Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userCHANG-ES XIII: Transport processes and the magnetic fields of NGC 4666 - indication of a reversing disk magnetic field
63
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We analyze the magnetic field geometry and the transport processes of the cosmic rays of the edge-on spiral starburst galaxy NGC 4666 from CHANG-ES radio continuum data in two frequencies; 6 GHz (C-band) and 1.5 GHz (L-band). Supplementary X-ray data are used to investigate the hot gas in NGC 4666. We determine the radio scale heights of total power emission at both frequencies for this galaxy. We show the magnetic field orientations derived from the polarization data. Using rotation measure (RM) synthesis we further study the behavior of the RM values along the disk in C-band to investigate the large-scale magnetic-field pattern. We use the revised equipartition formula to calculate a map of the magnetic field strength. Furthermore, we model the processes of cosmic-ray transport into the halo with the 1D SPINNAKER model. The extended radio halo of NGC 4666 is box-shaped and is probably produced by the previously observed supernova-driven superwind. This is supported by our finding of an advective cosmic-ray transport such as that expected for a galactic wind. The scaleheight analysis revealed an asymmetric halo above and below the disk as well as between the two sides of the major axis. A central point source as well as a bubble structure is seen in the radio data for the first time. Our X-ray data show a box-shaped hot halo around NGC 4666 and furthermore confirm the AGN nature of the central source. NGC 4666 has a large-scale X-shaped magnetic field in the halo, as has been observed in other edge-on galaxies. The analysis furthermore revealed that the disk of NGC 4666 shows hints of field reversals along its radius, which is the first detection of this phenomenon in an external galaxy.
rate research
Read More
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.
Using the Continuum HAloes in Nearby Galaxies - an EVLA Survey (CHANG-ES) radio continuum data from the Karl G. Jansky Very Large Array (VLA) in two frequency bands (C-band, L-band), we analyzed the radio properties, including polarization and the transport processes of the CR electrons (CREs), in the edge-on spiral galaxy NGC 4013. Supplementary LOw-Frequency ARray (LOFAR) data at 150MHz are used to study the low-frequency properties of this galaxy and X-ray (Chandra, XMM-Newton) data are used to investigate the central region. The central point source dominates the radio continuum in both CHANG-ES bands, but no clear AGN classification is possible at this time. The scale height analysis shows that Gaussian fits, with halo scale heights of 1.2 kpc in C-band, 2.0 kpc in L-band, and 3.1 kpc at 150 MHz, better represent the intensity profiles than do exponential fits. The radio continuum halo of NGC 4013 in C-band is rather small, while the low-frequency LOFAR data reveal a large halo. The polarization data reveal plane-parallel, regular magnetic fields within the entire disk and vertical halo components out to heights of about 6 kpc indicating the presence of an axisymmetric field having a radial component pointing outwards. The mean magnetic field strength of the disk of NGC 4013 of 6.6 $mu$G (using the revised equipartition formula) is rather small. The interaction and the low star formation rate (SFR) across the disk of NGC 4013 probably influence the appearance of its radio continuum. Several observable quantities give consistent evidence that the CR transport in the halo of NGC 4013 is diffusive: the frequency dependence of the synchrotron scale height, the disk/halo flux density ratio, the vertical profile of the synchrotron spectral index, the small propagation speed measured modeled with spinnaker, and the low temperature of the X-ray emitting hot gas.
NGC 4631 is an interacting galaxy which exhibits one of the largest gaseous halos observed among edge-on galaxies. We aim to examine the synchrotron and polarization properties of its disk and halo emission with new radio continuum data. Radio continuum observations of NGC 4631 were performed with the Karl G. Jansky Very Large Array at C-band (5.99 GHz) in the C & D array configurations, and at L-band (1.57 GHz) in the B, C, & D array configurations. The Rotation Measure Synthesis algorithm was utilized to derive the polarization properties. We detected linearly polarized emission at C-band and L-band. The magnetic field in the halo is characterized by strong vertical components above and below the central region of the galaxy. The magnetic field in the disk is only clearly seen in the eastern side of NGC 4631, where it is parallel to the plane of the major axis of the galaxy. We detected for the first time a large-scale, smooth Faraday depth pattern in a halo of an external spiral galaxy, which implies the existence of a regular (coherent) magnetic field. A quasi-periodic pattern in Faraday depth with field reversals was found in the northern halo of the galaxy. The field reversals in the northern halo of NGC 4631, together with the observed polarization angles, indicate giant magnetic ropes (GMRs) with alternating directions. To our knowledge, this is the first time such reversals are observed in an external galaxy.
Context. The magnetic field in spiral galaxies is known to have a large-scale spiral structure along the galactic disk and is observed as X-shaped in the halo of some galaxies. While the disk field can be well explained by dynamo action, the 3-dimensional structure of the halo field and its physical nature is still unclear. Aims. As first steps towards understanding the halo fields, we want to clarify whether the observed X-shaped field is a wide-spread pattern in the halos of spiral galaxies and whether these halo fields are just turbulent fields ordered by compression or shear (anisotropic turbulent fields), or have a large-scale regular structure. Methods. The analysis of the Faraday rotation in the halo is the tool to discern anisotropic turbulent fields from large-scale magnetic fields. This, however, has been challenging until recently because of the faint halo emission in linear polarization. Our sensitive VLA broadband observations C-band and L-band of 35 spiral galaxies seen edge-on (called CHANG-ES) allowed us to perform RM-synthesis in their halos and to analyze the results. We further accomplished a stacking of the observed polarization maps of 28 CHANG-ES galaxies at C-band. Results. Though the stacked edge-on galaxies were of different Hubble types, star formation and interaction activities, the stacked image clearly reveals an X-shaped structure of the apparent magnetic field. We detected a large-scale (coherent) halo field in all 16 galaxies that have extended polarized intensity in their halos. We detected large-scale field reversals in all of their halos. In six galaxies they are along lines about vertical to the galactic midplane (vertical RMTL) with about 2 kpc separation. Only in NGC 3044 and possibly in NGC 3448 we observed vertical giant magnetic ropes (GMRs) similar to those detected recently in NGC 4631.
NGC 4631 is an interacting galaxy that exhibits one of the largest, gaseous halos observed among edge-on galaxies. We aim to examine the synchrotron and cosmic-ray propagation properties of its disk and halo emission with new radio continuum data. Radio continuum observations of NGC 4631 were performed with the Karl G. Jansky Very Large Array at C-band (5.99 GHz) in the C and D array configurations, and at L-band (1.57 GHz) in the B, C, and D array configurations. Complementary observations of NGC 4631 with the Effelsberg telescope were performed at 1.42 and 4.85 GHz. The interferometric total intensity data were combined with the single-dish Effelsberg data in order to recover the missing large-scale total power emission. The thermal and nonthermal components of the total radio emission were separated by estimating the thermal contribution through the extinction-corrected H$alpha$ emission. The H$alpha$ radiation was corrected for extinction using a linear combination of the observed H$alpha$ and 24 $mu$m data. NGC 4631 has a global thermal fraction at 5.99 (1.57) GHz of 14$pm$3% (5.4$pm$1.1%). The mean scale heights of the total emission in the radio halo (thick disk) at 5.99 (1.57) GHz are $1.79pm0.54$ kpc ($1.75pm0.27$ kpc) and have about the same values for the synchrotron emission. The total magnetic field of NGC 4631 has a mean strength of $rm{langle B_{eq}rangle} simeq 9 rm{mu G}$ in the disk, and a mean strength of $rm{langle B_{eq}rangle}~simeq 7~rm{mu G}$ in the halo. We also studied a double-lobed background radio galaxy southwest of NGC 4631, which is an FR~II radio galaxy according to the distribution of spectral index across the lobes. From the halo scale heights we estimated that the radio halo is escape-dominated with convective cosmic ray propagation, and conclude that there is a galactic wind in the halo of NGC 4631.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
