arXiv subjects

Dominik J. Bomans

Publications and source records attributed to Dominik J. Bomans.

At least 37 records · Page 2Linked to original sources

arXiv2015

Shock structures of astrospheres

The interaction between a supersonic stellar wind and a (super-)sonic interstellar wind has recently been viewed with new interest. We here first give an overview of the modeling, which includes the heliosphere as an example of a special astrosphere. Then we concentrate on the shock structures of fluid models, especially of hydrodynamic (HD) models. More involved models taking into account radiation transfer and magnetic fields are briefly sketched. Even the relatively simple HD models show a rich shock structure, which might be observable in some objects. We employ a single fluid model to study these complex shock structures, and compare the results obtained including heating and cooling with results obtained without these effects. Furthermore, we show that in the hypersonic case valuable information of the shock structure can be obtained from the Rankine-Hugoniot equations. We solved the Euler equations for the single fluid case and also for a case including cooling and heating. We also discuss the analytical Rankine-Hugoniot relations and their relevance to observations. We show that the only obtainable length scale is the termination shock distance. Moreover, the so-called thin shell approximation is usually not valid. We present the shock structure in the model that includes heating and cooling, which differs remarkably from that of a single fluid scenario in the region of the shocked interstellar medium. We find that the heating and cooling is mainly important in this region and is negligible in the regions dominated by the stellar wind beyond an inner boundary.

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arXiv2014

The Mice at play in the CALIFA survey: A case study of a gas-rich major merger between first passage and coalescence

We present optical integral field spectroscopy (IFS) observations of the Mice, a major merger between two massive (>10^11Msol) gas-rich spirals NGC4676A and B, observed between first passage and final coalescence. The spectra provide stellar and gas kinematics, ionised gas properties and stellar population diagnostics, over the full optical extent of both galaxies. The Mice provide a perfect case study highlighting the importance of IFS data for improving our understanding of local galaxies. The impact of first passage on the kinematics of the stars and gas has been significant, with strong bars likely induced in both galaxies. The barred spiral NGC4676B exhibits a strong twist in both its stellar and ionised gas disk. On the other hand, the impact of the merger on the stellar populations has been minimal thus far: star formation induced by the recent close passage has not contributed significantly to the global star formation rate or stellar mass of the galaxies. Both galaxies show bicones of high ionisation gas extending along their minor axes. In NGC4676A the high gas velocity dispersion and Seyfert-like line ratios at large scaleheight indicate a powerful outflow. Fast shocks extend to ~6.6kpc above the disk plane. The measured ram pressure and mass outflow rate (~8-20Msol/yr) are similar to superwinds from local ULIRGs, although NGC4676A has only a moderate infrared luminosity of 3x10^10Lsol. Energy beyond that provided by the mechanical energy of the starburst appears to be required to drive the outflow. We compare the observations to mock kinematic and stellar population maps from a merger simulation. The models show little enhancement in star formation during and following first passage, in agreement with the observations. We highlight areas where IFS data could help further constrain the models.

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arXiv2013

The kinematics of the diffuse ionized gas in NGC 4666

The global properties of the interstellar medium with processes such as infall and outflow of gas and a large scale circulation of matter and its consequences for star formation and chemical enrichment are important for the understanding of galaxy evolution. In this paper we studied the kinematics and morphology of the diffuse ionized gas (DIG) in the disk and in the halo of the star forming spiral galaxy NGC 4666 to derive information about its kinematical properties. Especially, we searched for infalling and outflowing ionized gas. We determined surface brightness, radial velocity, and velocity dispersion of the warm ionized gas via high spectral resolution (R ~ 9000) Fabry-Pérot interferometry. This allows the determination of the global velocity field and the detection of local deviations from this verlocity field. We calculated models of the DIG distribution and its kinematics for comparison with the measured data. In this way we determined fundamental parameters such as the inclination and the scale height of NGC 4666, and established the need for an additional gas component to fit our observed data. We found individual areas, especially along the minor axis, with gas components reaching into the halo which we interpret as an outflowing component of the diffuse ionized gas. As the main result of our study, we were able to determine that the vertical structure of the DIG distribution in NGC 4666 is best modeled with two components of ionized gas, a thick and a thin disk with 0.8 kpc and 0.2 kpc scale height, respectively. Therefore, the enhanced star formation in NGC 4666 drives an outflow and also maintains a thick ionized gas layer reminiscent of the Reynold's layer in the Milky Way.

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arXiv2012

X-ray stacking of Lyman break galaxies in the 4\,Ms CDF-S - X-ray luminosities and star formation rates across cosmic time

Lyman Break Galaxies (LBGs) are widely thought to be prototypical young galaxies in the early universe, particularly representative of those undergoing massive events of star formation. Therefore, LBGs should produce significant amounts of X-ray emission. We aim to trace the X-ray luminosity of Lyman Break Galaxies across cosmic time and from that derive constraints on their star formation history. We utilize the newly released 4 Ms mosaic obtained with the Chandra X-ray Observatory, the deepest X-ray image to date, alongside with the superb spectroscopic data sets available in the CDF-S survey region to construct large but nearly uncontaminated samples of LBGs across a wide range of redshift (0.5 < z < 4.5) which can be used as input samples for stacking experiments. This approach allows us to trace the X-ray emission of Lyman Break Galaxies to even lower, previously unreachable, flux density limits (~10^-18 mW m^-2) and therefore to larger redshifts. We reliably detect soft-band X-ray emission from all our input redshift bins except for the highest redshift (z~4) one. From that we derive rest-frame 2-10 keV luminosities and infer star formation rates and stellar masses. We find that star formation in LBGs peaks at a redshift of z_peak~3.5 and then decreases quickly. We also see a characteristic peak in the specific star formation rate (sSFR=SFR/M_*) at this redshift. Furthermore, we calculate the contribution of LBGs to the total cosmic star formation rate density (SFRD) and find that the contribution of LBGs is negligible. Therefore, we conclude that most of the star formation in the early universe takes place in lower luminosity galaxies as suggested by hierarchical structure formation models.

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arXiv2012

On the nature of the peculiar superthin galaxy UGC 12281

UGC 12281 has been classified as having a pure disk and being a low surface brightness galaxy (LSBG), thus being an obvious member of the so-called superthin galaxies. At the same time it represents an extremely untypical type of LSBG due to its remarkable amount of current star formation and evidence for extraplanar ionized gas. This makes it become a perfect tool to investigate the triggering of star formation in LSB galaxies, located in an alleged isolated area. By means of deep photometry and long-slit spectroscopy we analyse the H$α$ halo and verify the existence of a potential dwarf companion which we found on processed SDSS images.

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arXiv2011

Supernovae without host galaxy? - Hypervelocity stars in foreign galaxies

Harvesting the SAI supernova catalog, we search for SNe that apparently do not occur within a distinct host galaxy but lie a great distance apart from their assigned host galaxy. Assuming two possible explanations for this host-lessness of a fraction of reported SNe, namely (i) a host galaxy which is too faint to be detected within the limits of currently available surveys or (ii) a hypervelocity star (HVS) as progenitor of the SN,we want to distinguish between these two cases. To do so, we use deep imaging to test explanation (i). If within our detection limit of 27 mag/arcsec^2, the central surface brightness of the faintest known LSB galaxy so far, no galaxy could be identified, we discard this explanation and regard the SN, after several other checks, to have had a hypervelocity star progenitor. Analyzing a selected subsample of five host-less SNe we find one, SN 2006bx in UGC5434, to be put in the hypervelocity progenitor category with a high probability, exhibiting a projected velocity of > 800 km/s. SN 1969L in NGC1058 is most likely an example for a very extended star-forming disk visible only in the far-UV, not in the optical wavebands. Therefore this SN is clearly due to in situ star formation. This mechanism may also apply for two other SNe we investigated (SN 1970L and SN 1997C), but this cannot be determined with final certainty. Another one, SN 2005nc associated with a gamma-ray burst (GRB 050525), is a special case not covered by our initial assumptions. Even with deep Hubble data, a host galaxy could not be unambiguously identified.

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arXiv2011

Tidal streams around galaxies in the SDSS DR7 archive

Context. Models of hierarchical structure formation predict the accretion of smaller satellite galaxies onto more massive systems and this process should be accompanied by a disintegration of the smaller companions visible, e.g., in tidal streams. Aims. In order to verify and quantify this scenario we have developed a search strategy for low surface brightness tidal structures around a sample of 474 galaxies using the Sloan Digital Sky Survey DR7 archive. Methods. Calibrated images taken from the SDSS archive were processed in an automated manner and visually inspected for possible tidal streams. Results. We were able to extract structures at surface brightness levels ranging from \sim 24 down to 28 mag arcsec-2. A significant number of tidal streams was found and measured. Their apparent length varies as they seem to be in different stages of accretion. Conclusions. At least 6% of the galaxies show distinct stream like features, a total of 19% show faint features. Several individual cases are described and discussed.

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arXiv2011

Magnetic fields in Local Group dwarf irregulars

We wish to clarify whether strong magnetic fields can be effectively generated in typically low-mass dwarf galaxies and to assess the role of dwarf galaxies in the magnetization of the Universe. We performed a search for radio emission and magnetic fields in an unbiased sample of 12 Local Group (LG) irregular and dwarf irregular galaxies with the 100m Effelsberg telescope at 2.64 and 4.85GHz. Magnetic fields in LG dwarfs are three times weaker than in the normal spirals (<4.2+-1.8muG). The production of total magnetic fields appears to be regulated mainly by the star-formation surface density, with the power-law exponent of 0.30+-0.04, or by the gas surface density (with the exponent 0.47+-0.09). In addition, we find systematically stronger fields in objects of higher global star-formation rate. The dwarf galaxies follow a similar far-infrared relationship (with a slope of 0.91+-0.08) to that determined for high surface brightness spiral galaxies. The magnetic field strength in dwarf galaxies does not correlate with their maximum rotational velocity, indicating a small-scale rather than a large-scale dynamo process. If magnetization of the Universe by galactic outflows is coeval with its metal enrichment, we show that more massive objects (such as Lyman Break Galaxies) can efficiently magnetize the intergalactic medium with a magnetic field strength of about 0.8nG out to a distance of 160-530kpc at redshifts 5-3, respectively. Several times weaker fields and shorter magnetization distances are expected from primordial dwarf galaxies. We also predict that most star-forming local dwarfs might have magnetized their surroundings up to about 0.1muG within about 5kpc distance. Strong magnetic fields (>6muG) are observed only in dwarfs of extreme characteristics while typical LG dwarfs are unsuitable objects for the efficient supply of magnetic fields to the intergalactic medium.

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arXiv2008

Compact stellar systems in the Fornax cluster: a UV perspective

In recent years, increasing evidence for chemical complexity and multiple stellar populations in massive globular clusters (GCs) has emerged, including extreme horizontal branches (EHBs) and UV excess. Our goal is to improve our understanding of UV excess in the regime of both massive GCs and ultra-compact dwarf galaxies (UCDs). To this end, we use deep archival GALEX data of the central Fornax cluster to measure NUV and FUV magnitudes of UCDs and massive GCs. We obtain NUV photometry for a sample of 35 compact objects with -13.5<M_V<-10 mag. Of those, 21 objects also have FUV photometry. Roughly half of the sources fall into the UCD luminosity regime (M_V <=-11 mag). We find that seven out of 17 massive Fornax GCs exhibit a NUV excess with respect to expectations from stellar population models, even for models with enhanced Helium abundance. This suggests that not only He-enrichment has contributed to forming the EHB population of these GCs. The GCs extend to stronger UV excess than GCs in M31 and massive GCs in M87, at the 97% confidence level. Most of the UCDs with FUV photometry also show evidence for UV excess, but their UV colours can be matched by isochrones with enhanced Helium abundances and old ages 12-14 Gyrs. We find that Fornax compact objects with X-ray emission detected from Chandra images are almost disjunct in colour from compact objects with GALEX UV detection, with only one X-ray source among the 35 compact objects. However, since this source is one of the three most UV bright GCs, we cannot exclude that the physical processes causing X-ray emission also contribute to some of the observed UV excess.

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arXiv2006

An optical search for Low Surface Brightness Galaxies in the Arecibo HI Strip Survey

In order to estimate the contribution of low surface brightness (LSB) galaxies to the local (z<0.1) galaxy number density, we performed an optical search for LSB candidates in a 15.5 deg^2 part of the region covered by the 65 deg^2 blind Arecibo HI Strip Survey (AHISS). Object detection and galaxy profile fitting were done with analytical algorithms. The detection efficiency and the selection effects were evaluated using large samples of artificial galaxies. Our final catalogue is diameter-limited and contains 306 galaxies with diameters >18" at the limiting surface brightness of mu_{B,lim}=25.2+-0.31 mag/arcsec^2. Of these 306 galaxies, 148 were not catalogued previously. Our results indicate that low surface brightness galaxies contribute at least to 30 % to the local galaxy number density. Without additional distance information, choosing the limiting diameter and the surface brightness at which the diameter is measured is crucial. Depending on these choices, diameter-limited optical catalogues are either biased against LSB galaxies, or contaminated with cosmologically dimmed high surface brightness galaxies, which affects the implied surface brightness distribution. The comparison to the AHISS showed that although optical surveys detect more galaxies per deg^2 than HI surveys, their drawback is the need for spectroscopic follow up observations to derive distances. Blind HI surveys have no diameter limits, but tend to miss gas-poor galaxies and all galaxies which lie outside their redshift limits. HI and optical surveys thus provide complementary information and sample different parts of the LSB galaxy population.

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arXiv2005

To see or not to see a Bow Shock: Identifying Bow Shocks with H-Alpha Allsky Surveys

OB-stars have the highest luminosities and strongest stellar winds of all stars, which enables them to interact strongly with their surrounding ISM, thus creating bow shocks. These offer us an ideal opportunity to learn more about the ISM. They were first detected and analysed around runaway OB-stars using the IRAS allsky survey by van Buren et al. (1995). Using the geometry of such bow shocks information concerning the ISM density and its fluctuations can be gained from such infrared observations. As to help to improve the bow shock models, additional observations at other wavelengths, e.g. H-Alpha, are most welcome. However due to their low velocity these bow shocks have a size of ~1 degrees, and could only be observed as a whole with great difficulties. In the light of the new H-Alpha allsky surveys (SHASSA/VTSS) this is no problem any more. We developed different methods to detect bow shocks, e.g. the improved determination of their symmetry axis with radial distance profiles. Using two H-Alpha-allsky surveys (SHASSA/VTSS), we searched for bow shocks and compared the different methods. From our sample we conclude, that the correlation between the direction of both proper motion and the symmetry axis determined with radial distance profile is the most promising detection method. We found eight bow shocks around HD 17505, HD 24430, HD 48099, HD 57061, HD 92206, HD 135240, HD 149757, and HD 158186 from 37 candidates taken from van Buren et al. (1995). Additionally to the traditional determination of ISM parameters using the standoff distance of the bow shock, another approach was chosen, using the thickness of the bow-shock layer. Both methods lead to the same results, yielding densities (~1 cm^{-3}) and the maximal temperatures (~10^4 K), that fit well to the up-to-date picture of the Warm Ionised Medium.

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arXiv2005

A spectroscopic event of eta Car viewed from different directions: The data and first results

We present spectroscopic observations with high spectral resolution of eta Car as seen by the SE lobe of the Homunculus nebula over the 2003.5 "spectroscopic event". The observed spectra represent the stellar spectrum emitted near the pole of the star and are much less contaminated with nebular emission lines than direct observations of the central object. The "event" is qualitatively similar near the pole to what is observed in direct spectra of the star (more equator-on at 45 degree), but shows interesting differences. The observations show that the equivalent width changes of H alpha emission and other lines are less pronounced at the pole than in the line of sight. Also the absorption components appear less variable. A pronounced high-velocity absorption is present near the event in the He I lines indicating a mass-ejection event. This feature is also seen, but less pronounced, in the hydrogen lines. HeII4686 emission is observed for a brief period of time near the event and appears, if corrected for light travel time, to precede similar emission in the direct view. Our observations indicate that the event is probably not only a change in ionization and excitation structure or a simple eclipse-like event.

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arXiv2004

VLT-UVES observations of the Balmer line variations of eta Carinae during the 2003 spectroscopic event

We present high spectral resolution echelle observations of the Balmer line variations during the 2003.5 ``spectroscopic event'' of eta Carinae. Spectra have been recorded of both eta Carinae and the Homunculus at the FOS4 position in its SE lobe. This spot shows a reflected stellar spectrum which is less contaminated by nebular emission lines than ground-based observations of the central object itself. Our observations show that the spectroscopic event is much less pronounced at this position than when seen directly on eta Car using HST/STIS. Assuming that the reflected spectrum is indeed latitude dependent this indicates that the spectral changes during the event seen pole-on (FOS4) are different from those closer to the equator (directly on the star). In contrast to the spectrum of the star, the scattered spectrum of FOS4 always shows pronounced P Cygni absorption with little variation across the ``spectroscopic event''. After that event an additional high-velocity absorption component appears. The emission profile is more peaked at FOS4 and consists of at least 3 distinct components, of which the reddest one shows the strongest changes through the event. The data seem to be compatible with changes in latitudinal wind structure of a single star, with or without the help of a secondary star, or the onset of a shell ejection during the spectroscopic event.

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arXiv2004

SN2002kg -- the brightening of LBV V37 in NGC 2403

SN2002kg is a type IIn supernova, detected in October 2002 in the nearby spiral galaxy NGC 2403. We show that the position of SN2002kg agrees within the errors with the position of the LBV V37. Ground based and HST ACS images however show that V37 is still present after the SN2002kg event. We compiled a lightcurve of V37 which underlines the variablity of the object, and shows that SN2002kg was the brightening of V37 and not a supernova. The recent brightening is not a giant eruption, but more likely part of an S Dor phase. V37 shows strong Halpha +[NII] emission in recent images and in the SN2002kg spectrum, which we interprete as the signature of the presence of an LBV nebula. A historic spectrum lacks emission, which may hint that we are witnessing the formation of an LBV nebula.

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arXiv2004

Probing The Multiphase Interstellar Medium Of The Dwarf Starburst Galaxy NGC 625 With FUSE Spectroscopy

We present new FUSE spectroscopy of the dwarf starburst galaxy NGC 625. These observations probe multiple phases of the interstellar medium, including the coronal, ionized, neutral and molecular gas. This nearby (D = 3.9 +/- 0.2 Mpc) system shows a clear detection of outflowing coronal gas as traced by OVI 1032 Angstrom absorption. The centroid of the OVI profile is blueshifted with respect to the galaxy systemic velocity by ~ 30 km/sec, suggesting a low-velocity outflow. The implied OVI velocity extent is found to be 100 +/- 20 km/sec, which is fully consistent with the detected HI outflow velocity found in radio synthesis observations. We detect multiple lines of diffuse H2 absorption from the ISM of NGC 625; this is one of only a few extragalactic systems with FUSE detections of H2 lines in the Lyman and Werner bands. We find a potential abundance offset between the neutral and nebular gas that exceeds the errors on the derived column densities. Since such an offset has been found in multiple dwarf galaxies, we discuss the implications of a lower-metallicity halo surrounding the central star forming regions of dwarf galaxies. The apparent offset may be due to saturation of the observed OI line, and higher S/N observations are required to resolve this issue.

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arXiv2004

BVR photometry of the resolved dwarf galaxy Ho IX

We present BVR CCD photometry down to limiting magnitude B=23.5 mag for 232 starlike objects and 11 diffuse objects in a 5.4' x 5.4' field of Ho IX. The galaxy is a gas-rich irregular dwarf galaxy possibly very close to M 81, which makes it especially interesting in the context of the evolution of satellite galaxies and the accretion of dwarf galaxies. Investigations of Ho IX were hampered by relatively large contradictions in the magnitude scale between earlier studies. With our new photometry we resolved these discrepancies. The color magnitude diagram (CMD) of Ho IX is fairly typical of a star-forming dwarf irregular, consistent with earlier results. Distance estimates from our new CMD are consistent with Ho IX being very close to M 81 and therefore being a definite member of the M 81 group, apparently in very close physical proximity to M 81.

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arXiv2003

A spectral and spatial analysis of eta Carinae's diffuse X-ray emission using CHANDRA

The luminous unstable star (star system) eta Carinae is surrounded by an optically bright bipolar nebula, the Homunculus and a fainter but much larger nebula, the so-called outer ejecta. As images from the EINSTEIN and ROSAT satellites have shown, the outer ejecta is also visible in soft X-rays, while the central source is present in the harder X-ray bands. With our CHANDRA observations we show that the morphology and properties of the X-ray nebula are the result of shocks from fast clumps in the outer ejecta moving into a pre-existing denser circumstellar medium. An additional contribution to the soft X-ray flux results from mutual interactions of clumps within the ejecta. Spectra extracted from the CHANDRA data yield gas temperatures kT of 0.6-0.76 keV. The implied pre-shock velocities of 670-760 km/s are within the scatter of the velocities we measure for the majority of the clumps in the corresponding regions. Significant nitrogen enhancements over solar abundances are needed for acceptable fits in all parts of the outer ejecta, consistent with CNO processed material and non-uniform enhancement. The presence of a diffuse spot of hard X-ray emission at the S condensation shows some contribution of the highest velocity clumps and further underlines the multicomponent, non-equilibrium nature of the X-ray nebula. The detection of an X-ray ``bridge'' between the northern and southern part of the X-ray nebula and an X-ray shadow at the position of the NN bow can be attributed to a large expanding disk, which would appear as an extension of the equatorial disk. No soft emission is seen from the Homunculus, or from the NN bow or the ``strings''.

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arXiv2003

The Recent Evolution of the Dwarf Starburst Galaxy NGC 625 from Hubble Space Telescope Imaging

New HST/WFPC2 imaging of the dwarf starburst galaxy NGC 625 is presented. These data, 80% complete to V and I magnitudes of 26.0 and 25.3, respectively, allow us to study the recent star formation history of NGC 625. We derive a tip of the red giant branch (TRGB) distance modulus of 27.95+/-0.07, corresponding to a distance of 3.89+/-0.22 Mpc, and a location on the far side of the Sculptor Group. NGC 625 has a well-defined radial stellar population gradient, evidenced by a central concentration of young MS stars and an RGB/AGB ratio that increases with galactocentric distance. The prominent AGB is very red, and RGB stars are detected far from the central star forming regions. Using H Alpha and H Beta narrow band imaging and previous optical spectroscopy we identify substantial and varying internal extinction (A_V = 0.0 to 0.6 mag) associated with the central active star formation regions. To better understand the effects of internal extinction on the analysis of young stellar populations, synthetic models are presented which, for the first time, examine and account for this effect. Using the luminous blue helium burning stars, we construct a simple model of the recent (< 100 Myr) star formation in which an elevated but declining star formation rate has been present over this entire period. This is at odds with the presence of spectroscopic W-R features in the major star formation region which imply a short duration (<= 5 Myr) for the recent starburst. This suggests that starbursts displaying W-R features are not necessarily all of a short duration. Finally, we speculate on the possible causes of the present burst of star formation in this apparently isolated galaxy, and compare it to other nearby, well-studied dwarf starburst systems.

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