Integral field optical spectroscopy of a representative sample of ULIRGs

et al. 2013

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We present a 2D study of the internal extinction on (sub)kiloparsec scales of a sample of local (z < 0.1) LIRGs (10) and ULIRGs (7), based on near-infrared Paα, Brδ, and Brγ line ratios, obtained with VLT-SINFONI integral-field spectroscopy (IFS). The 2D extinction (A V ) distributions of the objects, map regions of ∼3 × 3 kpc (LIRGs) and ∼12 × 12 kpc (ULIRGs), with average angular resolutions (FWHM) of ∼0.2 kpc and ∼0.9 kpc, respectively. The individual A V galaxy distributions indicate a very clumpy dust structure already on sub-kiloparsec scales, with values (per spaxel) ranging from A V ∼ 1 to 20 mag in LIRGs, and from A V ∼ 2 to 15 mag in ULIRGs. As a class, the median values of the distributions are A V = 5.3 mag and A V = 6.5 mag for the LIRG and ULIRG subsamples, respectively. In ∼70% of the objects, the extinction peaks at the nucleus with values ranging from A V ∼ 3 to 17 mag. Within each galaxy, the A V radial profile shows a mild decrement in LIRGs within the inner 2 kpc radius, while the same radial variation is not detected in ULIRGs, likely because of the lower linear scale resolution of the observations at the distance of ULIRGs. We evaluated the effects of the galaxy distance in the measurements of the extinction as a function of the linear scale (in kpc) of the spaxel (i.e. due to the limited angular resolution of the observations). If the distribution of the gas/dust and star-forming regions in local LIRGs (63 Mpc, 40 pc/spaxel on average) is the same for galaxies at greater distances, the observed median A V values based on emission line ratios would be a factor ∼0.8 lower at the average distance of our ULIRG sample (328 Mpc, 0.2 kpc/spaxel), and a factor ∼0.67 for galaxies located at distances of more than 800 Mpc (0.4 kpc/spaxel). This distance effect would have implications for deriving the intrinsic extinction in high-z star-forming galaxies and for subsequent properties such as star formation rate, star formation surface density, and KS-law, based on Hα line fluxes. If local LIRGs are analogues of the main-sequence star-forming galaxies at cosmological distances, the extinction values (A V ) derived from the observed emission lines in these high-z sources would need to be increased by a factor 1.4 on average.

Section: Introductionmentioning

confidence: 89%

VLT-SINFONI integral field spectroscopy of low-zluminous and ultraluminous infrared galaxies

López

et al. 2013

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“…Hα and the continuum emission; e.g. García-Marín et al 2009b). This method has been previously used in high-z samples (e.g.…”

Section: Hα Emitting Region Extension In Local U/lirgsmentioning

confidence: 99%

Integral field spectroscopy based Hαsizes of local luminous and ultraluminous infrared galaxies

Alonso‐Herrero

et al. 2012

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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). The comparison sample of local U/LIRGs also contains a larger fraction (59%) of objects with compact Hα 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 when comparing local and distant galaxies of similar SFRs. However, when using Hα as a tracer for both local and high-z samples, the differences are smaller than those reported using a variety of other tracers. In the L(Hα) -L(Hα) surface density (Σ Hα ) plane, most of the local U/LIRGs and high-z SF galaxies follow the same trend (i.e. higher luminosity for higher surface density) and cover a similar range, except for about 20-40% of local U/LIRGs, which have a higher Σ Hα by a factor of about 10. This is considerably smaller than the factors of 1000 or more reported in similar planes (i.e. L(TIR) versus Σ TIR ). Despite of the higher fraction of galaxies with compact Hα emission, a sizable group (about one-third) of local U/LIRGs are large (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.

“…It is well known that the bulk of luminosity produced in local (U)LIRGs is due to the large amount of dust that hides a large fraction of their star formation and nuclear activity (see Alonso-Herrero et al 2006;García-Marín et al 2009a, and references therein). This dust is responsible for the absorption of UV photons that are then re-emitted at FIR and submillimetre wavelengths.…”

Section: Hydrogen Lines and 2d Extinction Mapsmentioning

confidence: 99%

“…However, on scales of a few kpc or less, the distribution of dust in LIRGs and ULIRGs is not uniform, and it shows a patchy structure that includes almost transparent regions and very obscured ones (see García-Marín et al 2009a;Paper II). This non-uniform distribution of the dust implies that the correction from the extinction depends on the sampling scale, so that the correction to the SFR would depend on the scales where the Brγ (Paα) is sampled.…”

Section: Ionised Gasmentioning

confidence: 99%

“…In recent years, optical integral field spectroscopy (IFS) of representative samples of local LIRGs (Arribas et al 2008;Alonso-Herrero et al 2009) and ULIRGs (García-Marín et al 2009b) have been performed with the goal of investigating the nature of the ionisation sources (Monreal-Ibero et al 2010), the structure of the star-forming regions (Rodriguez-Zaurín et al 2011;Arribas et al 2012), the 2D internal dust/extinction distribution (García-Marín et al 2009a), and the gas kinematics (Colina et al 2005;Alonso-Herrero et al 2009). In parallel, a considerable effort has been made to investigate the nature of star-forming galaxies at redshifts between 1 and 3 (e.g.…”

Section: Introductionmentioning

confidence: 99%

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VLT-SINFONI integral field spectroscopy of low-zluminous and ultraluminous infrared galaxies

López

et al. 2012

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We present an atlas of a sample of local (z < 0.1) LIRGs (10) and ULIRGs (7) covering the luminosity range log(L IR /L ) = 11.1−12.4. The atlas is based on near-infrared H (1.45−1.85 μm) and K-band (1.95−2.45 μm) VLT-SINFONI integral field spectroscopy (IFS). The atlas presents the ionised, partially ionised, and warm molecular gas two-dimensional flux distributions and kinematics over an FoV of ∼3 × 3 kpc (LIRGs) and ∼12 × 12 kpc (ULIRGs) and with average linear resolutions of ∼0.2 kpc and ∼0.9 kpc, respectively. The different phases of the gas show a wide morphological variety with the nucleus as the brightest Brγ source for ∼33% of the LIRGs and ∼71% of the ULIRGs, whereas all the LIRGs and ULIRGs have their maximum H 2 emission in their nuclear regions. In LIRGs, the ionised gas distribution is dominated by the emission from the star-forming rings or giant HII regions in the spiral arms. The Brγ and [FeII] line at 1.644 μm trace the same structures, although the emission peaks at different locations in some of the objects, and the [FeII] seems to be more extended and diffuse. The ULIRG subsample is at larger distances and contains mainly pre-coalescence interacting systems. Although the peaks of the molecular gas emission and the continuum coincide in ∼71% of the ULIRGs, regions with intense Paα (Brγ) emission tracing luminous star-forming regions located at distances of 2−4 kpc away from the nucleus are also detected, usually associated with secondary nuclei or tidal tails. LIRGs have mean observed (i.e. uncorrected for internal extinction) SFR surface densities of about 0.4 to 0.9 M yr −1 kpc −2 over large areas (4−9 kpc 2 ) with peaks of about 2−2.5 M yr −1 kpc −2 in the smaller regions (0.16 kpc 2 ) associated with the nucleus of the galaxy or the brightest Brγ region. ULIRGs do have similar average SFR surface densities for the integrated emitting regions of ∼0.4 M yr −1 kpc −2 in somewhat larger areas (100−200 kpc 2 ) and for the Paα peak (∼2 M yr −1 kpc −2 in 4 kpc 2 ). The observed gas kinematics in LIRGs is primarily due to rotational motions around the centre of the galaxy, although local deviations associated with radial flows and/or regions of higher velocity dispersions are present. The ionised and molecular gas share the same kinematics (velocity field and velocity dispersion) to first order, showing slight differences in the velocity amplitudes (peak-to-peak) in some cases, whereas the average velocity dispersions are compatible within uncertainties. As expected, the kinematics of the ULIRG subsample is more complex, owing to the interacting nature of the objects of the sample.