Star Formation and Nuclear Activity in Local Starburst Galaxies: A Near-Infrared Perspective

Author: ORCID icon
Borish, Henry, Astronomy - Graduate School of Arts and Sciences, University of Virginia
Evans, Aaron, Department of Astronomy, University of Virginia

Near-Infrared spectroscopy provides a useful probe for viewing embedded nuclear activity in intrinsically dusty sources such as Luminous Infrared Galaxies (LIRGs). In addition, near-infrared spectroscopy is an essential tool for examining the late time evolution of type IIn supernovae as their ejected material cools through temperatures of a few thousand Kelvins. In this dissertation, I present observations and analysis of two distinct star-formation driven extragalactic phenomena: a luminous type IIn supernova and the nuclear activity of luminous galaxy mergers.

Near-infrared spectroscopy of a sample of 42 LIRGs from the Great Observatories All-Sky LIRG Survey (GOALS) were obtained in order to probe the excitation mechanisms as traced by near-infrared lines in the embedded nuclear regions of these energetic systems. The spectra are characterized by strong hydrogen recombination and forbidden line emission, as well as emission from ro-vibrational lines of H2 and strong stellar CO absorption. No evidence of broad recombination lines or [Si VI] emission indicative of AGN are detected in LIRGs without previously identified optical AGN, likely indicating that luminous AGN are not present or that they are obscured by more than 10 magnitudes of visual extinction. The [Fe II]/Pa-beta of the sample is well-correlated with the optical emission line classification, with LINERs and Seyferts having [Fe II]/Pa-beta > 0.3 and HII-region like objects having [Fe II]/Pa-beta < 0.3. Further, the optical classification of the comparison sample of nearby, normal star-forming galaxies, low luminosity LINERs and supernova remnants (SNRs) show a similar segregration, and is likely an indication that AGN and supernovae in LINERs and Seyferts are associated with shocks that liberate iron from grains into the gas phase. Comparison of these data with models of fast radiative shocks show an approximate agreement. The observed line ratios are not well fit by starburst photoionization models computed with MAPPINGS III. The discrepancy between the models and data are likely largely a function of the Fe abundances adopted for the models.

We fit non-linear least squares to the profiles from each detection of Paschen beta, Brackett gamma, and H2 S(1) 1-0 in our sample of near-IR spectra. Of the detected Pa-beta lines, 27(22) of 49 are fit by a single(double) Gaussian component. These fractions are 36(19) of 55 for Br gamma and 41(14) of 55 for H2. Faint broad components with velocity dispersions of ~ 650, 750, and 875 km/s are found in the Seyfert galaxies NGC 1275, NGC 7469, and NGC 7674 suggesting the presence of a broad line region. Five of the LIRGs show evidence of outflows in Pa-beta emission, with velocity offsets from the systemic velocity of > 100 km/s. The range in outflow velocities are significantly less than what is seen OH 119 micron - this difference may be a result of the ability of the far-infrared line to probe regions closer to the obscured starburst and/or AGN where the acceleration may be the strongest.

Temporal near-infrared (1 - 2.4 micron) observations of the Type IIn supernova SN 2010jl, which occurred in the tidally disrupted host galaxy UGC 5189A, were obtained to study the evolution of its near-infrared (NIR) emission. The data span the age range of 36 to 565 days from the earliest detection of the supernova. On day 36, the H lines show an unresolved narrow emission component along with a symmetric broad component. Over the next hundreds of days, the broad components of the H lines shift to the blue by 700 km/s, as is also observed in optical lines. The narrow lines do not show a shift, indicating they originate in a different region. We show the broad H emission is consistent with electron scattering in the circumstellar medium. In contrast to the optical He lines, the near-infrared He lines are strong with well-defined profiles. The He I 10830 feature has a narrow P Cygni line, with absorption extending to ~ 100 km/s and strengthening over the first 200 days, and an emission component which weakens with time. He I 10830 and 20587 lines both show an asymmetric broad emission component, with a shoulder on the blue side that varies in prominence and velocity from ~5500 km/s on day 108 to ~4000 km/s on day 219. The absence of the feature in the H lines suggests that this is from a He rich ejecta flow. This component may be associated with the higher velocity flow
indicated by X-ray observations of the supernova. At day 403, the continuum emission becomes dominated by a blackbody spectrum with a temperature of ~1900 K, suggestive of dust emission.

PHD (Doctor of Philosophy)
Galaxy Evolution, Star Formation, Active Galactic Nuclei
Issued Date: