The properties of the stellar populations in ULIRGs II: the star formation histories and evolution

This is the second of two papers presenting a detailed long-slit spectroscopic study of the stellar populations in a sample of 36 ULIRGs. In the previous paper we presented the sample, the data and the spectral synthesis modelling while in this paper, we carry out a detailed analysis of the modelling results. We find that the star formation histories of ULIRGs are complex, with at least two epochs of star formation activity and that the charcteristic timescale of the star formation acivity is <100Myr. These results are consistent with models that predict an epoch of enhanced star formation coinciding with the first pass of the merging nuclei, along with a further, more intense, episode of star formation occurring as the nuclei finally merge together. It is also found that the young stellar populations (YSPs) tend to be younger and more reddened in the nuclear regions of the galaxies. This is in good agreement with the merger simulations, which predict that the bulk of the star formation activity in the final stages of mergers will occur in the nuclear regions of the merging galaxies. In addition, our results show that ULIRGs have total stellar masses that are similar to, or smaller than, the break of the galaxy mass function (m* = 1.4 x 10^{11} Msolar). Finally, we find no significant differences between the ages of the YSP in ULIRGs with and without optically detected Seyfert nuclei, nor between those with warm and cool mid- to far-IR colours. While this results do not entirely rule out the idea that cool ULIRGs with HII/LINER spectra evolve into warm ULIRGs with Seyfert-like spectra, it is clear that the AGN activity in local Seyfert-like ULIRGs has not been triggered a substantial period (>=100 Myr) after the major merger-induced starbursts in the nuclear regions.Comment: Accepted for publication in MNRAS. The paper contains 16 pages, 6 figures and 7 table

HST emission-line images of nearby 3CR radio galaxies: two photoionization, accretion and feedback modes

We present HST/ACS narrow-band images of a low-z sample of 19 3C radio galaxies to study the H$\alpha$ and [OIII] emissions from the narrow-line region (NLR). Based on nuclear emission line ratios, we divide the sample into High and Low Excitation Galaxies (HEGs and LEGs). We observe different line morphologies, extended line emission on kpc scale, large [OIII]/H$\alpha$ scatter across the galaxies, and a radio-line alignment. In general, HEGs show more prominent emission line properties than LEGs: larger, more disturbed, more luminous, and more massive regions of ionized gas with slightly larger covering factors. We find evidence of correlations between line luminosities and (radio and X-ray) nuclear luminosities. All these results point to a main common origin, the active nucleus, which ionize the surrounding gas. However, the contribution of additional photoionization mechanism (jet shocks and star formation) are needed to account for the different line properties of the two classes. A relationship between the accretion, photoionization and feedback modes emerges from this study. For LEGs (hot-gas accretors), the synchrotron emission from the jet represents the main source of ionizing photons. The lack of cold gas and star formation in their hosts accounts for the moderate ionized-gas masses and sizes. For HEGs (cold-gas accretors), an ionizing continuum from a standard disk and shocks from the powerful jets are the main sources of photoionization, with the contribution from star formation. These components, combined with the large reservoir of cold/dust gas brought from a recent merger, account for the properties of their extended emission-line regions.Comment: accepted for publication on ApJ (22 pages, 12 figures

Integral Field Spectroscopy based H\alpha\ sizes of local Luminous and Ultraluminous Infrared Galaxies. A Direct Comparison with high-z Massive Star Forming Galaxies

Aims. We study the analogy between local U/LIRGs and high-z massive SFGs by comparing basic H{\alpha} structural characteristics, such as size, and luminosity (and SFR) surface density, in an homogeneous way (i.e. same tracer and size definition, similar physical scales). Methods. We use Integral Field Spectroscopy based H{\alpha} emission maps for a representative sample of 54 local U/LIRGs (66 galaxies). From this initial sample we select 26 objects with H{\alpha} luminosities (L(H{\alpha})) similar to those of massive (i.e. M\ast \sim 10^10 M\odot or larger) SFGs at z \sim 2, and observed on similar physical scales. Results. The sizes of the H{\alpha} emitting region in the sample of local U/LIRGs span a large range, with r1/2(H{\alpha}) from 0.2 to 7 kpc. However, about 2/3 of local U/LIRGs with Lir > 10^11.4 L\odot have compact H{\alpha} emission (i.e. r1/2 < 2 kpc). The comparison sample of local U/LIRGs also shows a higher fraction (59%) of objects with compact H{\alpha} emission than the high-z sample (25%). This gives further support to the idea that for this luminosity range the size of the star forming region is a distinctive factor between local and distant galaxies of similar SF rates. However, when using H{\alpha} as a tracer for both local and high-z samples, the differences are smaller than the ones recently reported using a variety of other tracers. Despite of the higher fraction of galaxies with compact H{\alpha} emission, a sizable group (\sim 1/3) of local U/LIRGs are large (i.e. r1/2 > 2 kpc). These are systems showing pre-coalescence merger activity and they are indistinguishable from the massive high-z SFGs galaxies in terms of their H{\alpha} sizes, and luminosity and SFR surface densities.Comment: Accepted for publication in A&A. (!5 pages, 7 figures, 2 tables

VLT-VIMOS integral field spectroscopy of luminous and ultraluminous infrared galaxies: III. the atlas of the stellar and ionized gas distribution

Context. Luminous and ultraluminous infrared galaxies (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. Local samples provide a unique opportunity to study these objects in detail. Aims. We aim to characterize the morphologies of the stellar continuum and the ionized gas (Hα) emissions from local sources, and investigate how they relate with the dynamical status and IR-luminosity of the sources. Methods. We use optical (5250-7450 Å) integral field spectroscopic (IFS) data for a representative sample of 38 sources (31 LIRGs and 7 ULIRGs), taken with the VIMOS instrument on the VLT. Results. We present an atlas of IFS images of continuum emission, Hα emission, and Hα equivalent widths for the sample. The morphologies of the Hα emission are substantially different from those of the stellar continuum. The Hα images frequently reveal extended structures that are not visible in the continuum, such as HII regions in spiral arms, tidal tails, rings, bridges, of up to few kpc from the nuclear regions. The morphologies of the continuum and Hα images are studied on the basis of the C2 kpc parameter, which measures the concentration of the emission within the central 2 kpc. The C2 kpc values found for the Hα 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 Hα emission outside the central 2 kpc. No clear trends are found between the values of C2 kpc 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. Finally, we compared the Hα and infrared emissions as tracers of the star-formation activity. We find that the star-formation rates derived using the Hα luminosities generally underpredict those derived using the IR luminosities, even after accounting for reddening effects. © 2011 ESO.Based on observations carried out at the European Southern observatory, Paranal (Chile), Programs 076.B- 0479(A), 078.B-0072(A) and 081.B-0108(A).The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166.This work has been supported by the Spanish Ministry of Science and Innovation (MICINN) under grant ESP2007-65475-C02-01. AM-I is supported by the Spanish Ministry of Science and Innovation (MICINN) under program “Specialization in International Organisms”, Ref. ES2006-0003.Peer Reviewe

Studying the kinematic asymmetries of disks and post-coalescence mergers using a new `kinemetry' criterion

We have obtained VIMOS/VLT optical integral field spectroscopy (IFS) data for a sample of 4 LIRGs which have been selected at a similar distance ($\sim$ 70 Mpc) to avoid relative resolution effects. They have been classified in two groups (isolated disk and post-coalescence mergers) according to their morphology. The $kinemetry$ method (developed by Krajnovic and coworkers) is used to characterize the kinematic properties of these galaxies and to discuss new criteria for distinguishing their status. We present and discuss new kinematic maps (i.e., velocity field and velocity dispersion) for these four galaxies. The morphological and kinematic classifications of these systems are consistent, with disks having lower kinematic asymmetries than post-coalescence mergers. We then propose and discuss a new kinematic criterion to differentiate these two groups. This criterion distinguishes better these two categories and has the advantage of being less sensitive to angular resolution effects. According to the previous criteria,the present post-coalescence systems would have been classified as disks. This indicates that the separation of disks from mergers is subjective to the definition of `merger'. It also suggests that previous estimates of the merger/disk ratio could have been underestimated, but larger samples are necessary to establish a firmer conclusion.Comment: 15 figures, 19 page

Higher prevalence of X-ray selected AGN in intermediate age galaxies up to z~1

We analyse the stellar populations in the host galaxies of 53 X-ray selected optically dull active galactic nuclei (AGN) at 0.34<z<1.07 with ultra-deep (m=26.5) optical medium-band (R~50) photometry from the Survey for High-z Absorption Red and Dead Sources (SHARDS). The spectral resolution of SHARDS allows us to consistently measure the strength of the 4000 AA break, Dn(4000), a reliable age indicator for stellar populations. We confirm that most X-ray selected moderate-luminosity AGN (L_X<10^44 erg/s) are hosted by massive galaxies (typically M*>10^10.5 M_sun) and that the observed fraction of galaxies hosting an AGN increases with the stellar mass. A careful selection of random control samples of inactive galaxies allows us to remove the stellar mass and redshift dependencies of the AGN fraction to explore trends with several stellar age indicators. We find no significant differences in the distribution of the rest-frame U-V colour for AGN hosts and inactive galaxies, in agreement with previous results. However, we find significantly shallower 4000 AA breaks in AGN hosts, indicative of younger stellar populations. With the help of a model-independent determination of the extinction, we obtain extinction-corrected U-V colours and light-weighted average stellar ages. We find that AGN hosts have younger stellar populations and higher extinction compared to inactive galaxies with the same stellar mass and at the same redshift. We find a highly significant excess of AGN hosts with Dn(4000)~1.4 and light weighted average stellar ages of 300-500 Myr, as well as a deficit of AGN in intrinsic red galaxies. We interpret failure in recognising these trends in previous studies as a consequence of the balancing effect in observed colours of the age-extinction degeneracy.Comment: Accepted for publication in MNRAS, 12 pages, 8 figures, 1 tabl

Integrated spectra extraction based on signal-to-noise optimization using Integral Field Spectroscopy

We propose and explore the potential of a method to extract high signal-to-noise (S/N) integrated spectra related to physical and/or morphological regions on a 2-dimensional field using Integral Field Spectroscopy (IFS) observations by employing an optimization procedure based on either continuum (stellar) or line (nebular) emission features. The optimization method is applied to a set of IFS VLT-VIMOS observations of (U)LIRG galaxies, describing the advantages of the optimization by comparing the results with a fixed-aperture, single spectrum case, and by implementing some statistical tests. We demonstrate that the S/N of the IFS optimized integrated spectra is significantly enhanced when compared with the single aperture unprocessed case. We provide an iterative user-friendly and versatile IDL algorithm that allows the user to spatially integrate spectra following more standard procedures. This is made available to the community as part of the PINGSoft IFS software package.Comment: Accepted for publication in Astronomy & Astrophysics, 12 pages, 7 figure

Quantifying the AGN-driven outflows in ULIRGs (QUADROS) – II. Evidencefor compact outflow regions from HST [O iii] imaging observations

The true importance of the warm, AGN-driven outflows for the evolution of galaxies remains uncertain. Measurements of the radial extents of the outflows are key for quantifying their masses and kinetic powers, and also establishing whether the AGN outflows are galaxywide. Therefore, as part of a larger project to investigate the significance of warm, AGNdriven outflows in the most rapidly evolving galaxies in the local universe, here we present deep Hubble Space Telescope (HST) narrow-band [O III] λ5007 observations of a complete sample of eight nearby ULIRGs with optical AGN nuclei. Combined with the complementary information provided by our ground-based spectroscopy, the HST images show that the warm gas outflows are relatively compact for most of the objects in the sample: in three objects, the outflow regions are barely resolved at the resolution of HST (0.065 < R[O III] < 0.12 kpc); in a further four cases, the outflows are spatially resolved but with flux-weighted mean radii in the range 0.65 < R[O III] < 1.2 kpc; and in only one object (Mrk273) is there clear evidence for a more extended outflow, with a maximum extent of R[O III] ∼ 5 kpc. Overall, our observations show little evidence for the galaxy-wide outflows predicted by some models of AGN feedback

Integral field spectroscopy based Hα sizes of local luminous and ultraluminous infrared galaxies. A direct comparison with high-z massive star-forming galaxies

[Aims]: We study the analogy between local luminous and ultraluminous infrared galaxies (U/LIRGs) and high-z massive star forming galaxies (SFGs) by comparing their basic Hα structural characteristics, such as size and luminosity surface density, in an homogeneous way (i.e. same tracer, size definition, and similar physical scales). [Methods]: We use integral field spectroscopy (IFS) based Hα emission maps for a representative sample of 54 local U/LIRGs (66 galaxies) observed with INTEGRAL/WHT and VIMOS/VLT. From this initial sample, we select 26 objects with similar Hα luminosities (L(Hα)) to those of massive (i.e. M * ∼ 10 10 M ⊙ or larger) SFGs at z ∼ 2, and observed on similar physical scales. We then directly compare the sizes, and luminosity (and SFR) surface densities of these local and high-z samples. [Results]: The size of the Hα emitting region in the local U/LIRGs that we study has a wide range of values, with r 1/2(Hα) from 0.2 kpc to 7 kpc. However, about two-thirds of local U/LIRGs with L ir > 10 11.4 L ⊙ have compact Hα emission (i.e. r 1/2 2 kpc). These are systems that show evidence of pre-coalescence merger activity and are indistinguishable from the massive high-z SFGs galaxies in terms of their Hα sizes, and luminosity and SFR surface densities. © 2012 ESO.MGM is supported by the German federal department for education and research (BMBF) under the project number 50OS1101. This work has been supported by the Spanish Ministry of Science and Innovation (MICINN) under grants ESP2007-65475- C02-01 and AYA2010-21161-C02-01.Peer Reviewe