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Register a new userContinuum Halos in Nearby Galaxies -- an EVLA Survey (CHANG-ES) -- I: Introduction to the Survey
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We introduce a new survey to map the radio continuum halos of a sample of 35 edge-on spiral galaxies at 1.5 GHz and 6 GHz in all polarization products. The survey is exploiting the new wide bandwidth capabilities of the Karl G. Jansky Very Large Array (i.e. the Expanded Very Large Array, or EVLA) in a variety of array configurations (B, C, and D) in order to compile the most comprehensive data set yet obtained for the study of radio halo properties. This is the first survey of radio halos to include all polarization products. In this first paper, we outline the scientific motivation of the survey, the specific science goals, and the expected improvements in noise levels and spatial coverage from the survey. Our goals include investigating the physical conditions and origin of halos, characterizing cosmic ray transport and wind speed, measuring Faraday rotation and mapping the magnetic field, probing the in-disk and extraplanar far-infrared - radio continuum relation, and reconciling non-thermal radio emission with high-energy gamma-ray models. The sample size allows us to search for correlations between radio halos and other properties, including environment, star formation rate, and the presence of AGNs. In a companion paper (Paper II) we outline the data reduction steps and present the first results of the survey for the galaxy, NGC 4631.
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We present the first results from the CHANG-ES survey, a new survey of 35 edge-on galaxies to search for both in-disk as well as extra-planar radio continuum emission. The motivation and science case for the survey are presented in a companion paper (Paper I). In this paper (Paper II), we outline the observations and data reduction steps required for wide-band calibration and mapping of EVLA data, including polarization, based on C-array test observations of NGC 4631. With modest on-source observing times (30 minutes at 1.5 GHz and 75 minutes at 6 GHz for the test data) we have achieved best rms noise levels of 22 and 3.5 $mu$Jy beam$^{-1}$ at 1.5 GHz and 6 GHz, respectively. New disk-halo features have been detected, among them two at 1.5 GHz that appear as loops in projection. We present the first 1.5 GHz spectral index map of NGC 4631 to be formed from a single wide-band observation in a single array configuration. This map represents tangent slopes to the intensities within the band centered at 1.5 GHz, rather than fits across widely separated frequencies as has been done in the past and is also the highest spatial resolution spectral index map yet presented for this galaxy. The average spectral index in the disk is $baralpha_{1.5 GHz},=,-0.84,pm,0.05$ indicating that the emission is largely non-thermal, but a small global thermal contribution is sufficient to explain a positive curvature term in the spectral index over the band. Two specific star forming regions have spectral indices that are consistent with thermal emission. Polarization results (uncorrected for internal Faraday rotation) are consistent with previous observations and also reveal some new features. On broad scales, we find strong support for the notion that magnetic fields constrain the X-ray emitting hot gas.
The CHANG-ES (Continuum Halos in Nearby Galaxies) survey of 35 nearby edge-on galaxies is revealing new and sometimes unexpected and startling results in their radio continuum emission. The observations were in wide bandwidths centered at 1.6 and 6.0 GHz. Unique to this survey is full polarization data showing magnetic field structures in unprecedented detail, resolution and sensitivity for such a large sample. A wide range of new results are reported here, some never before seen in any galaxy. We see circular polarization and variability in active galactic nuclei (AGNs), in-disk discrete features, disk-halo structures sometimes only seen in polarization, and broad-scale halos with reversing magnetic fields, among others. This paper summarizes some of the CHANG-ES results seen thus far. Released images can be found at https://www.queensu.ca/changes.
We detect 5 galaxies in the CHANG-ES (Continuum Halos in Nearby Galaxies -- an EVLA Survey) sample that show circular polarization (CP) at L-band in our high resolution data sets. Two of the galaxies (NGC~4388 and NGC~4845) show strong Stokes $V/I,equiv,m_C,sim,2$%, two (NGC~660 and NGC~3628) have values of $m_Csim ,0.3$%, and NGC~3079 is a marginal detection at $m_Csim ,0.2$%. The two strongest $m_C$ galaxies also have the most luminous X-ray cores and the strongest internal absorption in X-rays. We have expanded on our previous Faraday conversion interpretation and analysis and provide analytical expressions for the expected $V$ signal for a general case in which the cosmic ray electron energy spectral index can take on any value. We provide examples as to how such expressions could be used to estimate magnetic field strengths and the lower energy cutoff for CR electrons. Four out of our detections are {it resolved}, showing unique structures, including a {it jet} in NGC~4388 and a CP `conversion disk in NGC~4845. The conversion disk is inclined to the galactic disk but is perpendicular to a possible outflow direction. Such CP structures have never before been seen in any galaxy to our knowledge. None of the galaxy cores show linear polarization at L-band. Thus CP may provide a unique probe of physical conditions deep into radio AGNs.
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.
Differential K-band luminosity functions (LFs) are presented for a complete sample of 1613 nearby bright galaxies segregated by visible morphology. The LF for late-type spirals follows a power law that rises towards low luminosities whereas the LFs for ellipticals, lenticulars and bulge-dominated spirals are peaked and decline toward both higher and lower luminosities. Each morphological type (E, S0, S0/a-Sab, Sb-Sbc, Sc-Scd) contributes approximately equally to the overall K-band luminosity density of galaxies in the local universe. Type averaged bulge/disk ratios are used to subtract the disk component leading to the prediction that the K-band LF for bulges is bimodal with ellipticals dominating the high luminosity peak, comprising 60% of the bulge luminosity density in the local universe with the remaining 40% contributed by lenticulars and the bulges of spirals. Overall, bulges contribute 30% of the galaxy luminosity density at K in the local universe with spiral disks making up the remainder. If bulge luminosities indicate central black hole masses, then our results predict that the black hole mass function is also bimodal.
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