Engineering topics

Heesen, Volker

Publications and source records attributed to Heesen, Volker.

5 records

DOE OSTI2016

The magnetized galactic wind and synchrotron halo of the starburst dwarf galaxy IC 10

We aim to explore whether strong magnetic fields can be effectively generated in low-mass dwarf galaxies and, if so, whether such fields can be affected by galactic outflows and spread out into the intergalactic medium (IGM). We performed a radio continuum polarimetry study of IC 10, the nearest starbursting dwarf galaxy, using a combination of multifrequency interferometric (VLA) and single-dish (Effelsberg) observations. VLA observations at 1.43 GHz reveal an extensive and almost spherical radio halo of IC 10 in total intensity, extending twice more than the infrared-emitting galactic disk. The halo is magnetized with a magnetic field strength of 7 μG in the outermost parts. Locally, the magnetic field reaches about 29 μG in H ii complexes, becomes more ordered, and weakens to 22 μG in the synchrotron superbubble and to 7–10 μG within H i holes. At the higher frequency of 4.86 GHz, we found a large-scale magnetic field structure of X-shaped morphology, similar to that observed in several edge-on spiral galaxies. The X-shaped magnetic structure can be caused by the galactic wind, advecting magnetic fields injected into the interstellar medium by stellar winds and supernova explosions. The radio continuum scale heights at 1.43 GHz indicate the bulk speed of cosmic-ray electrons outflowing from H ii complexes of about 60 km s{sup −1}, exceeding the escape velocity of 40 km s{sup −1}. Hence, the magnetized galactic wind in IC 10 inflates the extensive radio halo visible at 1.43 GHz and can seed the IGM with both random and ordered magnetic fields. These are signatures of intense material feedback onto the IGM, expected to be prevalent in the protogalaxies of the early universe.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

HERSCHEL SPECTROSCOPIC OBSERVATIONS OF LITTLE THINGS DWARF GALAXIES

We present far-infrared (FIR) spectral line observations of five galaxies from the Little Things sample: DDO 69, DDO 70, DDO 75, DDO 155, and WLM. While most studies of dwarfs focus on bright systems or starbursts due to observational constraints, our data extend the observed parameter space into the regime of low surface brightness dwarf galaxies with low metallicities and moderate star formation rates. Our targets were observed with Herschel at the [C ii] 158 μm, [O i] 63 μm, [O iii] 88 μm, and [N ii] 122 μm emission lines using the PACS Spectrometer. These high-resolution maps allow us for the first time to study the FIR properties of these systems on the scales of larger star-forming complexes. The spatial resolution in our maps, in combination with star formation tracers, allows us to identify separate photodissociation regions (PDRs) in some of the regions we observed. Our systems have widespread [C ii] emission that is bright relative to continuum, averaging near 0.5% of the total infrared (TIR) budget—higher than in solar-metallicity galaxies of other types. [N ii] is weak, suggesting that the [C ii] emission in our galaxies comes mostly from PDRs instead of the diffuse ionized interstellar medium (ISM). These systems exhibit efficient cooling at low dust temperatures, as shown by ([O i]+[C ii])/TIR in relation to 60 μm/100 μm, and low [O i]/[C ii] ratios which indicate that [C ii] is the dominant coolant of the ISM. We observe [O iii]/[C ii] ratios in our galaxies that are lower than those published for other dwarfs, but similar to levels noted in spirals.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

High-resolution mass models of dwarf galaxies from LITTLE THINGS

We present high-resolution rotation curves and mass models of 26 dwarf galaxies from “Local Irregulars That Trace Luminosity Extremes, The H i Nearby Galaxy Survey” (LITTLE THINGS). LITTLE THINGS is a high-resolution (∼6″ angular; <2.6 km s{sup −1} velocity resolution) Very Large Array H i survey for nearby dwarf galaxies in the local volume within 11 Mpc. The high-resolution H i observations enable us to derive reliable rotation curves of the sample galaxies in a homogeneous and consistent manner. The rotation curves are then combined with Spitzer archival 3.6 μm and ancillary optical U, B, and V images to construct mass models of the galaxies. This high quality multi-wavelength data set significantly reduces observational uncertainties and thus allows us to examine the mass distribution in the galaxies in detail. We decompose the rotation curves in terms of the dynamical contributions by baryons and dark matter (DM) halos, and compare the latter with those of dwarf galaxies from THINGS as well as ΛCDM Smoothed Particle Hydrodynamic (SPH) simulations in which the effect of baryonic feedback processes is included. Being generally consistent with THINGS and simulated dwarf galaxies, most of the LITTLE THINGS sample galaxies show a linear increase of the rotation curve in their inner regions, which gives shallower logarithmic inner slopes α of their DM density profiles. The mean value of the slopes of the 26 LITTLE THINGS dwarf galaxies is α=−0.32±0.24 which is in accordance with the previous results found for low surface brightness galaxies (α=−0.2±0.2) as well as the seven THINGS dwarf galaxies (α=−0.29±0.07). However, this significantly deviates from the cusp-like DM distribution predicted by DM-only ΛCDM simulations. Instead our results are more in line with the shallower slopes found in the ΛCDM SPH simulations of dwarf galaxies in which the effect of baryonic feedback processes is included. In addition, we discuss the central DM distribution of DDO 210 whose stellar mass is relatively low in our sample to examine the scenario of inefficient supernova feedback in low mass dwarf galaxies predicted from recent ΛCDM SPH simulations of dwarf galaxies where central cusps still remain.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

The radio continuum-star formation rate relation in WSRT sings galaxies

We present a study of the spatially resolved radio continuum-star formation rate (RC-SFR) relation using state-of-the-art star formation tracers in a sample of 17 THINGS galaxies. We use SFR surface density (Σ{sub SFR}) maps created by a linear combination of GALEX far-UV (FUV) and Spitzer 24 μm maps. We use RC maps at λλ22 and 18 cm from the WSRT SINGS survey and Hα emission maps to correct for thermal RC emission. We compare azimuthally averaged radial profiles of the RC and FUV/mid-IR (MIR) based Σ{sub SFR} maps and study pixel-by-pixel correlations at fixed linear scales of 1.2 and 0.7 kpc. The ratio of the integrated SFRs from the RC emission to that of the FUV/MIR-based SF tracers is R{sub int}=0.78±0.38, consistent with the relation by Condon. We find a tight correlation between the radial profiles of the radio and FUV/MIR-based Σ{sub SFR} for the entire extent of the disk. The ratio R of the azimuthally averaged radio to FUV/MIR-based Σ{sub SFR} agrees with the integrated ratio and has only quasi-random fluctuations with galactocentric radius that are relatively small (25%). Pixel-by-pixel plots show a tight correlation in log-log diagrams of radio to FUV/MIR-based Σ{sub SFR}, with a typical standard deviation of a factor of two. Averaged over our sample we find (Σ{sub SFR}){sub RC}∝(Σ{sub SFR}){sub hyb}{sup 0.63±0.25}, implying that data points with high Σ{sub SFR} are relatively radio dim, whereas the reverse is true for low Σ{sub SFR}. We interpret this as a result of spectral aging of cosmic-ray electrons (CREs), which are diffusing away from the star formation sites where they are injected into the interstellar medium. This is supported by our finding that the radio spectral index is a second parameter in pixel-by-pixel plots: those data points dominated by young CREs are relatively radio dim, while those dominated by old CREs are slightly more RC bright than what would be expected from a linear extrapolation. We studied the ratio R of radio to FUV/MIR-based integrated SFR as a function of global galaxy parameters and found no clear correlation. This suggests that we can use RC emission as a universal star formation tracer for galaxies with a similar degree of accuracy as other tracers, if we restrict ourselves to global or azimuthally averaged measurements. We can reconcile our finding of an almost linear RC-SFR relation and sub-linear resolved (on 1 kpc scale) RC-Σ{sub SFR} relation by proposing a non-linear magnetic field-SFR relation, B∝SFR{sub hyb}{sup 0.30±0.02}, which holds both globally and locally.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

LITTLE THINGS

We present LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes, The H I Nearby Galaxy Survey), which is aimed at determining what drives star formation in dwarf galaxies. This is a multi-wavelength survey of 37 dwarf irregular and 4 blue compact dwarf galaxies that is centered around H I-line data obtained with the National Radio Astronomy Observatory (NRAO) Very Large Array (VLA). The H I-line data are characterized by high sensitivity ({<=}1.1 mJy beam{sup -1} per channel), high spectral resolution ({<=}2.6 km s{sup -1}), and high angular resolution ({approx}6''). The LITTLE THINGS sample contains dwarf galaxies that are relatively nearby ({<=}10.3 Mpc; 6'' is {<=}300 pc), that were known to contain atomic hydrogen, the fuel for star formation, and that cover a large range in dwarf galactic properties. We describe our VLA data acquisition, calibration, and mapping procedures, as well as H I map characteristics, and show channel maps, moment maps, velocity-flux profiles, and surface gas density profiles. In addition to the H I data we have GALEX UV and ground-based UBV and H{alpha} images for most of the galaxies, and JHK images for some. Spitzer mid-IR images are available for many of the galaxies as well. These data sets are available online.

79 ASTRONOMY AND ASTROPHYSICS↗