arXiv subjects

Fabian Walter

Publications and source records attributed to Fabian Walter.

425 records · Page 24Linked to original sources

arXiv2002

ROSAT X-ray observations of the dwarf galaxy Holmberg II

We present a study of the irregular dwarf galaxy Holmberg II based on ROSAT PSPC observations (total exposure time: 22 ksec). Holmberg II is a nearby (3.2 Mpc), well-studied dwarf irregular galaxy. It is famous for its interstellar medium which is dominated by expanding structures such as HI holes and shells. We search for X-ray emission from point sources as well as for diffuse emission, down to the detection limit of the ROSAT data. Using X-ray hardness ratio diagrams we differentiate between thermal plasma and power-law X-ray spectra which helps to determine the nature of the individual sources. Correlating the X-ray data with complementary observations ranging from the far-ultraviolet to the radio regime we increase the probability of correctly identifying sources belonging to Holmberg II. We did not detect soft X-ray emission originating from hot gas within supergiant HI shells above our luminosity sensitivity limit of (L_limit(0.1 - 2.1 keV) ~ 10^{37}erg/s). This finding can probably be attributed to blow-out in the case of the largest holes and insufficient sensitivity (due to strong photoelectric absorption) in case of the smaller HI holes. However we find faint X-ray sources well beyond the stellar body but within the HI distribution of Holmberg II, which suggests the presence of X-ray binaries. This indicates that star formation has taken place across the entire gaseous disk of Holmberg II in the past, some of which may have created the structures seen in the ISM at large galactocentric radii.

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arXiv2001

Discovery of Molecular Gas in the Outflow and Tidal Arms around M82

We present the first fully sampled map of 12CO (1-0) emission from M82 covering the entire galaxy. Our map contains a 12 x 15 kpc^2 area. We find that extraplanar CO emission, previously reported at short distances above the galactic plane, extends to heights of up to 6 kpc above the disk. Some of this emission is associated with tidal arms seen in HI, implying either that M82 contained substantial amounts of molecular gas in the outer disk, or that molecular gas formed after the tidal features. CO emission along the direction of the outflow extends to distances of 3 kpc above and below the disk. At this distance, the line is shifted in velocity about 100 km/s, and has the same sense as the galactic outflow from the central starburst. This implies that molecular gas may be entrained into the outflow.

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arXiv2001

Evidence for Blow-out in the Low-mass Dwarf Galaxy Holmberg I

We present radio and optical observations of Holmberg I (HoI), a member of the M81 group of galaxies (distance 3.6Mpc). HoI is a low-mass, low surface- brightness dwarf galaxy. High-resolution multi-array VLA HI observations reveal a supergiant shell (diameter: 1.7 kpc) which covers about half the optical extent of HoI and which comprises 75% of the total HI content (total HI mass: 1.1 10^8 M_o). We set a tentative upper limit to the dark matter content of < 3.1 10^8 M_o. The HI data are complemented by deep, optical UBV(RI)_c and Halpha observations obtained at the Calar Alto 2.2m telescope. We find M_HI/L_B = 1.1 M_o/L_B_o. The total visible (stars plus gas) mass of HoI adds up to 2.4 10^8 M_o. This leads to a total mass of < 5.5 10^8 M_o. The origin of HoI's peculiar HI morphology is discussed in terms of a supergiant shell created by strong stellar winds and supernova explosions (energy: equiv. 20-260 type II SN; age: 80+-20 Myr). The morphological center of HoI is offset by 0.75 kpc with respect to the dynamical center. Within the interior of the shell the light distribution is exponential with a rather shallow gradient and blue optical colors. Beyond a radius corresponding to an HI column density of 10^21 cm^-2, the putative star formation (SF) threshold, the disk becomes considerably redder and the slope for the exponential light distribution steepens. It is speculated that a generation of stars (age 15-30 Myr) is the result of secondary SF on the rim of the shell. Based on the global morphology and velocity dispersion as well as the location of the HII regions we find evidence for ram pressure within the M81 group. Finally, we discuss the likelihood of HoI having lost some of its interstellar material to the intergalactic medium (''blow-out'' scenario).

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arXiv2000

Atomic and Molecular Gas near NGC3077: The Making of a New Dwarf Galaxy?

Using the IRAM 30m radio telescope we have mapped the tidal arm feature south-east of NGC 3077 where we recently detected molecular gas in the CO (1-0) and (2-1) transitions. We find that the molecular gas is much more extended than previously thought (several kpc). The CO emission can be separated into at least 3 distinct complexes with equivalent radii between 250pc and 700pc - the newly detected complexes therefore range among the largest molecular complexes in the local universe. Mass estimates based on virialization and employing an X_CO factor yield a total mass for the complexes of order 4 x 10^7 M_sun, i.e. more than the estimated molecular mass within NGC 3077 itself. This implies that interactions between galaxies can efficiently remove heavy elements and molecules from a galaxy and enrich the intergalactic medium. A comparison of the distribution of HI and CO shows no clear correlation. However, CO is only found in regions where the HI column density exceeds 1.1 x 10^21 cm^-2. HI masses for the molecular complexes mapped are of the same order as the corresponding molecular masses. Since the complexes have all the ingredients to form stars in the future, we are thus presumably witnessing the birth of a dwarf galaxy. This process may have dominated the creation of dwarf galaxies at larger look-back times.

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arXiv2000

UV Observations of the Powering Source of the Supergiant Shell in IC2574

A multi-band analysis of the region containing the supergiant HI shell in the nearby dwarf irregular galaxy IC2574 presents evidence of a causal relationship between a central star cluster, the surrounding expanding HI shell, and secondary star formation sites on the rim of the HI shell. Comparisons of the far-UV (FUV, 1521 A), optical broad-band, H-alpha, X-ray, and HI morphologies suggest that the region is in an auspicious moment of star formation triggered by the central stellar cluster. The derived properties of the HI shell, the central stellar cluster, and the star forming regions on the rim support this scenario: The kinematic age of the HI shell is <14 Myr and in agreement with the age of the central stellar cluster derived from the FUV observations (sim 11 Myr). An estimate for the mechanical energy input from SN and stellar winds of the central stellar cluster made from FUV photometry and the derived cluster age is 4.1 x 10^52 erg, roughly a few times higher than the kinetic energy of the HI shell. The requisite energy input needed to create the HI shell, derived in the `standard' fashion from the HI observations (using the numerical models of Chevalier), is 2.6 x 10^53 erg which is almost an order of magnitude higher than the estimated energy input as derived from the FUV data. Given the overwhelming observational evidence that the central cluster is responsible for the expanding HI shell, this discrepancy suggests that the required energy input is overestimated using the `standard' method. This may explain why some other searches for remnant stellar clusters in giant HI holes have been unsuccessful so far. Our observations also show that stellar clusters are indeed able to create supergiant HI shells, even at large galactocentric radii, a scenario which has recently been questioned by a number of authors.

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arXiv2000

Extensive molecular gas in the tidal arms near NGC3077 - Birth of a dwarf galaxy?

Using the IRAM 30m radio telescope we have mapped the tidal arm feature south-east of NGC3077 where we recently detected molecular gas in the CO (J=1-0) and (2-1) transitions. We find that the molecular gas is much more extended than previously thought (several kpc). The CO emission can be separated into at least 3 distinct complexes with equivalent radii between 250pc and 700pc and all well confined over a narrow range in velocity. For one complex we have also obtained a CO (3-2) spectrum using the KOSMA 3m radio telescope; utilizing an LVG model we find that the kinetic temperature for this complex must be about 10K, and the H2 volume density between 600 and 10000cm^-3. Mass estimates based on virialization yield a total mass for the complexes of order 2-4 10^7 M_sun, i.e. more than the estimated molecular mass within NGC3077 itself. This implies that interactions between galaxies can efficiently remove heavy elements and molecules from a galaxy and enrich the intergalactic medium. A comparison of the distribution of HI and CO shows no clear correlation. However, CO is only found in regions where the HI column density exceeds 1.1 10^21 cm^-2. HI masses for the molecular complexes mapped are of the same order as the corresponding molecular masses. Since the tidal arm with its molecular complexes has all the ingredients to form stars in the future, we are thus presumably witnessing the birth of a dwarf galaxy. This process could be important for the formation of dwarf galaxies especially at larger look-back times in the universe where galaxy interactions may have been more frequent.

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arXiv1999

The Discovery of a Molecular Complex in the Tidal Arms near NGC 3077

We present the discovery of a giant molecular complex (r=350 pc, M_ vir=10^7 M_sun) in the tidal arms south-east of NGC 3077, a member of the M 81 triplet. The complex is clearly detected in the 12CO (J=1-0) transition at five independent positions. The position relative to NGC 3077, the systemic velocity (v_hel=14km/s) and the cloud averaged line width (15km/s) indicate that the object is not related to Galactic cirrus but is extragalactic. The tidal HI arm where the molecular complex is located has an total HI mass of M_HI=3x10^8 M_sun. This tidal material was presumably stripped off the outer parts of NGC 3077 during the closest encounter with M 81, about 3x10^8 years ago. After the complex detected along a torn-out spiral arm of M 81 by Brouillet et al., it is the second of its kind reported so far. Based on published optical observations, we have no evidence for on--going star formation in the newly detected molecular complex. Since the system has all the ingredients to form stars in the future, we speculate that it might eventually resemble the young dwarf galaxies in the M 81 group.

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arXiv1999

Evidence for an Expanding Molecular Superbubble in M82

We present evidence for an expanding superbubble in M82 (diameter: ~130 pc, expansion velocity: ~45 km/s, mass: ~8*10^6 SM). It is seen in the 12CO(J=1-0), 12CO(J=2-1), 13CO(J=1-0) and C18O(J=1-0) lines. The superbubble is centred around the most powerful supernova remnant in M82, 41.9+58. The CO observations show that the molecular superbubble already broke out of the disk. This scenario is supported by ROSAT HRI observations which suggest that hot coronal gas originating from inside the shell is the main contributor to the diffuse X-ray outflow in M82. We briefly discuss observations of the same region at other wavelengths (radio continuum, optical, HI, X-rays, ionized gas). From our spectral line observations, we derive a kinematic age of about 10^6 years for the superbubble. Using simple theoretical models, the total energy needed for the creation of this superbubble is of order 2*10^{54} ergs. The required energy input rate (0.001 SN yr^{-1}) is reasonable given the high supernova (SN) rate of ~0.1 SN yr^{-1} in the central part of M82. As much as 10% of the energy needed to create the superbubble is still present in form of the kinetic energy of the expanding molecular shell. Of order 10% is conserved in the hot X-ray emitting gas emerging from the superbubble into the halo of M82. This newly detected expanding molecular superbubble is believed to be powered by the same objects that also lie at the origin of the prominent X-ray outflow in M82. It can therefore be used as an alternative tool to investigate the physical properties of these sources.

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arXiv1999

Holes and Shells in the Interstellar Medium of the Nearby Dwarf Galaxy IC 2574

We present HI synthesis observations of the nearby dwarf galaxy IC 2574 (a member of the M81 group of galaxies) made with the NRAO Very Large Array at high spatial and velocity resolution (95 pc x 2.6 km/s). The VLA HI observations show a stunning amount of detail in the form of 48 mostly expanding HI shells and holes in its neutral interstellar medium. These features range in size from about 100 (a limit set by the size of the beam) to about 1000 pc dominating the appearance of the HI surface brightness map. Their dynamics clearly influence the velocity field of IC 2574. Current star formation, as traced by H-alpha emission, is predominantly found along the rims of the larger HI holes, suggesting propagating star formation. On linear scales of about 95 pc, star formation occurs if the HI surface density reaches values higher than 10^21 cm^-2. The scaleheight of the HI layer is found to be about 350 pc, considerably thicker than that in massive disk galaxies. This is due to a lower gravitational potential (for the same observed onedimensional velocity dispersion of about 7 km/s). A comparison with other galaxies shows that the energies needed to create the expanding HI structures are the same for all types of galaxies, at least to first order. The overall statistical properties of the HI holes and shells in galaxies show clear trends with Hubble type (or rather mass), such as in their diameter distribution, expansion velocities and ages.

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arXiv1999

The Violent Interstellar Medium of Nearby Dwarf Galaxies

High resolution HI observations of nearby dwarf galaxies (most of which are situated in the M 81 group at a distance of about 3.2 Mpc) reveal that their neutral interstellar medium (ISM) is dominated by hole-like features most of which are expanding. A comparison of the physical properties of these holes with the ones found in more massive spiral galaxies (such as M 31 and M 33) shows that they tend to reach much larger sizes in dwarf galaxies. This can be understood in terms of the galaxy's gravitational potential. The origin of these features is still a matter of debate. In general, young star forming regions (OB-associations) are held responsible for their formation. This picture, however, is not without its critics and other mechanism such as the infall of high velocity clouds, turbulent motions or even gamma ray bursters have been recently proposed. Here I will present one example of a supergiant shell in IC 2574 which corroborates the picture that OB associations are indeed creating these structures. This particular supergiant shell is currently the most promising case to study the effects of the combined effects of stellar winds and supernova-explosions which shape the neutral interstellar medium of (dwarf) galaxies.

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arXiv1998

X-ray Emission from an Expanding Supergiant Shell in IC 2574

We present a multi--wavelength study of a supergiant shell within the violent interstellar medium of the nearby dwarf galaxy IC 2574, a member of the M81 group of galaxies. Neutral hydrogen (HI) observations obtained with the Very Large Array (VLA) reveal a prominent expanding supergiant HI shell which is thought to be produced by the combined effects of stellar winds and supernova explosions. It measures roughly 1000 x 500 pc in size and is expanding at about 25 km/s. The HI data suggest an age of about 1.4 x 10^6 yrs; the energy input must have been of order (2.6\pm 1) x 10^53 ergs. Massive star forming regions, as traced by H$α$ emission, are situated predominantly on the rim of this HI shell. VLA radio continuum observations at 6 cm show that these star-forming regions are the main sources of radio continuum emission in this galaxy. Soft X-ray emission from within the HI hole is detected by a pointed ROSAT PSPC observation. The emission is resolved, coinciding in size and orientation with the HI shell. These spatial properties suggest that the emission is generated by an X-ray emitting plasma located within the HI shell although a contribution from X-ray binaries cannot be completely ruled out. The X-ray data are compatible with emission coming from a Raymond & Smith plasma at a temperature of about log(T[K]) = 6.8 and a density of 0.03 cm^-3.

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