Dominik J. Bomans

GaBoDS: The Garching-Bonn Deep Survey. IV. Methods for the image reduction of multi-chip cameras demonstrated on data from the ESO Wide-Field Imager

We present our image processing system for the reduction of optical imaging data from multi-chip cameras. In the framework of the Garching Bonn Deep Survey (GaBoDS; Schirmer et al. 2003) consisting of about 20 square degrees of high-quality data from WFI@MPG/ESO 2.2m, our group developed an imaging pipeline for the homogeneous and efficient processing of thislarge data set. Having weak gravitational lensing as the main science driver, our algorithms are optimised to produce deep co-added mosaics from individual exposures obtained from empty field observations. However, the modular design of our pipeline allows an easy adaption to different scientific applications. Our system has already been ported to a large variety of optical instruments and its products have been used in various scientific contexts. In this paper we give a thorough description of the algorithms used and a careful evaluation of the accuracies reached. This concerns the removal of the instrumental signature, the astrometric alignment, photometric calibration and the characterisation of final co-added mosaics. In addition we give a more general overview on the image reduction process and comment on observing strategies where they have significant influence on the data quality.

Interstellar Medium Abundances in the Pegasus Dwarf Irregular Galaxy

We report Hα imaging and optical spectrophotometry of a faint H II region in the Pegasus dwarf irregular galaxy, a low-luminosity, possible Local Group member. We find that the emission-line spectrum is consistent with excitation by a single star with a temperature of less than 35,000 K, in agreement with Aparicio & Gallart (1995). Although we cannot measure the electron temperature of the ionized gas directly, we can infer a narrow range in electron temperature by comparing the emission-line spectrum with photoionization models. Then we derive a relatively low oxygen abundance of approximately 10% of the solar value, in accordance with the well-known metallicity-luminosity relationship for dwarf irregular galaxies. We also find an N/O ratio of 6%, which is higher than the typical value of 3% found in more actively star-forming dwarf irregular galaxies. By comparing this high N/O ratio with the recent star formation history of the Pegasus dwarf irregular derived by Aparicio & Gallart, we hypothesize that this may be evidence of delayed N production.

Magnetic fields in Local Group dwarf irregulars

Aims. 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. Methods. 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 100-m Effelsberg telescope at 2.64 GHz. Three galaxies were detected. A higher frequency (4.85 GHz) was used to search for polarized emission in five dwarfs that are the most luminous ones in the infrared domain, of which three were detected. Results. Magnetic fields in LG dwarfs are weak, with a mean value of the total field strength of 6 μG) are observed only in dwarfs of extreme characteristics (e.g. NGC 4449, NGC 1569, and the LG dwarf IC 10). They are all starbursts and more evolved objects of statistically much higher metallicity and global star-formation rate than the majority of the LG dwarf population. Typical LG dwarfs are unsuitable objects for the efficient supply of magnetic fields to the intergalactic medium.

LUCIFER : a multi-mode NIR instrument for the LBT

LUCIFER (LBT NIR-Spectroscopic Utility with Camera and Integral-Field Unit for Extragalactic Research) is a NIR spectrograph and imager for the Large Binocular Telescope (LBT) on Mt. Graham, Arizona. It is built by a consortium of five German institutes and will be one of the first light instruments for the LBT. Later, a second copy for the second mirror of the telescope will follow. Both instruments will be mounted at the bent Gregorian foci of the two individual telescope mirrors. The final design of the instrument is presently in progress. LUCIFER will work at cryogenic temperature in the wavelength range from 0.9 μm to 2.5 μm. It is equipped with three exchangeable cameras for imaging and spectroscopy: two of them are optimized for seeing-limited conditions, the third camera for the diffraction-limited case with the LBT adaptive secondary mirror working. The spectral resolution will allow for OH suppression. Up to 33 exchangeable masks will be available for longslit and multi-object spectroscopy (MOS) over the full field of view (FOV). The detector will be a Rockwell HAWAII-2 HgCdTe-array.

Waiting in the wings: Reflected X-ray emission from the Homunculus nebula

We report the first detection of X-ray emission associated with the Homunculus nebula that surrounds the supermassive star η Car. The emission is characterized by a temperature in excess of 100 MK and is consistent with scattering of the time-delayed X-ray flux associated with the star. The nebular emission is bright in the northwestern lobe and near the central regions of the Homunculus, and fainter in the southeastern lobe. We also report the detection of an unusually broad Fe K fluorescent line, which may indicate fluorescent scattering off the wind of a companion star or some other high-velocity outflow. The X-ray Homunculus is the nearest member of the small class of Galactic X-ray reflection nebulae, and the only one in which both the emitting and reflecting sources are distinguishable.

EXPANSION OF THE SUPERNOVA REMNANT RCW 103

A new CCD image of the supernova remnant RCW 103 was taken in 1995 to compare with a photographic plate taken in 1970. We calculated the proper-motion expansion of the outer edge and several filaments by comparing radial slices through each of the data sets. The mean expansion rate of the outer edge is 1\farcs 8~±~0\farcs 2 per 25 years, or about 1100 km sec-1. At this velocity the remnant is most likely around 2000 years old. In addition, it must have made the transition from the double-shock phase of evolution to the point-blast phase fairly recently for the initial energy of the supernova explosion to be within a reasonable rang.

Tidal streams around galaxies in the SDSS DR7 archive - I. First results

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 ∼ 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.

LUMINOUS AND VARIABLE STARS IN M31 AND M33. II. LUMINOUS BLUE VARIABLES, CANDIDATE LBVs, Fe II EMISSION LINE STARS, AND OTHER SUPERGIANTS*

An increasing number of non-terminal eruptions are being found in the numerous surveys for optical transients. Very little is known about these giant eruptions, their progenitors and their evolutionary state. A greatly improved census of the likely progenitor class, including the most luminous evolved stars, the Luminous Blue Varaibles (LBVs), and the warm and cool hypergiants is now needed for a complete picture of the final pre-SN stages of very massive stars. We have begun a survey of the evolved and un stable luminous star populations in several nearby resolved galaxies. In this second paper on M31 and M33, we review the spectral characteristics, spectral energy distributions, circumstellar ejecta, and evidence for mass loss for 82 luminous and variable stars.We show that many of these stars have warm circumstellar dust including several of the Fe II emission line stars, but conclude that the confirmed LBVs in M31 and M33 do not. The confirmed LBVs have relatively low wind speeds even in their hot, quiescent or visual minimum state compared to the B-type supergiants and Of/WN stars which they spectroscopically resemble. The nature of the Fe II emis sion line stars and their relation to the LBV state remains uncertain, but some have properties in common with the warm hypergiants and the sgB[e] stars. Several individual stars are discussed in detail. We identify three possible candidate LBVs and three additional post-red supergiant candidates. We suggest that M33-013406.63 (UIT301,B416) is not an LBV/S Dor variable, but is a very luminous late O-type supergiant and one of the most luminous stars or pair of stars in M33.

High velocity structures in, and the X-ray emission from the LBV nebula around

The Luminous Blue Variable star η Carinae is one of the most massive stars known. It underwent a giant eruption in 1843 in which the Homunculus nebula was created. ROSAT and ASCA data indicate the existence of a hard and a soft X-ray component which appear to be spatially distinct: a softer diffuse shell of the nebula around η Carinae and a harder point-like source centered on the star η Car. Astonishingly the morphology of the X-ray emission is very different from the optical appearance of the nebula. We present a comparative analysis of optical morphology, the kinematics, and the diffuse soft X-ray structure of the nebula around η Carinae. Our kinematic analysis of the nebula shows extremely high expansion velocities. We find a strong correlation between the X-ray emission and the knots in the nebula and the largest velocities, i.e. the X-ray morphology of the nebula around η Carinae is determined by the interaction between material streaming away from η Car and the ambient medium.

\eta

We present optical integral field spectroscopy (IFS) observations of the Mice, a major merger between two massive (≳10^11 M_⊙) gas-rich spirals NGC 4676A 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 with ~1.6 kpc spatial resolution. The Mice galaxies provide a perfect case study that highlights 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 most likely induced in both galaxies. The barred spiral NGC 4676B exhibits a strong twist in both its stellar and ionised gas disk. The edge-on disk galaxy NGC 4676A appears to be bulge free, with a strong bar causing its “boxy” light profile. On the other hand, the impact of the merger on the stellar populations has been minimal thus far. By combining the IFS data with archival multiwavelength observations we show that star formation induced by the recent close passage has not contributed significantly to the total star formation rate or stellar mass of the galaxies. Both galaxies show bicones of high ionisation gas extending along their minor axes. In NGC 4676A the high gas velocity dispersion and Seyfert-like line ratios at large scaleheight indicate a powerful outflow. Fast shocks (vs ~ 350 km s^-1) extend to ~6.6 kpc above the disk plane. The measured ram pressure (P/k = 4.8 × 10^6 K cm^-3) and mass outflow rate (~8−20 M_⊙ yr^-1) are similar to superwinds from local ultra-luminous infrared galaxies, although NGC 4676A only has a moderate infrared luminosity of 3 × 10^10 L_⊙. Energy beyond what is provided by the mechanical energy of the starburst appears to be required to drive the outflow. Finally, we compare the observations to mock kinematic and stellar population maps extracted from a hydrodynamical 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.