arXiv subjects

R. Henriksen

Publications and source records attributed to R. Henriksen.

5 recordsLinked to original sources

arXiv2026

CHANG-ES XXXIX. Magnetic field structure in edge-on galaxies: Stacking Stokes parameters

Galactic magnetic fields regulate star formation and cosmic-ray (CR) transport, and understanding their three-dimensional structure, particularly in star-forming late-type galaxies, is key to constraining galactic CR transport. We explore the validity of stacking Stokes $Q$ and $U$ spectra, to infer about the intrinsic polarisation characteristics of star-forming galaxies. To prepare the stacking experiment, we align, scale, convolve, and reproject $C$-band (6 GHz) Stokes $Q$ and Stokes $U$ cubes of 27 star-forming late-type edge-on galaxies. On the stacked cubes, we perform RM-synthesis and discuss the derived polarised intensity (PI), polarisation angle ($\chi_0$), and RM maps. Synthetic data tests demonstrate that stacking Stokes $Q$ and $U$ spectra is valid for tightly constrained underlying distributions of PI, $\chi_0$, and RM. For underlying PI, $\chi_0$, and RM distributions that represent star-forming galaxies, stacking introduces a systematic uncertainty of $\delta_\mathrm{RM}^\mathrm{sys}=90 \mathrm{rad m^{-2}}$ and significantly underestimates the recovered PI. Stacking results reveal a clear X-shaped pattern in the polarisation plane, consistent with prior findings, detecting polarised emission up to 9 kpc above the galactic disc. We find stronger PI on the approaching side of galaxies. Furthermore, we find a decrease in PI in the galactic halo of $\sim 60$% near the galaxy's minor axis. A global RM pattern, as reported in a previous study, cannot be confirmed. Based on our analysis, we present stacking of Stokes $Q$ and Stokes $U$ cubes as an effective tool to recover faint polarised emission in the halo of nearby galaxies, if the underlying distributions of PI, $\chi_0$, and RM are tightly constrained. Our findings motivate future studies using broader-band data to increase the resolution in Faraday depth.

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arXiv2024

CHANG-ES XXXIV: a 20 kpc radio bubble in the halo of the star-forming galaxy NGC 4217

Cosmic rays may be dynamically very important in driving large-scale galactic winds. Edge-on galaxies give us an outsider's view of the radio halo, which shows the presence of extra-planar cosmic-ray electrons and magnetic fields. We present a new radio continuum imaging study of the nearby edge-on galaxy NGC 4217 in order to study the distribution of extra-planar cosmic rays and magnetic fields. We both observe with the Jansky Very Large Array (JVLA) in the S-band (2-4 GHz) and with LOw Frequency ARray (LOFAR) at 144 MHz. We measure vertical intensity profiles and exponential scale heights. We re-image both JVLA and LOFAR data at matched angular resolution in order to measure radio spectral indices between 144 MHz and 3 GHz. Confusing point-like sources were subtracted prior to imaging. Intensity profiles are then fitted with cosmic-ray electron advection models, where we use an isothermal wind model that is driven by a combination of pressure from the hot gas and cosmic rays. We discover a large-scale radio halo on one (northwestern) side of the galactic disc. The morphology is reminiscent of a bubble extending up to 20 kpc away from the disc. We find spectral ageing in the bubble which allows us to measure advection speeds of the cosmic-ray electrons accelerating from 300 to 600 $\rm km\, s^{-1}$ . Assuming energy equipartition between the cosmic rays and the magnetic field, we estimate the bubble can be inflated by a modest 10 per cent of the kinetic energy injected by supernovae over its dynamical time-scale of 35 Myr. While no active galactic nucleus (AGN) has been detected, such activity in the recent past cannot be ruled out. Non-thermal bubbles with sizes of tens of kiloparsec may be a ubiquitous feature of star-forming galaxies showing the influence of feedback. To determine possible contributions by AGN feedback, will require deeper observations.

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arXiv2021

CHANG-ES XXIII: Influence of a galactic wind in NGC 5775

We present new radio continuum images of the edge-on starburst galaxy NGC 5775, from LOFAR (140 MHz) and the Karl G. Jansky Very Large Array CHANG-ES survey (1500 MHz). We trace the non-thermal radio halo up to 13 kpc from the disc, measuring the non-thermal spectral index and estimating the total equipartition magnetic field strength ($\approx13μ$G in the disc and $\approx7μ$G above the plane). The radio halo has a similar extent at both frequencies, displays evidence for localized cosmic ray streaming coinciding with prominent H$α$ filaments and vertical extensions of the regular magnetic field, and exhibits a boxy morphology especially at 140 MHz. In order to understand the nature of the disc-halo flow, we extend our previous model of cosmic ray propagation by implementing an iso-thermal wind with a tunable `flux tube' (approximately hyperboloidal) geometry. This updated model is successful in matching the vertical distribution of non-thermal radio emission, and the vertical steepening of the associated spectral index, in a consistent conceptual framework with few free parameters. Our new model provides the opportunity to estimate the mass outflow driven by the star formation process, and we find an implied rate of $\dot{M}\approx3-6\,\mathrm{M_{\odot}\,yr^{-1}}$ ($\approx40-80$ per cent of the star formation rate) if the escape velocity is reached, with substantial uncertainty arising from the poorly-understood distribution of ISM material entrained in the vertical flow. The wind may play a role in influencing the vertical gradient in rotational velocity.

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arXiv2020

CHANG-ES XXI. Transport processes and the X-shaped magnetic field of NGC 4217: off-center superbubble structure

In order to gain a better understanding of the influence of cosmic rays (CRs) and magnetic fields in the disk-halo interface of edge-on spiral galaxies, we investigate the radio continuum halo, the magnetic field, and the transport processes of the CRs of the edge-on spiral galaxy NGC 4217 using CHANG-ES radio data at two frequencies, 6 GHz (C-band) and 1.5 GHz (L-band), and supplemental LOFAR data at 150 MHz and X-ray Chandra data. NGC 4217 shows a large-scale X-shaped magnetic field structure, covering a major part of the galaxy with a mean total magnetic field strength in the disk of 9 micro Gauss (via equipartition). Using rotation measure synthesis (RM-synthesis) at C-band, we found that the direction of the disk magnetic field is pointing inward. A helical outflow structure is furthermore present in the northwestern part of the galaxy, which is extended nearly 7 kpc into the halo. More polarized emission is observed on the approaching side of the galaxy. With a simplified galaxy disk model, we are able to explain that finding and predict that roughly 75% of edge-on spiral galaxies will show higher polarized intensity on the approaching side. Many loop and shell structures are found throughout the galaxy in total intensity at C-band. A superbubble-like structure is prominent in total and polarized intensity, as well as in Halpha and optical dust filaments, being a possible result of concentrated star formation in the disk. The flux density contribution of the disk in comparison to the halo decreases toward lower frequencies. Total intensity profiles at the three radio frequencies were fit with two-component exponential functions. The frequency dependence of the resulting scale heights between C-band and L-band suggests advection to be the main CR transport process. The 1D CR transport modeling (SPINNAKER) shows that advection appears to be more important than diffusion.

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arXiv1997

Time-dependent accretion and ejection implied by pre-stellar density profiles

Recent observations suggest that mass ejection and mass accretion both decline significantly with time during early protostellar evolution (Bontemps et al. 1996). In the present paper, we propose that this rapid decay of accretion/ejection activity is a direct result of the non-singular density profiles characterizing pre-collapse clouds. The radial density profiles of pre-stellar cores are indeed found to be much flatter than rho(r) ~ r(-2) at radii less than a few thousand AU (Ward-Thompson et al. 1994). Here we show, through Lagrangian analytical calculations, that the supersonic gravitational collapse of pre-stellar cloud cores with centrally peaked, but flattened density profiles leads to a transitory phase of energetic accretion immediately following the formation of the central hydrostatic protostar. Physically, the flat inner region collapses first nearly homologously to form a finite mass stellar nucleus, and the remaining cloud core material then accretes supersonically onto a non-zero point mass. Enhanced accretion persists as long as the gravitational pull of this initial point mass, which does not exist in the Shu singular solution, remains significant. We suggest that this epoch of vigorous accretion coincides with Class 0 protostars, which would explain their unusually powerful jets compared to the more evolved Class I objects. We also use a simple two-component power-law model to fit the diagrams of outflow power versus envelope mass observed by Bontemps et al. (1996), and suggest that rho Ophiuchi and Taurus young stellar objects follow different accretion histories because of differing initial conditions.

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