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Imaging and Photometric Studies of NGC 1316 (Fornax A) Using Astrosat/UVIT

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Imaging and Photometric Studies of NGC 1316 (Fornax A) Using Astrosat/UVIT J. Astrophys. Astr. (2021) 42:34 Ó Indian Academy of Sciences https://doi.org/10.1007/s12036-021-09708-4Sadhana(0123456789().,-volV)FT3](0123456789().,-volV) SCIENCE RESULTS Imaging and photometric studies of NGC 1316 (Fornax A) using Astrosat/UVIT NILKANTH D. VAGSHETTE1,*, SACHINDRA NAIK2, NEERAJ KUMARI2 and MADHAV K. PATIL3 1Department of Physics and Electronics, Maharashtra Udayagiri Mahavidyalaya, Udgir 413 517, India. 2Astronomy and Astrophysics Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India. 3School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded 431 606, India. *Corresponding Author. E-mail: [email protected] MS received 28 October 2020; accepted 13 January 2021 Abstract. We present imaging and photometric studies of the radio galaxy NGC 1316 (Fornax A) using high spatial resolution near-ultraviolet (NUV) and far-ultraviolet (FUV) imaging telescopes of the first Indian multi-wavelength space observatory AstroSat. The residual maps of UV emission obtained from the subtraction of smooth models witness peculiar features within the central few kpc (1–2 kpc) region. The spatial correspondence between the radio emission maps and FUV imaging study reveal that the UV emitting sources are displaced away from the centre by the AGN outburst (radio jet). The presence of rims and clumpy structures in the outskirt of this galaxy delineate that the galaxy has acquired a large fraction of gas through merger-like events and is still in the process of settling. The estimates of the star formation rates À1 À1 (SFR) using FUV and NUV luminosities are found to be 0.15 M yr and 0.36 M yr , respectively, and provide the lower limit due to the screen effect. The estimated lower rates of SFR in this galaxy probably represent its quenching due to the AGN driven outflows emanating from the central engine of NGC 1316. Keywords. Galaxies: formation—ultraviolet: galaxies—galaxies: evolution—galaxies: star formation. 1. Introduction into the intergalactic space through complex gravita- tional effects. Schweizer (1980) reported the presence NGC 1316 (Fornax A) is a nearby (z ¼ 0:00587) of a compact disk of gas near its center that has dif- giant peculiar S0 galaxy hosting numerous tidal tails, ferent orientation and much faster rotation relative to shells, unusual dust patches all embedded within a the stars. All of these signs of complex dust features, much larger outer envelop of stars and a prominent shells, loops and other sub-structures evident in dust lane oriented along its optical minor axis (Malin NGC 1316 collectively point to its violent past built- & Carter 1983; Deshmukh et al. 2013). In addition to up through the merger of several smaller dust rich the intricate dust patches and shells, NGC 1316 also galaxies in distant past (Terlevich & Forbes 2002). hosts filamentary, nebular emission features, ripples, Schweizer (1980) based on the expansion time of the arcs and several complex filamentary loops of other outer stellar loops and Goudfrooij et al. (2001) on the phases of inter stellar medium (ISM) (Schweizer & spread in the ages of globular clusters demonstrate Seitzer 1988). The faint tidal tails – wisps and shells that the mergers might have happened between 1 to of stars evident around the galaxy suggest that they 3 Gyr ago and then NGC 1316 has continued accretion have been torn from their original locations and flung of smaller satellite galaxies (Iodice et al. 2017). NGC 1316 is one of the nearest Central Dominant This article is part of the Special Issue on ‘‘AstroSat: Five (CD) giant radio galaxy, at a luminosity distance of Years in Orbit’’ 25 Mpc (Wright 2006) that exhibit filamentary low- 34 Page 2 of 8 J. Astrophys. Astr. (2021) 42:34 ionization nuclear emission, an unresolved bright However, the Earth’s atmosphere acts as a main hin- nucleus in UV bands, and interestingly hosts a strong drance in acquiring UV data on such galaxies, as a radio core with steep spectrum and dual jets. The result, we need to rely on space-based observatories double-lobe radio continuum source from this galaxy like GALEX and/or AstroSat for observing such extend well beyond its telescopic field-of-view, galaxies in UV bands. Therefore, the UV observations spanning over several degrees on the sky (Ekers et al. of NGC 1316 acquired using AstroSat observatory are 1983; Drinkwater et al. 2001), and exhibit sharp edges crucial for understanding the star formation and hence in its outer part (Ekers et al. 1983). The S-shaped evolution of the galaxy. nuclear radio jet in the central region of NGC 1316 The merger episodes in NGC 1316 fuel the central appears to bend at south of NW shell and north of SE supermassive black hole and transform it into a strong blob due to the interaction between the radio jet and radio source, an Active Galactic Nucleus (Croton the ISM (Morokuma-Matsui et al. 2019). In the pho- et al. 2006; Oogi & Habe 2013; Oogi et al. 2016; tographic plates. NGC 1316 appears to interact with a Rodriguez-Gomez et al. 2016; Vagshette et al. close companion spiral galaxy NGC 1317 on its 2016, 2017, 2019; Pandge et al. 2012; Sonkamble North, however, this companion is not dominant et al. 2015). The mergers drive the external gas inflow enough to distort NGC 1316 at the observed scales. into the central part of the galaxy and hence also Though formation of NGC 1316 through merging activate the star formation in central region (Hopkins episodes is unquestionable, the composition and et al. 2008). But the ignited AGN develop powerful characteristics of the multi-phase ISM in the galaxy is jets, which then heat and blow up the surrounding ISM not yet fully understood. Relative to its very large and hence suppress the star formation (Croton et al. 11 stellar content 6 Æ 2 Â 10 M , the dust content of 2006; Fabian 2012). As a result, the detailed study of 6 NGC 1316 is very low 2 Â 10 M (Draine et al. the star formation and interaction of radio jets with the 2007; Deshmukh et al. 2013), and is probably due to a surrounding environment in merger remnants is 10 : 1 merger between a dominant, dust-poor early- interesting. Therefore, NGC 1316 is one of the suit- type galaxy and a smaller, gas-rich spiral (Lanz et al. able candidates of recent mergers to investigate star 2010). formation and interplay between the AGN and ISM. Our earlier study (Deshmukh et al. 2013)of Using recent high resolution radio observations wavelength-dependent nature of dust extinction over with MeerKAT, Serra et al. (2019) have demonstrated the range near-infrared (NIR) to UV revealed that the that NGC 1316 hosts a significant fraction of H I dust grains in NGC 1316 posses identical physical and emission distributed in a variety of scales i.e. all the chemical characteristics as that of the canonical grains way from its center till the large-scale environment. in the Milky Way and was confirmed through the Serra et al. (2019) found that the H I emission parallel extinction curve. However, the smaller values detected in the central 5 kpc as additional compo- of Rk indicate that the dust grains in the environment nent to the complex interstellar medium in this region: of NGC 1316 are relatively smaller than the canonical including dust (Carlqvist 2010; Deshmukh et al. grains (Patil et al. 2007). This study also quantifies the 2013), dust emission (Lanz et al. 2010), molecular gas dust mass assuming screening effect of dust (Patil (Horellou et al. 2001; Morokuma-Matsui et al. 2019), et al. 2007; Vagshette et al. 2012), to be ionised gas (Schweizer 1980; Morokuma-Matsui et al. 5 2:12 Â 10 M , while that estimated from the 2019), and X-ray emitting gas (this paper). Serra et al. MIPS far-infrared flux was found to be (2019) have also revealed the extension of H I emis- 7 3:21 Â 10 M . sion up to 70–150 kpc and oriented along both the As ultraviolet (UV) and Far-Infrared (FIR) emission tails of NGC 1316 with a total mass content of 8 originate from the young stars, therefore, the UV and 7 Â 10 M , roughly 14 times larger than the past IR luminosities are often used in literature for inves- detection. Interestingly, both these H I tails of tigating star formation histories of external galaxies NGC 1316 show a spatial association with the com- (Buat et al. 2010; Kennicutt & Evans 2012). Particu- plex optical tidal features confirming their common larly, the UV continuum at k\2000 A˚ provides with origin through the merging of a gas-rich progenitor the more reliable estimates of star formation rates system. They further propose that the merger was so (Salim et al. 2007). As a result, several attempts have significant that the tidal forces pulled a large fraction been made in the past to better understand the star of gas and stars out to larger radii apparent in the form formation in external galaxies using UV observations. of optical tails. J. Astrophys. Astr. (2021) 42:34 Page 3 of 8 34 Further, the low frequency observation between 154 filters. The characteristics of the filters, their mean and 1510 MHz shows that both the radio lobes exhi- wavelength and band width, exposure time, zero point bits a steep spectrum (Mancuso et al. 2017). These and unit conversion factors are tabulated in Table 1. steep spectrum lobes are basically characterised by Photometric calibrations for FUV and NUV filters optically thin synchrotron radio emission from jets were carried out by following the procedure described and they are associated with very massive black holes in Tandon et al.
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