Star formation and nuclear activity of local luminous infrared galaxies

Resumen

Luminous infrared galaxies (LIRGs) and ultraluminous infrared galaxies (ULIRGs) are galaxies with infrared (IR) luminosities (LIR) in the range between 1011 and 1012 L¿, and >1012 L¿, respectively. Intense star-formation bursts and/or an active galactic nucleus (AGN) are the power sources of these galaxies. These galaxies are unusual in the local universe, they represent less than 6 % of the IR emission in the local universe. Nevertheless, at high-redshift, z ~ 1¿2, (U)LIRGs have an important contribution to the star-formation rate (SFR) density of the Universe. Consequently, the study of local LIRGs is a fundamental piece to understanding their more distant counterparts. This thesis is focused on the study of local LIRGs. For this purpose we de¿ned a complete volume-limited sample of 53 local galaxies (0.009 < z < 0.017) with log (LIR/L¿) >11.05. We analyzed mid-IR imaging and spectroscopic data obtained with Spitzer, and X-ray data from XMM-Newton and Chandra of these local LIRGs. The Spitzer spectroscopic data include observations from two GTO programs. Likewise the XMM-Newton X-ray data were obtained through two guest observer programs. In addition we used Spitzer, XMM-Newton, and Chandra archival data. As a ¿rst step in our study, we characterized the mid-IR high-ionization emission lines produced by AGN and star-formation for a large sample of local galaxies, including LIRGs, observed with Spitzer (Chapter 3). In Chapter 4 we studied the spatially resolved mid-IR properties of a sample of 15 local LIRGs using Spitzer/IRS spectral mapping observations. Finally, in Chapter 5, we present the analysis of the X-ray properties of a representative sample of 27 local LIRGs. The main results and conclusions of this thesis are: ¿ Mid-IR High-ionization Lines from AGN and Star-forming Galaxies We studied the Spitzer mid-IR spectra of a sample of 426 galaxies, including quasars, Seyferts, LINERs, and H II galaxies. According to their IR luminosities 15 % of them are LIRGs. We focused on the high-ionization emission lines ([Ne V], [O IV], and [NeIII]) originated principally in the AGN and commonly used as tracers of its luminosity. We con¿rmed the tight linear correlation between the [Ne V]14.3 and 24.3 µm and the [O IV] 25.89 µm emission lines for a large range of AGN luminosities and types. The correlation between the [O IV] 25.89 µm and [NeIII] 15.56 µm emission lines holds only for active galaxies. Galaxies with strong star-formation have an excess of [NeIII] 15.56 µm emission with respect to that expected from the [O IV] 25.89 µm versus [NeIII] 15.56 µm correlation seen in AGN. That is, it is likely that the [NeIII] emission includes a noticeable contribution from star-formation. We also quanti¿ed the star-formation contribution to the [O IV] luminosity. Only when the AGN bolometric luminosity is below 5 % of the LIR, the [O IV] is dominated by star-formation. ¿ Mid-IR Emission of Local LIRGs The mid-IR spectral range contains a large number of features that can be used to disentangle the star-formation and AGN contributions to the total galaxy emission. Among these spectral features we analyzed the emission of the polycyclic aromatic hydrocarbon bands (PAHs), the ionized gas emission ([NeII], [NeIII], [S III], and [S IV]), and the molecular gas emission (warm H2). We found that, in general, the mid-IR spectra of these LIRGs are similar to those of local starbursts. The PAH ratios and equivalent widths are comparable to the values observed in starbursts. Moreover, their 9.7 µm silicate absorptions are moderate and intermediate between those of local starbursts and the extremely deep silicate features of local ULIRGs. However the nuclear [NeIII] 15.56 µm/[NeII] 12.81 µm ratios are lower than that of H II regions and nearby star-forming galaxies. Photoionization models predictions suggest that the increased densities in the nuclei of LIRGs are responsible for the smaller nuclear ratios. This includes the possibility that a fraction of the most massive stars are hidden in ultra-compact H II regions or that the initial mass function (IMF) is truncated at 30 M¿ in the LIRG nuclei. The warm (300¿1000 K) molecular hydrogen, traced by the H2 S(1) and S(2) lines at 17.0 and 12.3 µm, is likely to be excited by the same mechanism as the PAHs, based on their similar morphologies and the small variation of the H2 S(1)/PAH ratios (a factor of 2) in the sample. In some galaxies we found enhanced H2 emission which can be attributed to large scale shocks associated to galaxy mergers. The derived warm molecular hydrogen gas masses are in the range 0.4¿3×108 M¿. This is similar to the values measured in ULIRGs and starbursts. ¿ X-ray Emission of Local LIRGs Galaxies with intense star-formation, such as LIRGs, are also strong X-ray emitters. The X-ray emission of starburst galaxies is mainly produced by high-mass X-ray binaries (HMXB), supernova remnants, and hot gas heated by the energy released by supernova explosions. Both the soft (0.5¿2 keV) and hard (2¿10 keV) X-ray emissions are well correlated with the SFR in local starbursts. We found that most LIRGs follow these soft and hard X-ray luminosity versus SFR correlations. Only about 15 % of the non-Seyfert LIRGs have an excess hard X-ray emission relative to that expected from star-formation that might indicate the presence of an obscured AGN. The detection of the Fe K¿ emission line at 6.4 keV with a large equivalent width is a signature of Compton-thick AGN. However we did not detect the Fe K¿ emission line in the H II LIRGs. The line ¿ux upper limits were used to put an upper limit to the bolometric luminosity for an obscured AGN in the star-forming LIRGs, Lbol(AGN) <1043 erg s¿1 . That is, the AGN contribution to the total luminosity in these galaxies is < 10 %. Likewise the AGN contribution to the total energy output in our sample is between 7 % and 10 %.