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

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

Shards: An optical spectro-photometric survey of distant galaxies

Pérez-González et al.We present the Survey for High-z Absorption Red and Dead Sources (SHARDS), an ESO/GTC Large Program carried out using the OSIRIS instrument on the 10.4 m Gran Telescopio Canarias (GTC). SHARDS is an ultra-deep optical spectro-photometric survey of the GOODS-N field covering 130 arcmin2 at wavelengths between 500 and 950 nm with 24 contiguous medium-band filters (providing a spectral resolution R ∿ 50). The data reach an AB magnitude of 26.5 (at least at a 3σ level) with sub-arcsec seeing in all bands. SHARDS' main goal is to obtain accurate physical properties of intermediate- and high-z galaxies using well-sampled optical spectral energy distributions (SEDs) with sufficient spectral resolution to measure absorption and emission features, whose analysis will provide reliable stellar population and active galactic nucleus (AGN) parameters. Among the different populations of high-z galaxies, SHARDS' principal targets are massive quiescent galaxies at z > 1, whose existence is one of the major challenges facing current hierarchical models of galaxy formation. In this paper, we outline the observational strategy and include a detailed discussion of the special reduction and calibration procedures which should be applied to the GTC/OSIRIS data. An assessment of the SHARDS data quality is also performed. We present science demonstration results on the detection and study of emission-line galaxies (star-forming objects and AGNs) at z = 0-5. We also analyze the SEDs for a sample of 27 quiescent massive galaxies with spectroscopic redshifts in the range 1.0 < z ≲ 1.4. We discuss the improvements introduced by the SHARDS data set in the analysis of their star formation history and stellar properties. We discuss the systematics arising from the use of different stellar population libraries, typical in this kind of study. Averaging the results from the different libraries, we find that the UV-to-MIR SEDs of the massive quiescent galaxies at z = 1.0-1.4 are well described by an exponentially decaying star formation history with scale Ï„ = 100-200 Myr, age around 1.5-2.0 Gyr, solar or slightly sub-solar metallicity, and moderate extinction, A(V) ∿ 0.5 mag. We also find that galaxies with masses above M* are typically older than lighter galaxies, as expected in a downsizing scenario of galaxy formation. This trend is, however, model dependent, i.e., it is significantly more evident in the results obtained with some stellar population synthesis libraries, and almost absent in others. © 2013. The American Astronomical Society. All rights reserved.We acknowledge support from the Spanish Programa Nacional de Astronomía y Astrofísica under grants AYA2009-07723-E and AYA2009-10368. SHARDS has been funded by the Spanish MICINN/MINECO under the Consolider-Ingenio 2010 Program grant CSD2006-00070: First Science with the GTC. O.G.-M., C.M.-T., J.M.R.-E., and J.R.-Z. wish to acknowledge support from grant AYA2010-21887-C04-04. A.A.-H. and A.H.-C. acknowledge financial support from the Universidad de Cantabria through the Augusto G. Linares Program.Peer Reviewe

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

Starburst radio galaxies: General properties, evolutionary histories and triggering

In this paper we discuss the results of a programme of spectral synthesis modelling of a sample of starburst radio galaxies in the context of scenarios for the triggering of the activity and the evolution of the host galaxies. New optical spectra are also presented for a subset of the objects discussed. The starburst radio galaxies - comprising ∼15-25 per cent of all powerful extragalactic radio sources - frequently show disturbed morphologies at optical wavelengths, and unusual radio structures, although their stellar masses are typical of radio galaxies as a class. In terms of the characteristic ages of their young stellar populations (YSPs), the objects can be divided into two groups: those with YSP ages tYSP≤ 0.1 Gyr, in which the radio source has been triggered quasi-simultaneously with the main starburst episode, and those with older YSP in which the radio source has been triggered or re-triggered a significant period after the starburst episode. Most of the former group are associated with a large mid- to far-IR (MFIR) continuum and [Oiii] emission-line luminosities (LIR > 1011L⊙, W), while most of the latter have lower luminosities. Combining the information on the YSP with that on the optical morphologies of the host galaxies, we deduce that the majority of the starburst radio galaxies have been triggered in galaxy mergers in which at least one of the galaxies is gas rich. However, the triggering (or re-triggering) of the radio jets can occur immediately before, around or a significant period after the final coalescence of the merging nuclei, reflecting the complex gas infall histories of the merger events. Although ∼25 per cent of starburst radio galaxies are sufficiently bright at MFIR wavelengths to be classified as ultraluminous infrared galaxies (ULIRGs), we show that only the most massive ULIRGs are capable of evolving into radio galaxies. Finally, for a small subset of starburst radio galaxies in rich clusters of galaxies, cooling flows associated with the hot X-ray haloes offer a viable alternative to mergers as a trigger for the radio jet activity. Overall, our results provide further evidence that a powerful radio jet activity can be triggered via a variety of mechanisms, including different evolutionary stages of major galaxy mergers; clearly, radio-loud AGN activity is not solely associated with a particular stage of a unique type of gas accretion event. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.This research has made use of the NASA/IPAC Extragalactic Data base (NED) which is operated by the Jet Propulsion Laboratory. Based on observations made with ESO Telescopes at the La Silla and Paranal observatories under programmes 70.B-0663(A), 71.B-0320(A), 078B-0660(A).Peer Reviewe

On the nature of the red, 2MASS-selected AGN in the local Universe I: an optical spectroscopic study

We present optical spectra for a representative sample of 27 nearby (z < 0.2) Two Micron All Sky Survey (2MASS)-selected active galactic nuclei (AGN) with red near-infrared colours (J − KS ≳ 2.0). The spectra were taken with the ISIS spectrograph on the William Herschel Telescope with the aim of determining the nature of the red 2MASS AGN, in particular whether they are young quasars obscured by their natal cocoons of gas and dust. We compare our findings with those obtained for comparison samples of PG quasars and unobscured type 1 AGN. The spectra show a remarkable variety, including moderately reddened type 1 objects (45 per cent), type 1 objects that appear similar to traditional ultraviolet (UV)-/optical-selected AGN (11 per cent), narrow-line type 1 Seyfert AGN (15 per cent), type 2 AGN (22 per cent) and H II/composite objects (7 per cent). The high Balmer decrements that we measure in many of the type 1 objects are consistent with their red J − KS colours being due to moderate levels of dust extinction (0.2 < E(B − V) < 1.2). However, we measure only modest velocity shifts and widths for the broader [O III]λ5007 emission-line components that are similar to those measured in the comparison samples. This suggests that the outflows in the red 2MASS objects are not unusual compared with those of optical-/UV-selected AGN of similar luminosity. In addition, the Eddington ratios for the 2MASS sample are relatively modest. Overall, based on their optical spectra, we find no clear evidence that the population of red, 2MASS-selected AGN at low redshifts represents young quasars. Most plausibly, these objects are normal type 1 AGN that are moderately obscured by material in the outer layers of the circumnuclear tori or in the discs of the host galaxies

PMAS Optical Integral Field Spectroscopy of Luminous Infrared Galaxies. II.-- Spatially resolved stellar populations and excitation conditions

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)Comment: Accepted for publication in A&

Optical spectroscopy of Arp220: the star formation history of the closest ULIRG

We present optical spectra of the merging system Arp 220,taken with the William Herschel Telescope(WHT) on La Palma. These data were taken with the aim of investigating the evolution and star formation history of this object. Spectral synthesis modelling has been used to estimate the ages of the stellar populations found in the diffuse light sampled by the spectra. The data show a remarkable uniformity in the stellar populations across the full 65 arcsec covered by our slit positions, sampling the measurable extent of the galaxy. The results are consistent with a dominant intermediate-age stellar population (ISP) with age 0.5 < t_{ISP} $\leq$ 0.9 Gyr that is present at all locations, with varying contributions from a young ($\leq$ 0.1 Gyr) stellar population (YSP) component. However, it is notable that while the flux contribution of the YSP component in the extended regions is relatively small ($\leq$ 40%), adequate fits in the nuclear region are only found for combinations with a significant contribution of a YSP component (22 - 63%). Moreover, while a low intrinsic reddening (E(B - V)\lsim 0.3) is found for the ISPs in the extended regions, intrinsic reddening values as high as E(B - V) ~ 1.0 are required in the galactic center. This clearly reflects the presence of a reddening gradient, with higher concentrations of gas and dust towards the nuclear regions, coinciding with dust lanes in the HST images. Overall, our results are consistent with models that predict an epoch of enhanced star formation coinciding with the first pass of the merging nuclei (represented by the ISP), with a further episode of star formation occurring as the nuclei finally merge together (represented by the YSP and ULIRG).Comment: 12 pages,3 Tables, 4 figures, accepted for publication in MNRA

A census of young stellar populations in the warm ULIRG PKS1345+12

We present a detailed investigation of the young stellar populations(YSP) in the radio-loud ultra luminous infrared galaxy (ULIRG) PKS1345+12, based on high resolution HST imaging and long slit spectra taken with the WHT. While the images clearly show bright knots suggestive of super star clusters(SSC), the spectra reveal the presence of YSP in the diffuse light across the full extent of the halo of the merging-double nucleus system. Spectral synthesis modelling has been used to estimate the ages of the YSP for both the SSC and the diffuse light sampled by the spectra. For the SSC we find ages t{SSC} < 6 Myr with reddenings 0.2 < E(B-V) < 0.5 and masses 10e6 < M{SSC} < 10e7 M{solar}. However, in some regions of the galaxy we find that the spectra of the diffuse light component can only be modelled with a relatively old post-starburst YSP (0.04 - 1.0 Gyr) or with a disk galaxy template spectrum. The results demonstrate the importance of accounting for reddening in photometric studies of SSC, and highlight the dangers of focussing on the highest surface brightness regions when trying to obtain a general impression of the star formation activity in the host galaxies of ULIRGs. The case of PKS1345+12 provides clear evidence that the star formation histories of the YSP in ULIRGs are complex. Intriguingly, our long-slit spectra show line splitting at the locations of the SSC, indicating that they are moving at up to 450km s-1 with respect to the local ambient gas. Given their kinematics, it is plausible that the SSC have been formed either in fast moving gas streams/tidal tails that are falling back into the nuclear regions as part of the merger process, or as a consequence of jet-induced star formation linked to the extended, diffuse radio emission detected in the halo of the galaxyComment: accepted for publication in MNRA