arXiv subjects

D. J. Bomans

Publications and source records attributed to D. J. Bomans.

87 records · Page 5Linked to original sources

arXiv2006

Magnetic fields and ionized gas in nearby late type galaxies

In order to analyze the importance of the star formation rate in generating and amplifying magnetic fields in the interstellar medium we obtained deep total power and polarization maps at 4.85 GHz of three late type spiral galaxies NGC4236, NGC4656 and IC2574 using the 100-m Effelsberg radio telescope. This was accompanied by imaging in the Halpha line. We also observed these objects at 1.4 GHz to obtain their integrated fluxes at this frequency and to determine their radio spectra. All galaxies were found to possess weak but detectable total power emission at 4.85 GHz, coincident with regions of recent star formation as traced by bright Halpha regions. The surface brightness of the radio-strongest object of our sample (NGC4656) is comparable to the radio-weakest objects in a sample of more than 50 normally star-forming spiral galaxies for which measurements at 4.8 GHz with the Effelsberg radio telescope are available. The fractional polarization of the 2 galaxies of our sample is less than 2%, significantly lower than for spiral galaxies of intermediate types, suggesting that the magnetic fields are not only weaker, but also less ordered than in spiral galaxies. The radio spectra of galaxies in our small sample are indicative of a substantial fraction of thermal emission, with a higher thermal fraction than in spirals with high star formation rates (SFR), while the nonthermal emission in our sample is relatively weak compared to spiral galaxies. We propose an equipartition model where the nonthermal emission increases ~SFR^(~1.4) and the ratio of nonthermal to thermal emission increases ~SFR^(~0.4). The objects of our sample still follow the radio-FIR correlation of surface brightness of the total emission, extending it towards the lowest values measured so far.

astro-ph↗

arXiv2005

Large-scale magnetized outflows from the Virgo Cluster spiral NGC4569

Using the Effelsberg radio telescope at 4.85GHz and 8.35 GHz we discovered large symmetric lobes of polarized radio emission around the strongly HI deficient Virgo cluster spiral galaxy NGC4569. These lobes extend up to 24 kpc from the galactic disk. Our observations were complemented by 1.4 GHz continuum emission from existing HI observations. This is the first time that such huge radio continuum lobes are observed in a cluster spiral galaxy. The eastern lobe seems detached and has a flat spectrum typical for in-situ cosmic ray electron acceleration. The western lobe is diffuse and possesses vertical magnetic fields over its whole volume. The lobes are not powered by an AGN, but probably by a nuclear starburst producing >10^5 supernovae which occurred ~30 Myr ago. Since the radio lobes are symmetric, they resist ram pressure due to the galaxy's rapid motion within the intracluster medium.

astro-ph↗

arXiv2005

GaBoDS: The Garching-Bonn Deep Survey - III. Lyman-Break Galaxies in the Chandra Deep Field South

We present first results of our search for high-redshift galaxies in deep CCD mosaic images. As a pilot study for a larger survey, very deep images of the Chandra Deep Field South (CDFS), taken withWFI@MPG/ESO2.2m, are used to select large samples of 1070 U-band and 565 B-band dropouts with the Lyman-break method. The data of these Lyman-break galaxies are made public as an electronic table. These objects are good candidates for galaxies at z~3 and z~4 which is supported by their photometric redshifts. The distributions of apparent magnitudes and the clustering properties of the two populations are analysed, and they show good agreement to earlier studies. We see no evolution in the comoving clustering scale length from z~3 to z~4. The techniques presented here will be applied to a much larger sample of U-dropouts from the whole survey in near future.

astro-ph↗

arXiv2005

The star formation history of the LSB Galaxy UGC 5889

We present HST photometry of the LSB galaxy UGC 5889 and derive its recent star formation history. In the last 200 Myr the star formation proceeded in modest bursts at a rate of the order of e-2 to e-3 solar masses masses per year, with periods of extremely low SFR or even quiescence. The rate derived from the present study for the last 20 Myr is in agreement with the Halpha emission from the galaxy. The presence of a consistent population older than 200 Myr is suggested by the data. However, observational errors and completeness correction prevent any firm conclusion on the oldest age. The total mass of stars is of the order of 5.5e7 solas masses. Even if the recent episodes of star formation have heated the gas and carved a hole in the disk, blow-away of the gas is unlikely to occur.

astro-ph↗

arXiv2004

Properties and environment of the molecular complex near Holmberg IX

This paper is aimed at providing new insight into the nature and origin of the molecular complex situated near the line of sight toward Holmberg IX in the M81 group of galaxies. The first high resolution CO maps of the complex as well as single dish 13CO(1-0), 12CO(3-2) and millimeter continuum observations and the results of a survey of 12CO in the region are presented. These data together with the available HI, optical and X-ray observations are analyzed to study the properties and environment of the complex. We confirm there is no unobscured massive star formation inside the complex and from the millimeter constraint on the extinction it must have a low star formation rate or be forming only low mass stars. According to the CO line ratios the abundances and physical conditions could be similar to that of cold gas in spirals. We find from its dynamics (no rotation) and its mass (2--6 million solar masses) that it resembles a massive GMC. Also, re-inspecting N-body simulations of the M81 group and the HI data we find that it might be located inside the extreme outer disk of M81 and be cospatial with the HI feature known as Concentration I. The negative result of the CO survey suggests that the complex is unique in this region and calls for a peculiar local formation process. We find that the distribution of the CO emission in the data cube is asymmetrical in a way similar to a cometary object. The optical observations of the nearby supershell MH9/10 suggest the existence of an outflow toward the complex. We consider the possibility that the molecular complex is related to this hypothetical outflow.

astro-ph↗

arXiv2004

The environment of Low Surface Brightness Galaxies

Using the Early Data Release of the Sloan Digital Sky Survey (SDSS) we investigated the clustering properties of Low Surface Brightness (LSB) galaxies in comparison to normal, High Surface Brightness (HSB) galaxies. We selected LSB galaxies and HSB galaxies with well measured redshifts from the SDSS data base and performed three-dimensional neighbour counting analysis within spheres of radii between 0.8 Mpc and 8.0 Mpc. As a second analysis method we used an N-th neighbour analysis with N varying from one to ten galaxies. Our results show significant differences between the galaxy densities of LSB galaxies and HSB galaxies on scales from 2 to 5 Mpc. At scales larger than 5 Mpc LSB and HSB galaxies share the same clustering properties. In the pie-slice diagrams the LSB galaxies appear to favour the inner rims of filaments as defined by the HSB galaxies, with a couple of LSB galaxies even being located inside the voids. Our results support the idea of gas-rich LSB galaxies forming and developing in low density regions without many galaxy interactions and just now reaching the filaments of the large scale structure.

astro-ph↗

arXiv2003

Star formation in gaseous galaxy halos: VLT-spectroscopy of extraplanar HII-regions in NGC55

We present first deep VLT-spectra of a new class of compact extraplanar objects located in the disk-halo interface of the edge-on spiral galaxy NGC55. Their spectra reveal continuum emission from stars and show typical emission-lines as observed in ordinary disk HII-regions. Similar to the Diffuse Ionized Gas (DIG) the prominent ionization stage of oxygen is O+, whereas the corresponding one for low metallicity HII-regions is O++. A comparison with CLOUDY model simulations reveals that the ionization mechanism of these compact objects is most likely photoionization by late OB stars (O9.5 to B0). Further analysis of diagnostic diagrams unambiguously confirms the HII-region character. This raises the question whether these extraplanar HII-regions (EHRs) originated from the prominent extraplanar gas of this galaxy or have just been expelled from the disk into the halo. From hydrodynamical considerations ejection from the disk can be ruled out. Therefore, these objects must have formed within the halo. Compared to the average abundance of the central disk HII-region (45% Z_solar) both EHRs reveal substantially lower [O/H] abundances of about 10% Z_solar. We could establish for the first time strong differences in the metal content along the minor axis of this galaxy. Oxygen appears to be less abundant in the halo by about a factor of 4. Since both EHRs are located above the central part of NGC55, it appears likely, that their formation was triggered by star formation activity in the disk below. In this environment the molecular gas clouds out of which EHRs have formed can survive and collapse only in the period between two successive bursts of star formation.

astro-ph↗

arXiv2003

Magnetic Fields and Ionized Gas in the Local Group Irregular Galaxies IC10 and NGC6822

We performed a high-sensitivity search for galaxy-scale magnetic fields by radio polarimetry at 10.45GHz and 4.85GHz with the Effelsberg 100m radio telescope, accompanied by Halpha imaging, for the two Local Group irregular galaxies IC10 and NGC6822. Their star-forming bodies are small and rotate slowly. IC10 is known to have a very high star-forming activity, resembling blue compact dwarfs, while NGC6822 has a low overall star-formation level. Despite very different current star formation rates, our Halpha imaging revealed a large web of diffuse Halpha filaments and shells in both IC10 and NGC6822. Some of them extend far away from the galaxy's main body. The total power emission of both objects shows bright peaks either at the positions of optically strong star-forming clumps (IC10) or individual HII regions or supernova remnants (NGC6822). However, in both cases we detect a smoothly distributed, extended component. In IC10 we found clear evidence for the presence of a diffuse, mostly random magnetic field of ~14muG strength, probably generated by a fluctuation dynamo. One of the Halpha-emitting filaments appears to be associated with enhanced magnetic fields. We also rediscuss the reddening of IC10 and its implications for its distance. In the case of NGC6822 we found only very weak evidence for nonthermal emission, except perhaps for some regions associated with local gas compression. We detect in both galaxies small spots of polarized emission, indicative of regular fields (~3muG), at least partly associated with local compressional phenomena.

astro-ph↗

arXiv1999

ORFEUS II echelle spectra: On the H_2/CO ratio in the LMC towards LH 10

ORFEUS far UV echelle spectra have been used to investigate HI, H_2 and CO absorption lines along the line of sight towards LH 10:3120 in the LMC, extending the study presented by de Boer et al. (1998). While H_2 absorption is clearly visible, no CO absorption at LMC velocities is detected, but an upper limit of N(CO) < 3.3 10^13 cm^-2 for the CO column density is derived for the C-X band near 1088 A. The detected H_2 absorption features were used to determine a limit for the H_2/CO ratio of N(H_2)/N(CO) > 2.0 10^5 for the LMC gas along this individual line of sight. Generally, the fraction of gas in molecular form in the LMC is low compared to interstellar gas in the Milky Way with the same total gas quantity. We compare the absorption spectroscopy with SEST CO emission line measurements. It is found that most of the CO emission comes from gas behind LH 10:3120. We discuss our results in view of the possible scenario, in which the low dust content limits the amount of molecular hydrogen in the diffuse LMC gas.

astro-ph↗

arXiv1998

ORFEUS II echelle spectra: Absorption by H_2 in the LMC

We present the first detection of molecular hydrogen (H_2) UV absorption profiles on the line of sight to the LMC. The star LH 10:3120 in the LMC was measured with the ORFEUS telescope and the Tuebingen echelle spectrograph during the space shuttle mission of Nov./Dec. 1996. 16 absorption lines from the Lyman band are used to derive the column densities of H_2 for the lowest 5 rotational states in the LMC gas. For these states we find a total column density of N(H_2)=6.6 x 10^18$ cm^-2 on this individual line of sight. We obtain equivalent excitation temperatures of T < 50 K for the rotational ground state and T = 470 K for 0 < J < 6 by fitting the population densities of the rotational states to theoretical Boltzmann distributions. We conclude that UV pumping dominates the population of the higher rotational levels, as known from the H_2 gas in the Milky Way. (Research supported in part by the DARA)

astro-ph↗

arXiv1997

NGC 1427A - an LMC type galaxy in the Fornax Cluster

We have discovered that the Fornax irregular galaxy NGC 1427A is in very many respects a twin of the Large Magellanic Cloud. Based on B, V, I, and H alpha images, we find the following. The light of the galaxy is dominated by high surface brightness regions in the south-west that are superimposed by a half-ring of OB associations and H II regions indicating recent star formation. The colors of the main stellar body are (V-I) = 0.8 mag and (B-V) = 0.4 mag, comparable to the LMC colors. A low surface brightness cloud north of the main body as well as a LSB tail in the west have colors (V-I = 0.2 mag) that are more typical for blue compact dwarf galaxies. We identified a system of cluster candidates with mean ages < 2 Gyr (assuming a LMC metallicity) comprising the richest cluster system in an irregular galaxy observed up to now outside the Local Group. In X-ray wavelengths NGC 1427A appears with a relatively soft and complex spectrum.

astro-ph↗

arXiv1997

Two Ring Nebulae around Blue Supergiants in the Large Magellanic Cloud

Ring nebulae are often found around massive stars such as Wolf-Rayet stars, OB and Of stars and Luminous Blue Variables (LBVs). In this paper we report on two ring nebulae around blue supergiants in the Large Magellanic Cloud. The star Sk-69 279 is classified as O9f and is surrounded by a closed shell with a diameter of 4.5 pc. Our echelle observations show an expansion velocity of 14 km/s and a high [N II]6583A/H alpha ratio. This line ratio suggests nitrogen abundance enhancement consistent with those seen in ejectas from LBVs. Thus the ring nebula around Sk-69 279 is a circumstellar bubble. The star Sk-69 271, a B2 supergiant, is surrounded by an H alpha arc resembling an half shell. Echelle observations show a large expanding shell with the arc being part of the approaching surface. The expansion velocity is about 24 km/s and the [N II]6583A/H alpha is not much higher than that of the background emission. The lack of nitrogen abundance anomaly suggests that the expanding shell is an interstellar bubble with a dynamic age of 2 10^5 yr.

astro-ph↗

arXiv1997

The bipolar structure of the LBV nebula around HR Carinae

HR Carinae is one of the few Luminous Blue Variables (LBVs) in the Galaxy. It has a nebula that appears bipolar. We have obtained imaging and high-dispersion, long-slit echelle data of the HR Car nebula, and confirmed that it is a bipolar nebula. Its polar axis lies along the position angle of 125 +- 5 degree; each lobe has, at a distance of 5 kpc a diameter of about 0.65 pc and a line-of-sight expansion velocity of 75-150 km/sec. Beside the expanding bipolar lobes, a number of [NII]-bright knots are detected. These knots have lower expansion velocities than the lobes and are detected only within the projected boundary of the lobes. These knots are most likely nitrogen-enriched material ejected by HR Car. On a larger scale, a funnel-shaped nebula is detected at 2.5" northwest of HR Car. The axis of the funnel is roughly aligned with the polar axis of the HR Car nebula, suggesting that HR Car may be responsible for the ionization and shaping of this nebula. Future observations of kinematics and abundances are needed to determine the nature of this nebula. We propose that the bipolar nebula of HR Car is a more evolved version of the Homunculus Nebula around eta Car. The recently developed theory of wind-compressed disks may explain the higher density of the equatorial plane and the formation of bipolar nebulae of LBVs.

astro-ph↗

arXiv1997

Hot Interstellar Gas in the Irregular Galaxy NGC 4449

NGC 4449 is an irregular galaxy with a moderately high star formation activity. The massive stars in NGC 4449 have given rise to many bright HII regions, superbubbles, supergiant shells, and "chimney-like" radial filaments. ROSAT X-ray observations of NGC 4449 have revealed four point-like sources and a wide-spread diffuse emission. The spectral properties of the diffuse component suggest that the emission originates from hot interstellar gas. We have compared deep ground-based H-alpha images with the X-ray images of NGC 4449 to determine the relationship between the hot (10^6 K) and the warm (10^4 K) components of the interstellar gas. We have also used an archival Hubble Space Telescope WFPC2 image of NGC 4449 taken through the F606W filter to examine the massive stellar content of the X-ray-emission regions. We find that hot interstellar gas exists in (1) active star forming regions, including the giant H II region CM 16, (2) probable outflows from star forming regions, and (3) the supergiant shell SGS2. The X-ray data have been used to derive the rms electron density, mass, and thermal energy of the hot interior of SGS2. Finally we discuss the origin of SGS2 and implications of the detection of diffuse X-rays in irregular galaxies.

astro-ph↗

arXiv1996

CIV absorption from hot gas inside supergiant shell LMC4 observed with HST and IUE

High resolution ultraviolet spectra with HST-GHRS of two stars in the direction of the supergiant shell LMC4 unambiguously show absorption by substantial quantities of CIV gas at velocities near the the systemic velocity of the LMC. In combination with the detection of diffuse X-rays from the LMC4 by ROSAT and other supporting data, this demonstrates that the interior of LMC4 is filled with tenuous hot gas. CIV interstellar absorption is seen over a large velocity range, having at least 2 components at about 280 and 320 km/s. The strong component at 280 km/s has a width of 40 km/s and a column density in the order of 3 times 10^13 cm^-2. The width of the absorption is best explained by bulk motions of CIV-containing gas clouds inside LMC4. These hot clouds or layers around cold clouds have to have a relatively high filling factor inside LMC4 to fit the observations. The characteristics of the CIV gas component at 320 km/s are such that they trace a blast wave from a recent supernova within the LMC4 cavity. Galactic CIV absorption is also present, as to be expected for these lines of sight.

astro-ph↗