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Register a new userPMAS Optical Integral Field Spectroscopy of Luminous Infrared Galaxies. II.-- Spatially resolved stellar populations and excitation conditions
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Almudena Alonso-Herrero
Publication date
2010
fields
Physics
and research's language is
English
Authors
Almudena Alonso-Herrero
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The general properties of luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) in the local universe are well known since large samples of these objects have been the subject of numerous spectroscopic works. There are, however, relatively few studies of large samples of LIRGs and ULIRGs using integral field spectroscopy (IFS). We analyze optical (3800-7200A) IFS data taken with the Potsdam Multi-Aperture Spectrophotometer (PMAS) of the central few kiloparsecs of 11 LIRGs. To study the stellar populations we fit the optical stellar continuum and the hydrogen recombination lines of selected regions. We analyze the excitation conditions of the gas using the spatially resolved properties of the brightest optical emission lines. The optical continua of the selected regions are well fitted with a combination of evolved (~0.7-10Gyr) and ionizing (1-20Myr) stellar populations. The latter is more obscured than the evolved population, and has visual extinctions in good agreement with those obtained from the Balmer decrement. Except for NGC 7771, there is no clear evidence for an important contribution to the optical light from an intermediate-aged population (~100-500Myr). Even after correcting for the presence of stellar absorption, a large fraction of spaxels with low observed equivalent widths of Halpha in emission still show enhanced [NII]/Halpha and [SII]/Halpha ratios. These ratios are likely to be produced by a combination of photoionization in HII regions and diffuse emission. These regions of enhanced ratios are generally coincident with low surface brightness HII regions and diffuse emission detected in the Halpha and Pa-alpha images. Using the PMAS line ratios and the NICMOS Pa-alpha photometry of HII regions we find that the fraction of diffuse emission in LIRGs varies from galaxy to galaxy, and it is generally less than 60% as found in other starburst galaxies. (Abridged)
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In this paper we present PMAS optical (3800-7200A) IFS of the northern hemisphere portion of a volume-limited sample of 11 LIRGs. The PMAS observations typically cover the central ~5kpc and are complemented with HST/NICMOS images. For most LIRGs in our sample, the peaks of the continuum and gas (e.g., Halpha, [NII]) emissions coincide, unlike what is observed in local, strongly interacting ULIRGs. The only exceptions are galaxies with circumnuclear rings of star formation where the most luminous Halpha emitting regions are found in the rings rather than in the nuclei, and the displacements are well understood in terms of differences in the stellar populations. A large fraction of the nuclei of these LIRGs are classified as LINER and intermediate LINER/HII, or composite objects. The excitation conditions of the integrated emission depend on the relative contributions of HII regions and the diffuse emission to the line emission over the PMAS FoV. Galaxies dominated by high surface-brightness HII regions show integrated HII-like excitation. A few galaxies show slightly larger integrated [NII]/Halpha and [SII]/Halpha line ratios than the nuclear ones, probably because of more contribution from the diffuse emission. The Halpha velocity fields over the central few kpc are generally consistent, at least to first order, with rotational motions. The velocity fields of most LIRGs are similar to those of disk galaxies, in contrast to the highly perturbed fields of most local, strongly interacting ULIRGs. The peak of the Halpha velocity dispersion coincides with the position of the nucleus and is likely to be tracing mass. All these results are similar to the properties of z~1 LIRGs, and they highlight the importance of detailed studies of flux-limited samples of local LIRGs. (Abridged)
We present near-infrared (NIR) color-magnitude diagrams (CMDs) for the resolved stellar populations within 26 fields of 23 nearby galaxies (<4 Mpc), based on F110W and F160W images from Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). The CMDs sample both old dormant and young star-forming populations. We match key NIR CMD features with their counterparts in optical CMDs, and identify the red core Helium burning (RHeB) sequence as a significant contributor to the NIR flux in stellar populations younger than a few 100 Myrs old, suggesting that star formation can drive surprisingly rapid variations in the NIR mass-to-light ratio. The NIR luminosity of star forming galaxies is therefore not necessarily proportional to the stellar mass. We note that these individual bright RHeB stars may be misidentified as old stellar clusters in low resolution imaging. We also discuss the CMD location of asymptotic giant branch (AGB) stars, and the separation of AGB sub-populations using a combination of optical and NIR colors. We empirically calibrate the NIR magnitude of the tip of the red giant branch (TRGB) as a function of color, allowing this widely adopted filter to be used for distance measurements. We find a clear trend between NIR RGB color and metallicity. However, it appears unlikely that the slope of the NIR RGB can be used as a metallicity indicator in extragalactic systems with comparable data. Finally, we discuss scattered light in the WFC3, which becomes significant for exposures taken close to a bright earth limb.
(Abridged) We perform integral field spectroscopy of a sample of Blue compact dwarf (BCD) galaxies with the aim of analyzing their morphology, the spatial distribution of some of their physical properties (excitation, extinction, and electron density) and their relationship with the distribution and evolutionary state of the stellar populations. Integral field spectroscopy observations of the sample galaxies were carried out with the Potsdam Multi-Aperture Spectrophotometer (PMAS) at the 3.5 m telescope at Calar Alto Observatory. An area 16 arcsec x 16 arcsec in size was mapped with a spatial sampling of 1 arcsec x 1 arcsec. We obtained data in the 3590-6996 Angstroms spectral range, with a linear dispersion of 3.2 Angstroms per pixel. From these data we built two-dimensional maps of the flux of the most prominent emission lines, of two continuum bands, of the most relevant line ratios, and of the gas velocity field. Integrated spectra of the most prominent star-forming regions and of whole objects within the FOV were used to derive their physical parameters and the gas metal abundances. Six galaxies display the same morphology both in emission line and in continuum maps; only in two objects, Mrk 32 and Tololo 1434+032, the distributions of the ionized gas and of the stars differ considerably. In general the different excitation maps for a same object display the same pattern and trace the star-forming regions, as expected for objects ionized by hot stars; only the outer regions of Mrk 32, I Zw 123 and I Zw 159 display higher [SII]/Halpha values, suggestive of shocks. Six galaxies display an inhomogeneous dust distribution. Regarding the kinematics, Mrk 750, Mrk 206 and I Zw 159 display a clear rotation pattern, while in Mrk 32, Mrk 475 and I Zw 123 the velocity fields are flat.
Luminous Infrared (IR) Galaxies (LIRGs) are an important cosmological class of galaxies as they are the main contributors to the co-moving star formation rate density of the universe at z=1. In this paper we present a GTO Spitzer IRS program aimed to obtain spectral mapping of a sample of 14 local (d<76Mpc) LIRGs. The data cubes map, at least, the central 20arcsec x 20arcsec to 30arcsec x 30arcsec regions of the galaxies, and use all four IRS modules covering the full 5-38micron spectral range. The final goal of this project is to characterize fully the mid-IR properties of local LIRGs as a first step to understanding their more distant counterparts. In this paper we present the first results of this GTO program. The IRS spectral mapping data allow us to build spectral maps of the bright mid-IR emission lines (e.g., [NeII], [NeIII], [SIII], H_2), continuum, the 6.2 and 11.3micron PAH features, and the 9.7micron silicate feature, as well as to extract 1D spectra for regions of interest in each galaxy. The IRS data are used to obtain spatially resolved measurements of the extinction using the 9.7micron silicate feature, and to trace star forming regions using the neon lines and the PAH features. We also investigate a number of AGN indicators, including the presence of high excitation emission lines and a strong dust continuum emission at around 6micron. We finally use the integrated Spitzer/IRS spectra as templates of local LIRGs. We discuss several possible uses for these templates, including the calibration of the star formation rate of IR-bright galaxies at high redshift. We also predict the intensities of the brightest mid-IR emission lines for LIRGs as a function of redshift, and compare them with the expected sensitivities of future space IR missions.
LIRGs and ULIRGs are much more numerous at higher redshifts than locally, dominating the star-formation rate density at redshifts ~1 - 2. Therefore, they are important objects in order to understand how galaxies form and evolve through cosmic time. We aim to characterize the morphologies of the stellar continuum and the ionized gas (H_alpha) emissions from local sources, and investigate how they relate with the dynamical status and IR-luminosity of the sources. We use optical (5250 -- 7450 AA) integral field spectroscopic (IFS) data for a sample of 38 sources, taken with the VIMOS instrument, on the VLT. We present an atlas of IFS images of continuum emission, H_alpha emission, and H_alpha equivalent widths for the sample. The H_alpha images frequently reveal extended structures that are not visible in the continuum, such as HII regions in spiral arms, tidal tails, rings, of up to few kpc from the nuclear regions. The morphologies of the continuum and H_alpha images are studied on the basis of the C_{2kpc} parameter, which measures the concentration of the emission within the central 2 kpc. The C_{2kpc} values found for the H_alpha images are higher than those of the continuum for the majority (85%) of the objects in our sample. On the other hand, most of the objects in our sample (~62%) have more than half of their H_alpha emission outside the central 2 kpc. No clear trends are found between the values of C_{2kpc} and the IR-luminosity of the sources. On the other hand, our results suggest that the star formation in advance mergers and early-stage interactions is more concentrated than in isolated objects. We compared the H_alpha and infrared emissions as tracers of the star-formation activity. We find that the star-formation rates derived using the H_alpha luminosities generally underpredict those derived using the IR luminosities, even after accounting for reddening effects.
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