67 research outputs found
Galaxy Disks
The formation and evolution of galactic disks is particularly important for
understanding how galaxies form and evolve, and the cause of the variety in
which they appear to us. Ongoing large surveys, made possible by new
instrumentation at wavelengths from the ultraviolet (GALEX), via optical (HST
and large groundbased telescopes) and infrared (Spitzer) to the radio are
providing much new information about disk galaxies over a wide range of
redshift. Although progress has been made, the dynamics and structure of
stellar disks, including their truncations, are still not well understood. We
do now have plausible estimates of disk mass-to-light ratios, and estimates of
Toomre's parameter show that they are just locally stable. Disks are mostly
very flat and sometimes very thin, and have a range in surface brightness from
canonical disks with a central surface brightness of about 21.5 -mag
arcsec down to very low surface brightnesses. It appears that galaxy
disks are not maximal, except possibly in the largest systems. Their HI layers
display warps whenever HI can be detected beyond the stellar disk, with
low-level star formation going on out to large radii. Stellar disks display
abundance gradients which flatten at larger radii and sometimes even reverse.
The existence of a well-defined baryonic Tully-Fisher relation hints at an
approximately uniform baryonic to dark matter ratio. Thick disks are common in
disk galaxies and their existence appears unrelated to the presence of a bulge
component; they are old, but their formation is not yet understood. Disk
formation was already advanced at redshifts of , but at that epoch
disks were not yet quiescent and in full rotational equilibrium. Downsizing is
now well-established. The formation and history of star formation in S0s is
still not fully understood.Comment: This review has been submitted for Annual Reviews of Astronomy &
Astrophysics, vol. 49 (2011); the final printed version will have fewer
figures and a somewhat shortened text. A pdf-version of this preprint with
high-resolution figures is available from
http://www.astro.rug.nl/~vdkruit/jea3/homepage/disks-ph.pdf. (table of
contents added; 71 pages, 24 figures, 529 references
IRAS observations of active galaxies
The IRAS survey gives an unbiased view of the infrared properties of the active galaxies. Seyfert galaxies occupy much the same area in color-color plots as to normal infrared bright galaxies, but extend the range towards flatter 60 to 25 mm slopes. Statistically the Seyfert 1 galaxies can be distinguished from the Seyfert 2 galaxies, lying predominantly closer to the area with constant slopes between 25 and 200 mm. The infrared measurements of the Seyfert galaxies cannot distinguish between the emission mechanisms in these objects although they agree with the currently popular ideas; they do provide a measure of the total luminosity of the Seyferts. The quasar's position in the color-color diagrams continue the trend of the Seyferts. The quasar 3C48 is shown to be exceptional among the radio loud quasars in that it has a high infrared luminosity which dominates the power output of the quasar and is most likely associated with the underlying host galaxy
Globular Cluster Distance Determinations
The present status of the distance scale to Galactic globular clusters is
reviewed. Six distance determination techniques which are deemed to be most
reliable are discussed in depth. These different techniques are used to
calibrate the absolute magnitude of the RR Lyrae stars. The various
calibrations fall into three groups. Main sequence fitting using Hipparcos
parallaxes, theoretical HB models and the RR Lyrae in the LMC all favor a
bright calibration, implying a `long' globular cluster distance scale. White
dwarf fitting and the astrometric distances yield a somewhat fainter RR Lyrae
calibration, while the statistical parallax solution yields faint RR Lyrae
stars implying a `short' distance scale to globular clusters. Various secondary
distance indicators discussed all favor the long distance scale. The `long' and
`short' distance scales differ by (0.31+/-0.16) mag. Averaging together all of
the different distance determinations yields Mv(RR) = (0.23+/-0.04)([Fe/H] +
1.6) + (0.56+/-0.12) mag.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in pres
The RR Lyrae Distance Scale
We review seven methods of measuring the absolute magnitude M_V of RR Lyrae
stars in light of the Hipparcos mission and other recent developments. We focus
on identifying possible systematic errors and rank the methods by relative
immunity to such errors. For the three most robust methods, statistical
parallax, trigonometric parallax, and cluster kinematics, we find M_V (at
[Fe/H] = -1.6) of 0.77 +/- 0.13, 0.71 +/- 0.15, 0.67 +/- 0.10. These methods
cluster consistently around 0.71 +/- 0.07. We find that Baade-Wesselink and
theoretical models both yield a broad range of possible values (0.45-0.70 and
0.45-0.65) due to systematic uncertainties in the temperature scale and input
physics. Main-sequence fitting gives a much brighter M_V = 0.45 +/- 0.04 but
this may be due to a difference in the metallicity scales of the cluster giants
and the calibrating subdwarfs. White-dwarf cooling-sequence fitting gives 0.67
+/- 0.13 and is potentially very robust, but at present is too new to be fully
tested for systematics. If the three most robust methods are combined with
Walker's mean measurement for 6 LMC clusters, V_{0,LMC} = 18.98 +/- 0.03 at
[Fe/H] = -1.9, then mu_{LMC} = 18.33 +/- 0.08.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 21
pages including 1 table; uses Kluwer's crckapb.sty LaTeX style file, enclose
The Age of the Galactic Disk
I review different methods devised to derive the age of the Galactic Disk,
namely the Radio-active Decay (RD), the Cool White Dwarf Luminosity Function
(CWDLF), old opne clusters (OOC) and the Color Magnitude Diagram (CMD) of the
stars in the solar vicinity. I argue that the disk is likely to be 8-10 Gyr
old. Since the bulk of globulars has an age around 13 Gyr, the possibility
emerges that the Galaxy experienced a minimum of Star Formation at the end of
the halo/bulge formation. This minimum might reflect the time at which the
Galaxy started to acquire material to form the disk inside-out.Comment: 10 pages, 4 figure, invited review, in "The chemical evolution of the
Milky Way : Stars vs Clusters, Vulcano (Italy), 20-24 September 199
Distances and ages of globular clusters using Hipparcos parallaxes of local subdwarfs
We discuss the impact of Population II and Globular Cluster (GCs) stars on
the derivation of the age of the Universe, and on the study of the formation
and early evolution of galaxies, our own in particular. The long-standing
problem of the actual distance scale to Population II stars and GCs is
addressed, and a variety of different methods commonly used to derive distances
to Population II stars are briefly reviewed. Emphasis is given to the
discussion of distances and ages for GCs derived using Hipparcos parallaxes of
local subdwarfs. Results obtained by different authors are slightly different,
depending on different assumptions about metallicity scale, reddenings, and
corrections for undetected binaries. These and other uncertainties present in
the method are discussed. Finally, we outline progress expected in the near
future.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 22
pages including 3 tables and 2 postscript figures, uses Kluwer's crckapb.sty
LaTeX style file, enclose
The contribution of microlensing surveys to the distance scale
In the early nineties several teams started large scale systematic surveys of
the Magellanic Clouds and the Galactic Bulge to search for microlensing
effects. As a by product, these groups have created enormous time-series
databases of photometric measurements of stars with a temporal sampling
duration and accuracy which are unprecedented. They provide the opportunity to
test the accuracy of primary distance indicators, such as Cepheids, RRLyrae
stars, the detached eclipsing binaries, or the luminosity of the red clump. We
will review the contribution of the microlensing surveys to the understanding
of the physics of the primary distance indicators, recent differential studies
and direct distance determinations to the Magellanic Clouds and the Galactic
Bulge.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 21
pages; uses Kluwer's crckapb.sty LaTeX style file, enclose
The Hubble Constant
I review the current state of determinations of the Hubble constant, which
gives the length scale of the Universe by relating the expansion velocity of
objects to their distance. There are two broad categories of measurements. The
first uses individual astrophysical objects which have some property that
allows their intrinsic luminosity or size to be determined, or allows the
determination of their distance by geometric means. The second category
comprises the use of all-sky cosmic microwave background, or correlations
between large samples of galaxies, to determine information about the geometry
of the Universe and hence the Hubble constant, typically in a combination with
other cosmological parameters. Many, but not all, object-based measurements
give values of around 72-74km/s/Mpc , with typical errors of 2-3km/s/Mpc.
This is in mild discrepancy with CMB-based measurements, in particular those
from the Planck satellite, which give values of 67-68km/s/Mpc and typical
errors of 1-2km/s/Mpc. The size of the remaining systematics indicate that
accuracy rather than precision is the remaining problem in a good determination
of the Hubble constant. Whether a discrepancy exists, and whether new physics
is needed to resolve it, depends on details of the systematics of the
object-based methods, and also on the assumptions about other cosmological
parameters and which datasets are combined in the case of the all-sky methods.Comment: Extensively revised and updated since the 2007 version: accepted by
Living Reviews in Relativity as a major (2014) update of LRR 10, 4, 200
The Distribution of Dust and Gas in Elliptical Galaxies
Results from IRAS and recent optical CCD surveys are examined to discuss the
distribution and origin of dust and ionized gas in elliptical galaxies. In
strong contrast with the situation among spiral galaxies, masses of dust in
elliptical galaxies as derived from optical extinction are an order of
magnitude LOWER than those derived from IRAS data. I find that this dilemma can
be resolved by assuming the presence of a diffusely distributed component of
dust which is not detectable in optical data.
The morphology of dust lanes and their association with ionized gas in
elliptical galaxies argues for an external origin of BOTH components of the
ISM.Comment: Invited talk given at conference on "NEW EXTRAGALACTIC PERSPECTIVES
IN THE NEW SOUTH AFRICA: Changing Perceptions of the Morphology, Dust Content
and Dust-Gas Ratios in Galaxies", Held in Johannesburg, South Africa, during
January 22-26, 1996. Proceedings will be edited by D.L. Block and published
by Kluwer, Dordrecht, The Netherlands. uuencoded, gzipped LaTeX file of 8
pages; figures included as PostScript files (enclosed). Uses crckapb.sty
(enclosed) and psfig.st
Extragalactic Radio Continuum Surveys and the Transformation of Radio Astronomy
Next-generation radio surveys are about to transform radio astronomy by
discovering and studying tens of millions of previously unknown radio sources.
These surveys will provide new insights to understand the evolution of
galaxies, measuring the evolution of the cosmic star formation rate, and
rivalling traditional techniques in the measurement of fundamental cosmological
parameters. By observing a new volume of observational parameter space, they
are also likely to discover unexpected new phenomena. This review traces the
evolution of extragalactic radio continuum surveys from the earliest days of
radio astronomy to the present, and identifies the challenges that must be
overcome to achieve this transformational change.Comment: To be published in Nature Astronomy 18 Sept 201
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