SHINING Science Goals and Results

The SHINING consortium performed a comprehensive far-infrared spectroscopic and photometric survey of infrared bright galaxies at local and intermediate redshifts. An important fraction of star formation and AGN activity in the universe takes place in such dusty, infrared bright galaxies. Our goal is to use the superior sensitivity, spatial and spectral resolution of Herschel to study these galaxies with minimal extinction effects. We aim to obtain a comprehensive view of the physical processes at work in the interstellar medium of local galaxies ranging from objects with moderately enhanced star formation to the most dense, energetic, and obscured environments in ultra-luminous infrared galaxies (ULIRGs) and around AGN. The objects cover a wide parameter range in luminosity, activity level, and metal enrichment, and will be complemented by a few objects at intermediate redshifts, i.e. at a more active epoch of star formation.

The main goals are:

  • Elucidate the energetics, obscuration, and physical conditions of the central regions of infrared galaxies, by obtaining full 60- to 670-micron PACS and SPIRE spectra of five template starbursts, AGN, and ULIRGs, and characterize for a larger local sample the state of the ionized medium and of photo dissociation regions through PACS observations of key diagnostic fine-structure and molecular lines. The densest and warmest region near AGN is being studied by probing for highly excited CO emission
  • Determine the role of metallicity in star formation processes through a spectroscopic study with PACS of a range of low-metallicity galaxies from the nearby LMC/SMC to more distant, less resolved galaxies, sampling a range of star formation activity
  • Search for evolution from the intermediate redshift population close to the peak of cosmic star formation up to the present time by comparing the far-infrared line emission of z ~ 1 infrared galaxies to the local analogs
  • Study the triggering mechanisms and temporal evolution of infrared activity by photometric mapping of a large sample of interacting galaxies with PACS
  • Determine the structure of a few local templates by spectroscopic mapping along characteristic axes. These PACS line maps will establish the physical conditions of nuclear region, spiral arms/disk, and wind regions and will be essential for a comprehensive view of the spatial and temporal evolution of infrared activity.

A list of SHINING-related publications can be found in Publications. Below, we list a few highlights.


Massive Molecular Outflows and Negative Feedback in ULIRGs

We have found unambiguous evidence for massive molecular outflows in the Herschel-PACS spectra of a large number of nearby ULIRGs, traced by the hydroxyl molecule (OH). These outflows may represent the long sought evidence for the strong QSO mode feedback onto the host galaxy required by galaxy evolution models. In some of these objects, the (terminal) outflow velocities exceed 1,000 km/s, and their outflow rates (up to ~1,200 M/year) are several times larger than their star formation rates. ULIRGs with a higher AGN luminosity have higher terminal velocities and shorter gas depletion time scales. We are pursuing an extensive follow-up program with Herschel, IRAM/PdBI, and ALMA, and we are now working toward a more holistic view of outflows. By expanding our OH studies to a sample of ~ 45 ULIRGs and ~ 50 BAT AGN, we strengthened the evidence that luminous AGN are the dominant drivers of fast winds (Stone et al. 2016). Another key aspect of our studies has been to show that broad wings of the [CII] emission line, which is the dominant coolant of the ISM over a wide range of physical conditions and observable at high redshift, correlate with the molecular outflows traced by OH in terms of velocity and mass (Janssen et al. 2016). This may be particularly relevant at high redshift, where the usual tracers of molecular gas (like low-J CO lines or OH absorption profiles) become hard to observe.

Related publications include: Fischer et al. 2010, Sturm et al. 2011, González-Alfonso et al. 2012, 2014, 2017, 2018, Veilleux et al. 2013, 2017, Contursi et al. 2013, Cicone et al. 2014, Janssen et al. 2016, Stone et al. 2016, 2018, and Calderon et al. 2016.


High-J CO Line SEDs in Nearby Infrared Bright Galaxies

Emission from high-J CO lines in galaxies has long been proposed as a tracer of X-ray-dominated regions (XDRs) produced by active galactic nuclei (AGN). We have detected far-infrared (FIR) CO rotational emission, in the Jupp = 14–30 range, from nearby AGN and starburst galaxies, as well as several merging systems and ultraluminous infrared galaxies (ULIRGs). The PACS CO data provide the first reference of well-sampled FIR extragalactic CO spectral line energy distributions (SLEDs) for this range. We find large variety in the overall SLED shape, even among galaxies of similar type, demonstrating the uncertainties in relying solely on high-J CO diagnostics to characterize the excitation source (PDR, XDR, shocks, cosmic rays) of a galaxy. Combining our data with low-J line intensities taken from the literature, we developed a CO ratio-ratio diagram as a tool for distinguishing excitation sources and physical properties of the molecular gas. From quantitative single-component and two-component large-velocity gradient radiative-transfer models fitting the CO SLEDs, we have derived the molecular gas mass and the corresponding CO-to-H2 conversion factor for each respective source.

It is particularly interesting to find out whether the obscuring torus, which is required by AGN unification models, can be observed via high-J CO cooling lines. The detection of 11 CO lines from upper levels as high as J = 30 enabled us to show that there were three distinct contributions to the CO spectral line energy distribution. The coolest component is known to be dominated by the circumnuclear ring, which has a radius of 1 kiloparsec. In contrast, the medium- and high-excitation components, at 170 K and 570 K respectively, arise from the central few hundred parsecs of NGC 1068. Models of their CO line intensities cannot robustly discriminate photo dissociation regions, X-ray-dominated regions, and shocks; however, arguments based on energetics strongly suggest that the highest excitation component is indeed X-ray-heated. The spectral diagnostic power of PACS was combined with spatially resolved kinematics of SINFONI. Matching the spectrally distinct CO components to the near-infrared H2 line emission from spatially distinct locations, we showed that the medium excitation emission arises primarily from the bright massive molecular region about 70 parsecs east of the AGN. At the same time, the highest excitation component only matches the tongue of H2 that lies just 20 parsecs north of the AGN. Based on a detailed analysis of the SINFONI kinematics, it is believed to be falling almost directly toward it. Taking this further, we obtained an extremely deep upper limit on CO (40–39), and we then applied an XDR model to investigate whether the upper limit constrains the properties of a molecular torus in NGC 1068. The XDR model predicts the CO spectral line energy distributions for various gas densities and illuminating X-ray fluxes. In our model, the CO (40–39) upper limit is matched by gas with densities of ~ 1E6–1E7 cm3, located at 1.6–5 parsecs from the AGN, with column densities of at least 1E25 cm2. At such high column densities, however, dust absorbs most of the CO (40–39) line emission. Therefore, even if NGC 1068 has a molecular torus that radiates in the CO (40–39) line, the dust can attenuate the line emission to below the PACS detection limit. The upper limit is thus consistent with the existence of a molecular torus in NGC 1068.

Related publications include: Hailey-Dunsheath et al. 2012, Mashian et al. 2015, Janssen et al. 2015.

FIR Line Deficits

From the far-infrared fine-structure line observations of our sample of 44 local starbursts, Seyfert galaxies, and infrared luminous galaxies, we were able to show that the ratio between the far-infrared luminosity and the molecular gas mass, L(FIR)/M(H2), is a much better proxy for the relative brightness of the far-infrared lines than L(FIR) alone. Galaxies with high L(FIR)/M(H2) ratios tend to have weaker fine-structure lines relative to their far-infrared continuum than galaxies with L(FIR)/M(H2) below approximately 80 L/M. A deficit of the [CII] 158 μm line relative to LFIR was previously found with the Infrared Space Observatory, but now we have seen for the first time that this is a general aspect of all far-infrared fine structure lines, regardless of their origin in the ionized or neutral phase of the interstellar medium. The L(FIR)/M(H2) value where these line deficits start to manifest is similar to the limit that separates the two modes of star formation recently found in galaxies on the basis of studies of their gas-star formation relations. Our finding that the properties of the interstellar medium are also significantly different in these regimes provides independent support for the different star-forming relations in normal disk galaxies and major merger systems. Using the spectral synthesis code Cloudy to model the emission of the lines, we found that the expected increase of the ionization parameter with L(FIR)/M(H2) can simultaneously explain the line deficits in the [CII], [NII], and [OI] lines. In a study of two-lensed, ultraluminous infrared galaxies at high redshift (MIPS J1428, a starburst-dominated system at z = 1.3, and IRAS F10214+4724, a source at z = 2.3 hosting both star formation and a luminous AGN), we were able to demonstrate the diagnostic power of these findings for determining the mode of star formation in high-redshift galaxies. We found that MIPS J1428, contrary to average local ULIRGs, does not show a deficit in [OI] relative to FIR. The combination of far-UV flux G0 and gas density n (derived from PDR models) as well as the star formation efficiency (derived from CO and FIR) is similar to normal or starburst galaxies, despite the high (ULIRG-like) infrared luminosity of this system. This is consistent with the findings that with an increased gas reservoir, star-forming galaxies at high redshifts can achieve ULIRG luminosities without being major mergers. In contrast, F10214 has stringent upper limits on [O IV] and [S III], and an [O III]/FIR ratio at least an order of magnitude lower than local starbursts or AGN, similar to local ULIRGs. This source might be a merger like local ULIRGs, and is probably in a ULIRG-like (enhanced) mode of star formation.

Related publications include: Sturm et al. 2010, Graciá-Carpio et al. 2011, González-Alfonso et al. 2015, and Herrera-Camus et al. 2018a, 2018b.

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