CHANG-ES (Continuum Halos in Nearby Galaxies—an EVLA Survey) is a project to observe 35 edge-on galaxies in the radio continuum with wide bandwidths centred at 1.6 GHz (L-band) and 6.0 GHz (C-band, using the Karl J. Jansky Very Large Array (hereafter, the VLA, formerly the Expanded Very Large Array or EVLA). The wide bandwidths,
, of the VLA (Table 1
) have made such a survey feasible since the theoretical signal-to-noise (S/N) improves as
, all else being equal. For CHANG-ES, this has meant an order of magnitude improvement, on average, compared to any similar survey. For example, [1
] achieved sensitivities ranging from 80 to 130
rms and [2
] achieved 45 to 90
rms at 5 GHz. By contrast, typical rms values of the CHANG-ES D-configuration images are
at the same frequency (range of values for all configurations and frequencies are summarized in Table 1
). Additionally, no previous survey has included polarization. This has opened up the possibility of exploring the magnetic fields in galaxy disks and halos for a well-defined galaxy sample.
Wide bandwidths, when broken up into many individual channels, also provide advantages other than improved S/N: (a) radio frequency interference (RFI) can be more readily identified and excised, (b) in-band
spectral index maps can be formed (e.g., Section 2.1.1
), (c) Rotation Measure (RM) analysis can be carried out (Section 2.4
and Section 2.5
), and (d) imaging can make use of the power of multi-frequency synthesis.
CHANG-ES has a variety of goals. We wanted to understand the incidence of radio halos in ‘normal’ spiral galaxies, determine their scale heights and correlate these results with other properties such as the underlying star formation rate (SFR) or SFR ‘surface density’ (SFR per unit area, SFR
). Measurements of outflow speeds can help to determine whether galaxies are more likely to experience winds or ‘fountain’ flow. Outflows in nearby galaxies could also represent low-energy analogues of the ‘feedback’ that appears to be required in galaxy formation scenarios in order to control the star formation rate [7
]. Since outflows can be observed with spectacular resolution in nearby galaxies, the physics of such systems may provide meaningful constraints on galaxy formation in the early universe. A consistent galaxy sample, with complementary data at other wavelengths, can also help us explore pressure balance in the ISM. For more information on our science goals, see [4
As indicated above, unique to the survey is that all polarization products were measured, namely Stokes I, Q, U, and V. With analysis of linear polarization (
), along with RMs, an in-depth understanding of the magnetic fields in these galaxies can be obtained, and several examples (Section 2.5
) have been very illuminating in this regard. Some of our galaxies show lagging halos (e.g., see others [8
]) either in optical emission lines or HI. The reason for such lags is not yet understood, but an association with magnetic fields is possible [9
A survey size that samples a sufficient number of galaxies in a reasonable (but large) total observing time resulted in 35 galaxies in total. Clearly, rms values that improve upon previous measurements were desired. Since vertical features in galaxies show structures on virtually all scales, we required observations over a range of VLA array configurations. Thus, we have observed at B, C, and D-configurations in L-band and in C and D-configurations in C-band. This also provides us with matching resolution for spectral index maps which are essential in understanding the origin of the emission (e.g., thermal or non-thermal fractions) as well as the propagation of cosmic ray electrons (CREs).
Our galaxy selection criteria were (1) an inclination
as given in the Nearby Galaxies Catalog [10
] in order to easily see a halo, (2) a declination
for detectability at the VLA, (3) an optical size between 4 and 15 arcmin for a good match to the array configurations, and (4) flux densities at L-band of at least 23 mJy in order for a detection to be more likely. Two more galaxies were added that did not meet these criteria but were included because of other interesting known halo properties (NGC 5775 which was smaller than the lower angular size limit, and NGC 4244 which had a flux density below the minimum cut-off).
Notice that selection effects should be minor, the main one being the possibility that, by adopting a radio flux density limit, we could inadvertently be choosing galaxies with high SFRs. However, Table 1
reveals that our SFR range is quite typical of ‘normal’ spirals. We do not
choose our galaxies based on whether or not they have halos (except for the two mentioned above) nor do we specifically choose any active galactic nuclei (AGNs) although several were found to be present once the sample was defined. Table 1
provides a summary of our data. In all, over 405 hours of VLA time were granted for this large project.
1.2. Techniques and Data Products
Data reduction was carried out using the Common Astronomy Software Applications (CASA) package1
. This allowed us to take advantage of a number of sophisticated imaging algorithms such as multi-scale multi-frequency synthesis (ms-mfs, [11
]). ‘Multi-scale’ assumes that any intensity feature can be represented by spatial scales of varying width prior to convolution with the synthesized beam, as opposed to the traditional assumption that an intensity feature is represented as a series of points. ‘Multi-frequency’ employs the technique of sampling differing spatial scales as the frequency varies across the band, thereby more thoroughly filling in the uv plane. In addition, these techniques, as employed in CASA, allow for the fitting of in-band spectral indices across any individual band. In principle, one can solve for curvature,
, in the spectral index as well (i.e.,
but our tests showed that the S/N for any given data set was not high enough to take advantage of the curvature option [12
]. Hence, a straight line in log space (
) was fitted across the band while imaging. Thus, a solution for the in-band spectral index,
, was obtained point-by-point, in each galaxy.
At the time of writing, D-configuration images [3
] have been released at our data release website2
, the B-configuration images have also been released [5
] and the C-configuration [6
] data release is imminent. Release data include total intensity images at two different uv weightings, i.e., ‘robust 0’ and ‘robust 0 + uv taper’, specified as high-res and low-res, respectively, in Table 1
. Both non-primary-beam-corrected and primary-beam-corrected versions are included. At D-configuration, linear polarization and polarization angle maps, and in-band spectral index maps are also available. At high resolution, where emission is weaker, we opted to include band-to-band spectral index maps, rather than in-band spectral index maps, i.e., C-configuration C-band to B-configuration L-band
Because the largest angular size detectable at the VLA is 16 arcmin at L-band and 4 arcmin at C-band, it was necessary to supplement the VLA data with single-dish data for the largest galaxies at L-band and all galaxies at C-band (Table 1
). This has been accomplished with over 200 h of supplementary Greenbank Telescope (GBT) observations. Because the GBT wide-band instrumentation was delayed and because data reduction algorithms had to be developed, all GBT observations are completed but the data reduction is currently in progress. For several galaxies (NGC 891, NGC 4565, and NGC 4631), Effelsberg data have been used for the large-scale total intensity emission.
Complementary to our radio data, we have obtained H
images using the Apache Point Observatory 3.5-m telescope [13
] for 21 galaxies, supplemented with images from the literature for the remaining galaxies. This data set was obtained to assist with the thermal/non-thermal separation analysis (Section 2.2
) and will also soon be publicly released. We also have WISE enhanced resolution images of all galaxies and XMM-Newton and Chandra data for a large fraction of the sample.