Understanding the two-dimensional ionization structure in luminous infrared galaxies

Abstract

We investigate the two-dimensional excitation structure of the interstellar medium (ISM) in a sample of luminous infrared galaxies (LIRGs) and Seyferts using near-IR integral field spectroscopy. This study extends to the near infrared the well-known optical and mid-IR emission line diagnostics used to classify activity in galaxies. Based on the spatially resolved spectroscopy of prototypes, we identify in the [FeII]1.64 µm/Brγ − H 2 2.12 µm/Brγ plane regions dominated by the different heating sources, i.e. active galactic nuclei (AGNs), young main-sequence massive stars, and evolved stars i.e. supernovae. The ISM in LIRGs occupy a wide region in the near-IR diagnostic plane from −0.6 to +1.5 and from −1.2 to +0.8 (in log units) for the [FeII]/Brγ and H 2 /Brγ line ratios, respectively. The corresponding median(mode) ratios are +0.18(0.16) and +0.02(−0.04). Seyferts show on average larger values by factors ∼2.5 and ∼1.4 for the [FeII]/Brγ and H 2 /Brγ ratios, respectively. New areas and relations in the near-IR diagnostic plane are defined for the compact, high surface brightness regions dominated by AGN, young ionizing stars, and supernovae explosions, respectively. In addition to these high surface brightness regions, the diffuse regions affected by the AGN radiation field cover an area similar to that of Seyferts, but with high values in [FeII]/Brγ that are not as extreme. The extended, non-AGN diffuse regions cover a wide area in the near-IR diagnostic diagram that overlaps that of individual excitation mechanisms (i.e. AGN, young stars, and supernovae), but with its mode value to that of the young star-forming clumps. This indicates that the excitation conditions of the extended, diffuse ISM are likely due to a mixture of the different ionization sources, weighted by their spatial distribution and relative flux contribution. The integrated line ratios in LIRGs show higher excitation conditions i.e. towards AGNs, than those measured by the spatially resolved spectroscopy. If this behaviour is representative, it would have clear consequences when classifying high-z, star-forming galaxies based on their near-infrared integrated spectra.

A SINFONI view of the nuclear activity and circumnuclear star formation in NGC 4303

“…3). [Fe ii]/Brγ is smaller than 2 for all starforming regions, being consistent with values expected for youngstellar clusters (Colina et al 2015). The H 2 /Brγ ratio for most starforming regions is smaller than 0.6, as expected for a young cluster, with exception of the regions G and H (identified in Fig.…”

“…For the nucleus, we obtain H 2 /Brγ= 1.4 ± 1.2 and [Fe ii]λ1.64/Brγ =3.6 ± 2.8 using the values listed in Table 1. These values are typical of Seyfert nuclei, but also of excitation by shocks from SNe (Reunanen, Kotilainen & Prieto 2002;Rodríguez-Ardila et al 2004;Rodríguez-Ardila, Riffel & Pastoriza 2005;Colina et al 2015). They are also significantly higher than those observed in the ring, for example, from regions A and B (H 2 /Brγ∼0.4 and [Fe ii]λ1.64/Brγ ∼1.0) where the presence of young (2.5-7.5 Myr) clusters dominating the ionization is well established from UV spectroscopy (Colina et al 2002).…”

“…The black dots correspond to all observed spaxels and are mostly located in the region of SNe and/or AGN, with some points covering part of the star-formation (SF) region. This behavior is typical of luminous star-forming infrared galaxies and interpreted as due to excitation by combined effects of the AGN and stellar ionizing radiation plus shocks in AGN outflows, stellar winds and supernovae explosions (Colina et al 2015).…”

“…Colina et al 2015;Dors et al 2012;Rodríguez-Ardila, Riffel & Pastoriza 2005;Rodríguez-Ardila et al 2004;Reunanen, Kotilainen & Prieto 2002). Both ratios present the smallest values at the locations of the star-forming clumps in the circumnuclear ring.…”

Shutting down or powering up a (U)LIRG? Merger components in distinctly different evolutionary states in IRAS 19115−2124 (the Bird)

“…The diagram progresses from young star formation ionisation at lower left, through AGN related ionisation in the middle to shock ionisation at upper right, with recent SN activity showing up at high [Fe II] / Paβ values. We also constructed a similar figure using the [Fe II] / Brγ ratio, as it is recently discussed in Colina et al (2015). It shows very similar information as Fig.…”