Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Enhanced UV emission knot in the giant radio galaxy NGC 315: Hint of patchy star formation?

Bannanje Ananthamoorthy, Debbijoy Bhattacharya, Dipanjan Mukherjee, P. Sreekumar

TL;DR

This study uses multiwavelength UV to optical imaging of NGC 315 to characterize a distinct UV knot ~1.7 kpc from the center. By modeling the host galaxy with GALFIT and correcting for heavy dust attenuation, the authors derive a localized star-formation rate of $0.23\pm0.10\,M_{\odot}\,\mathrm{yr}^{-1}$ and an age of $160-290$ Myr for the knot, embedded in filamentary dust with $M_{dust}\approx5.9\times10^{5}\,M_{\odot}$. The knot’s UV emission is not aligned with the AGN jet and H$\alpha$ is not detected there, while CO is undetected, implying relatively low gas density (${n}\sim30-46\,\mathrm{cm}^{-3}$) and a possible deviation from the KS relation; these observations point to episodic local SF triggered by either minor mergers delivering gas or jet-driven mechanical feedback, or a combination thereof. The findings suggest that AGN feedback and minor mergers can locally enhance SF in an elliptical galaxy harboring a large-scale jet, with dust formation potentially connected to AGN processes; however, higher-resolution spectroscopic data are needed to confirm the knot’s association and to better constrain the gas-dust physics in this environment.

Abstract

High-resolution AstroSat-UltraViolet Imaging Telescope (UVIT) observations revealed a knot of UV emission, $\sim 1.7$ kpc away from the centre of NGC 315, a nearby elliptical galaxy hosting a giant (Mpc scale) radio source with a jet. We suggest that this patchy and spatially extended UV emission is likely due to ongoing star formation (SF) in the galaxy. The estimated SF rate (SFR) averaged over 100 Myr for the UV knot ($0.23\pm0.10$ M$_{\odot}$ yr$^{-1}$) is significantly higher compared to a typical elliptical galaxy. As the galaxy does not show the signatures of recent major mergers, the possible mechanisms for the triggered SF include AGN feedback or minor mergers. Hubble Space Telescope} (HST) observations reveal dust filaments that extend through a UV knot. The origin of dusty filaments, though not clear, could be associated with gas clouds as a result of a minor merger, cooled gas falling into the central BCG and/or condensing of the gas uplifted by AGN jet. No significant clumpy UV emission is observed in other regions along the dust filament. We speculate that mechanical feedback from the AGN jet could be playing a role in triggering SF in the UV knot.

Enhanced UV emission knot in the giant radio galaxy NGC 315: Hint of patchy star formation?

TL;DR

This study uses multiwavelength UV to optical imaging of NGC 315 to characterize a distinct UV knot ~1.7 kpc from the center. By modeling the host galaxy with GALFIT and correcting for heavy dust attenuation, the authors derive a localized star-formation rate of and an age of Myr for the knot, embedded in filamentary dust with . The knot’s UV emission is not aligned with the AGN jet and H is not detected there, while CO is undetected, implying relatively low gas density () and a possible deviation from the KS relation; these observations point to episodic local SF triggered by either minor mergers delivering gas or jet-driven mechanical feedback, or a combination thereof. The findings suggest that AGN feedback and minor mergers can locally enhance SF in an elliptical galaxy harboring a large-scale jet, with dust formation potentially connected to AGN processes; however, higher-resolution spectroscopic data are needed to confirm the knot’s association and to better constrain the gas-dust physics in this environment.

Abstract

High-resolution AstroSat-UltraViolet Imaging Telescope (UVIT) observations revealed a knot of UV emission, kpc away from the centre of NGC 315, a nearby elliptical galaxy hosting a giant (Mpc scale) radio source with a jet. We suggest that this patchy and spatially extended UV emission is likely due to ongoing star formation (SF) in the galaxy. The estimated SF rate (SFR) averaged over 100 Myr for the UV knot ( M yr) is significantly higher compared to a typical elliptical galaxy. As the galaxy does not show the signatures of recent major mergers, the possible mechanisms for the triggered SF include AGN feedback or minor mergers. Hubble Space Telescope} (HST) observations reveal dust filaments that extend through a UV knot. The origin of dusty filaments, though not clear, could be associated with gas clouds as a result of a minor merger, cooled gas falling into the central BCG and/or condensing of the gas uplifted by AGN jet. No significant clumpy UV emission is observed in other regions along the dust filament. We speculate that mechanical feedback from the AGN jet could be playing a role in triggering SF in the UV knot.

Paper Structure

This paper contains 21 sections, 5 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Data
  3. AstroSat-UVIT
  4. Swift-UVOT
  5. GALEX
  6. HST
  7. Analysis and Results
  8. Determination of FUV flux from the UVIT knot
  9. SFR of the UV knot
  10. Age and mass of star-forming knot
  11. Dust distribution
  12. Optical dust extinction and dust mass:
  13. Discussions and Conclusions
  14. Origin of UV knot emission
  15. UV emission from the main body of the galaxy
  16. ...and 6 more sections

Figures (5)

  • Figure 1: Panel (a): RGB composite colour image of the central 25$^{\prime\prime}$ region of NGC 315 with FUV (from AstroSat-UVIT) in blue, NUV (from uvw1 filter of Swift-UVOT) in green, and optical (from HST F555W filter) in red. Solid white contours correspond to the radio emission observed with 1.4 GHz VLA observations (obtained from the VLA archive), representing the direction of the jet. The dashed yellow circle corresponds to a $3^{\prime \prime}$ region centred at the knot of UV emission in all panels. The cyan box corresponds to a box of width 20$^{\prime\prime}$ centred at NGC 315. Panels (b)–(g) show flux or colour maps of the central $10^{\prime\prime}$ region. Panel (b): FUV F154W filter. Panel (c): Swift-UVOT stacked image in $uvw1$, $uvw2$, and $uvm2$ filters. Panel (d): GALEX stacked image in FUV and NUV filters. Panel (e): Swift UVOT uvw1- uvw2 color map. The dotted white contour corresponds to a colour of -0.87, which is a $1\sigma$ contour level obtained by fitting a 2D Gaussian around the peak value. Panel (f): GALEX NUV-FUV colour map. The dotted white contour corresponds to a colour of -0.73, which is a $1\sigma$ contour level obtained by fitting a 2D Gaussian around the peak value. Panel (g): HST F555W-F814W colour map.
  • Figure 2: GALFIT modeling of NGC 315 in the central $100\times100$ pixels of UVIT F154W filter observations. Panel (a): Observed data. Panel (b): Best-fitting GALFIT model. Panel (c): 2D Residue image. The position of NGC 315 and the UV knot are shown in '$+$' and '$\times$' symbols, respectively. Panel (d): 1D profile of data and model components in a slice of 3 pixel width ($\sim$FWHM), along the box as shown in panel (b). For plotting the residual, a method similar to that in XSPEC delchi routine (https://heasarc.gsfc.nasa.gov/xanadu/xspec/manual//node113.html) is used.
  • Figure 3: Colour-colour diagram for UV knot overlaid on the stellar population synthesis tracks and colour excess, E(B-V), for different ages.
  • Figure 4: GALFIT modeling of NGC 315 in the central $440 \times 440$ pixels of HST F555W filter observations. Panel (a): Observed data. Panel (b): Best-fitting GALFIT model. The white region corresponds to the central dust disc region, which is masked during the fit. Panel (c): 2D Residue image. White contours correspond to dust patches detected in absorption. The dashed circle corresponds to 1.5$^{\prime\prime}$ centred around the UVIT knot. Lower right panel: 1D radial profile, model components, and residue. For plotting the residual, a method similar to that in XSPEC delchi routine is used.
  • Figure 5: Lightcurve for the UV knot from UVOT observation in uvw1, uvw2 and uvm2 filters.