1. Introduction
Interactions and mergers play a major role in the evolution of galaxies (Barnes & Hernquist Reference Barnes and Hernquist1991, Reference Barnes and Hernquist1996; Hopkins et al. Reference Hopkins, Hernquist, Cox and Kereš2008; Blumenthal & Barnes Reference Blumenthal and Barnes2018; Schawinski et al. Reference Schawinski, Dowlin, Thomas, Megan Urry and Edmondson2010; Yoon et al. Reference Yoon, Park, Chung and Lane2022). Interactions between galaxies can explain the ‘bridges’ and ‘tails’ seen in disturbed galaxy pairs. They also lead to galactic mergers that often trigger bursts of star formation (Zwicky Reference Zwicky1956; Toomre & Toomre Reference Toomre and Toomre1972; Larson Reference Larson1990; Barnes Reference Barnes, Kennicutt, Schweizer, Barnes, Friedli, Martinet and Pfenniger1998).
Violent encounters between galaxies can change the morphology, star formation rate (SFR), metallically and gas content, converting star-forming spiral galaxies to non-star-forming ellipticals (Schweizer & Seitzer Reference Schweizer and Seitzer1992; Kaviraj et al. Reference Kaviraj, Tan, Ellis and Silk2011). Interactions and mergers can trigger strong central starbursts and AGN activity in galaxies; during galaxy encounters, gravitational torques can remove the angular momentum from the gas in the disc systems resulting in large gas inflows into the galactic centres (Barnes & Hernquist Reference Barnes and Hernquist1991; Mihos, Christopher, & Hernquist Reference Mihos and Hernquist1996; Bournaud Reference Bournaud, Smith, Higdon, Higdon and Bastian2010). Compression of gas and dust clouds can speed up the rate of star formation. During interactions, tidal forces can generate long tidal structures like tails and bridges of gas, dust, and stars. These tidal features, around the galaxies, may contain self-gravitating star-forming clumps of dwarf galaxy masses and sizes, known as tidal dwarf galaxies (TDGs). TDGs are formed in situ from the gas and stellar matter that is pulled out of the discs of parent galaxies into the intergalactic space during interactions (Duc et al. Reference Duc, Brinks, Springel, Pichardo, Weilbacher and Mirabel2000, Reference Duc, Braine, Lisenfeld, Brinks and Boquien2007; Hancock et al. Reference Hancock, Smith, Struck, Giroux, Appleton, Charmandaris and Reach2007, Reference Hancock, Smith, Struck, Giroux and Hurlock2009). Though TDGs resemble normal independent dwarf irregulars (dIrrs) and Blue Compact Dwarf galaxies in the Universe (Duc & Mirabel Reference Duc, Mirabel, Barnes and Sanders1999) in terms of their size, mass, and SFR, they are still different from normal dwarf galaxies. Compared to normal dwarf galaxies that are of primordial origin, TDGs are generally metal rich and are considered to be free of dark matter (Duc & Mirabel Reference Duc, Mirabel, Barnes and Sanders1999; Duc et al. Reference Duc, Brinks, Springel, Pichardo, Weilbacher and Mirabel2000; Barnes & Hernquist Reference Barnes and Hernquist1992; Hunter, Hunsberger, & Roye Reference Hunter, Hunsberger and Roye2000).
Numerical simulations (Barnes & Hernquist Reference Barnes and Hernquist1992; Elmegreen, Kaufman, & Thomasson Reference Elmegreen, Kaufman and Thomasson1993) as well as observations (Mirabel, Lutz, & Maza. Reference Mirabel, Lutz and Maza1991; Mirabel, Dottori, & Lutz Reference Mirabel, Dottori and Lutz1992; Duc & Mirabel Reference Duc and Mirabel1994; Yoshida, Taniguchi, & Murayama Reference Yoshida, Taniguchi and Murayama1994; Duc & Mirabel Reference Duc and Mirabel1994; Duc et al. Reference Duc, Brinks, Springel, Pichardo, Weilbacher and Mirabel2000; Boquien et al. Reference Boquien2009; Schechtman-Rook & Hess Reference Schechtman-Rook and Hess2012; Sengupta et al. Reference Sengupta, Scott, Paudel, Dwarakanath, Saikia and Sohn2017) point to the possibility of dwarf galaxy formation during galaxy interactions and collisions. Thus, there are two possible scenarios for the formation of dwarf galaxies in the Universe: the formation from collapse of primordial gas clouds and tidal dwarf formation from galaxy–galaxy interactions (Hunter et al. Reference Hunter, Hunsberger and Roye2000). According to Hunsberger Charlton, & Zaritsky (2000), galaxy interactions produce at least one-third to one-half of dwarf galaxies in compact groups. Kaviraj et al. (Reference Kaviraj, Darg, Lintott, Schawinski and Silk2012) estimated that
$\sim$
6 percent of dwarf galaxies in cluster environments could have a tidal origin.
1.1. The post merger galaxy NGC 7252
The NGC 7252 galaxy (also known as Arp 226 or Atoms-for-Peace galaxy) is a post-merger system formed as a result of the merger of two massive gas-rich disc galaxies having similar masses. Each galaxy has an approximate disc mass of 1.87
$\times10^{10}$
M
$_\odot$
(Chien & Barnes Reference Chien and Barnes2010). Observations reveal that NGC 7252 possesses a single bright nucleus; both the light distribution and the inner gas disc are centred on this nucleus (Schweizer Reference Schweizer1982). TDGs are observed at the tip of both the two tidal tails of NGC 7252, and a number of small star-forming knots are seen at the base of the tails, close to the remnant. A preliminary study on star formation in the NGC 7252 system is given in George et al. (Reference George2018a) where they estimated the SFR (uncorrected for internal attenuation) of the knots and TDGs in the system.
1.2. The NGC 5291 interacting system
The NGC 5291 system is another system, that is considered to have formed due to a violent galaxy–galaxy collision. The NGC 5291 interacting system lies at the edge of the Abell 3574 cluster, and it consists of an early type galaxy (ETG) called NGC 5291 (morphological type: E/S0) and a companion galaxy known as Seashell. A huge collisional HI ring surrounds the two interacting galaxies.
The NGC 5291 ring structure hosts several young star-forming knots, several TDG candidates and 3 bonafide TDGs. These TDG candidates are formed in the HI ring due to collision with a high-velocity impactor (Bournaud et al. Reference Bournaud2007); being of different origin, they may not strictly be called TDGs. However, for simplicity, the term TDG is used to refer to these dwarfs. A detailed study of the star-forming knots and TDGs in the NGC 5291 interacting system, made by making use of FUV and NUV data of the NGC 5291 system from UVIT, is presented in Rakhi et al. (Reference Rakhi2023).
Several interacting systems have been identified in the local Universe that are observed to have dwarf-sized objects identified as candidate TDGs in collisional debris/tidally drawn out material around them. However, it is possible that these objects may either be pre-existing dwarf entities or objects just apparently associated with the interacting system due to projection effects. The observed TDGs in the NGC 5291 and NGC 7252 systems are genuine condensations of gas and stars, have a local potential well, and possess higher metallicities (near solar metallicities) than that observed for typical independent dwarf galaxies, confirming that they are bonafide TDGs formed during the interaction process (Bournaud et al. Reference Bournaud2007; Lelli et al. Reference Lelli2015).
Investigation of ongoing star formation due to merger events and other galaxy interactions in the nearby Universe is of great importance as it gives insights into the formation of galaxies as well as their evolution. Galaxy-galaxy collisions and mergers involving disc galaxies can result in the transformation of disc systems to early-type galaxies (S0/E). Both NGC 7252 and NGC 5291 central galaxies are observed to be ETGs (Longmore et al. Reference Longmore, Hawarden, Cannon, Allen, Mebold, Goss and Reif1979; Schweizer Reference Schweizer1982; Hibbard et al. Reference Hibbard, Guhathakurta, van Gorkom and Schweizer1994). Interactions between galaxies can drive AGN activity. An AGN has been discovered by optical spectroscopy in the galaxy NGC 5291 (Zaw, Chen, & Farrar Reference Zaw, Chen and Farrar2019b,a). Studies have identified signatures of low luminosity AGN activity in the centre of the NGC 7252 remnant (Weaver et al. Reference Weaver2018; George et al. Reference George2018b). Li et al. (Reference Li2023) describes the AGN activity to be fading in the post merger remnant phase.
1.3. Ultraviolet observations
The ultraviolet (UV) continuum is used as a direct tracer of recent star formation. Massive stars (O, B, A types) which emit UV radiation copiously have short lifetimes, typically below
$\sim$
hundred million years (Kennicutt & Evans Reference Kennicutt and Evans2012; Calzetti Reference Calzetti, Falcón-Barroso and Knapen2013). FUV flux and NUV flux, both are sensitive to stars in the age range of about 0–100 and 0–200 Myr, respectively [Kennicutt Evans (Reference Kennicutt and Evans2012), Table 1]. Thus, we can obtain information about the recent star formation activity of extra-galactic systems by examining their deep UV images (Kennicutt & Evans Reference Kennicutt and Evans2012). Since galaxy-galaxy collisions trigger star formation activity in the central regions of interacting galaxies/merger remnants and along their tidal features, interacting systems are good laboratories to observe and study such star formation activity which is considered to be more frequent in the earlier phases of the Universe. With the launch of the Ultraviolet Imaging Telescope (UVIT) on board AstroSat, a stream of new high-resolution observational information on star formation has emerged in the past few years (George et al. Reference George2018a; Mondal, Subramaniam, & George Reference Mondal, Subramaniam and George2018, Reference Mondal, Subramaniam and George2019; Poggianti et al. Reference Poggianti2019; Mondal, Subramaniam, & George Reference Mondal, Subramaniam and George2021a; Mondal et al. Reference Mondal, Subramaniam, George, Postma, Subramanian and Barway2021b; Hota et al. Reference Hota2021; Ujjwal et al. Reference Ujjwal, Kartha, Subramanian, George, Thomas and Mathew2022; Joseph et al. Reference Joseph, Sreekumar, Stalin, Paul, Mondal, George and Mathew2022; Mahajan et al. Reference Mahajan, Singh, Postma, Pradeep, George and Côté2022; George Reference George2023; Robin et al. Reference Robin, Kartha, Akhil Krishna, Krishnan, Mathew, Cysil, Patra and Shridharan2024).
In the present paper, we compare star formation activity in the interacting galaxy systems, NGC 5291 and NGC 7252. The sample selection is influenced by the fact that the two systems have intense star formation in their interaction debris and have bonafide TDGs. High-resolution UVIT NUV and FUV data are available, and both systems are observed through the same NUV and FUV filters. The distances to the two systems are roughly the same. Also, though their formation scenarios are different, substructure formation in the tidal tails of NGC 7252 and the collisional ring of NGC 5291 is observed to be similar, justifying a comparison of the star formation in their interaction debris. Attenuation-corrected SFR is determined with the help of observations made with UVIT.
The paper is structured as follows: Section 2 describes the details of data reduction, source extraction, and identification of star-forming knots. Results are presented in Section 3 while Section 4 presents the discussion on the results. Conclusions are presented in Section 5.
Throughout the paper,
$\Lambda_{CDM}$
cosmology with Hubble parameter,
$H_0= 71$
km s
$^{-1}$
Mpc
$^{-1}$
,
$\Omega_M$
= 0.27 and
$\Omega_{\Lambda}$
= 0.73 (Komatsu et al. Reference Komatsu2011) is adopted.
Table 1. NGC 7252 and NGC 5291: UVIT observations.
Note:
$\lambda_{mean}$
and
$\Delta\lambda$
respectively are the effective wavelength and bandwidth of the filters. Details on the UVIT filters can be found in Tandon et al. (Reference Tandon2017).
2. Data and analysis
2.1. Data
The post-merger galaxy, NGC 7252 and the interacting system NGC 5291 were observed with the Ultraviolet Imaging Telescope (UVIT) on board AstroSat.
The details of the observations are summarised in Table 1. Fig. 1 shows the NUV images of NGC 7252 and NGC 5291 systems.
Figure 1. UVIT NUV images of the (a) NGC 7252 and (b) NGC 5291 systems. North is up and east is towards the left of the image. The colour scale is in units of erg/s/cm
$^2/$
Å. (a) The NGC 7252 system: Two tidal tails, the eastern tail and the north-western tail, extend from the NGC 7252 remnant (labeled NGC 7252). Star-forming knots are visible along these tails. (b) The NGC 5291 system: Several star-forming knots extend toward the north, south and west tracing a fragmented ring structure.
2.2. Data reduction
Both NGC 7252 and NGC 5291 Level 1 (L1) UVIT data are reduced to Level 2 (L2) science ready images using CCDLAB pipeline (Postma & Leahy Reference Postma and Leahy2017, Reference Postma and Leahy2021). UVIT data is flat-fielded and corrected for fixed pattern noise, distortion, and drift using CCDLAB. The orbit-wise images are aligned to a common frame before merging. CCDLAB provides an option to optimise the Point Spread Function (PSF) of the source to get the best PSF, which corresponds to an improved, narrower radial profile (Postma & Leahy Reference Postma and Leahy2021). The NUV and FUV master images are PSF optimised ones. To optimise the PSF, CCDLAB pipeline analyses time-bins of photon centroids about the mean positions of their corresponding sources. In effect, this generates a residual drift series which can then be used to optimise the PSF (Postma et al. Reference Postma, Tandon, Leahy and Hutchings2023). The automated WCS solver in CCDLAB (Postma & Leahy Reference Postma and Leahy2020) is then used to align the images with respect to the sky coordinates. The final NUV and FUV master images have an array size of 4096
$\times$
4096 pixels where the size of a single square pixel corresponds to 0.416′′ in the sky. We computed the FWHM for 9 stars near to the galaxy by fitting a Gaussian to the light profile. The median value of which is then taken as the FWHM of the PSF of the imaging data. The final PSF of the NUV and FUV images thus obtained are
$\sim$
1.10′′ and
$\sim$
1.34′′ respectively.
Since both the NGC 7252 remnant and the NGC 5291 galaxy have regions with emission from AGN at the centre (Weaver et al. Reference Weaver2018; Zaw et al. Reference Zaw, Chen and Farrar2019a), source extraction is performed after masking the AGN in the images. For the NGC 7252 remnant, George et al. (Reference George2018b) estimated the size of the AGN-dominated region to be 1.3 kpc. However, there is no such estimate available for the NGC 5291 galaxy. Hence, for both the galaxies, the AGN contribution to the flux is removed by masking the central regions using an aperture with a diameter of 1.3 kpc. All fluxes provided in the present work are thus the values obtained after removing the AGN contribution.
2.3. Source extraction and identification of the knots in NGC 7252
Source extraction is performed using the ProFound source extraction package (Robotham et al. Reference Robotham, Davies, Driver, Koushan, Taranu, Casura and Liske2018). The package includes a suite of low, mid, and high-level functions for simple and advanced source extraction.Footnote a The highest level ProFound function, profoundProFound, detects sources, generates a segmentation map, and extracts photometry. The function detects pixels from the input image that are above a given threshold limit and uses a watershed de-blending algorithm for deblending the pixels. The deblended pixels make up a segment, and each segment corresponds to a source in the image. The segments are dilated iteratively until flux convergence is reached. The function generates a dilated segmentation map that shows the extent of each segment. The photometry is extracted from this map. Since the function does not assume a fixed aperture for extracting photometry, rather performs photometric extraction from the dilated segments, the ProFound source extraction package is most suitable for the extraction of flux from the sources with complex morphology (Robotham et al. Reference Robotham, Davies, Driver, Koushan, Taranu, Casura and Liske2018).
Figure 2. NGC 7252 FUV image with segment contours overlaid.
The function profoundProFound performs well with the default parameters (See the documentationFootnote b for the details of input parameters). The values of the input parameters, viz., sky (mean sky background in image units), skyRMS (standard deviation of the sky pixels), skycut (the minimum threshold for detecting pixels in units of skyRMS), sigma (the smoothness parameter used for generating segmentation map), pixcut (the minimum number of pixels for object identification), magzero (magnitude zero point of the filter) are optimised for better image segmentation, to ensure a signal to noise ratio (SNR) > 5 for the detected sources and to get accurate source statistics including flux and magnitudes. Sources are extracted from the coarser resolution FUV image using the profoundProFound function and the FUV image segmentation map generated by the function is used for extracting photometry from the NUV image by forcing the FUV segmentation map on the NUV image.
The star-forming knots in the NGC 7252 system are identified by overlaying the ProFound-extracted sources on the FUV image and selecting those sources corresponding to the tidal tails and the main body of the NGC 7252 system through a visual inspection. The selected sources are further compared with the Dark Energy Camera Legacy Survey (DECaLS) image of the NGC 7252 system. The DECaLS images the sky in g, r and z optical bands using the telescope at the Cerro Tololo Inter-American Observatory (Dey et al. Reference Dey2019). The DECaLS image of the NGC 7252 system from the Data Release 10 (DR10) of the survey is utilised in the present study for the confirmation of knots. We performed a visual inspection of DECaLS colour composite images and confirmed that the six selected regions (including the merger remnant) appear bright blue. Fig. 2 shows the FUV image of NGC 7252 system with segment contours overlaid in red for the identified star-forming regions. For NUV and FUV images, flux calibration is carried out using the zero point and unit conversion factors given in Tandon et al. (Reference Tandon2017) and updated in Tandon et al. (Reference Tandon2020).
George et al. (Reference George2018a) studied six star-forming regions in the NGC 7252 post-merger system and estimated their SFRs (uncorrected for internal attenuation). In the present study, we have identified these star-forming regions (marked as A, B, C, D, E, H in Fig. 3). These regions are considered for further analysis.
2.4. Source extraction and identification of the knots in NGC 5291
The steps involved in the extraction of the sources and the identification of star-forming knots are explained in detail by Rakhi et al. (Reference Rakhi2023). A total of 206 sources were extracted from the FUV image by the ProFound source extraction program. The FUV segmentation map was then forced on the NUV image to extract the NUV photometric parameters. To identify the knots belonging to the NGC 5291 system, the FUV-NUV colour distribution of the extracted sources was examined, and those sources that fall within one standard deviation from the mean were considered. Those sources that fall within the HI contour were then compared with the DECaLS optical image and, a total of 57 knots which were bright blue were selected for detailed analysis. The FUV image of the NGC 5291 system with segment contours overlaid in red for the identified star-forming regions are shown in Fig. 4.
Figure 3. NGC 7252 FUV image with selected knots marked. D is the main body NGC 7252 remnant. North is up and east is towards the left of the image.
2.5. Correction for Galactic extinction
Galactic extinction is estimated from the Cardelli extinction law (Cardelli, Clayton, & Mathis Reference Cardelli, Clayton and Mathis1989) by taking
$R_v=3.1$
.
For the UVIT FUV (
$\lambda_{mean}$
=1481 Å) and NUV (
$\lambda_{mean}$
=2 418 Å) bands, Galactic extinction is given by the following equations respectively:
\begin{align} A_{FUV}\,\,(Galactic) &= 8.34 \times E(B-V) \end{align}
\begin{align} A_{NUV}\,\,(Galactic) &= 7.75 \times E(B-V) \\[12pt]\nonumber \end{align}
We use the E(B-V) value of 0.0259
$\pm$
0.0002 mag from the Schlafly & Finkbeiner (Reference Schlafly and Finkbeiner2011) reddening mapFootnote
c
for the computation of Galactic extinction in the direction of NGC 7252. The foreground Galactic extinction thus estimated are:
$A_{FUV}(Galactic)$
= 0.216 mag and
$A_{NUV}(Galactic)$
= 0.201 mag.
Figure 4. NGC 5291 FUV image with segment contours overlaid.
The Galactic extinction-corrected FUV and NUV magnitudes are further corrected for internal attenuation, which is explained in Section 3.1.
For the resolved knots in the NGC 5291 system (Rakhi et al. Reference Rakhi2023), Galactic extinction in the FUV and NUV bands are estimated using Equations (1) and 2 respectively; E(B-V) = 0.0543
$\pm$
0.0013 from Schlafly & Finkbeiner (Reference Schlafly and Finkbeiner2011) reddening map. Both NGC 7252 and NGC 5291 systems are observed using the same FUV and NUV UVIT filters (see Table 1 for details). The estimated values for Galactic extinction are:
$A_{FUV}$
(Galactic) = 0.453 mag and
$A_{NUV}$
(Galactic) = 0.421 mag.
3. Results
3.1. Computation of attenuation for NGC 7252 and NGC 5291
One possible measure of dust attenuation in star-forming galaxies is the slope of the UV continuum (Boquien et al. Reference Boquien2012; Overzier et al. Reference Overzier2011) along with the Meurer relation [hereafter M99] (Meurer, Heckman, & Calzetti Reference Meurer, Heckman and Calzetti1999). The UV continuum of star-forming galaxies is characterised by the spectral index
$\beta$
with
$f_\lambda \propto \lambda^\beta$
(Calzetti, Kinney, & Storchi–Bergmann Reference Calzetti, Kinney and Storchi-Bergmann1994) for
$\lambda \gt1\,200$
Å;
$f_\lambda$
(erg cm
$^{-2}$
$s^{-1}$
Å
$^{-1}$
) is the flux density of the source. For UVIT FUV and NUV passbands, the slope of the UV continuum is given by:
\begin{equation} \def\lumina\hspace{1.5cm} \beta_{UVIT}=1.88(m_{FUV}-m_{NUV})-2.0 \end{equation}
where
$m_{FUV}$
and
$m_{NUV}$
are the Galactic extinction-corrected FUV and NUV magnitudes respectively (Rakhi et al. Reference Rakhi2023).
The internal attenuation is calculated using the M99 relation (for starburst case):
\begin{equation}\def\lumina\hspace{1.5cm} A_{FUV}\,\,(Internal)=4.43+1.99\;\beta \end{equation}
where
$\beta$
is given in Equation (3).
The
$\beta$
values are estimated from the Galactic extinction-corrected FUV and NUV magnitudes of the star forming regions identified in the NGC 7252 and NGC 5291 systems and from the estimated
$\beta$
values,
$A_{FUV}(Internal)$
values are estimated using Equation (4). It is to be noted that, in the present study, the Galactic extinction-corrected magnitudes are considered for estimating
$\beta$
and
$A_{FUV}(Internal)$
of star forming regions in the NGC 5291 system, whereas it was not considered in Rakhi et al. (Reference Rakhi2023). As a result, there is a difference of 0.12 mag, between the estimated values of
$A_{FUV}(Internal)$
reported in this work and those presented in Rakhi et al. (Reference Rakhi2023).
We note that the choice of star formation history can affect the computed internal attenuation. As demonstrated in Boquien et al. (Reference Boquien2012), using a starburst case can overestimate the attenuation and hence the computed SFR by an order of magnitude, compared to normal star formation. However, we used the same star formation history for all the knots in both galaxies studied in the present work, and hence, at the spatial scales probed by UVIT, the intrinsic
$\beta$
values of the knots could be slightly different from the values obtained here, due to slight differences between the knots in their star formation histories. This is particularly relevant for knots on the disks and at different distances from the centre along the tidal tails.
The
$\beta$
values of the star-forming regions in the NGC 7252 interacting system are given in
Fig. 5. The
$\beta$
values of the knots range from −1.66 to −1.06 whereas the NGC 7252 post merger remnant has a
$\beta$
value of −0.34. For the knots in the NGC 5291 system, the values of
$\beta$
range from −2.26 to −1.79 while the
$\beta$
values of the resolved star forming regions of the galaxy main body of the NGC 5291 ranges from −1.17 to −0.08. For the Seashell galaxy,
$\beta$
ranges from −0.54 to 0.01.
Fig. 6 gives a comparison of the
$\beta$
values of the star forming regions (including the galaxy main bodies) of the NGC 7252 and NGC 5291 systems. The
$\beta$
values and hence the
$A_{FUV}(Internal)$
derived from the
$\beta$
values are considerably higher for the main bodies of the two systems. The galaxy disk can contain stars of different stellar populations with the evolved population contributing to the UV continuum. To check this, we use NUV-r colour analysis, which is described in Section 3.2.
Figure 5. The
$\beta$
distribution of the star forming regions including the TDGs (diamonds) in the NGC 7252 system.
Figure 6. Distribution of
$\beta$
of the star forming regions in the NGC 7252 and NGC 5291 systems.
3.2. NUV-r colour analysis
The ultraviolet continuum can have a contribution from the evolved population of stars on the horizontal branch with ages > 8 Gyr. This is particularly true for the flux from the disk of galaxies where multiple populations of stars can be present. In collisional ring where in situ star formation is happening, there cannot be the presence of such a population. But if old stars are pulled from the disk there can be contribution from evolved population. We check for the presence of any evolved population using the NUV-r colour of the detected knots on the disk and collisional debris of both the systems. NUV-r < 5.4 is expected from a young population, while NUV-r > 5.4 is expected from an old population of stars (Schawinski et al. Reference Schawinski2007; Kaviraj et al. Reference Kaviraj2007). The DECaLS DR10 r-band (effective wavelength = 6 382.6 Å Schlafly & Finkbeiner Reference Schlafly and Finkbeiner2011) images, of the NGC 5291 and NGC 7252 interacting systems, are used to estimate the r-band magnitudes of the identified star forming knots and the galaxy main bodies. The r-band images from the Dark Energy Camera (DECam) have a PSF FWHM
$\sim$
1.18" which is lower than the PSF FWHM of the UVIT FUV images. The photometry is extracted by forcing the FUV segmentation map extracted from ProFound on the r-band images. The r-band fluxes given in units of nanomaggies are converted to magnitudes using the conversion equation:
$AB \, mag = 22.5 - 2.5 \times \log_{10}(nanomaggy)$
, given in the DECaLS fits image header. The magnitudes obtained are corrected for Galactic extinction following the O’Donnell extinction law (O’Donnell Reference O’Donnell1994) before estimating the NUV-r colour.
Figure 7. Distribution of NUV-r colour of the star-forming regions in the NGC 5291 and NGC 7252 systems. The regions with NUV-r
$\gt$
3 are the resolved star forming regions of the galaxy main bodies.
The histogram of NUV-r colour of the star-forming regions in the NGC 5291 and NGC 7252 systems are shown in
Fig. 7 and the NUV-r colour of the main body and the knots of the two systems are given in
Fig. 8. Contour corresponding to surface brightness level of 22 mag/arcsec
$^{2}$
from the DECaLS z-band imaging data is displayed to trace the stellar disc of the NGC 5291 and Seashell galaxies (
Fig. 8). It is observed that the regions coincident with the central interacting galaxies of the NGC 5291 system exhibit NUV-r colours between 4.5 and 5.28, while the identified knots in the NGC 5291 system show NUV-r colours between 0.20 and 1.9. For the NGC 7252 system, the NUV-r colour of the merger remnant is 3.64 while the NUV-r colours of the knots range from 1.00 to 2.56. All the star forming regions in both the systems exhibit NUV-r < 5.4 indicating star formation in the last 1-2 Gyr (recent star formation (RSF)) (Schawinski et al. Reference Schawinski2007; Kaviraj et al. Reference Kaviraj2007). Though the NUV-r colours indicate no significant contribution to the UV continuum from hot evolved stars, there could be contribution to NUV emission by stars older than 100 Myr (Hao et al. Reference Hao, Kennicutt, Johnson, Calzetti, Dale and Moustakas2011). This could be the reason for the higher values of
$\beta$
for the interacting galaxies of NGC 5291 and the merger remnant of NGC 7252, where the NUV-r colours are redder than 3.5 mag. We note that the
$\beta$
values and the extinction values of the main bodies of galaxies in NGC 5291 and the merger remnant of NGC 7252 could be over-estimated and hence internal attenuation is not estimated for main bodies in further analysis.
Figure 8. NUV-r colour of the star forming regions (including the main body) (a) NGC 7252 and (b) NGC 5291 systems. The markers are resized to show the relative sizes of the star-forming regions. Contour at level 22 mag/arcsec
$^{2}$
from the z-band image is overlaid.
Table 2. Estimated parameters of the star-forming regions in the NGC 7252 interacting system.
Note:
$^\mathrm{a}$
ID as given in George et al. (Reference George2018a);
$^\mathrm{b}$
Uncorrected SFR (uncorrected for both Galactic extinction and internal attenuation);
$^\mathrm{c}$
SFR corrected for Galactic extinction;
$^\mathrm{d}$
Corrected SFR (Corrected for both Galactic extinction and internal attenuation).
3.3. Star Formation Rate (
$SFR_{FUV}$
) of the NGC 7252 knots and the NGC 5291 knots
For the estimation of
$SFR_{FUV}$
, we use the following relation (Iglesias-Páramo et al. Reference Iglesias-Páramo2006; Cortese, Gavazzi, & Boselli Reference Cortese, Gavazzi and Boselli2008).
\begin{equation}SFR_{FUV}[\mathrm{M}_\odot/\mathrm{yr}] = \frac{L_{FUV}[\mathrm{erg/sec}]}{3.83 \times 10^{33}}\times 10^{-9.51} \end{equation}
where,
$L_{FUV}$
is the luminosity. The flux density is converted to flux and to luminosity using the effective wavelength of the filter,
$\lambda_{mean}$
and the distance to the source. This relation is based on the assumption of a constant rate of star formation over a timescale of
$10^8$
years, with a Salpeter initial mass function (IMF) (Salpeter Reference Salpeter1955) for stars with masses from 0.1 to 100 M
$_\odot$
as described in Iglesias-Páramo et al. (Reference Iglesias-Páramo2006) and in Cortese et al. (Reference Cortese, Gavazzi and Boselli2008).
The attenuation is calculated from the slope of the UV continuum (
$\beta$
) derived from the FUV-NUV colours of each of the selected knots (see Equation 3). The corrected SFR is then derived from the dust attenuation-corrected FUV fluxes.
It is to be noted that adopting a Milky Way-like IMF (Kroupa Reference Kroupa2001 or Chabrier Reference Chabrier2003 IMF), which has fewer low-mass stars compared to the Salpeter IMF, would tend to reduce our SFR estimates. Specifically, assuming a Kroupa IMF instead of the Salpeter IMF would decrease the SFR values by a factor of approximately 1.6 (Calzetti Reference Calzetti, Falcón-Barroso and Knapen2013).
The estimated parameters of the star-forming regions in the NGC 7252 system are given in Table 2. For the star-forming knots in the NGC 7252 system, the estimated values of
$\beta$
range from −1.66 to −1.06. The numerical value of the internal attenuation,
$A_{FUV}(Internal)$
derived from
$\beta$
using the M99 relation for the knots ranges from 1.12 to 2.33. The integrated
$SFR_{FUV}(corr)$
(SFR corrected for both Galactic extinction and internal attenuation) of the knots in the system is 0.46
$\pm$
0.03 M
$_\odot$
/yr.
The estimate for the total
$SFR_{FUV}(uncorr)$
(SFR without correction for both Galactic extinction and internal attenuation) for the knots, excluding the remnant, is 0.092
$\pm$
0.002 M
$_\odot$
/yr while the integrated
$SFR_{FUV}(Galactic)$
(SFR corrected only for Galactic extinction) for the knots, excluding the remnant, is 0.11
$\pm$
0.002 M
$_\odot$
/yr. The SFR surface density,
$\Sigma_{SFR}$
Footnote
d
across the location of the knots and TDGs in the NGC 7252 system is shown in
Fig. 9.
Figure 9. SFR surface density,
$\Sigma_{SFR_{FUV}}$
, across the location of the knots (circles) and TDGs (diamonds) in the NGC 7252 system. The black cross marks the location of the remnant. (a)
$\Sigma_{SFR_{FUV}(uncorr)}$
, uncorrected for Galactic extinction and internal attenuation and (b)
$\Sigma_{SFR_{FUV}(corr)}$
, corrected for both Galactic extinction and internal attenuation.
For the knots in the NGC 5291 system, the
$\beta$
values range from −2.26 to −1.79 and the
$A_{FUV}(Internal)$
values range from 0.0 to 0.87 mag. The total
$SFR_{FUV}(corr)$
of the knots in the NGC 5291 system is 2.4
$\pm$
0.06 M
$_\odot$
/yr. The total
$SFR_{FUV}(uncorr)$
of the knots is 1.0
$\pm$
0.005 M
$_\odot$
/yr while the integrated
$SFR_{FUV}(Galactic)$
is 1.6
$\pm$
0.008 M
$_\odot$
/yr.
4. Discussion
4.1. Comparison of NGC 7252 with NGC 5291 system
The NGC 7252 system is a notable example of a post-merger galaxy formed as a result of the merger of two spiral galaxies. The nuclei of the two parent galaxies have merged into one and the merger remnant has tidal tails, loops, and other features typical of mergers (Schweizer et al. Reference Schweizer, Seitzer, Kelson, Villanueva and Walth2013). NGC 5291 comprises of the parent galaxy which is interacting with a companion named the Seashell galaxy. The NGC 5291 system is believed to have formed as a result of a past collision between two galaxies (Bournaud et al. Reference Bournaud2007) and is known for its prominent ring structure in HI along which several TDGs and TDG candidates are seen.
Both NGC 7252 and NGC 5291 are examples of galaxy-galaxy interactions that exhibit unique star formation patterns. Both systems experienced a burst of star formation as a result of the collision and/or merger and host bonafide TDGs. In the NGC 7252 system, both the remnant body and the outskirts exhibit evidence of significant star formation. The central galaxies in the two systems have evolved into ETGs post-interaction. The NGC 7252 post-merger remnant shows indications of a gaseous disc and potential AGN feedback (Weaver et al. Reference Weaver2018; George et al. Reference George2018b). Similar to this, the NGC 5291 galaxy also hosts AGN (Zaw et al. Reference Zaw, Chen and Farrar2019a). These systems serve as prime examples of how galactic collisions and mergers may cause significant changes in the structure and dynamics of galaxies, resulting in bursts of star formation and the production of typical merger features. Such interacting systems are hence important laboratories to study both galaxy formation as well as evolution.
4.1.1 SFR in the main body of the NGC 7252 and NGC 5291 systems
The
$SFR_{FUV}(Galactic)$
in the main body of the NGC 7252 system is estimated to be 0.44
$\pm$
0.004 M
$_\odot$
/yr. For the NGC 5291 galaxy, the
$SFR_{FUV}(Galactic)$
is 0.053
$\pm$
0.001 M
$_\odot$
/yr. The
$\Sigma_{SFR}$
in the NGC 7252 main body is (5.4
$\pm$
0.05)
$\times$
10
$^{-3}$
M
$_\odot$
/yr/kpc
$^{2}$
while for the NGC 5291 galaxy, the
$\Sigma_{SFR}$
value is (1.7
$\pm$
0.04)
$\times$
10
$^{-3}$
M
$_\odot$
/yr/kpc
$^{2}$
. The main body of NGC 7252 system has a higher SFR and
$\Sigma_{SFR}$
than the main body of NGC 5291 system.
Figure 10. Histograms of area of the knots.
Figure 11. Comparison of (a) UV Slope
$\beta$
, (b)
$A_{FUV}(Internal)$
, (c)
$SFR_{FUV} (uncorr)$
, (d)
$SFR_{FUV}(corr)$
, (e)
$\Sigma _{SFR_{FUV}} (uncorr)$
and (f)
$\Sigma _{SFR_{FUV}} (corr)$
of the NGC 7252 post merger system with NGC 5291 interacting system.
Table 3. SFR of the knots and the main body of the interacting system NGC 5291 and post-merger system, NGC 7252.
4.1.2 Internal attenuation and SFR of the knots
The areas of the knots in the NGC 7252 and NGC 5291 systems are given in
Fig. 10.
Fig. 11 shows the comparison of estimated parameters-
$\beta$
,
$A_{FUV}(Internal)$
,
$SFR_{FUV} (uncorr)$
,
$SFR_{FUV}(corr)$
,
$\Sigma _{SFR_{FUV}} (uncorr)$
and
$\Sigma _{SFR_{FUV}} (corr)$
of the knots in the NGC 7252 post merger system with that of the knots in the NGC 5291 interacting system (Rakhi et al. Reference Rakhi2023). For the star-forming knots outside the NGC 7252 remnant, the estimated
$\beta$
values range from −1.66 to −1.06. For NGC 5291 system, the
$\beta$
range from −2.26 to −1.79. The numerical value of
$A_{FUV}(Internal)$
for the knots in the NGC 7252 system ranges from 1.12 to 2.33 while for the NGC 5291 system it ranges from 0.0 to 0.87. It is observed that the SFR values (both uncorrected and corrected for extinction) of individual knots in the NGC 7252 system lie within the range of SFR values exhibited by the knots in the NGC 5291 system.
Table 3 gives a comparison of the SFR in the tidal debris and main bodies of the NGC 5291 and NGC 7252 systems. The integrated
$SFR_{FUV}(corr)$
of the knots in the NGC 7252 and NGC 5291 systems are 0.46
$\pm$
0.03 and 2.4
$\pm$
0.06 M
$_\odot$
/yr respectively. It has to be noted that the total
$SFR_{FUV}(corr)$
of the knots in the NGC 5291 system is comparable to the total SFR in the disc of normal spiral galaxies i.e., SFR > 1 M
$_\odot$
/yr (Boquien et al. Reference Boquien2009).
Table 4. Atomic, molecular and total hydrogen gas content in NGC 5291 and NGC 7252 interacting systems (Malphrus et al. Reference Malphrus, Simpson, Gottesman and Hawarden1997; Braine et al. Reference Braine, Duc, Lisenfeld, Charmandaris, Vallejo, Leon and Brinks2001; Lelli et al. Reference Lelli2015).
Note: The
$M_{HI}$
corresponding to the NGC 7252 remnant is the mass of the so-called Western Loop (Dupraz et al. Reference Dupraz, Casoli, Combes and Kazes1990; Lelli et al. Reference Lelli2015).
The
$SFR_{FUV}(corr)$
values of the TDGs in NGC 7252 system – NGC 7252E and NGC 7252NW are 0.11
$\pm$
0.02 and 0.10
$\pm$
0.01 M
$_\odot$
/yr respectively while the same for the TDGs in NGC 5291 system – NGC 5291N, NGC 5291S and NGC5291SW are 0.40
$\pm$
0.03, 0.41
$\pm$
0.03 and 0.30
$\pm$
0.02 M
$_\odot$
/yr respectively. It is observed that TDGs in the NGC 7252 system show lower SFR and
$\Sigma_{SFR}$
than those in the NGC 5291 system (see also Table 5).
Table 5. Gas surface densities and SFR densities of TDGs of the NGC 5291 and NGC 7252 systems (Kovakkuni et al. Reference Kovakkuni2023).
Note: SFR and
$\Sigma_{SFR}$
correspond to the dust corrected values.
$M_{atom}$
and
$M_{mol}$
include contribution from helium and heavier elements. Note that the area corresponding to atomic gas, molecular gas, and SFR are the same and equal to the CO emitting area. The CO emitting area is given in Kovakkuni et al. (Reference Kovakkuni2023).
4.2. Observed trends in SFR in the NGC 7252 and NGC 5291 systems
While the main body of the NGC 7252 galaxy exhibits a higher SFR and
$\Sigma_{SFR}$
than that of NGC 5291/Seashell galaxies, the NGC 7252 TDGs along the tidal tails have lower SFR and
$\Sigma_{SFR}$
compared to the TDGs in the NGC 5291 collisional ring. A quantitative analysis of the gas content in the tidal debris and the main bodies of the NGC 7252 and NGC 5291 systems is essential for understanding these trends in the star formation activity. Estimates of the hydrogen gas content (atomic and molecular) is given in Table 4.
-
• NGC 5291
Malphrus et al. (Reference Malphrus, Simpson, Gottesman and Hawarden1997) performed a detailed high resolution study of the atomic hydrogen (HI) gas content in the interacting system NGC 5291 and estimated that the total HI mass of the entire system is 1.8
$\times$
10
$^{10}$
M
$_\odot$
. It was observed that approximately 9% of the total HI mass of the NGC 5291 system is found in its main body. The molecular hydrogen gas content in NGC 5291 main body is 1.7
$\times$
10
$^{9}$
M
$_\odot$
(Braine et al. Reference Braine, Duc, Lisenfeld, Charmandaris, Vallejo, Leon and Brinks2001) and the total molecular gas content in the TDGs is 7.4
$\times$
10
$^{8}$
M
$_\odot$
(Lelli et al. Reference Lelli2015). Approximately, 12% of the cooler gas is in molecular form and around 70% of the total molecular gas detected in the entire system is contained in the main body of NGC 5291. -
• NGC 7252
For the NGC 7252 post-merger system, the mass of HI detected in the tails is 5.2
$\times$
10
$^{9}$
M
$_\odot$
and the total HI mass of the entire system is 5.45
$\times$
10
$^{9}$
M
$_\odot$
. The remnant body is almost devoid of atomic gas (Hibbard et al. Reference Hibbard, Guhathakurta, van Gorkom and Schweizer1994). The mass of molecular hydrogen detected in the centre of NGC 7252 main body is 3.6
$\times$
10
$^{9}$
M
$_\odot$
and that detected in the tidal tails is greater than 0.02
$\times$
10
$^{9}$
M
$_\odot$
. For the entire system, the mass of molecular hydrogen detected is thus greater than 3.62
$\times$
10
$^{9}$
M
$_\odot$
(Lelli et al. Reference Lelli2015). Approximately, 40% of the cooler gas is in molecular form and the main body of NGC 7252 contains almost 99% of the total molecular gas found in the whole system.
There is more gas content (atomic + molecular) in the NGC 7252 main body than in the NGC 5291 main body and less gas in the outer tidal features of NGC 7252 than in the collisional ring of NGC 5291.
4.2.1 Star formation and gas content
The Kennicutt-Schmidt (KS) relation (Kennicutt Reference Kennicutt1998) is an empirical relation that connects the SFR and gas surface densities as
\[\Sigma_{SFR} \propto (\Sigma_{gas})^N\]
where N is the power law index,
$\Sigma_{SFR}$
is the SFR surface density and
$\Sigma_{\text{gas}}$
=
$\Sigma_{HI+H_{2}}$
is the total gas column density, which combines the contribution of atomic (
$\Sigma_{HI}$
) and molecular (
$\Sigma_{H_{2}}$
) gas.
The area, SFR densities, gas surface densities and the gas depletion time (
$\tau_{depletion}$
) for the TDGs in the NGC 5291 and NGC 7252 interacting systems are given in Table 5. The atomic and molecular masses are taken from Kovakkuni et al. (Reference Kovakkuni2023). High resolution CO data cubes (spatial resolution
$\sim$
2
$^{\prime\prime}$
) from ALMA are used for estimating molecular mass. Molecular mass is not detected in the TDGs NGC 5291SW and NGC 7252E. The atomic mass
$M_{atom}$
and the molecular mass
$M_{mol}$
include contribution from helium and heavier elements. The area over which the atomic mass, molecular mass and SFR are estimated is the same and corresponds to the CO emitting area (see Kovakkuni et al. Reference Kovakkuni2023). CO is confined to a small area within the TDGs. Consequently, the total CO emitting area is smaller than the size of the TDGs observed in the UV images. As a result, the SFR estimated from the CO emitting area (to ascertain the position of the TDGs in the KS plot) will be smaller than the actual SFR of the TDGs.
$\tau_{depletion}$
is estimated using the formula
$\tau_{depletion}=\Sigma_{gas}/\Sigma_{SFR}$
. From Table 5, it is seen that the TDGs in NGC 5291 system have higher
$\Sigma_{atom+mol}$
values compared to the TDGs in NGC 7252 system. The
$\Sigma_{SFR}$
values of the TDGs also exhibit the same trend.
Fig. 12 shows the location of the TDGs on the KS relation. The data for normal spirals and starburst galaxies are taken from Kennicutt (Reference Kennicutt1998). It is observed that the TDGs in the NGC 5291 and NGC 7252 systems are in the same regime with respect to the KS relation as normal spiral galaxies. This is in agreement with the recent results from Kovakkuni et al. (Reference Kovakkuni2023).
Figure 12. The location of TDGs on the Kennicutt-Schmidt relation (Gray markers, Kennicutt Reference Kennicutt1998). The blue dashed line with slope N = 1.4 is the original fit to the data (normal spirals and starbursts) from Kennicutt (Reference Kennicutt1998).
The NGC 5291 galaxy main body has a lower
$\Sigma_{H_{2}}$
(
$\Sigma_{H_{2}}$
= 49.7) than the NGC 7252 remnant (
$\Sigma_{H_{2}}$
= 92.5) (Braine et al. Reference Braine, Duc, Lisenfeld, Charmandaris, Vallejo, Leon and Brinks2001) and the
$\Sigma_{SFR}$
(Galactic extinction-corrected) value of NGC 5291 galaxy is lower than that of the NGC 7252 merger remnant. The significant star formation observed in the central part of NGC 7252 may be due to the compression of gas and thus the formation of dense molecular clouds in the central part, which in turn triggered the starburst. Previous studies (Mihos & Hernquist Reference Mihos and Hernquist1994; Di Matteo et al. Reference Di Matteo, Combes, Melchior and Semelin2007) show that gas inflows to the central areas occur in interacting systems. Also the gas from tidal tails is falling back into the NGC 7252 merger remnant body (Hibbard et al. Reference Hibbard, Guhathakurta, van Gorkom and Schweizer1994; Hibbard & Christopher Mihos. Reference Hibbard and Christopher Mihos1995; Chien & Barnes Reference Chien and Barnes2010). The lack of HI gas seen in the core of the NGC 7252 remnant body may be the result of a process that is changing the HI gas in the main body and in the tidal tails into other forms – molecular gas and stars. The HI gas gets compressed, condenses to molecular phase and forms stars. In the inner regions of the NGC 7252 merger remnant, our estimated SFR shows that gas is getting converted into stars at a high rate.
The observed trend in the SFR and
$\Sigma_{SFR}$
of the two systems can be attributed to several factors related to the merger history and subsequent environmental circumstances. The formation scenario, the age and strength of interaction, and the current evolutionary stage of interacting systems are some of the aspects that can explain the trend. The surface density of gas in the main body and in the tidal features, as well as the quantity of cold atomic and molecular gas available for star formation in the main body and tidal features are significantly influenced by these parameters.
5. Conclusion
The star formation activity in the tidal tails and main body of the NGC 7252 interacting system has been investigated using FUV and NUV data from the Ultra Violet Imaging Telescope (UVIT) on board AstroSat. A comparison of the results with a study based on UVIT data on star formation in the NGC 5291 interacting system (corrected here for Galactic extinction) is also presented.
The galaxies NGC 7252 and NGC 5291 are systems that have undergone interactions and/or collisions in the past, creating unique features. Star-forming knots of young, hot stars produced by the burst of star formation that followed the collision are found within the ring structure of the NGC 5291 system. The ring structure was generated as a result of collision. Tidal tails, loops and bridges are other features of galaxies that are remnants of gravitational interactions/merger. These are composed of stars and gas pulled out during the interaction. Both the systems studied have bonafide TDGs, are located at comparable distances from the Milky Way galaxy and the same FUV and NUV filters were used for both observations. The main results of this study are summarised as follows:
-
1. Six star-forming regions have been identified in the NGC 7252 system including the remnant body.
-
2. The NGC 7252 remnant exhibits significant star formation even in the absence of internal dust correction.
-
3. From the UV continuum slope,
$\beta$
, the internal attenuation towards each of the star-forming regions has been estimated for the NGC 7252 system. The total corrected SFR of the knots in the system is 0.46
$\pm$
0.03 M
$_\odot$
/yr. The total uncorrected SFR is estimated as 0.092
$\pm$
0.002 M
$_\odot$
/yr. -
4. A comparison of star formation in the NGC 7252 system with the star formation in the NGC 5291 system shows that the SFR of knots in NGC 7252 lie within the range of SFR observed for NGC 5291 knots.
-
5. For NGC 7252 system, the total SFR in the debris is 0.46
$\pm$
0.03 M
$_\odot$
/yr while for the NGC 5291 system it is 2.4
$\pm$
0.06 M
$_\odot$
/yr which is relatively high. The integrated corrected SFR of the star-forming knots along the NGC 5291 collisional ring is comparable to the total SFR in the disc of normal spiral galaxies. -
6. The NGC 7252 remnant exhibits a higher SFR and SFR density than the main body of NGC 5291 while the SFR and SFR density of the NGC 7252 TDGs is lower than that of the NGC 5291 TDGs. This observed reversal in star formation trends can be attributed to the formation scenario, strength and age of interaction, and their present state of evolution. All these factors are observed to have an effect on the spatial distribution of atomic and molecular gas available for star formation. The main body of NGC 7252 has more gas and greater molecular gas surface density than the main body of NGC 5291, which results in a higher SFR as well as SFR density. The TDGs in the collisional ring of NGC 5291, in contrast, contain more gas and higher gas surface density than the TDGs in the tidal tails of NGC 7252. This is consistent with the TDGs in the NGC 5291 system having a greater SFR and SFR density than those in the NGC 7252 system.
-
7. The TDGs in the NGC 5291 and NGC 7252 systems fall within the same KS relation regime as normal spiral galaxies.
Data availability statement
The Astrosat UVIT imaging data underlying this article are available in ISSDC Astrobrowse archive https://astrobrowse.issdc.gov.in/astro_archive/archive/Home.jsp
Funding statement
The authors acknowledge the financial support of ISRO under AstroSat archival data utilisation program (No. DS_2B-13013(2)/9/2020-Sec.2). This publication uses data from the AstroSat mission of the Indian Space Research Organisation (ISRO), archived at the Indian Space Science Data Centre (ISSDC).
Appendix
Table A1. Observed FUV, NUV and DECaLS r band fluxes for the star forming regions in NGC 5291 system (raw fluxes uncorrected for Galactic extinction and internal attenuation).
Note: Effective wavelengths of the filters: UVIT FUV: 1 481 Å, UVIT NUV: 2 418 Å, DECaLS r-band: 6 382.6 Å.
Table A2. Observed FUV, NUV and DECaLS r band fluxes for the star forming regions in NGC 5291 system (raw fluxes uncorrected for Galactic extinction and internal attenuation). The FUV and NUV fluxes are the same as given in the Appendix of Rakhi et al. (Reference Rakhi2023).
Note: Effective wavelengths of the filters: UVIT FUV: 1 481 Å, UVIT NUV: 2 418 Å, DECaLS r-band: 6 382.6 Å.
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