54 research outputs found
Planetary Nebula Studies of Face-On Spiral Galaxies: Is the Disk Mass-to-Light Ratio Constant?
When astronomers study the dark matter halos of spiral galaxies, they
normally assume that the disk mass-to-light ratio is *constant*. We describe a
method of analyzing the kinematics of planetary nebulae (PNe) in nearby face-on
spiral galaxies to test this assumption. Since the restoring force for stellar
motions perpendicular to the galactic disk is proportional to the disk mass
surface density, measurements of the vertical velocity dispersion can be used
to produce an independent measure of the total amount of matter in the disk.
Our steps are: (1) to identify a population of PNe by imaging the host spiral
in several filters, and (2) to isolate the vertical velocity dispersion from
spectroscopic observations of the PNe. Our first results for the PNe of M33
indicate that the mass-to-light ratio of the galaxy's disk actually *increases*
by more than a factor of 5 over the inner 6 disk scale lengths. We have begun
similar studies of the PNe in five more face-on galaxies: M83, M101, M94, NGC
6946, and M74. These data will also produce additional science such as galaxy
distances and constraints on the disk transparency.Comment: 4 pages, 4 figures, for "Planetary Nebulae as Astronomical Tools"
Conference Proceedings (in Gdansk, Poland June 28-July 2, 2005
The Shape of LITTLE THINGS Dwarf Galaxies DDO 46 and DDO 168: Understanding the stellar and gas kinematics
We present the stellar and gas kinematics of DDO 46 and DDO 168 from the
LITTLE THINGS survey and determine their respective Vmax/sigma_z,0 values. We
used the KPNO's 4-meter telescope with the Echelle spectrograph as a long-slit
spectrograph. We acquired spectra of DDO 168 along four position angles by
placing the slit over the morphological major and minor axes and two
intermediate position angles. However, due to poor weather conditions during
our observing run for DDO 46, we were able to extract only one useful data
point from the morphological major axis. We determined a central stellar
velocity dispersion perpendicular to the disk, sigma_z,0, of 13.5+/-8 km/s for
DDO 46 and of 10.7+/-2.9 km/s for DDO 168. We then derived the
maximum rotation speed in both galaxies using the LITTLE THINGS HI data. We
separated bulk motions from non-circular motions using a double Gaussian
decomposition technique and applied a tilted-ring model to the bulk velocity
field. We corrected the observed HI rotation speeds for asymmetric drift and
found a maximum velocity, Vmax, of 77.4 +/- 3.7 and 67.4 +/- 4.0 km/s for DDO
46 and DDO 168, respectively. Thus, we derived a kinematic measure,
Vmax/sigma_z,0, of 5.7 +/- 0.6 for DDO 46 and 6.3 +/- 0.3 for DDO 168.
Comparing these values to ones determined for spiral galaxies, we find that DDO
46 and DDO 168 have Vmax/sigma_z,0 values indicative of thin disks, which is in
contrast to minor-to-major axis ratio studies
A Survey for Planetary Nebulae in M31 Globular Clusters
We report the results of an [O III] 5007 spectroscopic survey for planetary
nebulae (PNe) located within the star clusters of M31. By examining R ~ 5000
spectra taken with the WIYN+Hydra spectrograph, we identify 3 PN candidates in
a sample of 274 likely globular clusters, 2 candidates in objects which may be
globular clusters, and 5 candidates in a set of 85 younger systems. The
possible PNe are all faint, between ~2.5 and ~6.8 mag down the PN luminosity
function, and, partly as a consequence of our selection criteria, have high
excitation, with [O III] 5007 to H-beta ratios ranging from 2 to ~12. We
discuss the individual candidates, their likelihood of cluster membership, and
the possibility that they were formed via binary interactions within the
clusters. Our data are consistent with the suggestion that PN formation within
globular clusters correlates with binary encounter frequency, though, due to
the small numbers and large uncertainties in the candidate list, this study
does not provide sufficient evidence to confirm the hypothesis.Comment: Accepted for publication in the Astrophysical Journal. 54 pages,
including 9 figures and 4 table
Planetary Nebulae in Face-On Spiral Galaxies. III. Planetary Nebula Kinematics and Disk Mass
Much of our understanding of dark matter halos comes from the assumption that
the mass-to-light ratio (M/L) of spiral disks is constant. The best way to test
this hypothesis is to measure the disk surface mass density directly via the
kinematics of old disk stars. To this end, we have used planetary nebulae (PNe)
as test particles and have measured the vertical velocity dispersion (sigma_z)
throughout the disks of five nearby, low-inclination spiral galaxies: IC 342,
M74 (NGC 628), M83 (NGC 5236), M94 (NGC 4736), and M101 (NGC 5457). By using HI
to map galactic rotation and the epicyclic approximation to extract sigma_z
from the line-of-sight dispersion, we find that, with the lone exception of
M101, our disks do have a constant M/L out to ~3 optical scale lengths.
However, once outside this radius, sigma_z stops declining and becomes flat
with radius. Possible explanations for this behavior include an increase in the
disk mass-to-light ratio, an increase in the importance of the thick disk, and
heating of the thin disk by halo substructure. We also find that the disks of
early type spirals have higher values of M/L and are closer to maximal than the
disks of later-type spirals, and that the unseen inner halos of these systems
are better fit by pseudo-isothermal laws than by NFW models.Comment: 18 pages, 15 figures, 5 tables; accepted to Ap
The Stellar and Gas Kinematics of the LITTLE THINGS Dwarf Irregular Galaxy NGC 1569
In order to understand the formation and evolution of dIm galaxies, one needs
to understand their three-dimensional structure. We present measurements of the
stellar velocity dispersion in NGC 1569, a nearby post-starburst dIm galaxy.
The stellar vertical velocity dispersion, , coupled with the
maximum rotational velocity derived from \ion{H}{1} observations, , gives a measure of how kinematically hot the galaxy is, and, therefore,
indicates its structure. We conclude that the stars in NGC 1569 are in a thick
disk with a = 2.4 0.7. In addition to the
structure, we analyze the ionized gas kinematics from \ion{O}{3} observations
along the morphological major axis. These data show evidence for outflow from
the inner starburst region and a potential expanding shell near supermassive
star cluster (SSC) A. When compared to the stellar kinematics, the velocity
dispersion of the stars increase in the region of SSC A supporting the
hypothesis of an expanding shell. The stellar kinematics closely follow the
motion of the gas. Analysis of high resolution \ion{H}{1} data clearly reveals
the presence of an \ion{H}{1} cloud that appears to be impacting the eastern
edge of NGC 1569. Also, an ultra-dense \ion{H}{1} cloud can be seen extending
to the west of the impacting \ion{H}{1} cloud. This dense cloud is likely the
remains of a dense \ion{H}{1} bridge that extended through what is now the
central starburst area. The impacting \ion{H}{1} cloud was the catalyst for the
starburst, thus turning the dense gas into stars over a short timescale,
1 Gyr. We performed a careful study of the spectral energy distribution using
infrared, optical, and ultraviolet photometry producing a state-of-the-art mass
model for the stellar disk. This mass modeling shows that stars dominate the
gravitational potential in the inner 1 kpc.Comment: 49 pages, 25 figures, accepted in A
Little Things
We present LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes,
The HI Nearby Galaxy Survey) that is aimed at determining what drives star
formation in dwarf galaxies. This is a multi-wavelength survey of 37 Dwarf
Irregular and 4 Blue Compact Dwarf galaxies that is centered around HI-line
data obtained with the National Radio Astronomy Observatory (NRAO) Very Large
Array (VLA). The HI-line data are characterized by high sensitivity (less than
1.1 mJy/beam per channel), high spectral resolution (less than or equal to 2.6
km/s), and high angular resolution (~6 arcseconds. The LITTLE THINGS sample
contains dwarf galaxies that are relatively nearby (less than or equal to 10.3
Mpc; 6 arcseconds is less than or equal to 300 pc), that were known to contain
atomic hydrogen, the fuel for star formation, and that cover a large range in
dwarf galactic properties. We describe our VLA data acquisition, calibration,
and mapping procedures, as well as HI map characteristics, and show channel
maps, moment maps, velocity-flux profiles, and surface gas density profiles. In
addition to the HI data we have GALEX UV and ground-based UBV and Halpha images
for most of the galaxies, and JHK images for some. Spitzer mid-IR images are
available for many of the galaxies as well. These data sets are available
on-line.Comment: In press in A
Planetary Nebulae in Face-On Spiral Galaxies. I. Planetary Nebula Photometry and Distances
As the first step to determine disk mass-to-light ratios for normal spiral
galaxies, we present the results of an imaging survey for planetary nebulae
(PNe) in six nearby, face-on systems: IC 342, M74 (NGC 628), M83 (NGC 5236),
M94 (NGC 4736), NGC 5068, and NGC 6946. Using Blanco/Mosaic II and WIYN/OPTIC,
we identify 165, 153, 241, 150, 19, and 71 PN candidates, respectively, and use
the Planetary Nebula Luminosity Function (PNLF) to obtain distances. For M74
and NGC 5068, our distances of 8.6 +/- 0.3 Mpc and 5.4 +0.2/-0.4 Mpc are the
first reliable estimates to these objects; for IC 342 (3.5 +/- 0.3 Mpc), M83
(4.8 +/- 0.1 Mpc), M94 (4.4 +0.1/-0.2 Mpc), and NGC 6946 (6.1 +/- 0.6 Mpc) our
values agree well with those in the literature. In the larger systems, we find
no evidence for any systematic change in the PNLF with galactic position,
though we do see minor field-to-field variations in the luminosity function. In
most cases, these changes do not affect the measurement of distance, but in one
case the fluctuations result in a ~0.2 mag shift in the location of the PNLF
cutoff. We discuss the possible causes of these small-scale changes, including
internal extinction in the host galaxies and age/metallicity changes in the
underlying stellar population.Comment: Accepted for publication in ApJ; 23 pages, 7 figures, 5 table
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