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Radio-Continuum Study of the Nearby Sculptor Group Galaxies. Part 2: NGC 55 at {\lambda}=20, 13, 6 and 3 cm

A series of new radio-continuum ({\lambda}=20, 13, 6 and 3 cm) mosaic images focused on the NGC55 galactic system were produced using archived observational data from the Australia Telescope Compact Array. These new images are both very sensitive (down to rms=33 {\mu}Jy) and feature high angular resolution (down to<4"). Using these newly created images, 66 previously unidentified discrete sources are identified. Of these sources, 46 were classified as background sources, 11 as HII regions and 6 as supernova remnant candidates. This relatively low number of SNR candidates detected coupled with the low number of large HII regions is consistent with the estimated low star formation rate of the galaxy at 0.06 solar masses per year. Our spectral index map shows that the core of galaxy appears to have a shallow spectral index between {\alpha} = -0.2 and -0.4. This indicates that the core of the galaxy is a region of high thermal radiation output.

. The proximity is an advantage as it allows for NGC 55 to be examined in great detail. Previous radio-continuum studies of NGC 55 (Condon et al. 1996;Puche et al. 1991) utilised the Karl G. Jansky Very Large Array (VLA) in compact array configurations as their primary instrument and thus suffer from low resolution. As a result, these studies did not provide source lists of objects within the NGC 55 field.
Until the next generation of radio telescopes such as the Australian Square Kilometre Array Pathfinder (ASKAP), Karoo Array Telescope (KAT & MeerKAT) and the Square Kilometre Array (SKA) become operational, we are restricted to consolidating a selection of NGC 55 radio observations. Part 1 of this paper (Galvin et al. (2012), Paper 1 hereafter) published a new set of highly sensitive and high-resolution radiocontinuum images of the NGC 300 field at λ=20 cm, created by combining data from the Australia Telescope Compact Array (ATCA) and the VLA (also see Payne et al. (2004)). In this paper, we examine all available archived radio-continuum observations of NGC 55 conducted with the ATCA and the VLA at λ=20, 13, 6 and 3 cm (ν=1.4, 2.3, 5.5, 9.0 GHz) with the intention of merging these observations to create a single radiocontinuum image following a similar methodology as presented in Paper 1. By combining a large amount of existing data using the latest generation of computing power we can create new images that feature both high angular resolution and excellent sensitivity. The newly constructed images are analysed and the differences between each map of NGC 55 created at the observed wavelengths are discussed.
In §2 we describe the observational data and reduction techniques. In §3 we present our new maps, a brief discussion and source list is given in §4, and §5 is the conclusion.

Observational Data
In order to create a high-resolution and sensitive radiocontinuum image, 29 observations from the ATCA and VLA were considered. These observations were selected from the Australian Telescope Online Archive (ATOA) and the National Radio Astronomy Observatory (NRAO) Science Data Archive. The observations which were selected and considered are summarised in Tables 1, 2, 3 and 4.
All ATCA projects, excluding C295, were conducted in mosaic mode with multiple pointings observed. All VLA observations and ATCA project C295 consisted of single pointings of NGC 55. All images are primary beam corrected.

Data Reduction and Image Creation
To create the best possible NGC 55 mosaic images, we included data from the fixed-position 6th antenna for all ATCA observations, as the large gaps in the uvplane that dominate compact array configurations were filled mainly with data from observations in other array configurations. The inclusion of these long baselines resulted in images of high resolution with good uv-plane coverage.
The miriad (Sault and Killeen 2006), aips (Greisen 2010) and karma (Gooch 2006) software packages were used for data reduction and analysis. Because of the large volume of data, the miriad package was compiled to run on a 16-processor high-performance computer system. miriad was used for all data reduction and imaging, however pre-processing in aips was required for the VLA data. This process consisted of importing the data into aips using the task fillm, and then splitting the sources into separate datasets with split. Using the task uvfix, source coordinates were converted from the B1950 to the J2000 reference frame. The task fittp was then used to export each source to a fits file. The miriad task fits was then used to import these fits files and convert them to miriad files. For the ATCA data, the task atlod was used to convert the raw observation files into miriad files.
Typical calibration and flagging procedures were then carried out (Sault and Killeen 2006), including the use of the guided automatic flagging task pgflag. Using the task invert with a robust weighting scheme, images were created for each ATCA and VLA project separately. Each image was then cleaned using the task mossdi. The mossdi task is a SDI clean algorithm designed for mosaic images (Steer et al. 1984). To convolve a clean model the task restor was then used on each of the cleaned maps. These images were created and visually inspected to assess the data quality.
Once the images had been verified to be free of errors, data observed at the same wavelength were combined in the uv-plane as mosaics by using the invert and providing the datasets of all observations as the input. This produced 4 dirty maps of all calibrated data of NGC 55 at 20, 13, 6 and 3 cm wavelengths, respectively. These dirty maps were then deconvolved using a more directed approach by providing mossdi with boxed regions around each visible source. This method produced superior images which were then restored to produce the combined images. The images were then cropped using imsub for analysis so that all images covered the same area of sky (∼ 0.63 • × 0.41 • ) centred at approximately 0h 15m 09s, −39 • 12 30 (J2000).

RESULTS
When visually inspecting the images produced from different observations, the effects of the different array configurations is apparent. As mentioned previously, compact arrays produce low resolution images with greater uv-coverage whereas larger arrays produce higher resolution images with less uv-coverage. While point sources are resolved to be much larger at lower resolutions, the advantage of the lower resolution is that they display extended emission throughout the field. This extended emission is lost in the high resolution images. The difference between an image produced using data from a compact array and an image produced using a larger configuration is obvious as can be seen in Figs. 1 and 2. Ideal images are those which have high resolution but also contain extended emission data, which is where the technique of combining observations is advantageous.
After the initial inspection, it was determined that the data from the ATCA project C295 contained very poor uv-coverage and very limited hour angle coverage (0.99 hours). As a consequence, the final image contained several errors. Normally, these errors would disappear or lessen significantly when combined with other data observed in varying array configurations, as the majority of the gaps in the uv-coverage would be filled by the combined data. Since C295 was observed in one of the largest configurations (6C) and was the only observation at this configuration, there was insufficient data to fill the missing uv-coverage. Therefore, the data from C295 was excluded from the final image. Similarly,    and, at present, there exists no wide-bandwidth deconvolution algorithm for mosaics in miriad, we had to merge the images in the image plane using immerge. We found that when using this method, the low resolution data appeared to be weighted incorrectly and much of the high resolution data was overwhelmed by the low resolution data. Thus, these data were not used to create the final 6 cm image.
In addition, the VLA observations were also excluded from the final 20 cm image (Fig. 3) as the inclusion of this data overpowered the extended emission from the ATCA observations. Excluding this data was of little consequence as the main advantage of VLA data is the higher resolution, however the largest VLA array configuration used to observe NGC 55 was a single observation in the the BC array which has a maximum baseline separation of approximately 7.6 km. This is relatively close to that of the 6 km maximum baseline at ATCA and so we decided the preservation of the extended emission data was worth more than the slight increase in resolution that would be gained by including the VLA data. Information on the final images used in this paper is given in Table 5.

Discrete Sources Within The Field Of NGC 55
A total of 66 sources above 5σ (0.45, 0.92, 0.15, 0.25 mJy/beam at wavelengths of 20, 13, 6 and 3 cm, respectively) were identified within the field of NGC 55. These sources were catalogued by position and flux density. 33 of these sources were detected at more than one of the observed wavelengths and consequently their spectral index, α, was estimated using the equation S ∝ ν α . Simple linear regression was then performed to determine the best-fit value for α between the measured frequencies. This catalogue was then compared to optical (Royal Observatory, Edinburgh et al. 1977), infrared (Dale et al. 2009;Jarrett et al. 2003) and Xray images (Pannuti et al. 2013;Stobbart et al. 2006) of the NGC 55 field. Source coincidences were accepted for sources within 1 beam of the highest resolution radio image being compared.
Based on their location and comparison with the images at other frequency bands, the sources were classified as either background (BKG) sources, supernova remnants (SNR), or ionised hydrogen regions (H ii). Fig. 3 were classified as background sources. Sources inside the boundary were classified as intrinsic (INTR) sources which are further classified as either SNRs or H ii regions. If X-ray radiation was detected from the intrinsic source, then it was classified as a SNR candidate since the violent explosions and shockwaves produced by a supernova are known to emit strong X-rays. Also, SNRs usually exhibit steeper spectral index of about α < −0.5 (Filipovic et al. 1998). Sources emitting infrared radiation but no detectable X-rays were classified as H ii regions. Of the 66 detected radiocontinuum sources, 46 were classified as background sources, 11 as H ii regions, and 6 as SNR candidates. The full list of sources with positions, flux densities, spectral index and classifications can be found in Table 6.

Sources outside of the extended emission boundary seen in
The low number of detected SNR candidates suggests that the star formation rate for NGC 55 is relatively low. To estimate the high mass star formation rate, we used the relation described by Kennicutt (1983). We adjusted the estimated Hα luminosity of NGC 55 given by Hoopes et al. (1996) to 4.08×10 40 ergs s −1 to reflect our adopted distance of 2.08 Mpc. The relation provided estimated high mass star formation rate of 0.06M year −1 . Compared to the star formation rates of other galaxies in the Sculptor Group (the highest being NGC 253 at 0.20M year −1 , and the lowest being NGC 45 at 0.01M year −1 ), the star formation rate of NGC 55 is comparatively low.
In Fig. 7 we show the spectral index distribution of all radio-continuum sources found in this study (Table 6; Col. 9). Here, we note that the spectral index alone cannot successfully distinguish between various type of sources. Of the 66 sources reported here, 10 have an estimated spectral index of steeper than −0.85, classifying them as candidates for compact steep spectrum (CSS) sources (Table 6; Col. 13).

Spectral Index Map
We show in Fig. 8 an image where each pixel represents the spectral index calculated across all 4 observed frequencies. This image was created based on measurements from Figs. 3, 4, 5 and 6 after convolving the images to the largest beam size. Pixels below the noise level are ignored.
The core of galaxy is shown to have a shallow spectral index of between −0.2 and −0.4 (shown as red pixels). This indicates that the core of the galaxy is a region of high thermal radiation output. The most likely reason for this is that the core of the galaxy is a dense star forming region. The spectral index becomes steeper moving further away from the centre of Table 5 Image details of ATCA single and merged projects of NGC 55 mosaics at 20, 13, 6 and 3 cm that were used to produce the merged images used for the measurements given in this paper. Merged images were created using jointdeconvolution of all projects.

ATCA
Centre ν Synthesised Beam r.m.s Figure  Project (      Table 6 List of point sources in the NGC 55 field at λ =20, 13, 6 and 3 cm. RA (3) and Dec (4) are in J2000 coordinates. Column 9 is the best fit spectral index for all flux measurements of a source. Blank cells indicate no measurement was detected. Asterisks (*) indicate no coverage was available. Daggers ( † ) indicate sources that were resolved as a single source at λ = 13 cm but as multiple sources at other wavelengths. These sources were therefore not included in the estimation of α. NGC 55, indicating a dominance of non-thermal radiation which could be caused by either synchrotron or inverse-Compton radiative mechanisms. Objects which radiate using this mechanism include SNRs and energetic jets.

CONCLUSION
We present and discuss a series of new highly sensitive, high resolution radio-continuum images of NGC 55 at wavelengths 20, 13, 6 and 3 cm. These images were created by combining data collected from observations using the ATCA radio interferometer telescope. As a result of combining the data, the final images had dramatically reduced levels of noise with higher angular resolution when compared with previous studies. From the new images, a total of 66 radio sources were detected within the field of NGC 55, 33 of which were detected over multiple wavelengths. Of these 66 sources, 46 have been classified as background sources, 11 as H ii regions, and 6 as SNR candidates. Spectral indices were also calculated for these multi-wavelengh sources. A spectral index map was produced for the galaxy, revealing a high level of thermal radiation emission from the core of the galaxy. Several concentrations of high nonthermal radiation emission were also detected within the plane of the galaxy. cility which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. This paper includes archived data obtained through the Australia Telescope Online Archive (http://atoa.atnf.csiro.au) and the NRAO Science Data Archive (http://archive.nrao.edu).