AstroSat-UVIT observations of a possibly interacting pair of galaxies in HCG 77

Abstract

We aim to study star-forming regions and the spectral energy distribution of two possibly interacting galaxies, PGC 56121 and PGC 56125, in the Hickson Compact Group 77. We utilized the far-ultraviolet (FUV) channel of the Ultra Violet Imaging Telescope (UVIT) on board AstroSat to observe and produce FUV images of the galaxies. Our FUV images are at a much higher resolution in comparison to those obtained from previous galaxy surveys by GALEX in the near-UV and those from PS-1, DSS. We have identified several star-forming regions in the two possibly interacting galaxies, PGC 56121 and PGC 56125. These two galaxies form a pair widely separated in redshift and are seen in projection. We also report the presence of a candidate tidal dwarf galaxy at the end of one of the tidal tails located to the east of the pair, and we identified three major star-forming regions in the tidal dwarf. The spectral energy distribution of three galaxies in the system is presented and analyzed to investigate the key physical properties, such as stellar mass, dust mass, total luminosity, and star formation history, of the three galaxies. Based on these observations and on a comparison with observations in radio, these three galaxies are probably part of a small group of interacting galaxies.

Figures (5)

• Figure 1: Far-UV images of galaxies in the field of view of the UVIT telescope while observing the HCG 77 region.
• Figure 2: Surface brightness contour map of three sources : PGC 56121, PGC 56125, and J15491758 +214951179. Left: The contour levels plotted are $4\sigma$, $5\sigma$, $7\sigma$, $11\sigma$, $16\sigma$, $22\sigma$, $34\sigma$, and $55\sigma$ above the background level on far-UV image. Regions highlighted in pink represent areas of higher flux density and are likely associated with active star-forming regions, while the green shaded areas denote more diffuse emission, possibly due to gas with lower luminosity. Objects analyzed in this study are outlined by white dashed regions. Right: Contours derived from the FUV data overlaid on the SDSS r-band image for a comparison and to identify star-forming and brighter regions.
• Figure 3: Star-forming regions identified within the diffuse gas of the system. Regions R1–R13 mark the star-forming sites detected through photometry. Background regions B1 and B2 were selected to estimate the background counts. Regions T1-T3 represent a gas trail that is also associated with star formation.
• Figure 4: Integrated SED of the system of two galaxies, PGC 56121 (top) and PGC 56125 (middle), and of J15491758 +214951179 (bottom) from FUV to mid-infrared region. In all the figures,the top panel shows the photometric points in green dots from Table \ref{['tab:photometry']}, and yellow circles are model photometric points at the same wavelength. The black line shows the best-fit model curve across all wavelengths in this regime, and the dashed blue line represents the unattenuated stellar emission. Here, $\chi^{2}$ depicts the total chi-square value of best-fit model and $z_{fit}$ is the redshift of the model used. The bottom panel shows the residuals of the photometric points in red dots along the black dashed line at y=0.
• Figure 5: Relative probability distributions of various physical parameters derived from SED fitting. The top row shows the likelihood curves of physical parameters for galaxy PGC 56121, and the middle and bottom rows show those for PGC 56125 and J15491758 +214951179, respectively. The best-fit values and their associated $16^{\text{th}}$ and $84^{\text{th}}$ percentile uncertainties are indicated.