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The extended molecular gas of the Circinus galaxy and NGC 1097 as seen by APEX
Cold molecular gas in the diffuse outer interstellar medium is an elusive and less-studied component of nearby galaxies, but it contains imprints of tidal tails, large-scale bars, spiral arms, inflows, and outflows: structures and processes that play an important role in galaxy evolution. Part of the challenge is efficiently mapping nearby galaxies with sub-millimeter telescopes, because they cover large angular extents on the sky. In this work, we study two iconic nearby galaxies - the Circinus galaxy and NGC 1097 - in CO(3-2) and CO(2-1), respectively, with the Atacama Pathfinder EXperiment (APEX) telescope, which we use to map their full optical extents. In Circinus, the extended molecular gas appears to be participating in a large-scale (r ~ 5 kpc) molecular bar, which we study using position-velocity diagrams and 3D tilted ring modelling using the 3DBAROLO tool. In NGC 1097, the observations reveal a tidal tail from the interaction with the neighboring NGC 1097A, which was previously unseen in the cold molecular gas phase. We also perform flux comparisons with archival ALMA data to underscore the utility of single dish observations in recovering flux missed by interferometers. This work also represents an ideal pathfinder science case for the large aperture, large field-of-view, Atacama Large Aperture Submillmeter Telescope (AtLAST). Read the full article: Lasrado, Cicone et al. (2025).
APEX observations of the Circinus galaxy and NGC 1097, from Lasrado et al. (2025). Left: CO(3-2) integrated intensity contours in green overlaid on the Spitzer 8 micron map. The large scale gas extends up to 6 kpc away from the center, and follows a barred structure possibly assisting gas flows towards the central molecular ring. Right: CO(2-1) integrated intensity contours in green overlaid on the Spitzer 8 micron map. Towards the northwest, a molecular tidal tail overlaps with the disturbed spiral arm seen in the Spitzer map, in the direction of the interacting companion NGC 1097A (yellow circle).
A theoretical investigation of far-infrared fine structure lines at z>6 and of the origin of the [OIII]88/[CII]158 enhancement
The [OIII]/[CII] and [OIII]/[NII] ratios have shown to be promising tracers of the ionisation state and gas-phase metallicity of the interstellar medium (ISM). Observations of galaxies at redshift z > 6 show unusually high [OIII]/[CII] luminosity ratios compared to local sources. The origin of the enhanced ratios has been investigated with different theoretical modelling approaches. However, no model has to date successfully managed to match the observed emission from both [OIII] and [CII], as well as their ratio. In this paper, we use Cloudy to model the [CII], [OIII], [NII], and [NIII] emission lines of a typical star-forming galaxy at redshift z ∼ 6.5. We modify the chemical abundances in Cloudy to obtain C/O and N/O abundance ratios more in line with recent high-z observational constraints. We find [OIII]/[CII] luminosity ratios that are a factor of ∼ 5 higher compared to models assuming solar abundances, and we find overall better agreement with high-z observational constraints of the [CII]-SFR and [OIII]-SFR relations. Our results showcase the importance of theoretical models combined with multi-tracer observations to understand the physical and chemical conditions of the ISM at z > 6. Read the full article: Nyhagen et al. (2025).
Left: [OIII]/[CII] luminosity ratio vs SFR for the model assuming high-z C/O and N/O abundance ratios (red), and the fiducial model from Schimek et al. (2024) which assumes solar abundances (purple). The top panel reports the comparison to observational data, and the bottom panel reports the comparison with simulation studies. Right: [OIII]/[CII] ratio maps comparing the Solar model at the top, and the High-z model in the bottom panel.
Observation of a flat trend between M* and molecular gas fraction down in the poorly explored low-M* regime of local star forming galaxies, enabled by a reanalysis of 353 galaxies observed in CO by the xCOLDGASS and ALLSMOG surveys.
Molecular gas scaling relations for local star forming galaxies in the low-M* regime
Molecular gas plays a central role in the evolution of galaxies as the fuel for star formation. We analysed the statistical correlations, or scaling relations, between the molecular gas content and other galaxy properties in a large representative sample of local, star-forming galaxies down to stellar masses of 10^8.5 solar masses. Due to the low CO detection fraction at these low masses, we employed analysis methods suited to dealing with upper limits derived from non-detections: survival analysis, spectral stacking, and a Bayesian linear regression technique. The results from all three methods show that the data are consistent with a flat trend between stellar mass and molecular gas fraction (molecular gas mass divided by stellar mass) in the low-mass regime, or even a downturn in the relation (see Figure on the left). The large sample size (353 galaxies) enabled us to repeat our analysis with galaxies split into environment classes (central, satellite, and isolated). We find no significant dependence of the molecular gas scaling relations on the galaxy (group) environment. Read the full article: Hagedorn, Cicone et al. (2024).
A possible relation between global CO excitation and massive molecular outflows in local ULIRGs
Local ultra luminous infrared galaxies (ULIRGs) 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 could only probe the global molecular gas conditions. Our ground-based high-S/N spectroscopic survey of multi-J CO emission lines (up to CO(7-6)) in a sample of ~17 local ULIRGs has revealed a statistically significant trend of increasing CO line width with Jup of the transition, seemingly driven by the ULIRGs showing higher CO excitation (i.e. with CO SLEDs peaking beyond Jup = 4). A spectral decomposition shows that the CO SLEDs of high-velocity and/or high-dispersion spectral components are significantly more excited than those of more dynamically quiescent CO components, as reported in the plot on the left. For the first time, we find evidence supporting that the exceptionally higher CO excitation of local ULIRGs is related to the presence of CO broad line wings, which in these sources are dominated by outflowing gas. Read the full article: Montoya Arroyave, Cicone et al. (2024).
Simulated sub-mm and FIR line emission from the halo of a typical star forming galaxy at z~6.5: the [CII]158micron and [OIII]88micron lines trace different regions of the circumgalactic medium (CGM)
High resolution modeling of [CII], [CI], [OIII] and CO line emission from the ISM and CGM of a star forming galaxy at z ~ 6.5
The circumgalactic medium (CGM) is a crucial component of galaxy evolution, but thus far its physical properties are highly unconstrained. As of yet, no cosmological simulation has reached convergence when it comes to constraining the cold and dense gas fraction of the CGM. Such components are also challenging to observe, and require sub-millimeter instruments with a high sensitivity to extended, diffuse emission, like the proposed Atacama Large Aperture Sub-millimetre telescope (AtLAST). In this paper we present the first results of a state-of-the-art theoretical effort at modeling sub-mm and FIR line emission from the halo of a typical star forming galaxy at z~6.5. We find that up to 10% and 20% of the total [CII]158micron and [OIII]88micron line emissions respectively originates in the CGM. [CII] and [OIII] however trace different regions of the CGM: [CII] arises from an accreting filament and from tidal tails, while [OIII] traces a puffy halo surrounding the main disc, probably linked to SN feedback. We discuss our results in the context of current and future sub-mm observations with ALMA and AtLAST. Read the full article: Schimek et al. (2024).
Carbon monoxide (CO) and Atomic Carbon ([CI](1-0)) emission line spectra of four ULIRGs of the sample
A sensitive APEX and ALMA multi-J CO and [CI](1-0) spectral survey of 40 local ULIRGs
We are undertaking a multi-J CO and atomic Carbon line survey of 40 local infrared luminous galaxies (ULIRGs) with the APEX telescope, complemented by public ALMA and ACA archival data. Among the results of this work, we highlight: (i) the measurement of average higher [CI]/CO luminosity ratios in ULIRGs than found in previous, less sensitive and less uniform surveys. This result shows the limitations of previous observations that either lacked low-J CO coverage (e.g. Herschel) and/or missed a significant [CI](1-0) line flux. (ii) The discovery that the [CI](1-0) lines are narrower than the CO lines in these ULIRGs, especially in the sources that show the broadest CO line profiles. Our investigation showcases the importance of single-dish spectroscopic observations at high frequencies (500 and 800 GHz) to cover the atomic carbon lines in nearby galaxies, and motivates the development of high-frequency, high spectral resolution receivers for the future AtLAST observatory. Read the full article: Montoya Arroyave, Cicone et al. (2023).
Left panel: Curve of growth of the CO(3-2) line emission in cid_346. Right panel: ACA CO(3-2) integrated line map
Discovery of a giant halo of molecular gas around one of the SUPER quasars at z~2
Using the ACA, we have detected a giant CO -emitting halo around cid_346, an X-ray selected z~2 SUPER quasar, where previous ALMA snapshot observations only detected a compact interstellar medium component. The ACA CO(3-2) spectrum is significantly broader than the ALMA one, the flux is ~14 times higher, and the ACA source extends out to r=200 kpc in projected size. This is the most extended molecular circumgalactic medium (CGM) reservoir that has ever been mapped. This work shows the crucial need of short baselines in high-z sub-millimeter observations, and the need for AtLAST that can enable detection of such cold CGM structures even at lower redshifts. Read the full article: Cicone et al. (2021). See also ALMA science highlight in the September 2021 EU ARC Newsletter.
Extreme CN/CO ratios measured in Mrk231's molecular outflow indicating enhancement of UV radiation
We found an exceptional boost of the CN/CO(1-0) line luminosity ratio in the massive molecular outflow of Mrk231, suggesting that the outflow material is exposed to a strong UV field. Additionally, we spectrally resolved the high-velocity HCN and HCO+(1-0) line wings into several narrow sub-structures with velocity dispersions of ~7-20 km/s, which we interpret as individual dense clumps entrained in the outflow. Read the full article: Cicone et al. (2020).
Testing the blast-wave AGN feedback scenario in a local Seyfert with an extremely powerful X-ray ultra fast outflow
Using ALMA CO(1-0) observations we ruled out the presence of a momentum-boosted molecular outflow in MCG-03-58-007, which hosts an extremely powerful X-ray ultra fast outflow. The residual emission shown in the figure is obtained by subtracting the rotation of the main disk from the ALMA data. The favoured interpretation of the residual CO emission in MCG-03-58-007 is a biconical molecular outflow, whose momentum rate is comparable to that of the X-ray outflow. Alternatively, the residuals may trace a circumnuclear disk (CND), which however would be a factor 10-100 larger in size and mass than typical CNDs observed in local Seyferts so far. Read the full article: Sirressi et al. (2019).
ALMA [CI](1-0) and CO line observations of NGC6240 and its massive molecular outflow
ALMA observations of the neutral Carbon line ([CI]1-0) in NGC6240 shed light on the state of the gas entrained in the massive molecular outflow, hence allowing us to constrain its extreme energetics. Furthermore, the high-resolution ALMA [CI] images surprisingly reveal that the outflow is launched between the two AGNs rather than from either of the two. Read the full article: Cicone et al. (2018).
Swift X-ray telescope data hint at a binary SMBH candidate at sub-pc separation
The 123-month Swift-BAT monitoring of the nearby Seyfert galaxy MCG+11-11-032 reveals a modular behaviour with peaks and dips occurring almost every 25 months. Under the hypothesis of an SMBH pair, such variability would imply a small sub-pc separation with an orbital velocity of ~0.06c. This value is in remarkable agreement with the energy offset of the two FeK emission lines detected in the Swift XRT spectrum of this source. Read the full article: Severgnini et al. (2018).
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