The starburst-active galactic nucleus connection in the merger galaxy Mrk 938: an infrared and X-ray view★

Abstract

Mrk 938 is a luminous infrared (IR) galaxy in the local Universe believed to be the remnant of a galaxy merger. It shows a Seyfert 2 nucleus and intense star formation according to optical spectroscopic observations. We have studied this galaxy using new Herschel far-IR imaging data in addition to archival X-ray, UV, optical, near-IR and mid-IR data. Mid-and far-IR data are crucial to characterize the starburst contribution, allowing us to shed new light on its nature and to study the coexistence of active galactic nuclei (AGN) and starburst activity in the local Universe. The decomposition of the mid-IR Spitzer spectrum shows that the AGN bolometric contribution to the mid-IR and total IR luminosity is small [L bol (AGN)/L IR ∼ 0.02], which agrees with previous estimations. We have characterized the physical nature of its strong IR emission and constrained it to a relatively compact emitting region of ≤2 kpc. It is in this obscured region where most of the current star formation activity is taking place as expected for luminous IR galaxies. We have used Herschel imaging data for the first time to constrain the cold dust emission with unprecedented accuracy. We have fitted the integrated far-IR spectral energy distribution and derived the properties of the dust, obtaining a dust mass of 3 × 10 7 M . The far-IR is dominated by emission at 35 K, consistent with dust heated by the ongoing star formation activity.

Alma Observations of Warm Dense Gas in NGC 1614—breaking of the Star Formation Law in the Central Kiloparsec

“…By comparison, there is a weak AGN in NGC 34 according to the X-ray data (Brightman & Nandra 2011;Esquej et al 2012), and the ALMA results ) are consistent with the AGN being Compton thick. Nevertheless, for both galaxies, the central kiloparsec is dominated by starburst activity and AGN contributions to both dust and gas heatings are insignificant Stierwalt et al 2013).…”

“…f Total mass of molecular hydrogen gas (assuming X CO = 3 × 10 20 cm −2 (K km s −1 ) −1 ); NGC 1614: Sanders et al (1991); NGC 34: Kruegel et al (1990). g Total dust mass; NGC 1614: this work; NGC 34: Esquej et al (2012). h Total flux of the 8.4 GHz radio continuum (Schmitt et al 2006).…”

Warm molecular gas temperature distribution in six local infrared bright Seyfert galaxies

“…For the most distant galaxies in our sample, IRASF 05189-2524 and UGC 05101, these apertures encompass their mid-IR emission. Likewise, most of the mid-IR emission of NGC 34 and NGC 7469 is produced in the compact nuclear region (few kpc, Esquej et al 2012;Díaz-Santos et al 2010), thus no aperture matching corrections are needed. For the more nearby NGC 5135 and NGC 7130, Spitzer/IRS spectral mapping observations are available and it is possible to extract their mid-IR spectra from a 15 × 15 aperture (5 × 5 kpc) comparable to the SPIRE/FTS beam size and large enough to include most of their bright nuclear mid-IR emission.…”

“…(d) Logarithm of the IR luminosity, L(8-1000 μm), from Sanders et al (2003) scaled to the adopted distance. (e) AGN fractional contribution to the total luminosity from Esquej et al (2012), Veilleux et al (2009), andPereira-Santaella et al (2011). ( f ) Herschel observation ID of the SPIRE/FTS data.…”

Diffuse X-Ray-emitting Gas in Major Mergers

“…NGC 34 has been variously listed as a Seyfert 2 galaxy (Dahari 1985;Veilleux et al 1995;Véron-Cetty & Véron 2006) or a LINER (Osterbrock & Dahari 1983). According to Esquej et al (2012), the far-infrared light is mainly powered by a starburst. However, a joint analysis of the Chandra, NuSTAR, and XMM-Newton data for NGC 34 indicates a highly obscured AGN (Ricci et al 2017).…”