Superwind evolution: the young starburst-driven wind galaxy NGC 2782

MNRAS 467, 3788–3800 (2017) doi:10.1093/mnras/stx327 Advance Access publication 2017 February 9 Superwind evolution: the young starburst-driven wind galaxy NGC 2782 Jimena Bravo-Guerrero1‹ and Ian R. Stevens2‹ 1 Universidad 2 School Iberoamericana, Prol. Paseo de la Reforma 880, 01219 Lomas de Santa Fe, D.F., Mexico of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Accepted 2017 February 3. Received 2017 February 2; in original form 2016 November 16 We present results from a 30-ks Chandra observation of the important starburst galaxy NGC 2782, covering the 0.3–10 keV energy band. We find evidence of a superwind of small extent, which is likely in an early stage of development. We find a total of 27 X-ray point sources within a region of radius 2D25 of the galaxy centre and that are likely associated with the galaxy. Of these, 13 are ultraluminous X-ray point sources (ULXs; LX ≥ 1039 erg s−1 ) and a number have likely counterparts. The X-ray luminosities of the ULX candidates are 1.2–3.9 × 1039 erg s−1 . NGC 2782 seems to have an unusually large number of ULXs. Central diffuse X-ray emission extending to ∼3 kpc from the nuclear region has been detected. We also find an X-ray structure to the south of the nucleus, coincident with Hα filaments and with a 5-GHz radio source. We interpret this as a blow-out region of a forming superwind. This X-ray bubble has a total luminosity (0.3–10 keV) of 5 × 1039 erg s−1 (around 15 per cent of the total luminosity of the extended emission), and an inferred wind mass of 1.5 × 106 M . We also discuss the nature of the central X-ray source in NGC 2782, and conclude that it is likely a low-luminosity active galactic nucleus, with a total X-ray luminosity of LX = 6 × 1040 erg s−1 , with strong Fe line emission at 6.4 keV. Key words: galaxies: haloes – galaxies: individual: NGC 2782 – galaxies: ISM – galaxies: starburst – X-rays: galaxies. 1 I N T RO D U C T I O N Starbursts are brief (∼108 yr) episodes of intense star formation in galaxies. In starburst galaxies, the star formation rate (SFR) is high enough to consume the star-forming gas on a relatively short time-scale. Starbursts produce important effects on their environments, and can influence the structure, evolution and formation of galaxies, by returning energy and heavy element-enriched gas to their surroundings via outflows from the starburst stellar populations (Weedman et al. 1981; Moorwood 1996). Starburst-driven galactic winds evolution can be briefly described as follows: when the kinetic energy of the collective effect of the supernovae and stellar wind has been efficiently thermalized via shocks, a high-pressure zone of hot gas will expand, sweep up interstellar material and form an X-ray emitting bubble, with low density and temperatures of T ∼ 107 –108 K (Chevalier & Clegg 1985). This bubble will carry on expanding, sweeping up more ambient medium, forming a superbubble, which will continue to expand until its swept-up shell fragments due to Rayleigh–Taylor instabilities, blowing out the hot gas into the intergalactic medium. This way, the superbubble in most star-forming galaxies results in a bipolar superwind, perpendicular to the disc, that blows out some of the  E-mail: (JB-G); (IRS) interstellar medium (ISM) of the galaxy and much of the energy and material ejected by massive stars in the starburst (Heckman, Lehnert & Armus 1993; Strickland & Stevens 2000). Therefore, galactic winds are considered the primary mechanism of energy and metal enrichment of the intergalactic medium (Veilleux, Cecil & Bland-Hawthorn 2005). Theoretical and hydrodynamic models (Chevalier & Clegg 1985; McCarthy, van Breugel & Heckman 1987; Heckman et al. 1993; Mac Low & Ferrara 1999; Strickland & Stevens 2000; Sofue & Vogler 2001; Strickland et al. 2004a,b; Veilleux et al. 2005; Cooper et al. 2009; Creasey, Theuns & Bower 2013) have been developed over the years for galactic superwinds; they agree with the conceptual model described above, and been validated observationally by several multiwavelength studies of superwind galaxies. The observed morphology of superwinds can be quite different in different galaxies and will depend on the specific conditions within the galaxy. We see structures ranging from nuclear superbubbles to a biconical structures with filamentary morphology, and the scales can range from ∼1 to 20 kpc. The models mentioned above have focused on the thermal expansion of hot gas generated by the supernova explosions in the starburst, to drive the galactic scale outflows. Recently, Thompson et al. (2015) have considered the role that radiation pressure on dust can have on outflows from galaxies [both starburst dominated and active galactic nucleus (AGN) dominated]. If dusty material, in  C 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society ABSTRACT The young starburst-driven wind galaxy NGC 2782 1 Northern Extended Millimetre Array. Figure 1. Schematic diagram of the structure of the starburst region of NGC 2782 from Yoshida et al. (1999). The nuclear starburst region is surrounded by the highly warped H I molecular disc. The collective effect of supernovae and stellar wind from the nuclear starburst drives the bipolar superbubble out of the inner galactic plane. The inner H I molecular disc is compressed radially by the wind and a dense gaseous ring is formed, then the gravitational instability in the ring leads to active star formation. that the star formation efficiency (SFE) is increased by gravitational compression, due to its tidal nature. In this paper, we present results from a ∼30-ks Chandra observation of the starburst galaxy NGC 2782. In Section 2, we discuss the Chandra observations and data reduction. In Section 3, we discuss the analysis of the point sources encountered, their counterparts and the ultraluminous X-ray candidates. In Section 4, we discuss the extended X-ray emission. We present the results of the spectral analysis of the extended emission, the central region and the southern bubble in Section 5, and summarize in Section 6. 2 O B S E RVAT I O N S A N D DATA R E D U C T I O N NGC 2782 was observed with the Chandra Advanced CCD Imaging Spectrometer (ACIS-S) on 2002 May 15 (ObsID 3014), for 29.96 ks, with the source located on the S3 chip. In this paper, we only include data from the S3 chip. These data were reduced and analysed with the CIAO2 version 4.7, CALDB (4.5.6), HEASOFT (6.12) and XSPEC (v12.7.0) software packages. The Chandra reprocessing script was used to run the data processing threads from CIAO ACIS data preparation, set the specific bad pixel files, filter light curves and to create a new level 2 event file. After filtering on good time intervals (GTI) the effective exposure time was 29.58 ks. The image analysis was restricted to the ACIS-S3 (ccd id = 7) and the energy range of 0.3–10 keV. In addition, we also searched the Galaxy (...truncated)