Theresa Wiegert

Continuum halos in nearby galaxies: an EVLA survey (CHANG-ES) - I. Introduction to the survey

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) 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 active galactic nuclei. 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.

CHANG-ES. IV. Radio Continuum Emission of 35 Edge-on Galaxies Observed with the Karl G. Jansky Very Large Array in D Configuration—Data Release 1

We present the first part of the observations made for the Continuum Halos in Nearby Galaxies, an EVLA Survey (CHANG-ES) project. The aim of the CHANG-ES project is to study and characterize the nature of radio halos, their prevalence as well as their magnetic fields, and the cosmic rays illuminating these fields. This paper reports observations with the compact D configuration of the Karl G. Jansky Very Large Array (VLA) for the sample of 35 nearby edge-on galaxies of CHANG-ES. With the new wide bandwidth capabilities of the VLA, an unprecedented sensitivity was achieved for all polarization products. The beam resolution is an average of 9.″6 and 36″ with noise levels reaching approximately 6 and 30 μJy beam-1 for C- and L-bands, respectively (robust weighting). We present intensity maps in these two frequency bands (C and L), with different weightings, as well as spectral index maps, polarization maps, and new measurements of star formation rates (SFRs). The data products described herein are available to the public in the CHANG-ES data release available at http://www.queensu.ca/changes. We also present evidence of a trend among galaxies with larger halos having higher SFR surface density, and we show, for the first time, a radio continuum image of the median galaxy, taking advantage of the collective signal-to-noise ratio of 30 of our galaxies. This image shows clearly that a “typical” spiral galaxy is surrounded by a halo of magnetic fields and cosmic rays.

Continuum Halos in Nearby Galaxies: An EVLA Survey (CHANG-ES). II. First Results on NGC 4631

We present the first results from the Continuum Halos in Nearby Galaxies—an EVLA Survey (CHANG-ES), a new survey of 35 edge-on galaxies to search for both in-disk and extraplanar radio continuum emission. CHANG-ES is exploiting the new wide-band, multi-channel capabilities of the Karl G. Jansky Very Large Array (i.e., the Expanded Very Large Array or EVLA) with observations in two bands centered at 1.5 and 6 GHz in a variety of array configurations with full polarization. The motivation and science case for the survey are presented in a companion paper (Paper I). These first results are based on C-array test observations in both observing bands of the well-known radio halo galaxy, NGC 4631. In this paper, we outline the observations and the data reduction steps that are required for wide-band calibration and mapping of EVLA data, including polarization. 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 μJy 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 \bar{\alpha }_{1.5 \,GHz} =-0.84+/- 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.

CHANG-ES V: NUCLEAR OUTFLOW IN A VIRGO CLUSTER SPIRAL AFTER A TIDAL DISRUPTION EVENT

We have observed the Virgo Cluster spiral galaxy, NGC 4845, at 1.6 and 6 GHz using the Karl G. Jansky Very Large Array, as part of the Continuum Halos in Nearby Galaxies—an EVLA Survey (CHANG-ES). The source consists of a bright unresolved core with a surrounding weak central disk (1.8 kpc diameter). The core is variable over the 6 month timescale of the CHANG-ES data and has increased by a factor of ≈6 since 1995. The wide bandwidths of CHANG-ES have allowed us to determine the spectral evolution of this core, which peaks between 1.6 and 6 GHz (it is a Gigahertz-peaked spectrum source). We show that the spectral turnover is dominated by synchrotron self-absorption and that the spectral evolution can be explained by adiabatic expansion (outflow), likely in the form of a jet or cone. The CHANG-ES observations serendipitously overlap in time with the hard X-ray light curve obtained by Nikolajuk & Walter (2013), which they interpret as due to a tidal disruption event (TDE) of a super-Jupiter mass object around a 10^5 M⊙ black hole. We outline a standard jet model, provide an explanation for the observed circular polarization, and quantitatively suggest a link between the peak radio and peak X-ray emission via inverse Compton upscattering of the photons emitted by the relativistic electrons. We predict that it should be possible to resolve a young radio jet via VLBI as a result of this nearby TDE.

CHANG-ES – VI. Probing Supernova energy deposition in spiral galaxies through multiwavelength relationships

How a galaxy regulates its supernovae (SNe) energy into different interstellar/circumgalactic medium components strongly affects galaxy evolution. Based on the JVLA D-configuration C- (6 GHz) and L-band (1.6 GHz) continuum observations, we perform statistical analysis comparing multiwavelength properties of the Continuum Haloes in Nearby Galaxies - an EVLA Survey galaxies. The high-quality JVLA data and edge-on orientation enable us for the first time to include the halo into the energy budget for a complete radio-flux-limited sample. We find tight correlations of Lradio with the mid-IR-based star formation rate (SFR). The normalization of our I1.6 GHz/W Hz-1-SFR relation is ˜2-3times of those obtained for face-on galaxies, probably a result of enhanced IR extinction at high inclination. We also find tight correlations between Lradio and the SNe energy injection rate dot{E}_SN(Ia+CC), indicating the energy loss via synchrotron radio continuum accounts for ˜1 of dot{E}_SN, comparable to the energy contained in cosmic ray electrons. The integrated C-to-L-band spectral index is α ˜ 0.5-1.1 for non-active galactic nucleus galaxies, indicating a dominance by the diffuse synchrotron component. The low-scatter Lradio-SFR/L_radio-dot{E}_{SN (Ia+CC)} relationships have superlinear logarithmic slopes at ˜2σ in L band (1.132 ± 0.067/1.175 ± 0.102) while consistent with linear in C band (1.057 ± 0.075/1.100 ± 0.123). The superlinearity could be naturally reproduced with non-calorimeter models for galaxy discs. Using Chandra halo X-ray measurements, we find sublinear LX-Lradio relations. These results indicate that the observed radio halo of a starburst galaxy is close to electron calorimeter, and a galaxy with higher SFR tends to distribute an increased fraction of SNe energy into radio emission (than X-ray).

Kinematic classification of non-interacting spiral galaxies

Using neutral hydrogen (HI) rotation curves of 79 galaxies, culled from the literature, as well as measured from HI data, we present a method for classifying disk galaxies by their kinematics. In order to investigate fundamental kinematic properties we concentrate on non-interacting spiral galaxies. We employ a simple parameterized form for the rotation curve in order to derive the three parameters: the maximum rotational velocity, the turnover radius and a measure of the slope of the rotation curve beyond the turnover radius. Our approach uses the statistical Hierarchical Clustering method to guide our division of the resultant 3D distribution of galaxies into five classes. Comparing the kinematic classes in this preliminary classification scheme to a number of galaxy properties we find that our class containing galaxies with the largest rotational velocities has a mean morphological type of Sb/Sbc while the other classes tend to later types. Other trends also generally agree with those described by previous researchers. In particular we confirm correlations between increasing maximum rotational velocity and the following observed properties: increasing brightness in B-band, increasing size of the optical disk (D25) and increasing star formation rate (as derived using radio continuum data). Our analysis also suggests that lower velocities are associated with a higher ratio of the HI mass over the dynamical mass. Additionally, three galaxies exhibit a drop in rotational velocity amplitude of & 20% after the turnover radius. However recent investigations suggest that they have interacted with minor companions which is a common cause for declining rotation curves. (Figures 12, 14, 16 and 17 are interactive in the electronic pdf version of this paper.)

CHANG-ES: IX. Radio scale heights and scale lengths of a consistent sample of 13 spiral galaxies seen edge-on and theircorrelations

The vertical halo scale height is a crucial parameter to understand the transport of cosmic-ray electrons (CRE) and their energy loss mechanisms in spiral galaxies. Until now, the radio scale height could only be determined for a few edge-on galaxies because of missing sensitivity at high resolution. We developed a sophisticated method for the scale height determination of edge-on galaxies. With this we determined the scale heights and radial scale lengths for a sample of 13 galaxies from the CHANG-ES radio continuum survey in two frequency bands. The sample average value for the radio scale heights of the halo are 1.1 +/- 0.3kpc in C-band and 1.4 +/- 0.7kpc in L-band. From the frequency dependence analysis of the halo scale heights we found that the wind velocities (estimated using the adiabatic loss time) are above the escape velocity. We found that the halo scale heights increase linearly with the radio diameters. In order to exclude the diameter dependence, we defined a normalized scale height which is quite similar for all sample galaxies at both frequency bands and does not depend on the star formation rate or the magnetic field strength. However, the normalized scale height shows a tight anticorrelation with the mass surface density. The sample galaxies with smaller scale lengths are more spherical in the radio emission, while those with larger scale lengths are flatter. The radio scale height depends mainly on the radio diameter of the galaxy. The sample galaxies are consistent with an escape-dominated radio halo with convective cosmic ray propagation, indicating that galactic winds are a widespread phenomenon in spiral galaxies. While a higher star formation rate or star formation surface density does not lead to a higher wind velocity, we deceleration of CRE outflow, e.g. a lowering of the wind velocity from the galactic disk.

Assessing the galaxy population out to z ~ 2 using the Hubble Deep Field South

In this work we use the Hubble Deep Field South (HDF-S) version 2 images to assess the galaxy population out to z ∼ 2. We have used two methods of templates fitting of the spectral energy distributions to obtain photometric redshifts and classify the objects. The Bayesian photometric redshifts gave better results when compared with 54 spectroscopic redshifts available in the literature. Analysis of the rest-frame colour distribution shows a bimodality out to z ∼ 1.4. We separated our sample in a blue and a red population at B - V = 0.29. At low redshifts (0.2 0.29 whereas at higher redshifts ∼60% of the galaxies are bluer than B - V < 0.29. Although in low numbers, a population of early-type galaxies (or heavily obscured low redshift galaxies) is seen out to z ∼ 2.

Compact Resolved Ejecta in the Nearest Tidal Disruption Event

Tidal disruption events (TDEs) occur when a star or substellar object passes close enough to a galaxys supermassive black hole to be disrupted by tidal forces. NGC 4845 (d = 17 Mpc) was host to a TDE, IGR J12580+0134, detected in 2010 November. Its proximity offers us a unique close-up of the TDE and its aftermath. We discuss new Very Long Baseline Array (VLBA) and Karl G. Jansky Very Large Array observations, which show that the radio flux from the active nucleus created by the TDE has decayed in a manner consistent with predictions from a jet-circumnuclear medium interaction model. This model explains the sources broadband spectral evolution, which shows a spectral peak that has moved from the submillimeter (at the end of 2010) to GHz radio frequencies (in 2011–2013) to <1 GHz in 2015. The milliarcsecond-scale core is circularly polarized at 1.5 GHz but not at 5 GHz, consistent with the model. The VLBA images show a complex structure at 1.5 GHz that includes an east–west extension that is ~40 mas (3 pc) long, as well as a resolved component that is 52 mas (4.1 pc) northwest of the flat-spectrum core, which is all that can be seen at 5 GHz. If ejected in 2010, the northwest component must have had ν = 0.96c over five years. However, this is unlikely, as our model suggests strong deceleration to speeds <0.5c within months and a much smaller, sub-parsec size. In this interpretation, the northwest component could have either a non-nuclear origin or be from an earlier event.

CHANG-ES – VIII. Uncovering hidden AGN activity in radio polarization

We report on C-band (5 - 7 GHz) observations of the galaxy, NGC~2992, from the CHANG-ES sample. This galaxy displays an embedded nuclear double-lobed radio morphology within its spiral disk, as revealed in linearly polarized emission but {\it not} in total intensity emission. The radio lobes are kpc-sized, similar to what has been observed in the past for other Seyfert galaxies, and show ordered magnetic fields. NGC~2992 has shown previous evidence for AGN-related activity, but not the linearly polarized radio features that we present here. We draw attention to this galaxy as the first clear example (and prototype) of bipolar radio outflow that is revealed in linearly polarized emission only. Such polarization observations, which are unobscured by dust, provide a new tool for uncovering hidden weak AGN activity which may otherwise be masked by brighter unpolarized emission within which it is embedded. The radio lobes observed in NGC~2992 are interacting with the surrounding interstellar medium and offer new opportunities to investigate the interactions between nuclear outflows and the ISM in nearby galaxies. We also compare the radio emission with a new CHANDRA X-ray image of this galaxy. A new CHANG-ES image of NGC~3079 is also briefly shown as another example as to how much more obvious radio lobes appear in linear polarization as opposed to total intensity.