S. Mühle

Joint Institute for VLBI in Europe, Hoogeveen, Drenthe, Netherlands

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Publications (17)18.73 Total impact

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    ABSTRACT: (abridged) NGC 1569 is a nearby dwarf irregular galaxy which underwent an intense burst of star formation 10 to 40 Myr ago. We present observations that reach surface brightnesses two to eighty times fainter than previous radio continuum observations and the first radio continuum polarization observations. These observations allow us to probe the relationship of the magnetic field of NGC 1569 to the rest of its interstellar medium. We confirm the presence of an extended radio continuum halo at 20 cm and see for the first time the radio continuum feature associated with the western Halpha arm at wavelengths shorter than 20cm. The spectral index trends in this galaxy support the theory that there is a convective wind at work in this galaxy. We derive a total magnetic field strength of 38 microG in the central regions and 10-15 microG in the halo. The magnetic field is largely random in the center of the galaxy; the uniform field is ~3-9 microG and is strongest in the halo. We find that the magnetic pressure is the same order of magnitude but, in general, a factor of a few less than the other components of the interstellar medium in this galaxy. The uniform magnetic field in NGC 1569 is closely associated with the Halpha bubbles and filaments. We suggest that a supernova-driven dynamo may be operating in this galaxy. The outflow of hot gas from NGC 1569 is clearly shaping the magnetic field, but the magnetic field in turn may be aiding the outflow by channeling gas out of the disk of the galaxy. Dwarf galaxies with extended radio continuum halos like that of NGC 1569 may play an important role in magnetizing the intergalactic medium. Comment: ApJ accepted. 56 pages, 14 figures (low resolution), 8 tables. Version with high resolution figures at http://www.astro.virginia.edu/~aak8t/data/n1569/ms.pdf
    The Astrophysical Journal 02/2010; · 6.73 Impact Factor
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    ABSTRACT: Irregular galaxies are the present day analogs of the high redshift building blocks of galaxies like the Milky Way. The shallow potential wells of irregular galaxies makes their interstellar medium a chaotic system prone to disruption by star formation, interactions, and mergers. An important, but oft-overlooked component of the interstellar medium of irregulars is their magnetic field. Previous observations suggest that irregulars have a wide range magnetic field strengths and properties. To increase the number of irregulars with detailed observations of their magnetic fields, we have observed three irregular galaxies with the VLA and WSRT: NGC 4214, NGC 1569, and NGC 1156. The magnetic field of NGC 1569 is shaped almost entirely by the outflow of gas from this galaxy, but the central magnetic field in this galaxy may be a dominant source of pressure in this galaxy. The magnetic field of NGC 4214 is mostly random and is not a dominant source of pressure. NGC 1156 is similar in size to NGC 4214, but has a much more extended radio continuum envelope. We will place these fields into the context of the interstellar medium of these galaxies. Finally, we will draw conclusions on the role of magnetic fields in irregular galaxies in general.
  • 01/2008;
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    ABSTRACT: Magnetic fields are an important component of the interstellar medium, especially in low-mass galaxies like irregulars where the magnetic pressure may be significant. However, few irregular galaxies have observed magnetic field structures. Using the VLA, the GBT, and the ATCA, we have observed several irregular galaxies in the radio continuum to determine their magnetic field structures. Here we report on our results for the galaxies NGC 4214 and NGC 1569.
    Proceedings of the International Astronomical Union 09/2007;
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    The Astronomical Journal 06/2006; 132(1):443. · 4.97 Impact Factor
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    ABSTRACT: The recent starburst in the nearby dwarf galaxy NGC 1569 has had a deep impact on the interstellar medium of the whole galaxy. In this paper, we discuss the consequences of this event for the distribution and kinematics of the neutral atomic hydrogen and demonstrate how strong star formation influences the properties of the surrounding molecular gas. The detection of a complex halo structure leads us to propose the accretion of an intergalactic cloud as the trigger and the gas reservoir of the remarkably strong starburst.
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    ABSTRACT: Photometric studies suggest that the nearby (D = 2 Mpc) dwarf irregular galaxy NGC 1569 has experienced a major burst of star formation for at least the last 25 Myr. Other pieces of evidence are numerous Ha filaments along the minor axis and a strong, metalrich galactic wind, which may lead to the loss of metal-enriched gas to the IGM. We show how the starburst has affected the properties of the neutral atomic and molecular gas in this metal-poor galaxy. In addition, we report the detection of the likely trigger and a possible gas reservoir of the unusually strong starburst.
    The Journal of the Royal Astronomical Society of Canada. Royal Astronomical Society of Canada 07/2005; 99:141.
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    ABSTRACT: As part of our study on the impact of violent star formation on the interstellar medium (ISM) of dwarf galaxies, we report observations of neutral atomic hydrogen (HI) in the post-starburst dwarf galaxy NGC 1569. High-resolution measurements with the VLA (B-, C- and D-array) are aimed at identifying morphological and kinematical signatures in the HI caused by the starburst. Our kinematical data suggest a huge hole in the HI distribution, probably due to the large number of supernovae explosions in the center of the galaxy over the past 20 Myr. Investigating the large-scale HI structure, we confirm the existence of a possible HI companion and a so-called HI bridge east of NGC 1569. Furthermore, we report the detection of additional low-intensity HI halo emission, which leads us to suggest a revised halo structure. Based on the new picture, we discuss the origin of the halo gas and possible implications for the evolution of the starburst in NGC 1569.
    The Astronomical Journal 05/2005; · 4.97 Impact Factor
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    ABSTRACT: The recent, very strong starburst in the nearby dwarf galaxy NGC 1569 has had a tremendous impact on both the ionized and the neutral interstellar gas in the galaxy and has led to the formation of a powerful galactic wind. In this contribution, we discuss the distribution and kinematics of the neutral atomic hydrogen using a high-resolution H I data cube and results from Chandra observations. We also report on our recent observations of the molecular gas morphology and velocity structure in the region near two very bright super star clusters and compare it to the distribution of the atomic gas. The spectra of several 12CO and 13CO transition lines have been used to derive an average kinetic temperature and gas density in the most prominent giant molecular association west of the super star clusters. In addition, we present evidence for an extended H I structure in the halo of NGC 1569. This, and several peculiar features in the disk of NGC 1569, leads us to suggest that NGC 1569 is currently merging with an intergalactic H I cloud, which may have been the trigger and a gas reservoir of the recent starburst.
    04/2005; 329:50P.
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    11/2004; 320:303.
  • Astronomische Nachrichten Supplement. 07/2003;
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    ABSTRACT: Without Abstract
    Astrophysics and Space Science 06/2002; 281(1):327-328. · 2.06 Impact Factor
  • 01/2002;
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    ABSTRACT: A tremendous starburst, that ended about 4 million years ago, has had a very strong impact on the dwarf galaxy NGC 1569. Evidence for this event is still prominent in the different components of the galaxy: For example, photometric studies of the resolved stellar population close to the very luminous super-star clusters hint at past star formation rates of up to 3 M_solar/yr. In ROSAT observations, extended X-ray emission has been found at the same location. And the spectrum of the strong synchrotron emission shows a kink characteristic for a recently stopped starburst. We study the impact of the starburst on the neutral atomic and molecular gas. Our CO(3--2) map obtained with the Heinrich-Hertz Telescope reveals 3 giant molecular associations as well as a large amount of extended CO gas. The comparison of this map with interferometric and single-dish data of the CO(2--1) and CO(1--0) transitions indicates an unusually warm molecular gas phase. The HI distribution is remarkably smooth for a star-forming dwarf galaxy. Only one huge bubble has been found. It is located near the position of the super-star clusters. With our naturally weighted VLA data cube, we can confirm the existence of a low-mass HI cloud and a bridge connecting it to NGC 1569. This might be an HI cloud in the process of merging with the galaxy and being tidally disrupted. We also report the detection of the remnants of a shell, that can be traced best in the HI velocity field.
  • 01/2001; -1:161.
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    ABSTRACT: The dwarf galaxy NGC 1569 has undergone a violent starburst with star formation rates of up to 3 Msolar/yr, that ended about 4 million years ago. Two bright super-star clusters are only two obvious pieces of evidence for this event. The distribution and the properties of the ISM in the galaxy have profoundly been affected by the starburst. With yet unpublished high-resolution HI data, the morphology of the neutral atomic gas in the galaxy and its halo is discussed. Comparing the HI morphology with other star-forming dwarf galaxies, e. g. IC 10, one notices that the distribution is remarkably smooth with only one prominent huge hole at the position of the super-star clusters. We also present new observations of CO in 5 lines, including the sub-mm (3-2) transition and the 13CO (1-0) and (2-1) transitions in a selected region and argue that the Giant Molecular Associations close to the starburst center seem to be unusually warm. LVG modeling with one gas component can not explain the extraordinary line ratios that we have derived in one particular Giant Molecular Cloud. Magnetic fields can play a crucial role in constraining or facilitating the outflow of hot, metal-rich gas into the halo in the aftermath of a starburst. Thus, we are engaged in a multi-frequency high-resolution study of the polarized radio continuum emission, of which we present our first results.
  • Stefanie Mühle, Susanne Hüttemeister, Uli Klein
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    ABSTRACT: A starburst usually has a dramatic impact on the distribution and evolution of the surrounding interstellar medium. The effects of a newly formed star cluster on its environment can best be studied in the shallow potential of dwarf galaxies. Here, HI shells can expand to large sizes, since there aren't any spiral density waves or strong shear forces that would destroy those structures within a short period of time. Furthermore, the possibility of propagating star formation is not in danger of being confused with the onset of star formation due to a density wave. Where the shallow potential of the galaxy can not contain the hot gas of an expanding bubble, huge outflows of gas are visible in Halpha, preferentially along the minor axis of the galaxy. Thus, in dwarf galaxies one starburst can determine the further evolution of the whole galaxy. Examples of dwarf galaxies that currently undergo a starburst are presented and the distribution of the surrounding atomic and molecular interstellar gas as traced by HI and CO is discussed.

Publication Stats

8 Citations
18.73 Total Impact Points


  • 2010
    • Joint Institute for VLBI in Europe
      Hoogeveen, Drenthe, Netherlands
  • 2002–2006
    • University of Bonn
      • Argelander-Institute of Astronomy
      Bonn, North Rhine-Westphalia, Germany