisms governing angular momentum transport in disks. Although outstanding questions remain, the framework is now in place to build a comprehensive theory of star formation that will be tested by the next generation of telescopes.
2,522 citations
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Space Telescope Science Institute1, University of California, Santa Cruz2, Johns Hopkins University3, Rutgers University4, Durham University5, University of Nottingham6, Harvard University7, University of Innsbruck8, University of Michigan9, DSM10, University of Edinburgh11, University of Massachusetts Amherst12, California Institute of Technology13, UK Astronomy Technology Centre14, University of California, Irvine15, Swinburne University of Technology16, University of Arizona17, The Catholic University of America18, Goddard Space Flight Center19, Hebrew University of Jerusalem20, University of Victoria21, University of California, Berkeley22, Texas A&M University23, University of Notre Dame24, Carnegie Institution for Science25, Smithsonian Institution26, Yale University27, University of Missouri–Kansas City28, University of California, Riverside29, Max Planck Society30, University of Pittsburgh31, Inter-University Centre for Astronomy and Astrophysics32, University of Barcelona33, European Southern Observatory34, University of Minnesota35, National Research Council36, Western Kentucky University37, Stanford University38, Atacama Large Millimeter Submillimeter Array39, University of Missouri40
TL;DR: The Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) as discussed by the authors was designed to document the first third of galactic evolution, from z approx. 8 - 1.5 to test their accuracy as standard candles for cosmology.
Abstract: The Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) is designed to document the first third of galactic evolution, from z approx. 8 - 1.5. It will image > 250,000 distant galaxies using three separate cameras on the Hubble Space Tele8cope, from the mid-UV to near-IR, and will find and measure Type Ia supernovae beyond z > 1.5 to test their accuracy as standard candles for cosmology. Five premier multi-wavelength sky regions are selected, each with extensive ancillary data. The use of five widely separated fields mitigates cosmic variance and yields statistically robust and complete samples of galaxies down to a stellar mass of 10(exp 9) solar mass to z approx. 2, reaching the knee of the UV luminosity function of galaxies to z approx. 8. The survey covers approximately 800 square arc minutes and is divided into two parts. The CANDELS/Deep survey (5(sigma) point-source limit H =27.7mag) covers approx. 125 square arcminutes within GOODS-N and GOODS-S. The CANDELS/Wide survey includes GOODS and three additional fields (EGS, COSMOS, and UDS) and covers the full area to a 50(sigma) point-source limit of H ? or approx. = 27.0 mag. Together with the Hubble Ultradeep Fields, the strategy creates a three-tiered "wedding cake" approach that has proven efficient for extragalactic surveys. Data from the survey are non-proprietary and are useful for a wide variety of science investigations. In this paper, we describe the basic motivations for the survey, the CANDELS team science goals and the resulting observational requirements, the field selection and geometry, and the observing design.
2,088 citations