1,110 research outputs found
Structure, mass and stability of galactic disks
In this review I concentrate on three areas related to structure of disks in
spiral galaxies. First I will review the work on structure, kinematics and
dynamics of stellar disks. Next I will review the progress in the area of
flaring of HI layers. These subjects are relevant for the presence of dark
matter and lead to the conclusion that disk are in general not `maximal', have
lower M/L ratios than previously suspected and are locally stable w.r.t.
Toomre's Q criterion for local stability. I will end with a few words on
`truncations' in stellar disks.Comment: Invited review at "Galaxies and their Masks" for Ken Freeman's 70-th
birthday, Sossusvlei, Namibia, April 2010. A version with high-res. figures
is available at
http://www.astro.rug.nl/~vdkruit/jea3/homepage/Namibiachapter.pd
Deep CCD Surface Photometry of the Edge-On Spiral NGC 4244
We have obtained deep surface photometry of the edge-on spiral galaxy NGC
4244. Our data reliably reach 27.5 R magnitude arcsec^{-2}, a significant
improvement on our earlier deep CCD surface photometry of other galaxies. NGC
4244 is a nearby Scd galaxy whose total luminosity is approximately one
magnitude fainter than the peak of the Sc luminosity function. We find that it
has a simple structure: a single exponential disk, with a scale height h_Z =
246 +/- 2 pc, a scale length h_R = 1.84 +/- 0.02 kpc and a disk cutoff at a
radius R(max) = 10.0 kpc (5.4 scale lengths). We confirm a strong cutoff in the
stellar disk at R(max), which happens over only 1 kpc. We do not see any
statistically significant evidence for disk flaring with radius. Unlike the
more luminous Sc galaxies NGC 5907 and M 33, NGC 4244 does not show any
evidence for a second component, such as a thick disk or halo, at mu(R) < 27.5
magnitude arcsec^{-2}.Comment: 36 pages, including 12 figures; accepted for publication in Sept 99
A
Stellar Disk Truncations: Where do we stand ?
In the light of several recent developments we revisit the phenomenon of
galactic stellar disk truncations. Even 25 years since the first paper on outer
breaks in the radial light profiles of spiral galaxies, their origin is still
unclear. The two most promising explanations are that these 'outer edges'
either trace the maximum angular momentum during the galaxy formation epoch, or
are associated with global star formation thresholds. Depending on their true
physical nature, these outer edges may represent an improved size
characteristic (e.g., as compared to D_25) and might contain fossil evidence
imprinted by the galaxy formation and evolutionary history. We will address
several observational aspects of disk truncations: their existence, not only in
normal HSB galaxies, but also in LSB and even dwarf galaxies; their detailed
shape, not sharp cut-offs as thought before, but in fact demarcating the start
of a region with a steeper exponential distribution of starlight; their
possible association with bars; as well as problems related to the
line-of-sight integration for edge-on galaxies (the main targets for truncation
searches so far). Taken together, these observations currently favour the
star-formation threshold model, but more work is necessary to implement the
truncations as adequate parameters characterising galactic disks.Comment: LaTeX, 10 pages, 6 figures, presented at the "Penetrating Bars
through Masks of Cosmic Dust" conference in South Africa, proceedings
published by Kluwer, and edited by Block, D.L., Freeman, K.C., Puerari, I., &
Groess, R; v3 to match published versio
A Structural and Dynamical Study of Late-Type, Edge-On Galaxies: I. Sample Selection and Imaging Data
We present optical (B & R) and infrared (K_s) images and photometry for a
sample of 49 extremely late-type, edge-on disk galaxies selected from the Flat
Galaxy Catalog of Karenchentsev et al. (1993). Our sample was selected to
include galaxies with particularly large axial ratios, increading the
likelihood that the galaxies in the sample are truly edge-on. We have also
concentrated the sample on galaxies with low apparent surface brightness, in
order to increase the representation of intrinisically low surface brightness
galaxies. Finally, the sample was chosen to have no apprarent bulges or optical
warps so that the galaxies represent undisturbed, ``pure disk'' systems. The
resulting sample forms the basis for a much larger spectroscopic study designed
to place constraints on the physical quantities and processes which shape disk
galaxies. The imaging data presented in this paper has been painstakingly
reduced and calibrated to allow accurate surface photometry of features as
faint as 30 mag/sqr-arcsec in B and 29 mag/sqr-arcsec in R on scales larger
than 10 arcsec. Due to limitations in sky subtraction and flat fielding, the
infrared data can reach only to 22.5 mag/sqr-arcsec in K_s on comparable
scales. As part of this work, we have developed a new method for quantifying
the reliability of surface photometry, which provides useful diagnostics for
the presence of scattered light, optical emission from infrared cirrus, and
other sources of non-uniform sky backgrounds.Comment: scheduled to appear in the Astronomical Journal, LaTeX, 36 pages
including 7 pages of figures (fig 1-2,4). A low resolution version of Figure
3 is included in JPEG format; contours are seriously degraded. A full
resolution Postscript version of Figure 3 (10.6Mb,gzipped) is available
through anonymous ftp at
ftp://ftp.astro.washington.edu/pub/users/jd/FGC/dalcanton.f3.ps.g
Star formation thresholds and galaxy edges: why and where
We study global star formation thresholds in the outer parts of galaxies by
investigating the stability of disk galaxies embedded in dark halos. The disks
are self-gravitating, contain metals and dust, and are exposed to UV radiation.
We find that the critical surface density for the existence of a cold
interstellar phase depends only weakly on the parameters of the model and
coincides with the empirically derived surface density threshold for star
formation. Furthermore, it is shown that the drop in the thermal velocity
dispersion associated with the transition from the warm to the cold gas phase
triggers gravitational instability on a wide range of scales. The presence of
strong turbulence does not undermine this conclusion if the disk is
self-gravitating. Models based on the hypothesis that the onset of thermal
instability determines the star formation threshold in the outer parts of
galaxies can reproduce many observations, including the threshold radii, column
densities, and the sizes of stellar disks as a function of disk scale length
and mass. Finally, prescriptions are given for implementing star formation
thresholds in (semi-)analytic models and three-dimensional hydrodynamical
simulations of galaxy formation.Comment: 16 pages, 6 figures, accepted for publication in the Astrophysical
Journal. Version 2: text significantly revised (major improvements), physics
unchanged. Version 3: minor correction
The Opacity of Spiral Galaxy Disks IV: Radial Extinction Profiles from Counts of Distant Galaxies seen through Foreground Disks
Dust extinction can be determined from the number of distant field galaxies
seen through a spiral disk. To calibrate this number for the crowding and
confusion introduced by the foreground image, Gonzalez et al.(1998) and
Holwerda et al. (2005) developed the ``Synthetic Field Method'' (SFM), which
analyses synthetic fields constructed by adding various deep exposures of
unobstructed background fields to the candidate foreground galaxy field.
The advantage of the SFM is that it gives the average opacity for area of
galaxy disk without assumptions about either the distribution of absorbers or
of the disk starlight. However it is limited by low statistics of the surviving
field galaxies, hence the need to combine a larger sample of fields. This paper
presents the first results for a sample of 32 deep HST/WFPC2 archival fields of
29 spirals.
The radial profiles of average dust extinction in spiral galaxies based on
calibrated counts of distant field galaxies is presented here, both for
individual galaxies as well as for composites from our sample. The effects of
inclination, spiral arms and Hubble type on the radial extinction profile are
discussed. (Abbreviated)Comment: 43 pages, 16 figures, 3 tables, accepted for publication in the
Astronomical Journal, (typos, table update, updates abstract
A New Method to Measure and Map the Gas Scale-Height of Disk Galaxies
We propose a new method to measure and map the gas scale height of nearby
disk galaxies. This method is applied successfully to the Australia Telescope
Compact Array interferometric HI survey of the Large Magellanic Cloud (LMC); it
could also be applied to a significant number of nearby disk galaxies, thanks
to the next generation of interferometric facilities, such as the extended VLA
and CARMA.
The method consists of computing the Spectral Correlation Function (SCF) for
a spectral-line map of a face-on galaxy. The SCF quantifies the correlation
between spectra at different map positions as a function of their separation,
and is sensitive to the properties of both the gas mass distribution and the
gas velocity field. It is likely that spatial correlation properties of the gas
density and velocity fields in a galactic disk are sensitive to the value of
the scale height of the gas disk. A scale-free turbulent cascade is unlikely to
extend to scales much larger than the disk scale height, as the disk dynamics
on those larger scales should be dominated by two dimensional motions.
We find a clear feature in the SCF of the LMC HI disk, on the scale of
approximately 180 pc, which we identify as the disk scale height. We are also
tentatively able to map variations of the scale height over the disk.Comment: 6 pages, submitted to ApJ
Structural Parameters of Thin and Thick Disks in Edge-On Disk Galaxies
We analyze the global structure of 34 late-type, edge-on, undisturbed, disk
galaxies spanning a wide range of mass. We measure structural parameters for
the galaxies using two-dimensional least-squares fitting to our -band
photometry. The fits require both a thick and a thin disk to adequately fit the
data. The thick disks have larger scale heights and longer scale lengths than
the embedded thin disks, by factors of ~2 and ~1.25, respectively. The observed
structural parameters agree well with the properties of thick and thin disks
derived from star counts in the Milky Way and from resolved stellar populations
in nearby galaxies. We find that massive galaxies' luminosities are dominated
by the thin disk. However, in low mass galaxies (Vc < 120 km/s), thick disk
stars contribute nearly half of the luminosity and dominate the stellar mass.
Thus, although low mass dwarf galaxies appear blue, the majority of their stars
are probably quite old.
Our data are most easily explained by a formation scenario where the thick
disk is assembled through direct accretion of stellar material from merging
satellites while the thin disk is formed from accreted gas. The baryonic
fraction in the thin disk therefore constrains the gas-richness of the merging
pre-galactic fragments. If we include the mass in HI as part of the thin disk,
the thick disk contains <10% of the baryons in high mass galaxies, and ~25-30%
of the baryons in low-mass galaxies. We discuss how our trends can be explained
by supernova-driven outflow at early times as well as the possibilities for
predicting abundance trends in thick disks, and for removing discrepancies
between semi-analytic galaxy formation models and the observed colors of low
mass galaxies. (abstract abridged)Comment: 25 pages, 24 figures, accepted for publication in A
van der Kruit to Spitzer: A New Look at the FIR-Radio Correlation
We present an initial look at the far infrared-radio correlation within the
star-forming disks of four nearby, nearly face-on galaxies (NGC~2403, NGC~3031,
NGC~5194, and NGC~6946). Using {\it Spitzer} MIPS imaging and WSRT radio
continuum data, we are able to probe variations in the logarithmic
70~m/22~cm () flux density ratios across each disk at sub-kpc
scales. We find general trends of decreasing with declining surface
brightness and with increasing radius. We also find that the dispersion in
within galaxies is comparable to what is measured {\it globally} among
galaxies at around 0.2 dex. We have also performed preliminary phenomenological
modeling of cosmic ray electron (CR) diffusion using an image-smearing
technique, and find that smoothing the infrared maps improves their correlation
with the radio maps. The best fit smoothing kernels for the two less active
star-forming galaxies (NGC~2403 and NGC~3031) have much larger scale-lengths
than that of the more active star-forming galaxies (NGC~5194 and NGC~6946).
This difference may be due to the relative deficit of recent CR
injection into the interstellar medium (ISM) for the galaxies having largely
quiescent disks.Comment: 6 pages, 3 figures, To appear in the proceedings of the "Island
Universes: Structure and Evolution of Disk Galaxies" conference held in
Terschelling, Netherlands, July 2005, ed. R. de Jong (Springer: Dordrecht
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