arXiv subjects

D. S. Meier

Publications and source records attributed to D. S. Meier.

At least 19 recordsLinked to original sources

arXiv2025

JWST Observations of Starbursts: Relations between PAH features and CO clouds in the starburst galaxy M 82

We present a study of new 7.7-11.3 $μ$m data obtained with the James Webb Space Telescope Mid-InfraRed Instrument in the starburst galaxy M 82. In particular, we focus on the dependency of the integrated CO(1-0) line intensity on the MIRI-F770W and MIRI-F1130W filter intensities to investigate the correlation between CO content and the 7.7 and 11.3 $μ$m features from polycyclic aromatic hydrocarbons (PAH) in M 82's outflows. To perform our analysis, we identify CO clouds using archival $^{12}$CO($J$=1-0) NOEMA moment 0 map within 2 kpc from the center of M 82, with sizes ranging between $\sim$21 and 270 pc; then, we compute the CO-to-PAH relations for the 306 validated CO clouds. On average, the power-law slopes for the two relations in M 82 are lower than what is seen in local main-sequence spirals. In addition, there is a moderate correlation between $I_{\rm CO(1-0)}$-$I_{\rm 7.7μm} /I_{\rm 11.3μm}$ for most of the CO cloud groups analyzed in this work. Our results suggest that the extreme conditions in M 82 translate into CO not tracing the full budget of molecular gas in smaller clouds, perhaps as a consequence of photoionization and/or emission suppression of CO molecules due to hard radiation fields from the central starburst.

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arXiv2024

An ALCHEMI inspection of sulphur-bearing species towards the central molecular zone of NGC 253

Sulphur-bearing species are detected in various environments within Galactic star-forming regions and are particularly abundant in the gas phase of outflows and shocks, and photo-dissociation regions. In this work, we aim to investigate the nature of the emission from the most common sulphur-bearing species observable at millimetre wavelengths towards the nuclear starburst of the galaxy NGC 253. We intend to understand which type of regions are probed by sulphur-bearing species and which process(es) dominate(s) the release of sulphur into the gas phase. We used the high-angular resolution (1.6" or 27 pc) observations from the ALCHEMI ALMA Large Program to image several sulphur-bearing species towards the central molecular zone (CMZ) of NGC 253. We performed local thermodynamic equilibrium (LTE) and non-LTE large velocity gradient (LVG) analyses to derive the physical conditions of the gas in which S-bearing species are emitted, and their abundance ratios across the CMZ. Finally, we compared our results with previous ALCHEMI studies and a few selected Galactic environments. We found that not all sulphur-bearing species trace the same type of gas: strong evidence indicates that H2S and part of the emission of OCS, H2CS, and SO, are tracing shocks whilst part of SO and CS emission rather trace the dense molecular gas. For some species, such as CCS and SO2, we could not firmly conclude on their origin of emission. The present analysis indicates that the emission from most sulphur-bearing species throughout the CMZ is likely dominated by shocks associated with ongoing star formation. In the inner part of the CMZ where the presence of super star clusters was previously indicated, we could not distinguish between shocks or thermal evaporation as the main process releasing the S-bearing species.

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arXiv2024

Molecular isotopologue measurements toward super star clusters and the relation to their ages in NGC253 with ALCHEMI

Determining the evolution of the CNO isotopes in the interstellar medium (ISM) of starburst galaxies can yield important constraints on the ages of superstar clusters (SSCs), or on other aspects and contributing factors of their evolution. Due to the time-dependent nature of the abundances of isotopes within the ISM as they are supplied from processes such as nucleosynthesis or chemical fractionation, this provides the possible opportunity to probe the ability of isotopes ratios to trace the ages of high star forming regions, such as SSCs. The goal of this study is to investigate whether the isotopic variations in SSC regions within NGC253 are correlated with their different ages as derived from stellar population modelling. We have measured abundance ratios of CO, HCN and HCO$^+$ isotopologues in six regions containing SSCs within NGC253 using high spatial resolution (1.6",$\sim 28$pc) data from the ALCHEMI (ALma Comprehensive High-resolution Extragalactic Molecular Inventory) ALMA Large program. We have then analysed these ratios using RADEX radiative transfer modelling, with the parameter space sampled using the nested sampling Monte Carlo algorithm MLFriends. These abundance ratios were then compared to ages predicted in each region via the fitting of observed star formation tracers (such as Br$γ$) to starburst stellar population evolution models. We do not find any significant trend with age for the CO and HCN isotopologue ratios on the timescales for the ages of the SSC* regions observed. The driving factors of these ratios within SSCs could be the Initial Mass Function as well as possibly fractionation effects. To further probe these effects in SSCs over time a larger sample of SSCs must be observed spanning a larger age range.

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arXiv2023

Reconstructing the shock history in the CMZ of NGC 253 with ALCHEMI

HNCO and SiO are well known shock tracers and have been observed in nearby galaxies, including the nearby (D=3.5 Mpc) starburst galaxy NGC 253. The simultaneous detection of these two species in regions where the star formation rate is high may be used to study the shock history of the gas. We perform a multi-line molecular study using these two shock tracers (SiO and HNCO) with the aim of characterizing the gas properties. We also explore the possibility of reconstructing the shock history in NGC 253's Central Molecular Zone (CMZ). Six SiO transitions and eleven HNCO transitions were imaged at high resolution $1''.6$ (28 pc) with the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the ALCHEMI Large Programme. Both non-LTE radiative transfer analysis and chemical modelling were performed in order to characterize the gas properties, and to investigate the chemical origin of the emission. The non-LTE radiative transfer analysis coupled with Bayesian inference shows clear evidence that the gas traced by SiO has different densities and temperatures than that traced by HNCO, with an indication that shocks are needed to produce both species. Chemical modelling further confirms such a scenario and suggests that fast and slow shocks are responsible for SiO and HNCO production, respectively, in most GMCs. We are also able to infer the physical characteristics of the shocks traced by SiO and HNCO for each GMC. Radiative transfer and chemical analysis of the SiO and HNCO in the CMZ of NGC 253 reveal a complex picture whereby most of the GMCs are subjected to shocks. We speculate on the possible shock scenarios responsible for the observed emission and provide potential history and timescales for each shock scenario. Higher spatial resolution observations of these two species are required in order to quantitatively differentiate between scenarios.

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arXiv2022

Methanol masers in NGC 253 with ALCHEMI

Context: Methanol masers of Class I (collisionally-pumped) and Class II (radiatively-pumped) have been studied in great detail in our Galaxy in a variety of astrophysical environments such as shocks and star-forming regions and are helpful to analyze the properties of the dense interstellar medium. However, the study of methanol masers in external galaxies is still in its infancy. Aims: Our main goal is to search for methanol masers in the central molecular zone (CMZ; inner 500 pc) of the nearby starburst galaxy NGC 253. Methods: Covering a frequency range between 84 and 373 GHz ($λ$ = 3.6 to 0.8 mm) at high angular (1.6"$\sim$27 pc) and spectral ($\sim$8--9 km s$^{-1}$) resolution with the ALMA large program ALCHEMI, we have probed different regions across the CMZ of NGC 253. In order to look for methanol maser candidates, we employed the rotation diagram method and a set of radiative transfer models. Results: We detect for the first time masers above 84 GHz in NGC 253, covering an ample portion of the $J_{-1}\rightarrow(J-$ 1)$_{0}-E$ line series (at 84, 132, 229, and 278 GHz) and the $J_{0}\rightarrow(J-$ 1)$_{1}-A$ series (at 95, 146, and 198 GHz). This confirms the presence of the Class I maser line at 84 GHz, already reported but now being detected in more than one location. For the $J_{-1}\rightarrow(J-$ 1)$_{0}-E$ line series, we observe a lack of Class I maser candidates in the central star-forming disk. Conclusions: The physical conditions for maser excitation in the $J_{-1}\rightarrow(J-$ 1)$_{0}-E$ line series can be weak shocks and cloud-cloud collisions as suggested by shock tracers (SiO and HNCO) in bi-symmetric shock/active regions located in the outskirts of the CMZ. On the other hand, the presence of photodissociation regions due to a high star-formation rate would be needed to explain the lack of Class I masers in the very central regions.

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arXiv2021

ALCHEMI: an ALMA Comprehensive High-resolution Extragalactic Molecular Inventory. Survey presentation and first results from the ACA array

We used the Atacama Large Millimeter/submillimeter Array (ALMA), covering a nearly contiguous 289 GHz frequency range between 84.2 and 373.2 GHz, to image the continuum and spectral line emission at 1.6\arcsec ($\sim 28$ pc) resolution down to a sensitivity of $30-50$ mK. This article describes the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) Large Program. We focus on the analysis of the spectra extracted from the $15''$ ($\sim255$ pc) resolution ALMA Compact Array data. We model the molecular emission assuming local thermodynamic equilibrium with 78 species detected. Additionally, multiple hydrogen and helium recombination lines are identified. Spectral lines contribute 5 to 36\% of the total emission in frequency bins of 50 GHz. We report the first extragalactic detections of C$_2$H$_5$OH, HOCN, HC$_3$HO, and several rare isotopologues. Isotopic ratios of carbon, oxygen, sulfur, nitrogen and silicon were measure with multiple species. Infrared pumped vibrationaly excited HCN, HNC, and HC$_3$N emission, originating in massive star formation locations, is clearly detected at low resolution, while we do not detect it for HCO$^+$. We suggest high temperature conditions in these regions driving a seemingly "carbon-rich" chemistry which may also explain the observed high abundance of organic species close to those in Galactic hot cores. The $L_{vib}/L_{IR}$ ratio is used as a proxy to estimate a $3\%$ contribution from proto super star cluster to the global infrared emission. Measured isotopic ratios with high dipole moment species agree with those within the central kiloparsec of the Galaxy, while those derived from $\rm^{13}C^{18}O$ are a factor of 5 larger, confirming the existence of multiple ISM components within NGC 253 with different degrees of nucleosynthesis enrichment. ALCHEMI provides a template for early Universe galaxies.

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arXiv2021

The Distribution and Origin of C$_2$H in NGC 253 from ALCHEMI

Observations of chemical species can provide an insight into the physical conditions of the emitting gas but it is important to understand how their abundances and excitation vary within different heating environments. C$_2$H is a molecule typically found in PDR regions of our own Galaxy but there is evidence to suggest it also traces other regions undergoing energetic processing in extragalactic environments. As part of the ALCHEMI ALMA large program, the emission of C$_2$H in the central molecular zone of the nearby starburst galaxy NGC 253 was mapped at 1.6 " (28 pc) resolution and characterized to understand its chemical origins. Spectral modelling of the N=1-0 through N=4-3 rotational transitions of C$_2$H was used to derive the C$_2$H column densities towards the dense clouds in NGC 253. Chemical modelling, including PDR, dense cloud, and shock models were then used to investigate the chemical processes and physical conditions that are producing the molecular emission. We find high C$_2$H column densities of $\sim 10^{15} cm^{-3}$ detected towards the dense regions of NGC 253. We further find that these column densities cannot be reproduced by assuming that the emission arises from the PDR regions at the edge of the clouds. Instead, we find that the C$_2$H abundance remains high even in the high visual extinction interior of these clouds and that this is most likely caused by a high cosmic-ray ionization rate.

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arXiv2021

Clustered Star Formation in the center of NGC 253 Contributes to Driving the Ionized Nuclear Wind

We present new 3 mm observations of the ionized gas toward the nuclear starburst in the nearby (D ~ 3.5 Mpc) galaxy NGC 253. With ALMA, we detect emission from the H40-alpha and He40-alpha lines in the central 200 pc of this galaxy on spatial scales of ~4 pc. The recombination line emission primarily originates from a population of approximately a dozen embedded super star clusters in the early stages of formation. We find that emission from these clusters is characterized by electron temperatures ranging from 7000-10000 K and measure an average singly-ionized helium abundance = 0.25 +/- 0.06, both of which are consistent with values measured for HII regions in the center of the Milky Way. We also report the discovery of unusually broad-linewidth recombination line emission originating from seven of the embedded clusters. We suggest that these clusters contribute to the launching of the large-scale hot wind observed to emanate from the central starburst. Finally, we use the measured recombination line fluxes to improve the characterization of overall embedded cluster properties, including the distribution of cluster masses and the fractional contribution of the clustered star formation to the total starburst, which we estimate is at least 50%.

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arXiv2020

ALMA Detects CO(3-2) within a Super Star Cluster in NGC5253

We present observations of CO(3-2) and $^{13}$CO(3-2) emission near the supernebula in the dwarf galaxy NGC 5253, which contains one of the best examples of a potential globular cluster in formation. The 0.3" resolution images reveal an unusual molecular cloud, "Cloud D1", coincident with the radio-infrared supernebula. The ~6-pc diameter cloud has a linewidth, $Δ$ v = 21.7 km/s, that reflects only the gravitational potential of the star cluster residing within it. The corresponding virial mass is 2.5 x 10$^5$ M$_\odot$. The cluster appears to have a top-heavy initial mass function, with $M_{low}$~1-2 M$_\odot$. Cloud D1 is optically thin in CO(3-2) probably because the gas is hot. Molecular gas mass is very uncertain but constitutes < 35% of the dynamical mass within the cloud boundaries. In spite of the presence of an estimated ~1500-2000 O stars within the small cloud, the CO appears relatively undisturbed. We propose that Cloud D1 consists of molecular clumps or cores, possibly star-forming, orbiting with more evolved stars in the core of the giant cluster.

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arXiv2020

The Star Formation in Radio Survey: 3 - 33 GHz Imaging of Nearby Galaxy Nuclei and Extranuclear Star-forming Regions

We present 3, 15, and 33 GHz imaging towards galaxy nuclei and extranuclear star-forming regions using the Karl G. Jansky Very Large Array as part of the Star Formation in Radio Survey. With $3-33$ GHz radio spectra, we measured the spectral indices and corresponding thermal (free-free) emission fractions for a sample of 335 discrete regions having significant detections in at least two radio bands. After removing 14 likely background galaxies, we find that the median thermal fraction at 33 GHz is $92 \pm 0.8\%$ with a median absolute deviation of $11\%$, when a two-component power-law model is adopted to fit the radio spectrum. Limiting the sample to 238 sources that are confidently identified as star-forming regions, and not affected by potential AGN contamination (i.e., having galactocentric radii $r_{\rm G} \geq 250$ pc), results in a median thermal fraction of $93 \pm 0.8 \%$ with a median absolute deviation of $10\%$. We further measure the thermal fraction at 33 GHz for 163 regions identified at 7" resolution to be $94 \pm 0.8 \%$ with a median absolute deviation of $8\%$. Together, these results confirm that free-free emission dominates the radio spectra of star-forming regions on scales up to $\sim$500 pc in normal star-forming galaxies. We additionally find a factor of $\sim$1.6 increase in the scatter of the measured spectral index and thermal fraction distributions as a function of decreasing galactocentric radius. This trend is likely reflective of the continuous star-formation activity occurring in the galaxy centers, resulting a larger contribution of diffuse nonthermal emission relative to star-forming regions in the disk.

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arXiv2019

Spatially resolved carbon and oxygen isotopic ratios in NGC 253 using optically thin tracers

To trace chemical evolution in a prototypical starburst environment, we spatially resolve the carbon and oxygen isotope ratios across the central molecular zone (full size ~$\sim 600$ pc) in the nearby starburst galaxy NGC 253. We imaged the emission of the optically thin isotopologues $^{13}$CO, C$^{18}$O, C$^{17}$O, $^{13}$C$^{18}$O at a spatial resolution $\sim50$ pc, comparable to the typical size of giant molecular associations. Optical depth effects and contamination of $^{13}$C$^{18}$O by C$_4$H is discussed and accounted for to derive column densities. This is the first extragalactic detection of the double isotopologue $^{13}$C$^{18}$O. Derived isotopic ratios $^{12}$C/$^{13}$C$\sim21\pm6$, $^{16}$O/$^{18}$O$\sim130\pm40$, and $^{18}$O/$^{17}$O$\sim4.5\pm0.8$ differ from the generally adopted values in the nuclei of galaxies. The molecular clouds in the central region of NGC 253 show similar rare isotope enrichment to those within the central molecular zone of the Milky way. This enrichment is attributed to stellar nucleosynthesis. Measured isotopic ratios suggest an enhancement of $^{18}$O as compared to our Galactic center, which we attribute to an extra $^{18}$O injection from massive stars. Our observations show evidence for mixing of distinct gas components with different degrees of processing. We observe an extra molecular component of highly processed gas on top of the already proposed less processed gas being transported to the central region of NGC 253. Such multicomponent nature and optical depth effects may hinder the use of isotopic ratios based on spatially unresolved line to infer the star formation history and/or initial stellar mass function properties in the nuclei of galaxies.

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arXiv2018

Full-disc $^{13}$CO(1-0) mapping across nearby galaxies of the EMPIRE survey and the CO-to-H$_2$ conversion factor

Carbon monoxide (CO) provides crucial information about the molecular gas properties of galaxies. While $^{12}$CO has been targeted extensively, isotopologues such as $^{13}$CO have the advantage of being less optically thick and observations have recently become accessible across full galaxy discs. We present a comprehensive new dataset of $^{13}$CO(1-0) observations with the IRAM 30-m telescope of the full discs of 9 nearby spiral galaxies from the EMPIRE survey at a spatial resolution of $\sim$1.5kpc. $^{13}$CO(1-0) is mapped out to $0.7-1r_{25}$ and detected at high signal-to-noise throughout our maps. We analyse the $^{12}$CO(1-0)-to-$^{13}$CO(1-0) ratio ($\Re$) as a function of galactocentric radius and other parameters such as the $^{12}$CO(2-1)-to-$^{12}$CO(1-0) intensity ratio, the 70-to-160$μ$m flux density ratio, the star-formation rate surface density, the star-formation efficiency, and the CO-to-H$_2$ conversion factor. We find that $\Re$ varies by a factor of 2 at most within and amongst galaxies, with a median value of 11 and larger variations in the galaxy centres than in the discs. We argue that optical depth effects, most likely due to changes in the mixture of diffuse/dense gas, are favored explanations for the observed $\Re$ variations, while abundance changes may also be at play. We calculate a spatially-resolved $^{13}$CO(1-0)-to-H$_2$ conversion factor and find an average value of $1.0\times10^{21}$ cm$^{-2}$ (K.km/s)$^{-1}$ over our sample with a standard deviation of a factor of 2. We find that $^{13}$CO(1-0) does not appear to be a good predictor of the bulk molecular gas mass in normal galaxy discs due to the presence of a large diffuse phase, but it may be a better tracer of the mass than $^{12}$CO(1-0) in the galaxy centres where the fraction of dense gas is larger.

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arXiv2017

The Star Formation in Radio Survey: Jansky Very Large Array 33 GHz Observations of Nearby Galaxy Nuclei and Extranuclear Star-Forming Regions

We present 33 GHz imaging for 112 pointings towards galaxy nuclei and extranuclear star-forming regions at $\approx$2" resolution using the Karl G. Jansky Very Large Array (VLA) as part of the Star Formation in Radio Survey. A comparison with 33 GHz Robert C. Byrd Green Bank Telescope single-dish observations indicates that the interferometric VLA observations recover $78\pm4 %$ of the total flux density over 25" regions ($\approx$ kpc-scales) among all fields. On these scales, the emission being resolved out is most likely diffuse non-thermal synchrotron emission. Consequently, on the $\approx30-300$ pc scales sampled by our VLA observations, the bulk of the 33 GHz emission is recovered and primarily powered by free-free emission from discrete HII regions, making it an excellent tracer of massive star formation. Of the 225 discrete regions used for aperture photometry, 162 are extranuclear (i.e., having galactocentric radii $r_{\rm G} \geq 250$ pc) and detected at $>3σ$ significance at 33 GHz and in H$α$. Assuming a typical 33 GHz thermal fraction of 90 %, the ratio of optically-thin 33 GHz-to-uncorrected H$α$ star formation rates indicate a median extinction value on $\approx30-300$ pc scales of $A_{\rm Hα} \approx 1.26\pm0.09$ mag with an associated median absolute deviation of 0.87 mag. We find that 10 % of these sources are "highly embedded" (i.e., $A_{\rm Hα}\gtrsim3.3$ mag), suggesting that on average HII regions remain embedded for $\lesssim1$ Myr. Finally, we find the median 33 GHz continuum-to-H$α$ line flux ratio to be statistically larger within $r_{\rm G}<250$ pc relative the outer-disk regions by a factor of $1.82\pm0.39$, while the ratio of 33 GHz-to-24 $μ$m flux densities are lower by a factor of $0.45\pm0.08$, which may suggest increased extinction in the central regions.

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arXiv2017

On the disappearance of a cold molecular torus around the low-luminosity active galactic nucleus of NGC 1097

We used the Atacama Large Millimeter/submillimeter Array (ALMA) to map the CO(3-2) and the underlying continuum emissions around the type 1 low-luminosity active galactic nucleus (LLAGN; bolometric luminosity $\lesssim 10^{42}$ erg~s$^{-1}$) of NGC 1097 at $\sim 10$ pc resolution. These observations revealed a detailed cold gas distribution within a $\sim 100$ pc of this LLAGN. In contrast to the luminous Seyfert galaxy NGC 1068, where a $\sim 7$ pc cold molecular torus was recently revealed, a distinctively dense and compact torus is missing in our CO(3-2) integrated intensity map of NGC 1097. Based on the CO(3-2) flux, the gas mass of the torus of NGC 1097 would be a factor of $\gtrsim 2-3$ less than that found for NGC 1068 by using the same CO-to-H$_2$ conversion factor, which implies less active nuclear star formation and/or inflows in NGC 1097. Our dynamical modeling of the CO(3-2) velocity field implies that the cold molecular gas is concentrated in a thin layer as compared to the hot gas traced by the 2.12 $μ$m H$_2$ emission in and around the torus. Furthermore, we suggest that NGC 1097 hosts a geometrically thinner torus than NGC 1068. Although the physical origin of the torus thickness remains unclear, our observations support a theoretical prediction that geometrically thick tori with high opacity will become deficient as AGNs evolve from luminous Seyferts to LLAGNs.

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arXiv2017

A 33 GHz Survey of Local Major Mergers: Estimating the Sizes of the Energetically Dominant Regions from High Resolution Measurements of the Radio Continuum

We present Very Large Array observations of the 33 GHz radio continuum emission from 22 local ultraluminous and luminous infrared (IR) galaxies (U/LIRGs). These observations have spatial (angular) resolutions of 30--720 pc (0.07"-0.67") in a part of the spectrum that is likely to be optically thin. This allows us to estimate the size of the energetically dominant regions. We find half-light radii from 30 pc to 1.7 kpc. The 33 GHz flux density correlates well with the IR emission, and we take these sizes as indicative of the size of the region that produces most of the energy. Combining our 33 GHz sizes with unresolved measurements, we estimate the IR luminosity and star formation rate per area, and the molecular gas surface and volume densities. These quantities span a wide range (4 dex) and include some of the highest values measured for any galaxy (e.g., $\mathrm{Σ_{SFR}^{33GHz} \leq 10^{4.1} M_{\odot} yr^{-1} kpc^{-2}}$). At least $13$ sources appear Compton thick ($\mathrm{N_{H}^{33GHz} \geq 10^{24} cm^{-2}}$). Consistent with previous work, contrasting these data with observations of normal disk galaxies suggests a nonlinear and likely multi-valued relation between SFR and molecular gas surface density, though this result depends on the adopted CO-to-H$_{2}$ conversion factor and the assumption that our 33 GHz sizes apply to the gas. 11 sources appear to exceed the luminosity surface density predicted for starbursts supported by radiation pressure and supernovae feedback, however we note the need for more detailed observations of the inner disk structure. U/LIRGs with higher surface brightness exhibit stronger [{\sc Cii}] 158$μ$m deficits, consistent with the suggestion that high energy densities drive this phenomenon.

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arXiv2017

Sub-arcsecond imaging of the water emission in Arp 220

Extragalactic observations of water emission can provide valuable insights into the excitation of the interstellar medium. In addition, extragalactic megamasers are powerful probes of kinematics close to active nuclei. Therefore, it is paramount to determine the true origin of the water emission, whether it is excited by processes close to an AGN or in star-forming regions. We use ALMA Band 5 science verification observations to analyse the emission of the 183 GHz water line in Arp 220 on sub-arcsecond scales, in conjunction with new ALMA Band 7 data at 325 GHz. Specifically, the nature of the process leading to the excitation of emission at these water lines is studied in this context. Supplementary 22 GHz VLA observations are used to better constrain the parameter space in the excitation modelling of the water lines. We detect 183 GHz H2O and 325 GHz water emission towards the two compact nuclei at the center of Arp 220, being brighter in Arp 220 West. The emission at these two frequencies is compared to previous single-dish data and does not show evidence of variability. The 183 and 325 GHz lines show similar spectra and kinematics, but the 22 GHz profile is significantly different in both nuclei due to a blend with an NH3 absorption line. Our findings suggest that the most likely scenario to cause the observed water emission in Arp 220 is a large number of independent masers originating from numerous star-forming regions.

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arXiv2016

Optical Depth Estimates and Effective Critical Densities of Dense Gas Tracers in the Inner Parts of Nearby Galaxy Discs

High critical density molecular lines like HCN(1-0) or HCO+(1-0) represent our best tool to study currently star-forming, dense molecular gas at extragalactic distances. The optical depth of these lines is a key ingredient to estimate the effective density required to excite emission. However, constraints on this quantity are even scarcer in the literature than measurements of the high density tracers themselves. Here, we combine new observations of HCN, HCO+ and HNC(1-0) and their optically thin isotopologues H13CN, H13CO+ and HN13C(1-0) to measure isotopologue line ratios. We use IRAM 30-m observations from the large program EMPIRE and new ALMA observations, which together target 6 nearby star-forming galaxies. Using spectral stacking techniques, we calculate or place strong upper limits on the HCN/H13CN, HCO+/H13CO+ and HNC/HN13C line ratios in the inner parts of these galaxies. Under simple assumptions, we use these to estimate the optical depths of HCN(1-0) and HCO+(1-0) to be τ~2-11 in the active, inner regions of our targets. The critical densities are consequently lowered to values between 5-20$\times 10^5$, 1-3$\times 10^5$ and 9$\times 10^4$ cm-3 for HCN, HCO+ and HNC, respectively. We study the impact of having different beam-filling factors, $η$, on these estimates and find that the effective critical densities decrease by a factor of $\frac{η_{12}}{η_{13}}\,τ_{12}$. A comparison to existing work in NGC 5194 and NGC 253 shows HCN/H13CN and HCO+/H13CO+ ratios in agreement with our measurements within the uncertainties. The same is true for studies in other environments such as the Galactic Centre or nuclear regions of AGN-dominated nearby galaxies.

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arXiv2016

AGN feedback in the nucleus of M51

AGN feedback is invoked as one of the most relevant mechanisms that shape the evolution of galaxies. Our goal is to understand the interplay between AGN feedback and the interstellar medium in M51, a nearby spiral galaxy with a modest AGN and a kpc-scale radio jet expanding through the disc of the galaxy. For that purpose, we combine molecular gas observations in the CO(1-0) and HCN(1-0) lines from the Plateau de Bure interferometer with archival radio, X-ray, and optical data. We show that there is a significant scarcity of CO emission in the ionisation cone, while molecular gas emission tends to accumulate towards the edges of the cone. The distribution and kinematics of CO and HCN line emission reveal AGN feedback effects out to r~500pc, covering the whole extent of the radio jet, with complex kinematics in the molecular gas which displays strong local variations. We propose that this is the result of the almost coplanar jet pushing on molecular gas in different directions as it expands; the effects are more pronounced in HCN than in CO emission, probably as the result of radiative shocks. Following previous interpretation of the redshifted molecular line in the central 5" as caused by a molecular outflow, we estimate the outflow rates to be Mdot_H2~0.9Msun/yr and Mdot_dense~0.6Msun/yr, which are comparable to the molecular inflow rates (~1Msun/yr); gas inflow and AGN feedback could be mutually regulated processes. The agreement with findings in other nearby radio galaxies suggests that this is not an isolated case, and probably the paradigm of AGN feedback through radio jets, at least for galaxies hosting low-luminosity active nuclei.

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