[show abstract][hide abstract] ABSTRACT: In a laboratory experiment, water-ice aggregates are trapped in a vacuum
chamber at a pressure of 2 mbar due to photophoresis and thermophoresis. The
particles are located between a Peltier element at the bottom at 250 K and a
reservoir of liquid nitrogen at the top at 77 K. Particle sizes vary between 20
micrometres and a few hundred micrometres. It is found that 95% of all the
particles rotate about their vertical axis. A qualitative model is developed
which explains why particles should mainly align to and rotate around the
vertical. The results imply that rotation does not decrease the vertical
strength of photophoretically driven motion in, e.g., protoplanetary discs.
Monthly Notices of the Royal Astronomical Society 10/2011; · 5.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: (Abridged) We present new, deep HI line and 20-cm radio continuum data of the
very puzzling blue compact dwarf galaxy NGC 5253, obtained with the ATCA as
part of the `Local Volume HI Survey' (LVHIS). Our low-resolution HI maps show
the disturbed HI morphology that NGC 5253 possesses, including tails, plumes
and detached HI clouds. The high-resolution map reveals an HI plume at the SE
and an HI structure at the NW that surrounds an Ha shell. We confirm that the
kinematics of the neutral gas are highly perturbed and do not follow a rotation
pattern. We discuss the outflow and infall scenarios to explain such disturbed
kinematics, analyze the environment in which it resides, and compare it
properties with those observed in similar star-forming dwarf galaxies. The
radio-continuum emission of NGC 5253 is resolved and associated with the
intense star-forming region at the center of the galaxy. We complete the
analysis using multiwavelength data extracted from the literature. We estimate
the SFR using this multiwavelength approach. NGC 5253 does not satisfy the
Schmidt-Kennicutt law of star-formation, has a very low HI mass-to-light ratio
when comparing with its stellar mass, and seems to be slightly metal-deficient
in comparison with starbursts of similar baryonic mass. Taking into account all
available multiwavelength data, we conclude that NGC 5253 is probably
experiencing the infall of a diffuse, low-metallicity HI cloud along the minor
axis of the galaxy, which is comprising the ISM and triggering the powerful
starburst. The tidally disturbed material observed at the east and north of the
galaxy is a consequence of this interaction, which probably started more than
100 Myr ago. The origin of this HI cloud may be related with a strong
interaction between NGC 5253 and the late-type spiral galaxy M 83 in the past.
Monthly Notices of the Royal Astronomical Society 09/2011; 419. · 5.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: Observations of the stellar and gaseous components in disc galaxies often reveal asymmetries in the morphological and kinematic
distribution. However, the origin of this effect is not well known to date, and quantitative studies are rare. Here, we present
the first statistical investigation of a sample of 76 Hi discs using the WHISP survey. We perform a Fourier analysis to study the morphological lopsidedness. This allows to trace
the degree of asymmetry with radius. We further investigate the dependence on, e.g., the morphological type and environment.
[show abstract][hide abstract] ABSTRACT: The distribution of stars and gas in many galaxies is asymmetric. This
so-called lopsidedness is expected to significantly affect the dynamics and
evolution of the disc, including the star formation activity. Here, we measure
the degree of lopsidedness for the gas distribution in a selected sample of 70
galaxies from the Westerbork HI Survey of Spiral and Irregular Galaxies. This
complements our earlier work (Paper I) where the kinematic lopsidedness was
derived for the same galaxies. The morphological lopsidedness is measured by
performing a harmonic decomposition of the surface density maps. The amplitude
of lopsidedness A_1, the fractional value of the first Fourier component, is
typically quite high (about 0.1) within the optical disc and has a constant
phase. Thus, lopsidedness is a common feature in galaxies and indicates a
global mode. We measure A_1 out to typically one to four optical radii,
sometimes even further. This is, on average, four times larger than the
distance to which lopsidedness was measured in the past using near-IR as a
tracer for the old stellar component, and will therefore provide a new, more
stringent constraint on the mechanism for the origin of lopsidedness.
Interestingly, the value of A_1 saturates beyond the optical radius.
Furthermore, the plot of A_1 vs. radius shows fluctuations which we argue are
due to local spiral features. We also try to explain the physical origin of
this observed disc lopsidedness. No clear trend is found when the degree of
lopsidedness is compared to a measure of the isolation or interaction
probability of the sample galaxies. However, this does not rule out a tidal
origin if the lopsidedness is long-lived. Additionally, we find that the
early-type galaxies tend to be more morphologically lopsided than late-type
galaxies. Both results together indicate a tidal origin for the lopsidedness.
Astronomy and Astrophysics 03/2011; 530. · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: The frequently observed lopsidedness of the distribution of stars and gas in
disc galaxies is still considered as a major problem in galaxy dynamics. It is
even discussed as an imprint of the formation history of discs and the
evolution of baryons in dark matter haloes. Here, we analyse a selected sample
of 70 galaxies from the Westerbork HI Survey of Spiral and Irregular Galaxies.
The HI data allow us to follow the morphology and the kinematics out to very
large radii. In the present paper, we present the rotation curves and study the
kinematic asymmetry. We extract the rotation curves of receding and approaching
sides separately and show that the kinematic behaviour of disc galaxies can be
classified by five different types: symmetric velocity fields where the
rotation curves of receding and approaching sides are almost identical; global
distortions where the rotation velocities of receding and approaching side have
an offset which is constant with radius; local distortions which lead to large
deviations in the inner and negligible deviations in the outer parts (and vice
versa); and distortions which split the galaxies into two kinematic systems,
visible in the different behaviour of the rotation curves of receding and
approaching sides, which leads to a crossing and a change in side. The
kinematic lopsidedness is measured from the maximum rotation velocities,
averaged over the plateau of the rotation curves. This gives a good estimate of
global lopsidedness in the outer parts of the sample galaxies. We find that the
mean value of the perturbation parameter denoting the lopsided potential as
obtained from the kinematic data is 0.056. 36% of all sample galaxies are
globally lopsided, which can be interpreted as the disc responding to a halo
that was distorted by a tidal encounter. In Paper II, we study the
morphological lopsidedness for the same sample of galaxies.
Astronomy and Astrophysics 03/2011; 530. · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: The feedback between massive stars and the interstellar medium is one of the most important processes in the evolution of dwarf galaxies. This interaction results in numerous neutral and ionized gas structures that have been found both in the disc and in the halo of these galaxies. However, their origin and fate are still poorly understood.We here present new H i and optical data of two Magellanic irregular dwarf galaxies in the Local Volume: IC 4662 and NGC 5408. The H i line data were obtained with the Australia Telescope Compact Array and are part of the ‘Local Volume H i Survey’. They are complemented by optical images and spectroscopic data obtained with the European Southern Observatory (ESO) New Technology Telescope and the ESO 3.6-m telescope. Our main aim is to study the kinematics of the neutral and ionized gas components in order to search for outflowing gas structures and to make predictions about their fate. Therefore, we perform a Gaussian decomposition of the H i and Hα line profiles.We find the H i gas envelopes of IC 4662 and NGC 5408 to extend well beyond the optical discs, with H i to optical diameter ratios of above 4. The optical disc is embedded into the central H i maximum in both galaxies. However, higher resolution H i maps show that the H i intensity peaks are typically offset from the prominent H ii regions.While NGC 5408 shows a fairly regular H i velocity field, which allows us to derive a rotation curve, IC 4662 reveals a rather twisted H i velocity field, possibly caused by a recent merger event. We detect outflows with velocities between 20 and 60 km s−1 in our Hα spectra of both galaxies, sometimes with H i counterparts of similar velocity. We suggest the existence of expanding superbubbles, especially in NGC 5408. This is also supported by the detection of full width at half-maxima as high as 70 km s−1 in Hα, which cannot be explained by thermal broadening alone. In the case of NGC 5408, we compare our results with the escape velocity of the galaxy, which shows that the measured expansion velocities are in all cases too low to allow the gas to escape from the gravitational potential of NGC 5408. This result is consistent with studies of other dwarf galaxies.
Monthly Notices of the Royal Astronomical Society 08/2010; 407(1):113 - 132. · 5.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: Abridged. The feedback between massive stars and the interstellar medium is one of the most important processes in the evolution of dwarf galaxies. This interaction results in numerous neutral and ionised gas structures that have been found both in the disc and in the halo of these galaxies. However, their origin and fate are still poorly understood. We here present new HI and optical data of two nearby irregular dwarf galaxies: IC 4662 and NGC 5408. The HI line data were obtained with the ATCA and are part of the Local Volume HI Survey. They are complemented by optical images and spectroscopic data obtained with the ESO NTT and the ESO 3.6m telescope. Our main aim is to study the kinematics of the neutral and ionised gas components in order to search for outflowing gas structures and to make predictions about their fate. We find the HI gas envelopes of both galaxies to extend well beyond the optical discs. The optical disc is embedded into the central HI maximum in both galaxies. However, higher resolution HI maps show that the HI intensity peaks are typically offset from the prominent HII regions. While NGC 5408 shows a fairly regular HI velocity field, which allows us to derive a rotation curve, IC 4662 reveals a rather twisted HI velocity field, possibly caused by a recent merger event. We detect outflows with velocities between 20 and 60 km/s in our Halpha spectra of both galaxies, sometimes with HI counterparts of similar velocity. We suggest the existence of expanding superbubbles, especially in NGC 5408. This is also supported by the detection of FWHMs as high as 70 km/s in Halpha. In case of NGC 5408, we compare our results with the escape velocity of the galaxy, which shows that the measured expansion velocities are in all cases too low to allow the gas to escape from the gravitational potential of NGC 5408. This result is consistent with studies of other dwarf galaxies. Comment: 23 pages, 16 figures, accepted for publication by MNRAS
[show abstract][hide abstract] ABSTRACT: Deep H(alpha) images of irregular dwarf galaxies show a wealth of expanding filamentary and shell-like structures whose fate is still poorly understood. Theoretical models which try to explain the formation and evolution of these galaxies require galactic winds. However, no clear evidence of a galactic wind in a local star-forming dwarf galaxy has been found, so far. This apparent contradiction appears to be based on the technical difficulties of such an observation. We here propose a new approach using the WIYN telescope equipped with SparsePak in Echelle mode and centred at H(alpha) in order to spectroscopically resolve the H(alpha) line and to measure the kinematics of even the faintest shell structures in two well-studied dwarf galaxies, NGC 2366 and NGC 4861. SparsePak is an excellent instrument for this kind of analysis as it has a large field of view and as it provides us with a sufficiently high spectral resolution. We want to decompose the H(alpha) line and search for expanding gas. A comparison with the escape velocity of the host galaxies will reveal its fate.
[show abstract][hide abstract] ABSTRACT: We are obtaining deep multiwavelength data of a sample of nearby blue compact dwarf galaxies (BCDGs) combining broad-band optical/NIR and H$\alpha$ photometry, optical spectroscopy and 21-cm radio observations. Here we present HI results obtained with the Australia Telescope Compact Array for some BCDGs, all showing evident interaction features in their neutral gas component despite the environment in which they reside. Our analysis strongly suggests that interactions with or between low-luminosity dwarf galaxies or HI clouds are the main trigger mechanism of the star-forming bursts in BCDGs; however these dwarf objects are only detected when deep optical images and complementary HI observations are performed. Are therefore BCDGs real isolated systems? Comment: 4 pages, 2 figures, proceedings to "Galaxies in Isolation: Exploring Nature vs. Nurture" conference
[show abstract][hide abstract] ABSTRACT: Context. The cusp-core discrepancy is one of the major problems in astrophysics. It results from comparing the observed mass distribution of galaxies with the predictions of Cold Dark Matter simulations. The latter predict a cuspy density profile in the inner parts of galaxies, whereas observations of dwarf and low surface brightness galaxies show a constant density core. Aims. We want to determine the shape of the dark matter potential in the nuclear regions of a sample of six nearby irregular dwarf galaxies. Methods. In order to quantify the amount of non-circular motions which could potentially affect a mass decomposition, we first perform a harmonic decomposition of the HI Hermite velocity fields of all sample galaxies. We then decompose the HI rotation curves into different mass components by fitting NFW and pseudo-isothermal halo models to the HI rotation curves using a chi^2 minimisation. We model the minimum-disc, the minimum-disc+gas, and the maximum-disc cases. Results. The non-circular motions are in all cases studied here of the order of only a few km/s (generally corresponding to less than 25% of the local rotation velocity), which means that they do not significantly affect the rotation curves. The observed rotation curves can better be described by the cored pseudo-isothermal halo than by the NFW halo. The slopes of the dark matter density profiles confirm this result and are in good agreement with previous studies. The quality of the fits can often be improved when including the baryons, which suggests that they contribute significantly to the inner part of the density profile of dwarf galaxies. Comment: 21 pages, 17 figures, accepted for publication by A&A
Astronomy and Astrophysics 06/2009; · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: Context: Outflows powered by the injection of kinetic energy from massive stars can strongly affect the chemical evolution of galaxies, in particular of dwarf galaxies, as their lower gravitational potentials enhance the chance of a galactic wind. Aims: We therefore performed a detailed kinematic analysis of the neutral and ionised gas components in the nearby star-forming irregular dwarf galaxy NGC 4861. Similar to a recently published study of NGC 2366, we want to make predictions about the fate of the gas and to discuss some general issues about this galaxy. Methods: Fabry-Perot interferometric data centred on the Halpha line were obtained with the 1.93 m telescope at the Observatoire de Haute-Provence. They were complemented by H i synthesis data from the VLA. We performed a Gaussian decomposition of both the Halpha and the H i emission lines in order to search for multiple components indicating outflowing gas. The expansion velocities of the detected outflows were compared to the escape velocity of NGC 4861, which was modelled with a pseudo-isothermal halo. Results: Both in Halpha and H i the galaxy shows several outflows, three directly connected to the disc and probably forming the edges of a supergiant shell, and one at kpc-distance from the disc. We measured velocity offsets of 20 to 30 km s-1, which are low in comparison to the escape velocity of the galaxy and therefore minimise the chance of a galactic wind.
Astronomy and Astrophysics 01/2009; 505:105-116. · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: Abridged. Context. The metal content of dwarf galaxies and the metal enrichment of the intergalactic medium both suggest that mass loss from galaxies is a significant factor for the chemical evolution history of galaxies, in particular of dwarf galaxies. However, no clear evidence of a blow-away in local dwarf galaxies has been found so far. Aims. We therefore performed a detailed kinematic analysis of the neutral and ionised gas in the nearby star-forming irregular dwarf galaxy NGC 2366 in order to make predictions about the fate of the gas and to get a more complete picture of this galaxy. Methods. A deep Halpha image and Fabry-Perot interferometric data of NGC 2366 were obtained. They were complemented by HI synthesis data from the THINGS survey. We searched for line-splitting both in Halpha and HI by performing a Gaussian decomposition. To get an idea whether the expansion velocities are high enough for a gas blow-away, we used the pseudo-isothermal halo model, which gives us realistic values for the escape velocities of NGC 2366. The good data quality also allowed us to discuss some peculiarities of the morphology and the dynamics in NGC 2366. Results. A large red-shifted outflow north west of the giant extragalactic HII region with an expansion velocity of up to 50 km/s is found in Halpha, but not in HI. Additionally, a blue-shifted component north of the giant extragalactic HII region was detected both in Halpha and HI with an expansion velocity of up to 30 km/s. A comparison with the escape velocities of NGC 2366 reveals that the gas does not have enough kinetic energy to leave the gravitational potential. Comment: 15 pages, 14 figures, accepted for publication by A&A
Astronomy and Astrophysics 11/2008; · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: We propose to complete the `Local Volume HI Survey' (LVHIS) of all
nearby (D < 10 Mpc) galaxies south of declination -30 degrees that
were detected in the HI Parkes All-Sky Survey (HIPASS). The main goals
of this project are to study the immediate environment, velocity field,
alignment, star formation rate and mass function of gas-rich galaxies in
the Local Volume and, with accurate distance information from the TRGB
brightness as well as a parallel NIR survey at the AAT, derive the true
Tully-Fisher relation. Project webpage:
[show abstract][hide abstract] ABSTRACT: We propose to carry out full-synthesis (12-h) HI ATCA observations of 8 bright Blue Compact Dwarf Galaxies (BCDGs), for which we have already got interesting results. Some are apparently isolated, others belong to galaxy groups. We want to estimate the distribution of the HI in each BCDG and its surrounding, looking for independent HI clouds or morphological features that reveal interaction processes. We will also analyze the kinematics of the neutral gas and, using 1.4 GHz radio continuum maps, derive an extinction-free star formation rate. We will combine the data with the results obtained using UV/optical/NIR images and spectroscopy. We remark that 4 of the galaxies have been observed at the 8.2m VLT and 5 will be observed at the 10.4m GTC and the Herschel observatory.
[show abstract][hide abstract] ABSTRACT: Fabry-Pérot (FP) interferometry centred on the Hα line was performed to analyse the kinematics of the most prominent ionised
gas structures in a sample of nearby irregular dwarf galaxies. The complete spatial coverage of the galaxy in one exposure
enables us to follow already known expanding gas structures further out than before. Additionally, we use these 3d images
to determine the velocities of structures which could not be kinematically analysed, yet. We here present as an example our
results of NGC 2366.
[show abstract][hide abstract] ABSTRACT: We obtained deep multi-wavelength data of a sample of nearby blue compact dwarf galaxies (BCDGs) combining broad-band optical/NIR and H photometry, optical spectroscopy and 21-cm radio continuum and line observations. The selected BCDGs are found in di erent environments, from apparently isolated to compact groups. Our aims are to analyze the chemical and physical properties of their gas, their star formation activity, the kinematics, the stellar populations and environment. Here we present new exciting H i results obtained with the Australia Telescope Compact Array for some BCDGs, all showing interaction features. This analysis strongly suggests that interactions of BCDGs with low- luminosity dwarf galaxies are the main trigger mechanism of the observed star-forming bursts.
[show abstract][hide abstract] ABSTRACT: 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&A
Astronomy and Astrophysics 09/2007; · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: Galactic winds are mayor ingredients of our current paradigms of galaxy formation and evolution. They are needed to explain the chemical evolution of galaxies and the intergalactic medium, the luminosity function of galaxies, the formation of galaxies, and the reionization of the universe, just to name a few topics. While the observational support for the presence of galactic winds in massive galaxies and gas-rich mergers is quite strong, the case for galactic winds in dwarf galaxies is much weaker. This is rather surprising, given their shallow potential well and the many examples of star bursts in local dwarf galaxies. Apparently, the physics is complicated in the low-mass case. In this paper we first comment on the theoretical framework for galactic winds in dwarf galaxies and the problems of the observational verification in massive, intermediate mass, and true dwarf galaxies. We then present new observations underlining the complexity of the gas structure of gas halos of dwarf galaxies and the resulting effects on the formation of galactic winds.
New Astronomy Reviews - NEW ASTRON REV. 01/2007; 51(1):141-145.
[show abstract][hide abstract] ABSTRACT: High-resolution long-slit echelle spectroscopy of the two irregular dwarf galaxies NGC 2366 and NGC 4861 was performed with the 4 m telescope of the Kitt Peak National Observatory to examine the structure and the kinematics of the ionized gas. Additionally, we use Hα images from the 3.5 m telescope of the Calar Alto Observatory (NGC 2366) and from the 3.6 m Canada France Hawaii Telescope (NGC 4861). We are particularly looking for expanding shells and outflows in Hα. In both galaxies giant filaments are detected at distances of several kpcs from the center of current star formation. They expand with velocities up to 110 km/s. A special feature is the outflow to the north-west of the Giant HII Region NGC 2363. Sometimes it even seems that these outflows emanate from regions with only weak star formation. That means that they probably descend from an older star formation event.