Survey plan

The use of VST for this project has two main reasons: 

the large field of view (FOV) and the high arcsec-level angular resolution of OmegaCam.

The large FOV is necessary to map the structure of the galaxies’ outskirts out to an unprecedented radius, more than 10 Re (see Figure below). A bunch (18) of the Arp galaxies have been observed with the HST. Although HST images have better angular resolution than ground-based VST images, the FOV of the HST is significantly smaller than 1 square degree. Specifically, Arp images taken with ACS/WFC have a FOV approximately 300 times smaller than that of the VST, while those taken with WFC3 and WFPC2 are about 500 times smaller. Therefore, they provide a limited view of the faint features in the galaxies’ outskirts of the Arp targets. With the large area covered with VST, we can extend the detection and analysis of these features.

The large FOV, combined with the resolution of 0.21 arcsec/pixels is fundamental to improving the detection of the satellite galaxies in the outskirts and to resolving the fine features in the tidal tails, like the star-forming clumps. Furthermore, VST mounts a narrow band H𝛼 filter (NB_659) divided into 4 quadrants (see Drew et al. 2014), which are all centered at ~ 6590Å. Since our targets are all in the local Universe (z ~ 0), it will allow us to take advantage of the full VST FoV while searching for H𝛼 emission.

Observing strategy

Data in the g and r bands are acquired in dark and grey time (FLI < 0.3 - 0.5), respectively, under an average seeing ≤ 1.3 arcsec conditions. With the total integration times of 2.5 hrs in both bands, result in the measured surface brightness depth for a point source at 5 sigma, over an area of FWHM~1 arcsec, reaches μg ~ 27.3 mag/arcsec2 and μr ~ 28.9 mag/arcsec2, respectively.

Given the larger collecting area with the galactocentric radius, the limiting surface brightness of the azimuthally averaged surface brightness profiles are deeper: μg ~ 28 - 29 mag/arcsec2, μg ~ 27 - 30 mag/arcsec2, in the g and r bands, respectively.

Shallower data, with 1 hr of integration time, are acquired in the i band during the bright time, which allows us to reach azimuthally averaged surface brightness of μg ~ 25 mag/arcsec2.

The observing strategy for H𝛼 observations aims to detect H𝛼 emission with a surface brightness of  Σ(H𝛼) = 5 x 10-17 ergs/s/cm2/arcsec2. This level of depth guarantees the detection of H𝛼 emission both in the inner star-forming regions of galaxies and possibly diffuse gas emission (e.g. Boselli et al. 2018, Kleiner et al. 2021).

Since all targets have a major axis diameter D ≤ 3 arcmin, the sky background can be estimated directly on the science frame, by using a polynomial surface fit over the entire frame (see Iodice et al. 2022).

Summary of the total integration time for each filter

• g band: 2.5 hrs; FLI<0.3

• r band: 2.5 hrs; FLI<0.5

• i band: 1.5 hrs; FLI<0.9

• H𝛼 (NB_659): 1.0 hr; FLI<0.9

Since the selected targets span almost the whole range in RA, and the constraints set up cover all possible cases, i.e. dark time for the g band, grey time for the r band, and bright time for the i and H𝛼 filters, this project is a filler program.