14 research outputs found
Secular Evolution and the Formation of Pseudobulges in Disk Galaxies
We review internal processes of secular evolution in galaxy disks,
concentrating on the buildup of dense central features that look like
classical, merger-built bulges but that were made slowly out of disk gas. We
call these pseudobulges. As an existence proof, we review how bars rearrange
disk gas into outer rings, inner rings, and gas dumped into the center. In
simulations, this gas reaches high densities that plausibly feed star
formation. In the observations, many SB and oval galaxies show central
concentrations of gas and star formation. Star formation rates imply plausible
pseudobulge growth times of a few billion years. If secular processes built
dense central components that masquerade as bulges, can we distinguish them
from merger-built bulges? Observations show that pseudobulges retain a memory
of their disky origin. They have one or more characteristics of disks: (1)
flatter shapes than those of classical bulges, (2) large ratios of ordered to
random velocities indicative of disk dynamics, (3) small velocity dispersions,
(4) spiral structure or nuclear bars in the bulge part of the light profile,
(5) nearly exponential brightness profiles, and (6) starbursts. These
structures occur preferentially in barred and oval galaxies in which secular
evolution should be rapid. So the cleanest examples of pseudobulges are
recognizable. Thus a large variety of observational and theoretical results
contribute to a new picture of galaxy evolution that complements hierarchical
clustering and merging.Comment: 92 pages, 21 figures in 30 Postscript files; to appear in Annual
Review of Astronomy and Astrophysics, Vol. 42, 2004, in press; for a version
with full resolution figures, see
http://chandra.as.utexas.edu/~kormendy/ar3ss.htm
A review of elliptical and disc galaxy structure, and modern scaling laws
A century ago, in 1911 and 1913, Plummer and then Reynolds introduced their
models to describe the radial distribution of stars in `nebulae'. This article
reviews the progress since then, providing both an historical perspective and a
contemporary review of the stellar structure of bulges, discs and elliptical
galaxies. The quantification of galaxy nuclei, such as central mass deficits
and excess nuclear light, plus the structure of dark matter halos and cD galaxy
envelopes, are discussed. Issues pertaining to spiral galaxies including dust,
bulge-to-disc ratios, bulgeless galaxies, bars and the identification of
pseudobulges are also reviewed. An array of modern scaling relations involving
sizes, luminosities, surface brightnesses and stellar concentrations are
presented, many of which are shown to be curved. These 'redshift zero'
relations not only quantify the behavior and nature of galaxies in the Universe
today, but are the modern benchmark for evolutionary studies of galaxies,
whether based on observations, N-body-simulations or semi-analytical modelling.
For example, it is shown that some of the recently discovered compact
elliptical galaxies at 1.5 < z < 2.5 may be the bulges of modern disc galaxies.Comment: Condensed version (due to Contract) of an invited review article to
appear in "Planets, Stars and Stellar
Systems"(www.springer.com/astronomy/book/978-90-481-8818-5). 500+ references
incl. many somewhat forgotten, pioneer papers. Original submission to
Springer: 07-June-201
Dwarf Galaxies of the Local Group
The Local Group (LG) dwarf galaxies offer a unique window to the detailed
properties of the most common type of galaxy in the Universe. In this review, I
update the census of LG dwarfs based on the most recent distance and radial
velocity determinations. I then discuss the detailed properties of this sample,
including (a) the integrated photometric parameters and optical structures of
these galaxies, (b) the content, nature and distribution of their ISM, (c)
their heavy-element abundances derived from both stars and nebulae, (d) the
complex and varied star-formation histories of these dwarfs, (e) their internal
kinematics, stressing the relevance of these galaxies to the dark-matter
problem and to alternative interpretations, and (f) evidence for past, ongoing
and future interactions of these dwarfs with other galaxies in the Local Group
and beyond. To complement the discussion and to serve as a foundation for
future work, I present an extensive set of basic observational data in tables
that summarize much of what we know, and what we still do not know, about these
nearby dwarfs. Our understanding of these galaxies has grown impressively in
the past decade, but fundamental puzzles remain that will keep the Local Group
at the forefront of galaxy evolution studies for some time.Comment: 66 pages; 9 figures; 8 table
The Mass Distribution and Rotation Curve in the Galaxy
The mass distribution in the Galaxy is determined by dynamical and
photometric methods. Rotation curves are the major tool for determining the
dynamical mass distribution in the Milky Way and spiral galaxies. The
photometric (statistical) method utilizes luminosity profiles from optical and
infrared observations, and assumes empirical values of the mass-to-luminosity
(M/L) ratio to convert the luminosity to mass. In this chapter the dynamical
method is described in detail, and rotation curves and mass distribution in the
Milky Way and nearby spiral galaxies are presented. The dynamical method is
categorized into two methods: the decomposition method and direct method. The
former fits the rotation curve by calculated curve assuming several mass
components such as a bulge, disk and halo, and adjust the dynamical parameters
of each component. Explanations are given of the mass profiles as the de
Vaucouleurs law, exponential disk, and dark halo profiles inferred from
numerical simulations. Another method is the direct method, with which the mass
distribution can be directly calculated from the data of rotation velocities
without employing any mass models. Some results from both methods are
presented, and the Galactic structure is discussed in terms of the mass.
Rotation curves and mass distributions in external galaxies are also discussed,
and the fundamental mass structures are shown to be universal.Comment: 54 pages, 25 figures, in 'Planets, Stars and Stellar Systems',
Springer, Vol. 5, ed. G. Gilmore, Chap. 19. Note: Preprint with full figures
is available from http://www.ioa.s.u-tokyo.ac.jp/~sofue/htdocs/2013psss
The expansion field: The value of H_0
Any calibration of the present value of the Hubble constant requires
recession velocities and distances of galaxies. While the conversion of
observed velocities into true recession velocities has only a small effect on
the result, the derivation of unbiased distances which rest on a solid zero
point and cover a useful range of about 4-30 Mpc is crucial. A list of 279 such
galaxy distances within v<2000 km/s is given which are derived from the tip of
the red-giant branch (TRGB), from Cepheids, and from supernovae of type Ia (SNe
Ia). Their random errors are not more than 0.15 mag as shown by
intercomparison. They trace a linear expansion field within narrow margins from
v=250 to at least 2000 km/s. Additional 62 distant SNe Ia confirm the linearity
to at least 20,000 km/s. The dispersion about the Hubble line is dominated by
random peculiar velocities, amounting locally to <100 km/s but increasing
outwards. Due to the linearity of the expansion field the Hubble constant H_0
can be found at any distance >4.5 Mpc. RR Lyr star-calibrated TRGB distances of
78 galaxies above this limit give H_0=63.0+/-1.6 at an effective distance of 6
Mpc. They compensate the effect of peculiar motions by their large number.
Support for this result comes from 28 independently calibrated Cepheids that
give H_0=63.4+/-1.7 at 15 Mpc. This agrees also with the large-scale value of
H_0=61.2+/-0.5 from the distant, Cepheid-calibrated SNe Ia. A mean value of
H_0=62.3+/-1.3 is adopted. Because the value depends on two independent zero
points of the distance scale its systematic error is estimated to be 6%.
Typical errors of H_0 come from the use of a universal, yet unjustified P-L
relation of Cepheids, the neglect of selection bias in magnitude-limited
samples, or they are inherent to the adopted models.Comment: 44 pages, 4 figures, 6 tables, accepted for publication in the
Astronony and Astrophysics Review 15
Structure and Kinematics of Early-Type Galaxies from Integral Field Spectroscopy
Observations of galaxy isophotes, longs-slit kinematics and high-resolution
photometry suggested a possible dichotomy between two distinct classes of E
galaxies. But these methods are expensive for large galaxy samples. Instead,
integral-field spectroscopic can efficiently recognize the shape, dynamics and
stellar population of complete samples of early-type galaxies (ETGs). These
studies showed that the two main classes, the fast and slow rotators, can be
separated using stellar kinematics. We showed there is a dichotomy in the
dynamics of the two classes. The slow rotators are weakly triaxial and dominate
above . Below , the
structure of fast rotators parallels that of spiral galaxies. There is a smooth
sequence along which, the metals content, the enhancement in -elements,
and the "weight" of the stellar initial mass function, all increase with the
CENTRAL mass density slope, or bulge mass fraction, while the molecular gas
fraction correspondingly decreases. The properties of ETGs on galaxy scaling
relations, and in particular the diagram, and their
dependence on environment, indicate two main independent channels for galaxy
evolution. Fast rotators ETGs start as star forming disks and evolve trough a
channel dominated by gas accretion, bulge growth and quenching. While slow
rotators assemble near the center of massive halos via intense star formation
at high redshift, and remain as such for the rest of their evolution via a
channel dominated by gas poor mergers. This is consistent with independent
studies of the galaxies redshift evolution.Comment: 67 pages, 30 figures, LaTeX. To appear in Annual Review of Astronomy
and Astrophysics (ARA&A), Vol. 54 (2016