E. M. Xilouris

Black hole accretion and star formation as drivers of gas excitation and chemistry in Markarian 231

We present a full high resolution SPIRE FTS spectrum of the nearby ultraluminous infrared galaxy Mrk 231. In total 25 lines are detected, including CO J = 5-4 through J = 13-12, 7 rotational lines of H2O, 3 of OH+ and one line each of H2O+, CH+, and HF. We find that the excitation of the CO rotational levels up to J = 8 can be accounted for by UV radiation from star formation. However, the approximately flat luminosity distribution of the CO lines over the rotational ladder above J = 8 requires the presence of a separate source of excitation for the highest CO lines. We explore X-ray heating by the accreting supermassive black hole in Mrk 231 as a source of excitation for these lines, and find that it can reproduce the observed luminosities. We also consider a model with dense gas in a strong UV radiation field to produce the highest CO lines, but find that this model strongly overpredicts the hot dust mass in Mrk 231. Our favoured model consists of a star forming disk of radius 560 pc, containing clumps of dense gas exposed to strong UV radiation, dominating the emission of CO lines up to J = 8. X-rays from the accreting supermassive black hole in Mrk 231 dominate the excitation and chemistry of the inner disk out to a radius of 160 pc, consistent with the X-ray power of the AGN in Mrk 231. The extraordinary luminosity of the OH+ and H2O+ lines reveals the signature of X-ray driven excitation and chemistry in this region.

The molecular gas in luminous infrared galaxies – I. CO lines, extreme physical conditions and their drivers

We report results from a large molecular line survey of luminous infrared galaxies (L-IR greater than or similar to 10(11) L-circle dot) in the local Universe (z less than or similar to 0.1), conducted during the last decade with the James Clerk Maxwell Telescope and the IRAM 30-m telescope. This work presents the CO and 13CO line data for 36 galaxies, further augmented by multi-J total CO line luminosities available for other infrared (IR) bright galaxies from the literature. This yields a combined sample of N = 70 galaxies with the star formation (SF) powered fraction of their IR luminosities spanning L-IR((+))similar to(10(10)-2 x10(12))L-circle dot and a wide range of morphologies. Simple comparisons of their available CO spectral line energy distributions (SLEDs) with local ones, as well as radiative transfer models, discern a surprisingly wide range of average interstellar medium (ISM) conditions, with most of the surprises found in the high-excitation regime. These take the form of global CO SLEDs dominated by a very warm (T-kin greater than or similar to 100 K) and dense (n >= 10(4) cm(-3)) gas phase, involving galaxy-sized (similar to(few) x 10(9) M-circle dot) gas mass reservoirs under conditions that are typically found only for similar to(13) per cent of mass per typical SF molecular cloud in the Galaxy. Furthermore, some of the highest excitation CO SLEDs are found in ultraluminous infrared galaxies (ULIRGs; L-IR >= 10(12)L(circle dot)) and surpass even those found solely in compact SF-powered hot spots in Galactic molecular clouds. Strong supersonic turbulence and high cosmic ray energy densities rather than far-ultraviolet/optical photons or supernova remnant induced shocks from individual SF sites can globally warm the large amounts of dense gas found in these merger-driven starbursts and easily power their extraordinary CO line excitation. This exciting possibility can now be systematically investigated with Herschel and the Atacama Large Milimeter Array (ALMA). As expected for an IR-selected (and thus SF rate selected) galaxy sample, only few cold CO SLEDs are found, and for fewer still a cold low/moderate-density and gravitationally bound state (i.e. Galactic type) emerges as the most likely one. The rest remain compatible with a warm and gravitationally unbound low-density phase often found in ULIRGs. Such degeneracies, prominent when only the low-J SLED segment (J = 10, 21 and 32) is available, advise against using its CO line ratios and the so-called X-co = M(H-2)/L-co(10) factor as SF mode indicators, a practice that may have led to the misclassification of the ISM environments of IR-selected gas-rich discs in the distant Universe. Finally, we expect that the wide range of ISM conditions found among LIRGs will strongly impact the X-co factor, an issue we examine in detail in Paper II.

THE MOLECULAR GAS IN LUMINOUS INFRARED GALAXIES. II. EXTREME PHYSICAL CONDITIONS AND THEIR EFFECTS ON THE X-co FACTOR

In this work, we conclude the analysis of our CO line survey of luminous infrared galaxies (LIRGs: L-IR greater than or similar to 10(11) L-circle dot) in the local universe (Paper I) by focusing on the influence of their average interstellar medium (ISM) properties on the total molecular gas mass estimates via the so-called X-co = M(H-2)/L-co,L-1-0 factor. One-phase radiative transfer models of the global CO spectral line energy distributions (SLEDs) yield an X-co distribution with similar to (0.6 +/- 0.2) M-circle dot (K km s(-1) pc(2))(-1) over a significant range of average gas densities, temperatures, and dynamic states. The latter emerges as the most important parameter in determining X-co, with unbound states yielding low values and self-gravitating states yielding the highest ones. Nevertheless, in many (U)LIRGs where available higher-J CO lines (J = 3-2, 4-3, and/or J = 6-5) or HCN line data from the literature allow a separate assessment of the gas mass at high densities (>= 10(4) cm(-3)) rather than a simple one-phase analysis, we find that near-Galactic X-co similar to (3-6) M-circle dot (K km s(-1) pc(2))(-1) values become possible. We further show that in the highly turbulent molecular gas in ULIRGs, a high-density component will be common and can be massive enough for its high X-co to dominate the average value for the entire galaxy. Using solely low-J CO lines to constrain X-co in such environments (as has been the practice up until now) may have thus resulted in systematic underestimates of molecular gas mass in ULIRGs, as such lines are dominated by a warm, diffuse, and unbound gas phase with low X-co but very little mass. Only well-sampled high-J CO SLEDs (J = 3-2 and higher) and/or multi-J observations of heavy rotor molecules (e.g., HCN) can circumvent such a bias, and the latter type of observations may have actually provided early evidence of it in local ULIRGs. The only way that the global X-co of such systems could be significantly lower than Galactic is if the average dynamic state of the dense gas is strongly gravitationally unbound. This is an unlikely possibility that must nevertheless be examined, with lines of rare isotopologues of high gas density tracers (e.g., (HCN)-C-13, high-J (CO)-C-13 lines) being very valuable in yielding (along with the lines of the main isotopes) such constraints. For less IR-luminous, disk-dominated systems, we find that the galaxy-averaged X-co deduced by one-phase models of global SLEDs can also underestimate the total molecular gas mass when much of it lies in an star-formation-quiescent phase extending beyond a central star-forming region. This is because such a phase (and its large X-co) remains inconspicuous in global CO SLEDs. Finally, detailed studies of a subsample of galaxies find ULIRGs with large amounts (similar to 10(9) M-circle dot) of very warm (>= 100 K) and dense gas (>= 10(5) cm(-3)), which could represent a serious challenge to photon-dominated regions as the main energy portals in the molecular ISM of such systems.

DUST HEATING SOURCES IN GALAXIES: THE CASE OF M33 (HERM33ES)

Dust emission is one of the main windows to the physics of galaxies and to star formation as the radiation from young, hot stars is absorbed by the dust and reemitted at longer wavelengths. The recently launched Herschel satellite now provides a view of dust emission in the far-infrared at an unequaled resolution and quality up to 500 mu m. In the context of the Herschel HERM33ES open time key project, we are studying the moderately inclined Scd local group galaxy M33 which is located only 840 kpc away. In this article, using Spitzer and Herschel data ranging from 3.6 mu m to 500 mu m, along with H I, H alpha maps, and Galaxy Evolution Explorer ultraviolet data, we have studied the emission of the dust at the high spatial resolution of 150 pc. Combining Spitzer and Herschel bands, we have provided new, inclination-corrected, resolved estimators of the total infrared brightness and of the star formation rate from any combination of these bands. The study of the colors of the warm and cold dust populations shows that the temperature of the former is, at high brightness, dictated by young massive stars but, at lower brightness, heating is taken over by the evolved populations. Conversely, the temperature of the cold dust is tightly driven by the evolved stellar populations.

PACS and SPIRE photometer maps of M33: First results of the HERschel M33 Extended Survey (HERM33ES)

Context. Within the framework of the HERM33ES key program, we are studying the star forming interstellar medium in the nearby, metal-poor spiral galaxy M33, exploiting the high resolution and sensitivity of Herschel. Aims. We use PACS and SPIRE maps at 100, 160, 250, 350, and 500 mu m wavelength, to study the variation of the spectral energy distributions (SEDs) with galacto-centric distance. Methods. Detailed SED modeling is performed using azimuthally averaged fluxes in elliptical rings of 2 kpc width, out to 8 kpc galacto-centric distance. Simple isothermal and two-component grey body models, with fixed dust emissivity index, are fitted to the SEDs between 24 mu m and 500 mu m using also MIPS/Spitzer data, to derive first estimates of the dust physical conditions. Results. The far-infrared and submillimeter maps reveal the branched, knotted spiral structure of M33. An underlying diffuse disk is seen in all SPIRE maps (250-500 mu m). Two component fits to the SEDs agree better than isothermal models with the observed, total and radially averaged flux densities. The two component model, with beta fixed at 1.5, best fits the global and the radial SEDs. The cold dust component clearly dominates; the relative mass of the warm component is less than 0.3% for all the fits. The temperature of the warm component is not well constrained and is found to be about 60 K +/- 10 K. The temperature of the cold component drops significantly from similar to 24 K in the inner 2 kpc radius to 13 K beyond 6 kpc radial distance, for the best fitting model. The gas-to-dust ratio for beta = 1.5, averaged over the galaxy, is higher than the solar value by a factor of 1.5 and is roughly in agreement with the subsolar metallicity of M33.

CO SPECTRAL LINE ENERGY DISTRIBUTIONS OF INFRARED-LUMINOUS GALAXIES AND ACTIVE GALACTIC NUCLEI

We report on new sensitive CO J = 6-5 line observations of several luminous infrared galaxies (LIRGs; L IR(8-1000 ?m) 1011?L ?), 36% (8/22) of them ultraluminous infrared galaxies (ULIRGs) (L IR>1012?L ?), and two powerful local active galactic nuclei (AGNs)?the optically luminous QSO PG?1119+120 and the powerful radio galaxy 3C?293?using the James Clerk Maxwell Telescope on Mauna Kea in Hawaii. We combine these observations with existing low-J CO data and dust emission spectral energy distributions in the far-infrared-submillimeter from the literature to constrain the properties of the star-forming interstellar medium (ISM) in these systems. We then build the first local CO spectral line energy distributions (SLEDs) for the global molecular gas reservoirs that reach up to high J-levels. These CO SLEDs are neither biased by strong lensing (which affects many of those constructed for high-redshift galaxies), nor suffer from undersampling of CO-bright regions (as most current high-J CO observations of nearby extended systems do). We find: (1) a significant influence of dust optical depths on the high-J CO lines, suppressing the J = 6-5 line emission in some of the most IR-luminous LIRGs, (2) low global CO line excitation possible even in vigorously star-forming systems, (3) the first case of a shock-powered high-excitation CO SLED in the radio galaxy 3C?293 where a powerful jet-ISM interaction occurs, and (4) unusually highly excitated gas in the optically powerful QSO PG?1119+120. In Arp?220 and possibly other (U)LIRGs very faint CO J = 6-5 lines can be attributed to significant dust optical depths at short submillimeter wavelengths immersing those lines in a strong dust continuum, and also causing the C+ line luminosity deficit often observed in such extreme starbursts. Re-analysis of the CO line ratios available for submillimeter galaxies suggests that similar dust opacities also may be present in these high-redshift starbursts, with genuinely low excitation of large amounts of SF-quiescent gas being the only other possibility for their often low CO (high-J)/(low-J) line ratios. We then present a statistical method of separating these two almost degenerate possibilities, and show that high dust optical depths at submillimeter wavelengths can impede the diagnostic potential of submillimeter/IR lines (e.g., starbursts versus AGNs as gas excitation agents), which is of particular importance for the upcoming observations of the Herschel Space Observatory and the era of ALMA.

The Herschel Comprehensive (U)LIRG Emission Survey (HERCULES): CO Ladders, Fine Structure Lines, and Neutral Gas Cooling

(Ultra) luminous infrared galaxies ((U)LIRGs) are objects characterized by their extreme infrared (8-1000 mu m) luminosities (L-LIRG > 10(11) L-circle dot and L-ULIRG > 10(12) L-circle dot). The Herschel Comprehensive ULIRG Emission Survey (PI: van derWerf) presents a representative flux-limited sample of 29 (U)LIRGs that spans the full luminosity range of these objects (10(11)L(circle dot) <= L-IR <= 10(13)L(circle dot)). With the Herschel Space Observatory, we observe [CII] 157 mu m, [O I] 63 mu m, and [O I] 145 mu m line emission with Photodetector Array Camera and Spectrometer, CO J = 4-3 through J = 13-12, [C I] 370 mu m, and [C I] 609 mu m with SPIRE, and low-J CO transitions with ground-based telescopes. The CO ladders of the sample are separated into three classes based on their excitation level. In 13 of the galaxies, the [O I] 63 mu m emission line is self absorbed. Comparing the CO excitation to the InfraRed Astronomical Satellite 60/100 mu m ratio and to far infrared luminosity, we find that the CO excitation is more correlated to the far infrared colors. We present cooling budgets for the galaxies and find fine-structure line flux deficits in the [C II], [Si II], [O I], and [C I] lines in the objects with the highest far IR fluxes, but do not observe this for CO 4 <= J(upp) <= 13. In order to study the heating of the molecular gas, we present a combination of three diagnostic quantities to help determine the dominant heating source. Using the CO excitation, the CO J = 1-0 linewidth, and the active galactic nucleus (AGN) contribution, we conclude that galaxies with large CO linewidths always have high-excitation CO ladders, and often low AGN contributions, suggesting that mechanical heating is important.

The emissivity of dust grains in spiral galaxies

We use the radiation transfer simulation of Xilouris et al. (1999) to constrain the quantity of dust in three nearby spiral galaxies (NGC 4013, NGC 5907 and NGC 4565). The predicted visual optical depth from the model is compared with the thermal continuum radiation detected from NGC 4013 and NGC 5907 at 850 µm and from NGC 4565 at 1.2 mm. The former is based on SCUBA images of NGC 4013 and NGC 5907, reduced and presented for the first time in this work. The comparison of visual optical depth and 850 µm (1.2 mm) emission yields the emissivity of dust grains in the submillimeter (millimeter) waveband. We infer a value of 1.2 × 10 4 for the emissivity at 850 µm which is a factor 4 higher than the benchmark, semi- empirical model of Draine & Lee (1984). At 1.2 mm our values are a factor 1.5 higher than this model. Our estimates are more closely aligned with recent measurements carried out in the laboratory on amorphous carbon and silicate particulates. A comparison between the distribution of 850 µm (1.2 mm) surface brightness and the intensity levels in the 12 CO(1-0) and 21 cm lines underlines the spatial association between dust detected in the submillimeter/millimeter waveband and molecular gas clouds. We suggest that the relatively high emissivity values that we derive may be attributable to amorphous, fluff yg rains situated in denser gas environments.

MOLECULAR GAS HEATING MECHANISMS, AND STAR FORMATION FEEDBACK IN MERGER/STARBURSTS: NGC 6240 AND Arp 193 AS CASE STUDIES

We used the SPIRE/FTS instrument aboard the Herschel Space Observatory to obtain the Spectral Line Energy Distributions (SLEDs) of CO from J= 4-3 to J= 13-12 of Arp 193 and NGC 6240, two classical merger/starbursts selected from our molecular line survey of local Luminous Infrared Galaxies (L-IR >= 10(11) L-circle dot). The high-J CO SLEDs are then combined with ground-based low-J CO, (CO)-C-13, HCN, HCO+, CS line data and used to probe the thermal and dynamical states of their large molecular gas reservoirs. We find the two CO SLEDs strongly diverging from J = 4-3 onward, with NGC 6240 having a much higher CO line excitation than Arp 193, despite their similar low-J CO SLEDs and LFIR/LCO, 1-0, LHCN/LCO (J = 1-0) ratios ( proxies of star formation efficiency and dense gas mass fraction). In Arp 193, one of the three most extreme starbursts in the local universe, the molecular SLEDs indicate a small amount (similar to 5%-15%) of dense gas (n >= 10(4) cm(-3)) unlike NGC 6240 where most of the molecular gas (similar to 60%-70%) is dense (n similar to (10(4)-10(5)) cm(-3)). Strong star-formation feedback can drive this disparity in their dense gas mass fractions, and also induce extreme thermal and dynamical states for the molecular gas. In NGC 6240, and to a lesser degree in Arp 193, we find large molecular gas masses whose thermal states cannot be maintained by FUV photons from Photon-Dominated Regions. We argue that this may happen often in metal-rich merger/starbursts, strongly altering the initial conditions of star formation. ALMA can now directly probe these conditions across cosmic epoch, and even probe their deeply dust-enshrouded outcome, the stellar initial mass function averaged over galactic evolution.

TRACING MOLECULAR GAS MASS IN EXTREME EXTRAGALACTIC ENVIRONMENTS: AN OBSERVATIONAL STUDY

U.L. acknowledges financial support from the research project AYA2007-67625-C02-02 from the Spanish Ministerio de Ciencia y Educacion and from the Junta de Anaducia.