1,111 research outputs found
Outflow or galactic wind: The fate of ionized gas in the halos of dwarf galaxies
Context: H\alpha images of star bursting irregular galaxies reveal a large
amount of extended ionized gas structures, in some cases at kpc-distance away
from any place of current star forming activity. A kinematic analysis of
especially the faint structures in the halo of dwarf galaxies allows insights
into the properties and the origin of this gas component. This is important for
the chemical evolution of galaxies, the enrichment of the intergalactic medium,
and for the understanding of the formation of galaxies in the early universe.
Aims: We want to investigate whether the ionized gas detected in two
irregular dwarf galaxies (NGC 2366 and NGC 4861) stays gravitationally bound to
the host galaxy or can escape from it by becoming a freely flowing wind.
Methods: Very deep H\alpha images of NGC 2366 and NGC 4861 were obtained to
detect and catalog both small and large scale ionized gas structures down to
very low surface brightnesses. Subsequently, high-resolution long-slit echelle
spectroscopy of the H\alpha line was performed for a detailed kinematic
analysis of the most prominent filaments and shells. To calculate the escape
velocity of both galaxies and to compare it with the derived expansion
velocities of the detected filaments and shells, we used dark matter halo
models.
Results: We detected a huge amount of both small scale (up to a few hundred
pc) and large scale (about 1-2 kpc of diameter or length) ionized gas
structures on our H\alpha images. Many of the fainter ones are new detections.
The echelle spectra reveal outflows and expanding bubbles/shells with
velocities between 20 and 110 km/s. Several of these structures are in
accordance with filaments in the H\alpha images. A comparison with the escape
velocities of the galaxies derived from the NFW dark matter halo model shows
that all gas features stay gravitationally bound.Comment: 15 pages, 13 figures, accepted for publication in A&
On the Dynamical and Physical State of the `Diffuse Ionized Medium' in Nearby Spiral Galaxies
We have analyzed deep narrow-band H images and high-resolution
long-slit spectra for a sample of the nearest and brightest late-type galaxies
to study the morphology, physical state, and kinematics of the `Diffuse Ionized
Medium' (`DIM'). We find that the DIM covers most of the star-forming disk, and
is morphologically related to the presence of the giant HII regions. In
addition, the DIM and the giant HII regions differ systematically in their
physical and dynamical state. The DIM is characterized by enhanced emission in
the low-ionization forbidden lines ([OI], [NII], and [SII]), and even the
high-ionization [OIII]5007 line is moderately strong in the DIM. We
verify the inference made by Lehnert & Heckman that the DIM contributes
significantly to the global emission-line ratios measured in late-type
galaxies. We also find that the DIM is more disturbed kinematically than the
gas in the giant HII regions. The intrinsic FWHMs of the H and
[NII]6584 lines range from 30 to 100 km s in the DIM compared
to 20-50 km s in HII regions. The high-ionization gas in the DIM is even
more kinematically disturbed than the low-ionization gas: the
[OIII]5007 lines have intrinsic FWHMs of 70-150 km s. The
differing kinematics implies that `the DIM' is not a single monolithic phase of
the ISM. Instead, it may consist of a `quiescent DIM' with a low
ionization-state and small scale-height (few hundred pc) and a `disturbed DIM'
with a high ionization state and moderate scale-height (0.5 to 1 kpc). We argue
that the quiescent DIM is most likely photoionized by radiation leaking out of
giant HII regions, while the disturbed DIM is most likely heated by the
mechanical energy supplied by supernovae and stellar winds.Comment: 37 pages(including 7 tables) and 12 figures. To appear in the Dec 10,
1997 issue of The Astrophysical Journa
M82 - A radio continuum and polarisation study I. Data reduction and cosmic ray propagation
The potential role of magnetic fields and cosmic ray propagation for feedback
processes in the early Universe can be probed by studies of local starburst
counterparts with an equivalent star-formation rate. Archival data from the
WSRT was reduced and a new calibration technique introduced to reach the high
dynamic ranges needed for the complex source morphology of M82. This data was
combined with archival VLA data, yielding total power maps at 3cm, 6cm, 22cm
and 92cm. The data shows a confinement of the emission at wavelengths of 3/6cm
to the core region and a largely extended halo reaching up to 4kpc away from
the galaxy midplane at wavelengths of 22/92cm up to a sensitivity limit of
90muJy and 1.8mJy respectively. The results are used to calculate the magnetic
field strength in the core region to 98muG and to 24muG in the halo regions.
From the observation of free-free losses the filling factor of the ionised
medium could be estimated to 2%. We find that the radio emission from the core
region is dominated by very dense HII-regions and supernova remnants, while the
surrounding medium is filled with hot X-ray and neutral gas. Cosmic rays
radiating at frequencies higher than 1.4 GHz are suffering from high
synchrotron and inverse Compton losses in the core region and are not able to
reach the halo. Even the cosmic rays radiating at longer wavelengths are only
able to build up the observed kpc sized halo, when several starbursting periods
are assumed where the photon field density varies by an order of magnitude.
These findings together with the strong correlation between Halpha, PAH+, and
our radio continuum data suggests a magnetic field which is frozen into the
ionised medium and driven out of the galaxy kinematically.Comment: 17 pages, 17 figures, to be published in A&
Optical Spectroscopy of Diffuse Ionized Gas in M31
We have obtained sensitive long-slit spectra of Diffuse Ionized Gas (DIG) in
the Andromeda Galaxy, M31, covering the wavelength range of 3550-6850 Angs. By
co-adding extracted DIG spectra, we reached a 1 sigma uncertainty of 9.3E-19
ergs/s/cm^{2}/arcsec^{2} corresponding to .46 pc/cm^{6} in Emission Measure. We
present average spectra of DIG at four brightness levels with Emission Measures
ranging from 9 to 59 pc/cm^{6}. We present the first measurements of
[OII]\lambda3727 and [OIII]\lambda5007 of the truly diffuse ionized medium in
the disk of an external spiral galaxy. We find that I_[OII]/I_H\alpha=.9-1.4.
The [OIII] line is weak (I_[OIII]/I_H\beta = .5), but stronger than in the
Galactic DIG. Measurements of [NII]\lambda6583 and
[SII](\lambda6717+\lambda6731) are also presented. The [SII] lines are clearly
stronger than typical HII regions (I_[SII]/I_H\alpha = .5 compared to .2).
Overall, the line ratios are in agreement with predictions of photoionization
models for diffuse gas exposed to a dilute stellar radiation field, but the
line ratios of the DIG in M31 are somewhat different than observed for Galactic
DIG. The differences indicate a less diluted radiation field in the DIG of
M31's spiral arms compared to DIG in the Solar Neighborhood of the Milky Way.
We have also detected HeI\lambda5876 emission from the brightest DIG in M31.
The HeI line appears to be stronger than in the Galactic DIG, possibly
indicating that most of the Helium in the bright DIG in M31 is fully ionized.
However, this result is somewhat tentative.Comment: Fig. 5 corrected and other minor changes. Paper accepted to ApJ. 21
pages, Latex, incl. 5 fig. & 1 tab., submitted to Ap
Low surface brightness galaxies around the HDF-S - I. Object extraction and photometric results
This study reports on photometric results of a search for LSB galaxies in a
0.76deg^2 field centered on the HDF-S. We present results from photometric
analysis of the derived sample galaxies and compare number densities to results
of former surveys. We used public data from the NOAO Deep Wide-Field survey and
the multi-wavelength Goddard Space Flight Center survey. The former reaches a
limiting surface brightness of mu_BW~29 magarcsec^-2 and is therefore one of
the most sensitive ground based data sets systematically analyzed for LSB
galaxies. To reduce the contamination by High Surface Brightness (HSB) galaxies
at higher redshift, mimicking LSBs due to the ''Tolman Dimming'' effect, we
placed a lower diameter limit of 10.8 arcsec and compared the colors of our
candidate galaxies with the redshift tracks of 5 ''standard'' HSB galaxy types.
We report the detection of 37 galaxies with low apparent central surface
brightness (mu_BW>=22 magarcsec^-2). Using color-color diagrams we were able to
derive a subsample of 9 LSB galaxy candidates with intrinsic central surface
brightnesses below mu_(0,BW)=22.5 magarcsec^-2 and diameters larger than the
preselected size limit of 10.8 arcsec. We selected three additional LSB
candidates due to there extreme low blue central surface brightness (mu_BW>=25
magarcsec^-2). These galaxies were only found in the larger and more sensitive
NOAO data. So finally we derived a sample of 12 LSB galaxy candidates and
therfore this survey results in a four times higher surface density than other
CCD based surveys for field galaxies before.Comment: 22 pages, 23 figures, A&A in pres
GaBoDS: The Garching-Bonn Deep Survey -- II. Confirmation of EIS cluster candidates by weak gravitational lensing
We report the first confirmation of colour-selected galaxy cluster candidates
by means of weak gravitational lensing. Significant lensing signals were
identified in the course of the shear-selection programme of dark matter haloes
in the Garching-Bonn Deep Survey, which currently covers 20 square degrees of
deep, high-quality imaging data on the southern sky. The detection was made in
a field that was previously covered by the ESO Imaging Survey (EIS) in 1997. A
highly significant shear-selected mass-concentration perfectly coincides with
the richest EIS cluster candidate at z~0.2, thus confirming its cluster nature.
Several other shear patterns in the field can also be identified with cluster
candidates, one of which could possibly be part of a filament at z~0.45.Comment: 4 pages, 4 figures, submitted to A&A Letter
Radio haloes in nearby galaxies modelled with 1D cosmic-ray transport using SPINNAKER
We present radio continuum maps of 12 nearby (), edge-on
(), late-type spiral galaxies mostly at and 5 GHz,
observed with the Australia Telescope Compact Array, Very Large Array,
Westerbork Synthesis Radio Telescope, Effelsberg 100-m and Parkes 64-m
telescopes. All galaxies show clear evidence of radio haloes, including the
first detection in the Magellanic-type galaxy NGC 55. In 11 galaxies, we find a
thin and a thick disc that can be better fitted by exponential rather than
Gaussian functions. We fit our SPINNAKER (SPectral INdex Numerical Analysis of
K(c)osmic-ray Electron Radio-emission) 1D cosmic-ray transport models to the
vertical model profiles of the non-thermal intensity and to the non-thermal
radio spectral index in the halo. We simultaneously fit for the advection speed
(or diffusion coefficient) and magnetic field scale height. In the thick disc,
the magnetic field scale heights range from 2 to 8 kpc with an average across
the sample of ; they show no correlation with either
star-formation rate (SFR), SFR surface density () or rotation
speed (). The advection speeds range from 100 to and display correlations of and
; they agree remarkably well with the
escape velocities (), which can be explained by
cosmic-ray driven winds. Radio haloes show the presence of disc winds in
galaxies with
that extend over several kpc and are driven by processes related to the
distributed star formation in the disc.Comment: 39 pages, 20 colour figures, 10 tables. Accepted by MNRA
The multi-phase gaseous halos of star forming late-type galaxies - II. Statistical analysis of key parameters
In Paper I we showed that multi-phase gaseous halos of late-type spiral
galaxies, detected in the radio continuum, in Halpha, and in X-rays, are
remarkably well correlated regarding their morphology and spatial extent. In
this work we present new results from a statistical analysis in order to
specify and quantify these phenomenological relations. This is accomplished by
investigating soft X-ray (0.3-2.0keV) luminosities, FIR, radio continuum,
Halpha, B-band, and UV luminosities for a sample of 23 edge-on late-type spiral
galaxies. Typical star formation indicators, such as SFRs, are determined and a
statistical correlation analysis is carried out. We find strong linear
correlations, covering at least two orders of magnitude, between star formation
indicators and integrated (disk+halo) luminosities in all covered wavebands. In
addition to the well established L_FIR/L_1.4GHz-relation, we show new and
highly significant linear dependencies between integrated soft X-ray
luminosities and FIR, radio continuum, Halpha, B-band, and UV luminosities.
Moreover, integrated soft X-ray luminosities correlate well with SFRs and the
energy input into the ISM by SNe. The same holds if these quantities are
plotted against soft halo X-ray luminosities. Only a weak correlation exists
between the dust mass of a galaxy and the corresponding X-ray luminosity. Among
soft X-ray luminosities, baryonic, and HI-gas masses, no significant
correlations are found. There seems to exist a critical input energy by SNe
into the ISM or a SFR threshold for multi-phase halos to show up. It is still
not clear whether this threshold is a physical or an instrument dependent
sensitivity limit. These findings strongly support our previous results, but
conflict with the concept of halos being due to infalling gas from the IGM.Comment: 8 pages, 3 figures, 1 table, accepted for publication in A&
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