R. Meijerink

Explaining the [C II]157.7 μm Deficit in Luminous Infrared Galaxies : First Results from a Herschel/PACS Study of the GOALS Sample

We present the first results of a survey of the [C II] 157.7 μm emission line in 241 luminous infrared galaxies (LIRGs) comprising the Great Observatories All-sky Survey (GOALS) sample, obtained with the PACS instrument on board the Herschel Space Observatory. The [C II] luminosities, L_([C II]), of the LIRGs in GOALS range from ∼ 10^7 to 2×10^9 L_⊙. We find that LIRGs show a tight correlation nof [C II]/FIR with far-IR flux density ratios, with a strong negative trend spanning from ∼ 10^(−2) to 10^(−4), as the average temperature of dust increases. We find correlations between the [C II]/FIR ratio and the strength of the 9.7 μm silicate absorption feature as well as with the luminosity surface density of the mid-IR emitting region (∑_(MIR)), suggesting that warmer, more compact starbursts have substantially smaller [C II]/FIR ratios. Pure star-forming LIRGs have a mean [C II]/FIR∼ 4 × 10^(−3), nwhile galaxies with low 6.2 μm PAH equivalent widths (EWs), indicative of the presence of active galactic nuclei (AGN), span the full range in [C II]/FIR. However, we show that even when only pure star-forming galaxies are considered, the [C II]/FIR ratio still drops by an order of magnitude, from 10^(−2) to 10^(−3), with ∑_(MIR) and ∑_(IR), implying that the [C II] 157.7 μm luminosity is not a good indicator nof the star formation rate (SFR) for most LIRGs, for it does not scale linearly with the warm dust emission most likely associated to the youngest stars. Moreover, even in LIRGs in which we detect an AGN in the mid-IR, the majority (2/3) of galaxies show [C II]/FIR≥ 10^(−3) typical of high 6.2 μm PAH EW sources, suggesting that most AGNs do not contribute significantly to the far-IR emission. We provide an empirical relation between the [C II]/FIR and the specific SFR (SSFR) for star-forming LIRGs. Finally, we present predictions for the starburst size based on the observed [C II] and far-IR luminosities which should be useful for comparing with results from future surveys of high-redshift galaxies with ALMA and CCAT.

Irradiated ISM: Discriminating between Cosmic Rays and X-Rays

The interstellar medium (ISM) at the centers of active galaxies is exposed to a combination of cosmic-ray, far-ultraviolet (FUV), and X-ray radiation. We apply photodissociation region (PDR) models to this ISM with both normal and highly elevated (5 × 10-15 s-1) cosmic-ray (CR) rates and compare the results to those obtained for X-ray dissociation regions (XDRs). Our existing PDR-XDR code is used to construct models over a 103-105 cm-3 density range and for 0.16-160 ergs s-1 cm-2 impingent fluxes. We obtain larger high-J (J > 10) CO ratios in PDRs when we use the highly elevated CR rate, but these are always exceeded by the corresponding XDR ratios. The [C I] 609 μm/13CO (2-1) line ratio is boosted by a factor of a few in PDRs with n ~ 103 cm-3 exposed to a high CR rate. At higher densities, ratios become identical irrespective of CR flux, while XDRs always show elevated [C I] emission per CO column. The HCN/CO and HCN/HCO+ line ratios, combined with high-J CO emission lines, are good diagnostics to distinguish between PDRs, under either low or high CR irradiation conditions, and XDRs. Hence, the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory, which can detect these CO lines, will be crucial in the study of active galaxies.

Gas lines from the 5-Myr old optically thin disk around HD141569A - Herschel observations and modeling

Context. The gas- and dust dissipation processes in disks around young stars remain uncertain despite numerous studies. At the distance of∼ 99‐116 pc, HD141569A is one of the nearest HerbigAe stars that is surrounded by a tenuous disk, probably in transition between a massive primordial disk and a debris disk. Atomic and molecular gases haves been found in the structured 5-Myr old HD141569A disk, making HD141569A the perfect object within which to directly study the gaseous atomic and molecular component. Aims. We wish to constrain the gas and dust mass in the disk around HD141569A. Methods. We observed the fine-structure lines of Oi at 63 and 145µm and the Cii line at 157µm with the PACS instrument onboard the Herschel Space Telescopeas part of the open-time large programme GASPS. We complemented the atomic line observations with archival Spitzer spectroscopic and photometric continuum data, a ground-based VLT-VISIR image at 8.6µm, and 12 CO fundamental ro-vibrational and pure rotational J=3‐2 observations. We simultaneously modeled the continuum emission and the line fluxes with the Monte Carlo radiative transfer code MCFOST and the thermo-chemical code ProDiMo to derive the disk gas- and dust properties assuming no dust settling. Results. The models suggest that the oxygen lines are emitted from the inner disk around HD141569A, whereas the [Cii] line emission is more extended. The CO submillimeter flux is emitt ed mostly by the outer disk. Simultaneous modeling of the photometric and line data using a realistic disk structure suggests a dus t mass derived from grains with a radius smaller than 1 mm of∼ 2.1× 10 −7 M⊙ and from grains with a radius of up to 1 cm of 4.9× 10 −6 M⊙. We constrained the polycyclic aromatic hydrocarbons (PAH) mass to be between 2×10 −11 and 1.4×10 −10 M⊙ assuming circumcircumcoronene (C150H30) as the representative PAH. The associated PAH abundance relative to hydrogen is lower than those found in the interstellar medium (3×10 −7 ) by two to three orders of magnitude. The disk around HD141569A is less massive in gas (2.5 to 4.9× 10 −4 M⊙ or 67 to 164 M⊕) and has a flat opening angle (< 10%). Conclusions. We constrained simultaneously the silicate dust grain, PAH, and gas mass in a∼5-Myr old Herbig Ae disk. The diskaveraged gas-to-dust-mass is most likely around 100, which is the assumed value at the disk formation despite the uncertainties due to disagreements between the different gas tracers. If the disk was originally massive, the ga s and the dust would have dissipated at the same rate.

Diagnostics of the molecular component of photon-dominated regions with mechanical heating

Context. Multitransition CO observations of galaxy centers have revealed that significant fractions of the dense circumnuclear gas have high kinetic temperatures, which are hard to explain by pure photon excitation, but may be caused by dissipation of turbulent energy. Aims: We aim to determine to what extent mechanical heating should be taken into account while modeling PDRs. To this end, the effect of dissipated turbulence on the thermal and chemical properties of PDRs is explored. Methods: Clouds are modeled as 1D semi-infinite slabs whose thermal and chemical equilibrium is solved for using the Leiden PDR-XDR code, where mechanical heating is added as a constant term throughout the cloud. An extensive parameter space in hydrogen gas density, FUV radiation field and mechanical heating rate is considered, covering almost all possible cases for the ISM relevant to the conditions that are encountered in galaxies. Effects of mechanical heating on the temperature profiles, column densities of CO and H2O and column density ratios of HNC, HCN and HCO+ are discussed. Results: In a steady-state treatment, mechanical heating seems to play an important role in determining the kinetic temperature of the gas in molecular clouds. Particularly in high-energy environments such as starburst galaxies and galaxy centers, model gas temperatures are underestimated by at least a factor of two if mechanical heating is ignored. The models also show that CO, HCN and H2O column densities increase as a function of mechanical heating. The HNC/HCN integrated column density ratio shows a decrease by a factor of at least two in high density regions with n ~ 105 cm-3, whereas that of HCN/HCO+ shows a strong dependence on mechanical heating for this same density range, with boosts of up to three orders of magnitude. Conclusions: The effects of mechanical heating cannot be ignored in studies of the molecular gas excitation whenever the ratio of the star formation rate to the gas density (SFR/n3/2) is close to, or exceeds, 7 × 10-6 M⊙ yr-1 cm4.5. If mechanical heating is not included, predicted column densities (such as those of CO) are underestimated, sometimes (as in the case of HCN and HCO+) even by a few orders of magnitude. As a lower bound to its importance, we determined that it has non-negligible effects already when mechanical heating is as little as 1% of the UV heating in a PDR. Appendix A is available in electronic form at http://www.aanda.org

Radiative and mechanical feedback into the molecular gas of NGC 253

Starburst galaxies are galaxies or regions of galaxies undergoing intense periods of star formation. Understanding the heating and cooling mechanisms in these galaxies can give us insight to the driving mechanisms that fuel the starburst. Molecular emission lines play a crucial role in the cooling of the excited gas. With Herschel Spectral and Photometric Imaging Receiver we have been able to observe the rich molecular spectrum towards the central region of NGC 253. Carbon monoxide (CO, J = 4 - 3 to 13-12) is the brightest molecule in the Herschel wavelength range and together with ground-based low-J observations, the line fluxes trace the excitation of CO. By studying the CO excitation ladder and comparing the intensities to models, we investigate whether the gas is excited by UV radiation, X-rays, cosmic rays, or turbulent heating. Comparing the 12CO and 13CO observations to large velocity gradient models and photon-dominated region (PDR) models we find three main interstellar medium (ISM) phases. We estimate the density, temperature, and masses of these ISM phases. By adding 13CO, HCN, and HNC line intensities, we are able to constrain these degeneracies and determine the heating sources. The first ISM phase responsible for the low-J CO lines is excited by PDRs, but the second and third phases, responsible for the mid to high-J CO transitions, require an additional heating source. We find three possible combinations of models that can reproduce our observed molecular emission. Although we cannot determine which of these is preferable, we can conclude that mechanical heating is necessary to reproduce the observed molecular emission and cosmic ray heating is a negligible heating source. We then estimate the mass of each ISM phase; 6 × 107M⊙ for phase 1 (low-J CO lines), 3 × 107M⊙ for phase 2 (mid-J CO lines), and 9 × 106M⊙ for phase 3 (high-J CO lines) for a total system mass of 1 × 108M⊙.

A submillimeter exponential disk in M 51: Evidence for an extended cold dust disk

Received ????; accepted ???? Abstract. A 850 µm map of the interacting spiral galaxy M 51 shows well-defined spiral arms, closely resembling the struc- tures seen in CO and HI emission. However, most of the 850 µm emission originates in an underlying exponential disk, a component that has not been observed before in a face-on galaxy at these wavelengths. The scale-length of this disk is 5.45 kpc, which is somewhat larger than the scale-length of the stellar disk, but somewhat smaller than that of atomic hydrogen. Its profile can not be explained solely by a radial disk temperatu re gradient but requires the underlying dust to have an exponential distribution as well. This reinforces the view that the submm emission from spiral galaxy disks traces total hydrogen column density, i.e. the sum of H2 and H I. A canonical gas-to-dust ratio of 100±26 is obtained for �850 = 1.2 g −1 cm 2 , where �850 is the dust opacity at 850 µm.

Uncertainties in water chemistry in disks: An application to TW Hydrae

Context. This paper discusses the sensitivity of water lines to chemical processes and radiative transfer for the protoplanetary disk around TW Hya. The study focuses on the Herschel spectral range in the context of new line detections with the PACS instrument from the Gas in Protoplanetary Systems project (GASPS). Aims: The paper presents an overview of the chemistry in the main water reservoirs in the disk around TW Hya. It discusses the limitations in the interpretation of observed water line fluxes. Methods: We use a previously published thermo-chemical Protoplanetary Disk Model (ProDiMo) of the disk around TW Hya and study a range of chemical modeling uncertainties: metallicity, C/O ratio, and reaction pathways and rates leading to the formation of water. We provide results for the simplified assumption of Tgas = Tdust to quantify uncertainties arising for the complex heating/cooling processes of the gas and elaborate on limitations due to water line radiative transfer. Results: We report new line detections of p-H2O (322-211) at 89.99 μm and CO J = 18-17 at 144.78 μm for the disk around TW Hya. Disk modeling shows that the far-IR fine structure lines ([O i], [C ii]) and molecular submm lines are very robust to uncertainties in the chemistry, while the water line fluxes can change by factors of a few. The water lines are optically thick, sub-thermally excited and can couple to the background continuum radiation field. The low-excitation water lines are also sensitive to uncertainties in the collision rates, e.g. with neutral hydrogen. The gas temperature plays an important role for the [O i] fine structure line fluxes, the water line fluxes originating from the inner disk as well as the high excitation CO, CH+ and OH lines. Conclusions: Due to their sensitivity on chemical input data and radiative transfer, water lines have to be used cautiously for understanding details of the disk structure. Water lines covering a wide range of excitation energies provide access to the various gas phase water reservoirs (inside and outside the snow line) in protoplanetary disks and thus provide important information on where gas-phase water is potentially located. Experimental and/or theoretical collision rates for H2O with atomic hydrogen are needed to diminish uncertainties from water line radiative transfer. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.org

The molecular circumnuclear disk (CND) in Centaurus A - A multi-transition CO and [CI] survey with Herschel, APEX, JCMT, and SEST

This paper presents emission line intensities of CO and C° from the compact circumnuclear disk in the center of NGC 5128 (Centaurus A) obtained with the Herschel Space Observatory in the 400-1000 GHz range as well as previously unpublished measurements obtained with the ground-based observatories SEST, JCMT and APEX in the 90-800 GHz range. The results show that the Cen A center has an emission ladder of CO transitions quite different from those of either star-burst galaxies or (Seyfert) AGNs. In addition, the neutral carbon ([CI]) emission lines from the Cen A center are much stronger relative to the adjacent CO lines than in any other galaxy. The CO surface brightness of the compact circumnuclear disk (CND) is significantly higher than that of the much more extended thin disk (ETD) in the same line of sight. LVG analysis of the CO line profiles decomposed into the constituent contributions show that the ETD is relatively cool and of low excitation, wheres the brighter CND is hotter and more highly excited. Our PDR/XDR models suggest that most of the CND gas is relatively cool (temperatures 25 K-80 K) and not very dense (≈300 cm-3) if it is primarily heated by UV photons. A small fraction of the gas in both the CND and the ETD has a much higher density (typically 30 000 cm-3). A more highly excited, high-density phase is present in the CND, either in the form of an extreme PDR or more likely in the form of an XDR. Such a phase does not occur in the part of the ETD sampled. We have determined, for the first time, the molecular mass parameters of the CND. The total gas mass of the CND is MCND = 8.4 × 107 M⊙, uncertain by a factor of two. The CO-H2 conversion factor (XCND) is 4 × 1020 (K km s-1)-1 also within a factor of two.

The Response of Metal-rich Gas to X-Ray Irradiation from a Massive Black Hole at High Redshift: Proof of Concept

Observational studies show that there is a strong link between the formation and evolution of galaxies and the growth of their supermassive black holes. However, the underlying physics behind this observed relation is poorly understood. In order to study the effects of X-ray radiation on black hole surroundings, we implement X-ray-dominated region physics into Enzo and use the radiation transport module Moray to calculate the radiative transfer for a polychromatic spectrum. In this work, we investigate the effects of X-ray irradiation, produced by a central massive black hole (MBH) with a mass of Mxa0= 5 × 104 M ☉, on ambient gas with solar and zero metallicity. We find that in the solar metallicity case, the energy deposition rate in the central region (≤20xa0pc) is high due to the high opacity of the metals. Hence, the central temperatures are on the order of 105-107xa0K. Moreover, due to the cooling ability and high intrinsic opacity of solar metallicity gas, column densities of 1024 cm–2 are reached at a radius of 20xa0pc from the MBH. These column densities are about three orders of magnitudes higher than in the zero metallicity case. Furthermore, in the zero metallicity case, an X-ray-induced H II region is already formed after 5.8xa0Myr. This causes a significant outflow of gas (~8 × 106 M ☉) from the central region; the gas reaches outflow velocities up to ~100xa0km s–1. At later times, ~23 Myr after we insert the MBH, we find that the solar metallicity case also develops an X-ray-induced H II region, but it is delayed by ~17xa0Myr compared to the zero metallicity case.

PROBING THE GASEOUS DISK OF T Tau N WITH CN 5-4 LINES

We present spectrally resolved observations of the young multiple system T Tau in atomic and molecular lines obtained with the Heterodyne Instrument for the Far Infrared on board Herschel. While CO, H2O, [C II], and SO lines trace the envelope and the outflowing gas up to velocities of 33 km s-1 with respect to systemic, the CN 5-4 hyperfine structure lines at 566.7, 566.9 GHz show a narrow double-peaked profile centered at systemic velocity, consistent with an origin in the outer region of the compact disk of T Tau N. Disk modeling of the T Tau N disk with the thermo-chemical code ProDiMo produces CN line fluxes and profiles consistent with the observed ones and constrain the size of the gaseous disk (R_out=110^{+10}_{-20} AU) and its inclination (i = 25°± 5°). The model indicates that the CN lines originate in a disk upper layer at 40-110 AU from the star, which is irradiated by the stellar UV field and heated up to temperatures of 50-700 K. With respect to previously observed CN 2-1 millimeter lines, the CN 5-4 lines appear to be less affected by envelope emission, due to their larger critical density and excitation temperature. Hence, high-J CN lines are a unique confusion-free tracer of embedded disks, such as the disk of T Tau N.