17 research outputs found
HI 21cm imaging of a nearby Damped Lyman-alpha system
We present Giant Metrewave Radio Telescope (GMRT) HI 21cm emission images of
the z=0.009 damped Lyman-alpha (DLA) absorber towards the QSO HS 1543+5921. The
DLA has been earlier identified as a low surface brightness galaxy SBS 1543+593
at an impact parameter of ~ 400 pc to the QSO line of sight. The extremely low
redshift of the absorber allows us to make spatially resolved images of the
21cm emission; besides the HI mass, this also enables us to determine the
velocity field of the galaxy and, hence, to estimate its dynamical mass.
We obtain a total HI mass of ~ 1.4x10^9 Msun, considerably smaller than the
value of M*(HI) determined from blind 21cm emission surveys. This continues the
trend of low HI mass in all low redshift DLAs for which HI emission
observations have been attempted. We also find that the QSO lies behind a
region of low local HI column density in the foreground galaxy. This is
interesting in view of suggestions that DLA samples are biased against high HI
column density systems. The dynamical mass of the galaxy is found to be Mdyn ~
5x10^9 Msun.Comment: Accepted for publication in A&
Magnetic fields in barred galaxies. IV. NGC 1097 and NGC 1365
We present 3.5cm and 6.2cm radio continuum maps in total and polarized
intensity of the barred galaxies NGC 1097 and NGC 1365. Both galaxies exhibit
radio ridges roughly overlapping with the massive dust lanes in the bar region.
The contrast in total intensity across the radio ridges is compatible with
compression and shear of an isotropic random magnetic field. The contrast in
polarized intensity is significantly smaller than that expected from
compression and shearing of the regular magnetic field; this could be the
result of decoupling of the regular field from the dense molecular clouds. The
regular field in the ridge is probably strong enough to reduce significantly
shear in the diffuse gas (to which it is coupled) and hence to reduce magnetic
field amplification by shearing. This contributes to the misalignment of the
observed field orientation with respect to the velocity vectors of the dense
gas. Our observations, for the first time, indicate that magnetic forces can
control the flow of the diffuse interstellar gas at kiloparsec scales. The
total radio intensity reaches its maximum in the circumnuclear starburst
regions, where the equipartition field strength is about 60\mu G, amongst the
strongest fields detected in spiral galaxies so far. The regular field in the
inner region has a spiral shape with large pitch angle, indicating the action
of a dynamo. Magnetic stress leads to mass inflow towards the centre,
sufficient to feed the active nucleus in NGC 1097. We detected diffuse X-ray
emission, possibly forming a halo of hot gas around NGC 1097.Comment: 32 pages with 45 PostScript figures. Accepted for publication in A&A;
Typos corrected 12/10/200
Gas distribution, kinematics and star formation in faint dwarf galaxies
We compare the gas distribution, kinematics and the current star formation in
a sample of 10 very faint (-13.37 < M_B < -9.55) dwarf galaxies. For 5 of these
galaxies we present fresh, high sensitivity, GMRT HI 21cm observations. For all
our galaxies we construct maps of the HI column density at a constant linear
resolution of ~300 pc; this forms an excellent data set to check for the
presence of a threshold column density for star formation. We find that while
current star formation (as traced by Halpha emission) is confined to regions
with relatively large (N_HI > (0.4 -1.7) X 10^{21} atoms cm^{-2}) HI column
density, the morphology of the Halpha emission is in general not correlated
with that of the high HI column density gas. Thus, while high column density
gas may be necessary for star formation, in this sample at least, it is not
sufficient to ensure that star formation does in fact occur. We examine the
line profiles of the HI emission, but do not find a simple relation between
regions with complex line profiles and those with on-going star formation.
Finally, we examine the very fine scale (~20-100 pc) distribution of the HI
gas, and find that at these scales the emission exhibits a variety of shell
like, clumpy and filamentary features. The Halpha emission is sometimes
associated with high density HI clumps, sometimes the Halpha emission lies
inside a high density shell, and sometimes there is no correspondence between
the Halpha emission and the HI clumps. In summary, the interplay between star
formation and gas density in these galaxy does not seem to show the simple
large scale patterns observed in brighter galaxies (abridged).Comment: 15 pages, 6 tables, 13 figures. Accepted for publication in MNRA
Rings and spirals in barred galaxies. III. Further comparisons and links to observations
In a series of papers, we propose a theory to explain the formation and
properties of rings and spirals in barred galaxies. The building blocks of
these structures are orbits guided by the manifolds emanating from the unstable
Lagrangian points located near the ends of the bar. In this paper, the last of
the series, we present more comparisons of our theoretical results to
observations and also give new predictions for further comparisons. Our theory
provides the right building blocks for the rectangular-like bar outline and for
ansae. We consider how our results can be used to give estimates for the
pattern speed values, as well as their effect on abundance gradients in barred
galaxies. We present the kinematics along the manifold loci, to allow
comparisons with the observed kinematics along the ring and spiral loci. We
consider gaseous arms and their relations to stellar ones. We discuss several
theoretical aspects and stress that the orbits that constitute the building
blocks of the spirals and rings are chaotic. They are, nevertheless, spatially
well confined by the manifolds and are thus able to outline the relevant
structures. Such chaos can be termed `confined chaos' and can play a very
important role in understanding the formation and evolution of galaxy
structures and in galactic dynamics in general. This work, in agreement with
several others, argues convincingly that galactic dynamic studies should not be
limited to the study of regular motions and orbits.Comment: 17 pages, 12 figures; accepted in MNRA
The impact of bars on the mid-infrared dust emission of spiral galaxies: global and circumnuclear properties
We study the mid-infrared properties of a sample of 69 nearby spiral
galaxies, selected to avoid Seyfert activity contributing a significant
fraction of the central energetics, or strong tidal interaction, and to have
normal infrared luminosities. These observations were obtained with ISOCAM,
which provides an angular resolution of the order of 10 arcsec (half-power
diameter of the point spread function) and low-resolution spectro-imaging
information. Between 5 and 18 microns, we mainly observe two dust phases,
aromatic infrared bands and very small grains, both out of thermal equilibrium.
On this sample, we show that the global F15/F7 colors of galaxies are very
uniform, the only increase being found in early-type strongly barred galaxies,
consistent with previous IRAS studies. The F15/F7 excesses are unambiguously
due to galactic central regions where bar-induced starbursts occur. However,
the existence of strongly barred early-type galaxies with normal circumnuclear
colors indicates that the relationship between a distortion of the
gravitational potential and a central starburst is not straightforward. As the
physical processes at work in central regions are in principle identical in
barred and unbarred galaxies, and since this is where the mid-infrared activity
is mainly located, we investigate the mid-infrared circumnuclear properties of
all the galaxies in our sample. We show how surface brightnesses and colors are
related to both the available molecular gas content and the mean age of stellar
populations contributing to dust heating. Therefore, the star formation history
in galactic central regions can be constrained by their position in a
color-surface brightness mid-infrared diagram.Comment: 22 pages, 25 figures, accepted for publication in A&A ; small errors
corrected and references update
Simulating Observations of Dark Matter Dominated Galaxies: Towards the Optimal Halo Profile
Low Surface Brightness (LSB) galaxies are dominated by dark matter, and their
rotation curves thus reflect their dark matter distribution. Recent
high-resolution rotation curves suggest that their dark matter mass-density
distributions are dominated by a constant-density core. This seems inconsistent
with the predictions of Cold Dark Matter (CDM) models which produce halos with
compact density cusps and steep mass-density profiles. However, the
observationally determined mass profiles may be affected by non-circular
motions, asymmetries and offsets between optical and dynamical centres, all of
which tend to lower the observed slopes. Here we determine the impact of each
of these effects on a variety of halo models, and compare the results with
observed mass-density profiles. Our simulations suggest that no single
systematic effect can reconcile the data with the cuspy CDM halos. The data are
best described by a model with a soft core with an inner power-law mass-density
slope alpha = -0.2 +/- 0.2. However, no single universal halo profile provides
a completely adequate description of the data.Comment: Accepted for publication in MNRAS; small problem with figure
numbering fixe
Dynamics of Barred Galaxies
Some 30% of disc galaxies have a pronounced central bar feature in the disc
plane and many more have weaker features of a similar kind. Kinematic data
indicate that the bar constitutes a major non-axisymmetric component of the
mass distribution and that the bar pattern tumbles rapidly about the axis
normal to the disc plane. The observed motions are consistent with material
within the bar streaming along highly elongated orbits aligned with the
rotating major axis. A barred galaxy may also contain a spheroidal bulge at its
centre, spirals in the outer disc and, less commonly, other features such as a
ring or lens. Mild asymmetries in both the light and kinematics are quite
common. We review the main problems presented by these complicated dynamical
systems and summarize the effort so far made towards their solution,
emphasizing results which appear secure. (Truncated)Comment: This old review appeared in 1993. Plain tex with macro file. 82 pages
18 figures. A pdf version with figures at full resolution (3.24MB) is
available at http://www.physics.rutgers.edu/~sellwood/bar_review.pd
Magnetic fields in barred galaxies I. The atlas
The total and polarized radio continuum emission of 20 barred galaxies was
observed with the Very Large Array (VLA) at 3, 6, 18 and 22 cm and with the
Australia Telescope Compact Array (ATCA) at 6 cm and 13 cm. Maps at 30 arcsec
angular resolution are presented here. Polarized emission (and therefore a
large-scale regular magnetic field) was detected in 17 galaxies. Most galaxies
of our sample are similar to non-barred galaxies with respect to the
radio/far-infrared flux correlation and equipartition strength of the total
magnetic field. Galaxies with highly elongated bars are not always
radio-bright. We discuss the correlation of radio properties with the aspect
ratio of the bar and other measures of the bar strength. We introduce a new
measure of the bar strength, \Lambda, related to the quadrupole moment of the
bar's gravitational potential. The radio surface brightness I of the barred
galaxies in our sample is correlated with \Lambda, I \propto \Lambda^0.4+/-0.1,
and thus is highest in galaxies with a long bar where the velocity field is
distorted by the bar over a large fraction of the disc. In these galaxies, the
pattern of the regular field is significantly different from that in non-barred
galaxies. In particular, field enhancements occur upstream of the dust lanes
where the field lines are oriented at large angles to the bar's major axis.
Polarized radio emission seems to be a good indicator of large-scale
non-axisymmetric motions.Comment: 29 pages with 66 PostScript figures. Accepted for publication in A&A.
Figures 5-24 also available at http://www.mpifr-bonn.mpg.d
Secular Evolution in Barred Galaxies
A strong bar rotating within a massive halo should lose angular momentum to
the halo through dynamical friction, as predicted by Weinberg. We have
conducted fully self-consistent, numerical simulations of barred galaxy models
with a live halo population and find that bars are indeed braked very rapidly.
Specifically, we find that the bar slows sufficiently within a few rotation
periods that the distance from the centre to co-rotation is more than twice the
semi-major axis of the bar. Observational evidence (meagre) for bar pattern
speeds seems to suggest that this ratio typically lies between 1.2 to 1.5 in
real galaxies. We consider, a number of possible explanations for this
discrepancy between theoretical prediction and observation, and conclude that
no conventional alternative seems able to account for it.Comment: To appear in the proceedings of Nobel Symposium 98 "Barred Galaxies
and Circumnuclear Activity", edited by Aa. Sandquist, P.O. Lindblad and S.
J\"ors\"ater. 17 pages, latex, 6 figure
Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic properties
We performed a compilation of more than 1000 published spectra of HII regions
in spiral galaxies. The oxygen and nitrogen abundances in each HII region were
recomputed in a homogeneous way, using the P-method. The radial distributions
of oxygen and nitrogen abundances were derived. The correlations between oxygen
abundance and macroscopic properties are examined. There is a significant
difference between the L-Z relationship obtained here and that based on the
oxygen abundances determined through the R_23-calibrations. The oxygen
abundance of NGC 5457 recently determined using direct measurements of Te
(Kennicutt, Bresolin & Garnett 2003) agrees with the L-Z relationship derived
here, but is in conflict with the L-Z relationship derived with the R_23-based
oxygen abundances. The obtained L-Z relation for spirals is compared to that
for irregulars. Our sample of galaxies shows evidence that the slope of the
O/H-M_B relationship for spirals is slightly more shallow than that for
irregulars. The effective oxygen yields were estimated for spiral and irregular
galaxies. The effective oxygen yield increases with increasing luminosity from
M_B=-11 to M_B=-18 (or with increasing rotation velocity from Vrot=10 km/s to
Vrot=100 km/s) and then remains approximately constant. Irregular galaxies from
our sample have effective oxygen yields lowered by a factor of 3 at maximum,
i.e. irregular galaxies usually keep at least 1/3 of the oxygen they
manufactured during their evolution.Comment: Accepted for publication in Astronomy and Astrophysics (Figures 2-5,
Tables 2,6 and Appendix will only be published in the electronic version of
the Journal