Ultraluminous Infrared Galaxies Research Articles

Characterizing the Molecular Gas in Infrared Bright Galaxies with CARMA

We present the CO(1–0) maps of 28 infrared-bright galaxies from the Great Observatories All-Sky Luminous Infrared Galaxy Survey (GOALS) taken with the Combined Array for Research in Millimeter Astronomy (CARMA). We detect 100 GHz continuum in 16 of the 28 CARMA GOALS galaxies, which trace both active galactic nuclei (AGNs) and compact star-forming cores. The GOALS galaxies show a variety of molecular gas morphologies, though in the majority of cases the average velocity fields show a gradient consistent with rotation. We fit the full continuum spectral energy distributions (SEDs) of each of the sources using either magphys or SED3FIT (if there are signs of an AGN) to derive the total stellar mass, dust mass, and SFRs of each object. We adopt a value determined from luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) of α CO = 1.5−0.8+1.3 M ⊙ (K km s−1 pc2)−1, which leads to more physical values for f mol and the gas-to-dust ratio. Mergers tend to have the highest gas-to-dust ratios. We assume the cospatiality of the molecular gas and star formation and plot the CARMA GOALS sample on the Schmidt–Kennicutt relation, where we find that they preferentially lie above the line set by normal star-forming galaxies. This hyper-efficiency is likely due to the increased turbulence in these systems, which decreases the freefall time compared to star-forming galaxies, leading to “enhanced” star formation efficiency. Line wings are present in a non-negligible subsample (11/28) of the CARMA GOALS sources and are likely due to outflows driven by AGNs or star formation, gas inflows, or additional decoupled gas components.

Evidence of a Forming Nucleus in the Fourcade–Figueroa Galaxy

We analyze data from the IRAS, Wide-field Infrared Survey Explorer, and Planck satellites, revealing an unresolved dust condensation at the center of the Fourcade–Figueroa galaxy (ESO270-G017), which may correspond to a forming nucleus. We model the condensation’s continuum spectrum in the spectral range from 3 to 1300 μm using the DUSTY code. The best-fit model, based on the chi-square test, indicates that the condensation is a shell with an outer temperature of T out ≈ 12 K and an inner boundary temperature of T i ≈ 500 K. The shell’s outer radius is r o = 86.2 pc, and the inner cavity radius is r i = 0.082 pc. The condensation produces an extinction A V = 50 mag, and its luminosity is L c = 1.08 × 1034 W, which would correspond to a burst of massive star formation approximately similar to the central 5 pc of R 136 in the LMC and NGC 3603, the ionizing cluster of a giant Carina arm H ii region. The comparison with normal, luminous, and ultraluminous IR galaxies leads us to consider this obscured nucleus as the nearest and weakest object of this category.

Molecular gas excitation in the circumgalactic medium of MACS1931–26

The evolution of galaxies is largely affected by exchanging material with their close environment, the circumgalactic medium (CGM). In this work, we investigate the CGM and the interstellar medium (ISM) of the bright central galaxy (BCG) of the galaxy cluster, MACS1931−26 at z ∼ 0.35. We detected [CI](2−1), 12CO(1−0), and 12CO(7−6) emission lines with the APEX 12-m and NRO 45-m telescopes. We complemented these single-dish observations with 12CO(1−0), 12CO(3−2), and 12CO(4−3) ALMA interferometric data and inferred the cold molecular hydrogen physical properties. Using a modified large velocity gradient (LVG) model, we modelled the CO and CI emission of the CGM and BCG to extract the gas thermodynamical properties, including the kinetic temperature, the density, and the virialisation factor. Our study shows that the gas in the BCG is highly excited, comparable to the gas in local ultra luminous infrared galaxies (ULIRGs), while the CGM is likely less excited, colder, less dense, and less bound compared to the ISM of the BCG. The molecular hydrogen mass of the whole system derived using [CI](2−1) is larger than the mass derived from 12CO(1−0) in literature, showing that part of the gas in this system is CO-poor. Additional spatially resolved CI observations in both transitions, [CI](1−0) and [CI](2−1), and the completion of the CO SLED with higher CO transitions are crucial to trace the different phases of the gas in such systems and constrain their properties.

A GMRT 610 MHz radio survey of the North Ecliptic Pole (NEP, ADF-N) / Euclid Deep Field North

Abstract This paper presents a 610 MHz radio survey covering 1.94 deg2 around the North Ecliptic Pole (NEP), which includes parts of the AKARI (ADF-N) and Euclid, Deep Fields North. The median 5σ sensitivity is 28 μJy beam−1, reaching as low as 19 μJy beam−1, with a synthesised beam of 3.6″× 4.1″. The catalogue contains 1675 radio components, with 339 grouped into multi-component sources and 284 ’isolated’ components likely part of double radio sources. Imaging, cataloguing, and source identification are presented, along with preliminary scientific results. From a non-statistical sub-set of 169 objects with multi-wavelength AKARI and other detections, luminous infrared galaxies (LIRGs) represent 66$\%$ of the sample, ultra-luminous infrared galaxies (ULIRGs) 4$\%$, and sources with LIR < 1011 L⊙ 30$\%$. In total, 56$\%$ of sources show some AGN presence, though only seven are AGN-dominated. ULIRGs require three times higher AGN contribution to produce high-quality SED fits compared to lower luminosity galaxies, and AGN presence increases with AGN fraction. The PAH mass fraction is not significant, although ULIRGs have about half the PAH strength of lower IR-luminosity galaxies. Higher luminosity galaxies show gas and stellar masses an order of magnitude larger, suggesting higher star formation rates. For LIRGs, AGN presence increases with redshift, indicating that part of the total luminosity could be contributed by AGN activity rather than star formation. Simple cross-matching revealed 13 ROSAT QSOs, 45 X-ray sources, and 61 sub-mm galaxies coincident with GMRT radio sources.

A Comparative Study of Ultraluminous Infrared Galaxies in the IRAS and SDSS Surveys

We present a comprehensive study of ultraluminous infrared galaxies (ULIRGs), leveraging data from the Infrared Astronomical Satellite Faint Source Catalogue and the spectroscopic catalog in the Sloan Digital Sky Survey Data Release 16. Our meticulous crossmatching technique significantly enhances the reliability of ULIRG identification, resulting in the identification of 283 reliable ULIRGs, including 102 new detections, while discarding 120 previously reported false sources. Covering a redshift range of z = 0.018–0.996, with a median redshift of z¯=0.259 , our uniform sample reveals apparent interaction features in approximately 40% of ULIRGs, increasing to 92% for those with z < 0.1. Through optical spectra analysis, it is indicated that over 58% of ULIRGs host an active galactic nucleus (AGN), which is twice as high as detections based solely on infrared colors. Moreover, a pronounced excess of radio emissions associated with AGN activity results in a steeper radio–far-infrared correlation. Notably, Type I ULIRGs exhibit properties similar to those of narrow-line Seyfert 1 galaxies, with an elevated incidence rate of Mg ii broad absorption lines (16.7%), surpassing that of typical optically selected quasars by over tenfold, consistent with current evolutionary models. We anticipate that forthcoming telescopes such as the China Space Station Telescope and Leighton Chajnantor Telescope will provide deeper insights into ULIRG morphology, dust distribution, molecular gas, and AGN activity.

Black Hole Mass and Eddington Ratio Distribution of Hot Dust-obscured Galaxies

Hot dust-obscured galaxies (Hot DOGs) are a rare population of hyperluminous infrared galaxies discovered by the Wide-field Infrared Survey Explorer mission. Despite the significant obscuration of the active galactic nucleus (AGN) by dust in these systems, pronounced broad and blueshifted emission lines are often observed. Previous work has shown that eight Hot DOGs, referred to as blue-excess Hot DOGs (BHDs), present a blue excess consistent with type 1 quasar emission in their UV–optical spectral energy distributions (SEDs), which has been shown to originate from the light of the obscured central engine scattered into the line of sight. We present an analysis of the rest-frame optical emission characteristics for 172 Hot DOGs through UV–mid-IR SED modeling and spectroscopic details, with a particular focus on the identification of BHDs. We find that while the optical emission observed in Hot DOGs is in most cases dominated by a young stellar population, 26% of Hot DOGs show a significant enough blue excess emission to be classified as BHDs. Based on their broad C iv and Mg ii lines, we find that the mass of the black hole M BH in a BHD ranges from 108.7 to 1010 M ⊙. When using the same emission lines in regular Hot DOGs, we find the M BH estimates cover the entire range found for BHDs while also extending to somewhat lower values. This agreement may imply that the broad lines in regular Hot DOGs also originate from scattered light from the central engine, just as in BHDs, although a more detailed study would be needed to rule out an outflow-driven nature. Similar to z ∼ 6 quasars, we find that Hot DOGs sit above the local relation between stellar and black hole masses, suggesting either that AGN feedback has not yet significantly suppressed the stellar mass growth in the host galaxies or that they will be outliers of the relation when reaching z = 0.

The Relationships of Dust and Total Masses of Luminous Infrared Galaxies with Their Far Infrared and Total Luminosities Using Two Methods of Distance Measurements

This research aims to determine the nature of the relationships among luminous infrared galaxies (LIRGs) by using different methods of distance measurement (distance modulus “dm” and luminosity distance “dl”). We used data from the following sources: NASA Extragalactic Database (NED) and the HYPERLEDA database, where this data was used to study the nature of the relationships and the extent to which they relate to each other. The program (statistica-win-program) was employed, which helped us know the strength of the association between the variables. The results of the statistical analysis showed that there was a strong correlation between dust mass (Mdust dm &amp; Mdust dl) and luminous infrared (LFIRdm, LIRdm) &amp; (LFIRdl, LIRdl), where the partial correlation coefficient was equal to (R⋍0.7) and the slope was linear. There was also a positive correlation between (LFIRdm, LFIRdl)&amp; ( ) where the partial correlation coefficient was (R⋍0.4) with a linear slope. The relationship between (LIR)&amp; ( ) was a positive relation in the modulus distance case (dm), where its correlation was equal to (R⋍0.4), but in another case (luminosity distance), the relation was very weak, and the correlation was (R⋍0.2). The relationship between total masses ( ) &amp; (LFIRdm, dl­­) in both cases of distance (dm) &amp; (dl) was good, and the partial correlation coefficient was equal to (R⋍0.4) with a linear slope. The relationship between (LIR) &amp; ( ) in both cases of distance was also positive, and the partial correlation coefficient was equal to (R⋍0.3). There was an inverse relationship between the ratio fluxes (F100/F60) &amp; ( ), as well as a very tight partial correlation coefficient, which was equal to (R⋍-0.9) with a slope of (-1).

H i content at cosmic noon – a millimetre-wavelength perspective

ABSTRACT In order to understand galaxy growth evolution, it is critical to constrain the evolution of its building block: gas. Mostly comprised by Hydrogen in its neutral (H i) and molecular (H$_2$) phases, the latter is the one mostly directly associated to star formation, while the neutral phase is considered the long-term gas reservoir. In this work, we make use of an empirical relation between dust emission at millimetre wavelengths and total gas mass in the interstellar medium (M$_{\rm HI}$ plus M$_{\rm H_2}$) in order to retrieve the H i content in galaxies. We assemble an heterogeneous sample of 335 galaxies at $0.01\lt z\lt 6.4$ detected in both mm-continuum and carbon monoxide (CO), with special focus on a blindly selected sample to retrieve H i cosmological content when the Universe was $\sim 2-6\,$ Gyr old ($1\lt z\lt 3$). We find no significant evolution with redshift of the M$_{\rm HI}$/M$_{\rm H_2}$ ratio, which is about $1-3$ (depending on the relation used to estimate M$_{\rm HI}$). This also shows that M$_{\rm H_2}$-based gas depletion times are underestimated overall by a factor of $2-4$. Compared to local Universe H i mass functions, we find that the number density of galaxies with M$_{\rm HI}\gtrsim 10^{10.5}\,$M$_\odot$ significantly decreased since 8–12 Gyr ago. The specific sample used for this analysis is associated to 20–50 per cent of the total cosmic H i content as estimated via Damped Lyman-$\alpha$ Absorbers. In IR luminous galaxies, H i mass content decreases between $z\sim 2.5$ and $z\sim 1.5$, while H$_2$ seems to increase. We also show source detection expectations for SKA surveys.

Detailed study of a rare hyperluminous rotating disk in an Einstein ring 10 billion years ago

Hyperluminous infrared galaxies (HyLIRGs) are the rarest and most extreme starbursts and found only in the distant Universe (z ≳ 1). They have intrinsic infrared (IR) luminosities LIR ≥ 1013 L⊙ and are commonly found to be major mergers. Recently, the Planck All-Sky Survey to Analyze Gravitationally-lensed Extreme Starbursts project (PASSAGES) searched ~104 deg2 of the sky and found ~20 HyLIRGs. We describe a detailed study of PJ0116-24, the brightest (μLIR ≈ 2.6 × 1014 L⊙, magnified with μ ≈ 17) Einstein-ring HyLIRG in the southern sky, at z = 2.125, with observations from the near-IR integral-field spectrograph VLT/ERIS and the submillimetre interferometer ALMA. We detected Hα, Hβ, [N ii] and [S ii] lines and obtained an extreme Balmer decrement (Hα/Hβ ≈ 8.73 ± 1.14). We modelled the molecular-gas and ionized-gas kinematics with CO(3–2) and Hα data at ~100–300 pc and (sub)kiloparsec delensed scales, respectively, finding consistent regular rotation. We found PJ0116-24 to be highly rotationally supported (vrot/σ0, mol. gas ≈ 9.4) with a richer gaseous substructure than other known HyLIRGs. Our results imply that PJ0116-24 is an intrinsically massive (Mbaryon ≈ 1011.3 M⊙) and rare starbursty disk (star-formation rate, SFR = 1,490 M⊙ yr−1) probably undergoing secular evolution. This indicates that the maximal SFR (≳1,000 M⊙ yr−1) predicted by simulations could occur during a galaxy’s secular evolution, away from major mergers.

Very Long Baseline Interferometry Detection of an Active Radio Source Potentially Driving 100 kpc Scale Emission in the Ultraluminous Infrared Galaxy IRAS F01004–2237

The nearby ultraluminous infrared galaxy (ULIRG) IRAS F01004−2237 exhibits 100 kpc scale continuum emission at radio wavelengths. The absence of extended X-ray emission in IRAS F01004−2237 has suggested an active galactic nucleus (AGN) origin for the extended radio emission, whose properties and role in merging systems still need to be better understood. We present the results of multifrequency observations of IRAS F01004−2237 conducted by the Very Long Baseline Array at 2.3 and 8.4 GHz. Compact 8.4 GHz continuum emission was detected on a 1 pc scale in the nuclear region with an intrinsic brightness temperature of 108.1 K suggesting that the radio source originated from an AGN, potentially driving the extended emission. In contrast, no significant emission was observed at 2.3 GHz, indicating the presence of low-frequency absorption. This absorption cannot be attributed solely to synchrotron self-absorption; alternatively, free–free absorption due to thermal plasma is mainly at work in the spectrum. From combined perspectives, including mid-infrared and X-ray data, the AGN is obscured in a dense environment. The kinetic power of the nonthermal jet, as inferred from the extended emission, can play a more important role in dispersing the surrounding medium than the thermal outflow in IRAS F01004−2237. These findings hint that jet activities in ULIRGs may contribute to AGN feedback during galaxy evolution induced by merger events.

A Complete 16 μm Selected Galaxy Sample at z ∼ 1. II. Morphological Analysis

We present a morphological analysis of the 16 μm flux-density-limited galaxy sample at 0.8 < z < 1.3 from Huang et al. (2021). At the targeted redshift, the 16 μm emission corresponds to the polycyclic aromatic hydrocarbon feature from intense star formation, or dust heated by active galactic nuclei (AGN). Our sample of 479 galaxies are dominated by luminous infrared galaxies (LIRGs; 67%) in three CANDLES fields (Extended Groth Strip, ​​​​Great Observatories Origins Deep Survey North and South fields), and are further divided into AGN-dominated, star-forming dominated, composite, and blue compact galaxies by their spectral energy distribution types. The majority of our sample (71%) have disk morphologies, with the few AGN-dominated galaxies being more bulge-dominated than the star-forming-dominated and composite galaxies. The distribution of our sample on the Gini versus M 20 plane is consistent with previous studies, where the Sérsic index n shows an increasing trend toward the smaller M 20 and higher Gini region below the dividing line for mergers. The subsample of ultraluminous infrared galaxies (ULIRGs) follows a steep size–mass relation that is closer to the early-type galaxies. In addition, as the 4.5 μm luminosity excess ( L4.5exc , proxy for AGN strength) increases, our sample appears to be more bulge dominated (i.e., higher n). Based on the specific star formation rate and compactness ( log10Σ1.5,Σ1.5=M*/Re1.5 ) diagram, the majority of our LIRG-dominated galaxy sample follows a secular evolution track, and their distribution can be explained without involving any merging activities. Out of the 16 ULIRGs in our sample, six are compact with strong AGN contributions, likely evolving along the fast track from more violent activities.

Finding dusty AGNs from the JWST CEERS survey with mid-infrared photometry

ABSTRACT The nature of the interaction between active galactic nuclei (AGNs) and their host galaxies remains an unsolved question. Therefore, conducting an AGN census is valuable for AGN research. Nevertheless, a significant fraction of AGNs are obscured by their environment, which blocks UV and optical emissions due to the dusty torus surrounding the central supermassive black hole (SMBH). To overcome this challenge, mid-infrared (IR) surveys have emerged as a valuable tool for identifying obscured AGNs, as the obscured light is re-emitted in this range. With its high sensitivity, the JWST uncovered more fainter objects than previous telescopes. By applying the SED fitting, this work investigates AGN candidates in JWST Cosmic Evolution Early Release Science (CEERS) fields. We identified 42 candidates; 30 of them are classified as composites ($0.2\le f_{\text{AGN, IR}} &amp;lt; 0.5$), and 12 of them are AGNs ($f_{\text{AGN, IR}}\ge 0.5$). We report the AGN luminosity contributions and AGN number fractions as a function of redshift and total infrared luminosity, showing that previously reported increasing relations are not apparent in our sample due to the sample size. We also extend the previous results on ultraluminous infrared galaxies (ULIRGs, $L_{\rm TIR}\ge 10^{12} L_{\odot }$) to less luminous AGNs, highlighting the power of JWST.

A possible relation between global CO excitation and massive molecular outflows in local ULIRGs

Local ultra-luminous infrared galaxies (ULIRGs) have been observed to host ubiquitous molecular outflows, including the most massive and powerful ever detected. These sources have also exceptionally excited global, galaxy-integrated CO ladders. A connection between outflows and molecular gas excitation has however never been established, since previous multi-J CO surveys were limited in spectral resolution and sensitivity and so they could only probe the global molecular gas conditions. In this work, we address this question using new, ground-based, sensitive heterodyne spectroscopy of multiple CO rotational lines (up to CO(7−6)) in a sample of 17 local ULIRGs. We used the Atacama Pathfinder Experiment (APEX) telescope to survey the CO(Jup ≥ 4) lines at a high signal-to-noise ratio, and complemented these data with CO(Jup ≤ 3) APEX and Atacama Large Millimeter Array (ALMA and ACA) observations presented in Montoya Arroyave et al. (2023, A&amp;A, 673, A13). We detected a total of 74 (out of 75) CO lines, with up to six transitions per source. The resulting CO spectral line energy distributions (SLEDs) show a wide range in gas excitation, in agreement with previous studies on ULIRGs. Some CO SLEDs peak at Jup ∼ 3, 4, which we classify as “lower excitation”, while others plateau or keep increasing up to the highest-J CO transition probed, and we classify these as “higher excitation”. Our analysis includes for completeness the results of CO SLED fits performed with a single large velocity gradient component, but our main focus is the investigation of possible links between global CO excitation and the presence of broad and/or high-velocity CO spectral components that can contain outflowing gas. We discovered an increasing trend of line width as a function of Jup of the CO transition, which is significant at the 4σ level and appears to be driven by the eight sources that we classified as higher excitation. We further analyzed such higher-excitation ULIRGs, by performing a decomposition of their CO spectral profiles into multiple components, and we derived CO ladders that are clearly more excited for the spectral components characterized by higher velocities and/or velocity dispersion. Because these sources are known to host widespread molecular outflows, we favor an interpretation whereby the highly excited CO-emitting gas in ULIRGs resides in galactic-scale massive molecular outflows whose emission fills a large fraction of the beam of our APEX high-J CO observations. On the other hand, our results challenge alternative scenarios for which the high CO excitation in ULIRGs can be explained by classical component of the interstellar medium, such as photon- or X-ray dominated regions around the nuclear sources.

CON-quest

Context. Nuclear regions of ultraluminous and luminous infrared galaxies (U/LIRGs) are powered by starbursts and/or active galactic nuclei (AGNs). These regions are often obscured by extremely high columns of gas and dust. Molecular lines in the submillimeter windows have the potential to determine the physical conditions of these compact obscured nuclei (CONs). Aims. We aim to reveal the distributions of HCN and HCO+ emission in local U/LIRGs and investigate whether and how they are related to galaxy properties. Methods. Using the Atacama Large Millimeter/submillimeter Array (ALMA), we have conducted sensitive observations of the HCN J = 3−2 and HCO+J = 3−2 lines toward 23 U/LIRGs in the local Universe (z &lt; 0.07) with a spatial resolution of ∼0.3″ (∼50−400 pc). Results. We detected both HCN and HCO+ in 21 galaxies, only HCN in one galaxy, and neither in one galaxy. The global HCN/HCO+ line ratios, averaged over scales of ∼0.5−4 kpc, range from 0.4 to 2.3, with an unweighted mean of 1.1. These line ratios appear to have no systematic trend with bolometric AGN luminosity or star formation rate. The line ratio varies with position and velocity within each galaxy, with an average interquartile range of 0.38 on a spaxel-by-spaxel basis. In eight out of ten galaxies known to have outflows and/or inflows, we found spatially and kinematically symmetric structures of high line ratios. These structures appear as a collimated bicone in two galaxies and as a thin spherical shell in six galaxies. Conclusions. Non-LTE analysis suggests that the high HCN/HCO+ line ratio in outflows is predominantly influenced by the abundance ratio. Chemical model calculations indicate that the enhancement of HCN abundance in outflows is likely due to high-temperature chemistry triggered by shock heating. These results imply that the HCN/HCO+ line ratio can aid in identifying the outflow geometry when the shock velocity of the outflows is sufficiently high to heat the gas.

The kinematics of Polycyclic Aromatic Hydrocarbons (PAHs) in Galaxies revealed by Principal Component Analysis (PCA) tomography with JWST/NIRSpec

ABSTRACT Polycyclic Aromatic Hydrocarbons (PAHs) are organic molecules which comprise the smallest particles of dust in the interstellar medium (ISM). Due to their broad/complex emission profiles, obtaining kinematics is a challenge with traditional methods, especially before the advent of the JWST. In this work, we employ Principal Component Analysis (PCA) tomography to analyse JWST/NIRSpec IFU data of three nearby luminous infrared galaxies (LIRGs), namely, NGC 3256 N, NGC 3256 S, and NGC 7469. We detect the signature of rotation in the second principal component of the 3.3 $\mu$m PAH feature in all three targets. We construct velocity maps from the principal components for the 3.3 $\mu$m PAH feature, Br β (2.625 $\mu$m) and molecular hydrogen, H2 1–0 S(1) (2.12 $\mu$m). We find that in each target, the PAHs qualitatively follow the rotation of the galaxy, consistent with the rotational signature derived from Br β and H2. There are however some differences, with the PAH rotation in NGC 3256 N appearing at a different position angle, which suggest differences in the motion of the dust as compared to the gas. This kind of analysis opens a new window into this key component of the ISM.

GOALS-JWST: Mid-infrared Molecular Gas Excitation Probes the Local Conditions of Nuclear Star Clusters and the Active Galactic Nucleus in the LIRG VV 114

The enormous increase in mid-IR sensitivity and spatial and spectral resolution provided by the JWST spectrographs enables, for the first time, detailed extragalactic studies of molecular vibrational bands. This opens an entirely new window for the study of the molecular interstellar medium in luminous infrared galaxies (LIRGs). We present a detailed analysis of rovibrational bands of gas-phase CO, H2O, C2H2, and HCN toward the heavily obscured eastern nucleus of the LIRG VV 114, as observed by NIRSpec and the medium resolution spectrograph on the Mid-InfraRed Instrument (MIRI MRS). Spectra extracted from apertures of 130 pc in radius show a clear dichotomy between the obscured active galactic nucleus (AGN) and two intense starburst regions. We detect the 2.3 μm CO bandheads, characteristic of cool stellar atmospheres, in the star-forming regions, but not toward the AGN. Surprisingly, at 4.7 μm, we find highly excited CO (T ex ≈ 700–800 K out to at least rotational level J = 27) toward the star-forming regions, but only cooler gas (T ex ≈ 200 K) toward the AGN. We conclude that only mid-infrared pumping through the rovibrational lines can account for the equilibrium conditions found for CO and H2O in the deeply embedded starbursts. Here, the CO bands probe regions with an intense local radiation field inside dusty young massive star clusters or near the most massive young stars. The lack of high-excitation molecular gas toward the AGN is attributed to geometric dilution of the intense radiation from the bright point source. An overview of the relevant excitation and radiative transfer physics is provided in an appendix.

SMART: Spectral energy distributions Markov chain Analysis with Radiative Transfer models

Abstract In this paper we present the publicly available open-source spectral energy distribution (SED) fitting code SMART (Spectral energy distributions Markov chain Analysis with Radiative Transfer models). Implementing a Bayesian Markov chain Monte Carlo (MCMC) method, SMART fits the ultraviolet to millimetre SEDs of galaxies exclusively with radiative transfer models that currently constitute four types of pre-computed libraries, which describe the starburst, active galactic nucleus (AGN) torus, host galaxy and polar dust components. An important novelty of SMART is that, although it fits SEDs exclusively with radiative transfer models, it takes comparable time to popular energy balance methods to run. Here we describe the key features of SMART and test it by fitting the multi-wavelength SEDs of the 42 local ultraluminous infrared galaxies (ULIRGs) that constitute the HERschel Ultraluminous Infrared Galaxy Survey (HERUS) sample. The Spitzer spectroscopy data of the HERUS ULIRGs are included in the fitting at a spectral resolution, which is matched to that of the radiative transfer models. We also present other results that highlight the performance and versatility of SMART. SMART promises to be a useful tool for studying galaxy evolution in the James Webb Space Telescope (JWST) era. SMART is developed in Python and is available at https://github.com/ch-var/SMART.git.

Radio-only and Radio-to-far-ultraviolet Spectral Energy Distribution Modeling of 14 ULIRGs: Insights into the Global Properties of Infrared Bright Galaxies

We present detailed spectral energy distribution (SED) modeling of 14 local ultraluminous infrared galaxies (ULIRGs) with outstanding photometric data from the literature covering the ultraviolet–infrared (FIR) and radio bands (∼50 MHz to ∼30 GHz). We employ the CIGALE SED fitting code to model the ultraviolet–FIR–radio SED. For the radio-only SED modeling, we use the UltraNest package, leveraging its nested sampling algorithm. Combining the results from our previous study on 11 luminous infrared galaxies (LIRGs), we discuss the global astrophysical properties of a sample of 25 starburst galaxies (z < 0.5). Their radio spectra are frequently characterized by bends and turnovers, with no indication of ULIRGs exhibiting more complicated SEDs than LIRGs despite showing more signs of interactions. Including radio measurements in the CIGALE modeling constrained the dust luminosity and star formation rate (SFR) estimates by more than 1 order of magnitude better than previously reported for starburst galaxies. We show that total and nonthermal radio luminosity at 1.4 and 4.8 GHz frequencies can be good estimators of recent SFRs for all LIRGs and those ULIRGS with an insignificant influence of active galactic nuclei. A weaker but still significant correlation is observed between radio SFRs at 1.4 GHz and old (averaged over 100 Myr) SFRs based on SED modeling, indicative of multiple episodes of starburst activity during their lifetime. The thermal radio luminosity at 4.8 GHz is a better tracer of recent star formation than the thermal luminosity at 1.4 GHz. Statistically, our modeled nonthermal radio spectral indices do not significantly correlate with redshift, stellar mass, SFR, specific SFR, and dust mass.

Exploring the hardness of the ionising radiation with the infrared softness diagram

Aims. We explored the softness parameter in the infrared, ηIR′, whose main purpose is the characterisation of the hardness of the incident ionising radiation in emission-line nebulae. This parameter is obtained from the combination of mid-infrared wavelength range transitions corresponding to consecutive ionisation stages in star-forming regions. Methods. We compiled observational data from a sample of star-forming galaxies (SFGs), including luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs), to study the ηIR′ and its equivalent expression in two dimensions, the softness diagram. We compared them with predictions from photoionisation models to determine the shape of the ionising continuum energy distribution in each case. We also used the measured emission-line ratios as input for HCMISTRY-TEFF-IR, a code that performs a Bayesian-like comparison with photoionisation model predictions in order to quantify the equivalent effective temperature (T*) and the ionisation parameter. Results. We found similar average values within the errors of ηIR′ in (U)LIRGs (−0.57) in the rest of the SFGs (−0.51), which could be interpreted as indicative of a similar incident radiation field. This result is confirmed from the analysis using HCM-TEFF-IR, which simultaneously points to a slightly lower, although similar within the errors, T* scale for (U)LIRGs, even when a higher dust-to-gas mass ratio is considered in the models for these objects. These derived T* values are compatible with the ionisation from massive stars, without any need of harder ionising sources, both for (U)LIRGs and the rest of the SFGs. However, the derived T* in (U)LIRGs do not show any correlation with metallicity. This could be interpreted as a sign that their similar average T* values are due to the attenuation of the energetic incident flux from massive stars by the heated dust mixed with the gas. This is supported by the known very large amounts of small grains associated with the very high star formation rates measured in galaxies of this type.

The cold molecular gas in z ≳ 6 quasar host galaxies

Context. Probing the molecular gas reservoirs of z ≳ 6 quasar (QSO) host galaxies is fundamental to understanding the coevolution of star formation and black hole growth in these extreme systems. Yet, there is still an inhomogeneous coverage of molecular gas tracers for z ≳ 6 QSO hosts. Aims. To measure the average excitation and mass of the molecular gas reservoirs in the brightest z &gt; 6.5 QSO hosts, we combined new observations of CO(2–1) emission with existing observations of CO(6–5), CO(7–6), [C I] (2–1), [C II] 158 μm, and dust-continuum emission. Methods. We reduced and analysed observations of CO(2–1), taken with the Karl G. Jansky Very Large Array, in three z = 6.5 − 6.9 QSO hosts – the highest redshift observations of CO(2–1) to date. By combining these with the nine z = 5.7 − 6.4 QSO hosts for which CO(2–1) emission has already been observed, we studied the spread in molecular gas masses and CO excitation of z ≳ 6 QSOs. Results. Two of our three QSOs, P036+03 and J0305–3150, were not detected in CO(2–1), implying more highly excited CO than in the well-studied z = 6.4 QSO J1148+5251. However, we detected CO(2–1) emission at 5.1σ for our highest-redshift target, J2348–3054, yielding a molecular gas mass of (1.2 ± 0.2)×1010 M⊙, assuming αCO = 0.8 (K km s−1 pc2)−1 and r2, 1 = 1. This molecular gas mass is equivalent to the lower limit on the dynamical mass measured previously from resolved [C II] 158 μm observations, implying that there is little mass in stars or neutral gas within the [C II]-emitting region and that a low CO-to-H2 conversion factor is applicable. On average, these z ≳ 6 QSO hosts have far higher CO(6–5)-, CO(7–6)-, and [C II] 158 μm versus CO(2–1) line ratios than the local gas-rich and IR-luminous galaxies that host active galactic nuclei, but with a large range of values, implying some variation in their interstellar medium conditions. We derived a mean CO(6–5)-to-CO(1–0) line luminosity ratio of r6, 1 = 0.9 ± 0.2. Conclusions. Our new CO(2–1) observations show that even at 780 Myr after the Big Bang, QSO host galaxies can already have molecular gas masses of 1010 M⊙, consistent with a picture in which these z ≳ 6 QSOs reside in massive starbursts that are coevolving with the accreting supermassive black holes. Their high gas versus dynamical masses and extremely high line excitation imply the presence of extremely dense and warm molecular gas reservoirs illuminated by strong interstellar radiation fields.