arXiv subjects

L. Armus

Publications and source records attributed to L. Armus.

At least 19 recordsLinked to original sources

arXiv2026

The Multi-phase Biconical Outflow in the local IR-Luminous Merger IRASF01364-1042

We investigate the spatially-resolved ISM properties of the local ($z = 0.048$), IR-luminous ($L_{\rm IR} = 10^{11.87}$\,L$_\odot$), late-stage galaxy merger IRAS F01364-1042, combining multi-wavelength IFU observations from \textit{JWST/MIRI-MRS}, ALMA and Keck/KCWI. Using these datasets, we construct emission line maps of several key tracers of the ionized (e.g., [Ne\,II]\,12.8$\mu$m, [O\,III]$\lambda5007$), warm molecular (e.g., \ce{H2}\,0-0\,S(3)), and cold molecular gas (e.g., CO (J$=2-1$)), and perform detailed decomposition of spectra extracted in resolved regions across the areas of emission. We confirm the presence of a multi-phase galactic biconical outflow along the minor axis of a highly inclined rotating disk. The inferred outflow velocities are $\sim\,$500 - 600\,km\,s$^{-1}$ $\sim\,$350\,km\,s$^{-1}$, and $\sim\,$200 - 300\,km\,s$^{-1}$, in the ionized, warm and cold molecular phase, respectively, with corresponding mass outflow rates of $\sim 0.3 - 2.3$, $\sim 31$, and $\sim 38 - 240$\,M$_\odot\,$yr$^{-1}$. The cold molecular phase dominates both the total mass outflow rate and the associated kinetic energy ($\sim\,2 - 8 \times 10^{42}$\,erg\,s$^{-1}$). We confirm, for the first time, a dust-obscured AGN in IRAS\,F01364-1042, via detection of the [Ne\,V]\,14.3$\mu$m line. The low inferred AGN bolometric luminosity ($1.2 - 1.8 \times 10^{43}$\,erg\,s$^{-1}$) suggests that the nuclear starburst alone, with a star formation rate of $\sim 40 - 60$\,M$_\odot$\,yr$^{-1}$, can account for the energy required to drive the outflow, though a more active AGN phase in the recent past may have also played a role. Our work showcases the necessity of multi-wavelength observations for interpreting the gas dynamics in merger-driven dusty starbursts, and the capability of \textit{JWST/MIRI-MRS} to uncover obscured, low-luminosity AGN that may be common in these systems.

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arXiv2026

GOALS-JWST: Resolved multi-phase molecular gas in IRAS 20551-4250 using JWST and ALMA

Studying the content and distribution of molecular gas provides key insights into how feedback from Active Galactic Nuclei (AGN) and star formation influences galaxy evolution, since molecular gas is the primary fuel for star formation. Ultra-Luminous Infrared Galaxies (ULIRGs) are ideal candidates to study how AGN and/or starbursts affect the interstellar medium due to their intense AGN and star forming activity. We present spatially-resolved multi-phase molecular gas study of IRAS20551-4250, a nearby ($z=0.0429$) ULIRG, using JWST/MIRI-MRS and ALMA. Mid-infrared diagnostics do not rule out the presence of AGN in IRAS20551-4250. [OIII]$\lambda$5007 in VLT/MUSE data reveal ionised gas outflows with $w_{80}^{\rm [OIII]} \sim 790$ km s$^{-1}$ and $\dot{M}_{\rm out}^{\rm[OIII]}<0.01$ M$_{\odot}$ yr$^{-1}$. No outflows are observed in either molecular phases. JWST/MIRI-MRS data reveal several rotational transitions of warm H$_{2}$ (T$\sim500-1400$ K) within the central $\sim4\times4$ kpc$^{2}$ region. Excitation temperature maps suggest that the warm H$_{2}$ is primarily heated by UV radiation from the central source. The CO-based cold molecular component dominates the molecular gas mass, accounting for $>$95% of the total molecular gas mass. Warm H$_{2}$ maps show two tidal tails and the velocity centroid maps show disturbed, non-rotational motions and a systematic gradient across the field-of-view, similar to that of ALMA CO-based cold molecular gas and consistent with a late-stage merger. Together, our analysis indicate that the molecular gas composition in IRAS20551-4250 is consistent with ongoing star formation in the host galaxy and the outflows observed in ionised gas phase appear insufficient to expel the molecular gas or quench ongoing star formation.

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arXiv2026

Spatially Resolved Star Formation relations in local LIRGs along the complete merger sequence

We investigate the properties of the interstellar medium (ISM) at ~100 pc scales in a sample of 27 nearby luminous infrared galaxies (LIRGs) spanning the entire merger sequence. In particular, we study the relations between star-formation (SF) and molecular gas surface density as a function of the interaction stage using two complementary approaches: beam-sized (unresolved, line-of-sight) regions and physically identified molecular gas clumps. To map the distribution of molecular gas we use ALMA CO(2-1) observations, while SF is traced using HST Pa-alpha or Pa-beta images. We derive spatially resolved Kennicutt-Schmidt (KS) relations for each galaxy. When using beam-sized regions, we find that 67% of galaxies follow a single relation between Sigma_SFR and Sigma_H2. However, in the remaining galaxies, the relation splits into two branches, indicating the presence of a duality in this relation. In contrast, when using physical gas clumps, the duality disappears and all galaxies show a single trend. We also study other ISM/clump properties as a function of the merger stage. We find that isolated galaxies and systems in early stages of interaction exhibit lower amounts of gas and SF. As the merger progresses, however, the amount of gas in the central kpc of the galaxy undergoing the merger increases, along with the SFR, and the slope of the KS relation becomes steeper, indicating an increase in SF efficiency of the gas clumps. Clumps in late-stage mergers are predominantly located at small distances from the nucleus, confirming that most of the activity is concentrated in the central regions. Finally, the relation between the SF efficiency and the boundedness parameter evolves from being roughly flat in the early stages of the merger to becoming positive in the final phases, indicating that clump self-gravity only starts to regulate the SF process between the early- and mid-merger stages.

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arXiv2025

Detection of Deuterated Hydrocarbon Nanoparticles in the Whirlpool Galaxy, M51

Deuteration of hydrocarbon material, including polycyclic aromatic hydrocarbons (PAHs), has been proposed to account for the low gas-phase abundances of D in the interstellar medium. JWST spectra of four star-forming regions in M51 show an emission feature, with central wavelength $\sim$4.647$\mu$m and FWHM 0.0265$\mu$m, corresponding to the C-D stretching mode in aliphatic hydrocarbons. The emitting aliphatic material is estimated to have (D/H)$_{\rm aliph}\approx 0.17\pm0.02$ -- a factor $\sim$$10^4$ enrichment relative to the overall interstellar medium (ISM). On $\sim$$50\,$pc scales, deuteration levels toward four H$\,$II regions in M51 are 2-3 times higher than in the Orion Bar photodissociation region (PDR), with implications for the processes responsible for the formation and evolution of hydrocarbon nanoparticles, including PAHs. The deuteration of the aliphatic material is found to anticorrelate with helium ionization in the associated H$\,$II, suggesting that harsh FUV radiation may act to lower the deuteration of aliphatics in PDRs near massive stars. No evidence is found for deuteration of aromatic material, with (D/H)$_{\rm arom} \lesssim 0.016$: deuteration of the aliphatic material exceeds that of the aromatic material by at least a factor 10. The observed levels of deuteration may account for the depletion of D observed in the Galactic interstellar medium. If so, the $4.65\mu$m feature may be detectable in absorption.

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arXiv2024

A case study of gas impacted by black-hole jets with the JWST: outflows, bow shocks, and high excitation of the gas in the galaxy IC5063

We present James Webb Space Telescope MIRI data of the inner 3x2kpc^2 of the galaxy IC5063, in which the jets of a supermassive black hole interact with the gaseous disk they are crossing. Jet-driven outflows were known to be initiated along or near the jet path and to modify the stability of molecular clouds, possibly altering their star formation properties. The MIRI data, of unprecedented resolution and sensitivity in the infrared, now reveal that there are more than ten discrete regions with outflows, nearly doubling the number of such known regions. Outflows exist near the radio lobes, at the nucleus, in a biconical structure perpendicular to the jet, and in a bubble moving against the disk. In some of them, velocities above escape velocity are observed. Stratification is also observed, with higher ionization or excitation gas attaining higher velocities. More outflows and bow shocks, found further away from the nucleus than the radio lobes, in regions without significant radio emission, reveal the existence of past or weak radio jets that interacted with the interstellar medium. The coincidence of the bow shocks with the optical extended emission line region (EELR) suggests that the jets also contributed to the gas ionization. Maps of the H2 gas excitation temperature, T_ex, indicate that the molecular gas is most excited in regions with radio emission. There, T_ex is >100 K higher than in the EELR interior. We argue that a combination of jet-related shocks and cosmic rays is likely responsible for this excess molecular gas excitation.

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arXiv2024

Disentangling the co-evolution of galaxies and supermassive black holes with PRIMA

The most active phases of star formation and black hole accretion are strongly affected by dust extinction, making far-infrared (far-IR) observations the best way to disentangle and study the co-evolution of galaxies and super massive black holes. The plethora of fine structure lines and emission features from dust, ionised and neutral atomic and warm molecular gas in the rest-frame mid- and far-IR provide unmatched diagnostic power to determine the properties of gas and dust, measure gas-phase metallicities and map cold galactic outflows in even the most obscured galaxies. By combining multi-band photometric surveys with low and high-resolution far-IR spectroscopy, the PRobe far-Infrared Mission for Astrophysics (PRIMA), a concept for a far-IR, 1.8m-diameter, cryogenically cooled observatory, will revolutionise the field of galaxy evolution by taking advantage of this IR toolkit to find and study dusty galaxies across galactic time. In this work, we make use of the phenomenological simulation SPRITZ and the Santa Cruz semi-analytical model to describe how a moderately deep multi-band PRIMA photometric survey can easily reach beyond previous IR missions to detect and study galaxies down to $10^{11}\,L_{\odot}$ beyond cosmic noon and at least up to z=4, even in the absence of gravitational lensing. By decomposing the spectral energy distribution (SED) of these photometrically selected galaxies, we show that PRIMA can be used to accurately measure the relative AGN power, the mass fraction contributed by polycyclic aromatic hydrocarbon (PAH) and the total IR luminosity. At the same time, spectroscopic follow up with PRIMA will allow to trace both the star formation and black hole accretion rates (SFR, BHAR), the gas phase metallicities and the mass outflow rates of cold gas in hundreds to thousands of individual galaxies to z=2.

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arXiv2024

The Impact of an AGN on PAH Emission in Galaxies: the Case of Ring Galaxy NGC 4138

We present a focused study of radially-resolved varying PAH emission in the low-luminosity AGN-host NGC 4138 using deep Spitzer/IRS spectral maps. Using new model PAH spectra, we investigate whether these variations could be associated with changes to the PAH grain size distribution due to photodestruction by the AGN. Separately, we model the effects of the varying radiation field within NGC 4138, and we use this model to predict the corresponding changes in the PAH emission spectrum. We find that PAH band ratios are strongly variable with radius in this galaxy with short-to-long wavelength band ratios peaking in the starburst ring. The changing mix of starlight appears to have a considerable effect on the trends in these band ratios, and our radiation model predicts the shapes of these trends. However, the amplitude of observed variation is ~2.5 times larger than predicted for some ratios. A cutoff of small grains in the PAH size distribution, as has been suggested for AGN, together with changes in PAH ionization fraction could explain the behavior of the shorter bands, but this model fails to reproduce longer band behaviors. Additionally, we find that short-to-long wavelength PAH band ratios increase slightly within ~270pc of the center, suggesting that the AGN may directly influence PAH emission there.

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arXiv2023

GOALS-JWST: Pulling Back the Curtain on the AGN and Star Formation in VV 114

We present results from the James Webb Space Telescope (JWST) Director's Discretionary Time Early Release Science (ERS) program 1328 targeting the nearby, Luminous Infrared Galaxy (LIRG), VV 114. We use the MIRI and NIRSpec instruments to obtain integral-field spectroscopy of the heavily obscured Eastern nucleus (V114E) and surrounding regions. The spatially resolved, high-resolution, spectra reveal the physical conditions in the gas and dust over a projected area of 2-3 kpc that includes the two brightest IR sources, the NE and SW cores. Our observations show for the first time spectroscopic evidence that the SW core hosts an AGN as evidenced by its very low 6.2 μm and 3.3 μm PAH equivalent widths (0.12 and 0.017 μm respectively) and mid and near-IR colors. Our observations of the NE core show signs of deeply embedded star formation including absorption features due to aliphatic hydrocarbons, large quantities of amorphous silicates, as well as HCN due to cool gas along the line of sight. We detect elevated [Fe II]/Pfα consistent with extended shocks coincident with enhanced emission from warm H$_{2}$, far from the IR-bright cores and clumps. We also identify broadening and multiple kinematic components in both H$_{2}$ and fine structure lines caused by outflows and previously identified tidal features.

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arXiv2022

GOALS-JWST: Mid-Infrared Spectroscopy of the Nucleus of NGC 7469

We present mid-infrared spectroscopic observations of the nucleus of the nearby Seyfert galaxy NGC 7469 taken with the MIRI instrument on the James Webb Space Telescope (JWST) as part of Directors Discretionary Time Early Release Science (ERS) program 1328. The high resolution nuclear spectrum contains 19 emission lines covering a wide range of ionization. The high ionization lines show broad, blueshifted emission reaching velocities up to 1700 km s$^{-1}$ and FWHM ranging from $\sim500 - 1100$ km s$^{-1}$. The width of the broad emission and the broad to narrow line flux ratios correlate with ionization potential. The results suggest a decelerating, stratified, AGN driven outflow emerging from the nucleus. The estimated mass outflow rate is one to two orders of magnitude larger than the current black hole accretion rate needed to power the AGN. Eight pure rotational H$_{2}$ emission lines are detected with intrinsic widths ranging from FWHM $\sim 125-330$ km s$^{-1}$. We estimate a total mass of warm H$_{2}$ gas of $\sim1.2\times10^{7}$M$_{\odot}$ in the central 100 pc. The PAH features are extremely weak in the nuclear spectrum, but a $6.2μ$m PAH feature with an equivalent width $\sim0.07μ$m and a flux of $2.7\times10^{-17}$ W m$^{-2}$ is detected. The spectrum is steeply rising in the mid-infrared, with a silicate strength $\sim0.02$, significantly smaller than seen in most PG QSOs, but comparable to other Seyfert 1's. These early MIRI mid-infrared IFU data highlight the power of JWST to probe the multi-phase interstellar media surrounding actively accreting supermassive black holes.

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arXiv2022

Characterizing Compact 15-33 GHz Radio Continuum Sources in Local U/LIRGs

We present the analysis of $\sim 100$pc-scale compact radio continuum sources detected in 63 local (Ultra) Luminous Infrared Galaxies (U/LIRGs; $L_{\rm IR} \ge 10^{11} L_\odot$), using FWHM $\lesssim 0''.1 - 0''.2$ resolution 15 and 33 GHz observations with the Karl G. Jansky Very Large Array. We identify a total of 133 compact radio sources with effective radii of 8 - 170pc, which are classified into four main categories -- "AGN" (AGN), "AGN/SBnuc" (AGN-starburst composite nucleus), "SBnuc" (starburst nucleus) and "SF" (star-forming clumps) -- based on ancillary datasets and the literature. We find that "AGN" and "AGN/SBnuc" more frequently occur in late-stage mergers and have up to 3 dex higher 33 GHz luminosities and surface densities compared with "SBnuc" and "SF", which may be attributed to extreme nuclear starburst and/or AGN activity in the former. Star formation rates (SFRs) and surface densities ($Σ_{\rm SFR}$) are measured for "SF" and "SBnuc" using both the total 33 GHz continuum emission (SFR $\sim 0.14 - 13$ M$_\odot$ yr$^{-1}$, $Σ_{\rm SFR} \sim 13 - 1600$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$) and the thermal free-free emission from HII regions (median SFR$_{\rm th} \sim 0.4$ M$_\odot$ yr$^{-1}$, $Σ_{\rm SFR_{th}} \sim 44$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$). These values are 1 - 2 dex higher than those measured for similar-sized clumps in nearby normal (non-U/LIRGs). The latter also have much flatter median 15 - 33 GHz spectral index ($\sim -0.08$) compared with "SBnuc" and "SF" ($\sim -0.46$), which may reflect higher non-thermal contribution from supernovae and/or ISM densities in local U/LIRGs that directly result from and/or lead to their extreme star-forming activities on 100\,pc scales.

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arXiv2022

Preparing for low surface brightness science with the Vera C. Rubin Observatory: characterisation of tidal features from mock images

Tidal features in the outskirts of galaxies yield unique information about their past interactions and are a key prediction of the hierarchical structure formation paradigm. The Vera C. Rubin Observatory is poised to deliver deep observations for potentially of millions of objects with visible tidal features, but the inference of galaxy interaction histories from such features is not straightforward. Utilising automated techniques and human visual classification in conjunction with realistic mock images produced using the NEWHORIZON cosmological simulation, we investigate the nature, frequency and visibility of tidal features and debris across a range of environments and stellar masses. In our simulated sample, around 80 per cent of the flux in the tidal features around Milky Way or greater mass galaxies is detected at the 10-year depth of the Legacy Survey of Space and Time (30-31 mag / sq. arcsec), falling to 60 per cent assuming a shallower final depth of 29.5 mag / sq. arcsec. The fraction of total flux found in tidal features increases towards higher masses, rising to 10 per cent for the most massive objects in our sample (M*~10^{11.5} Msun). When observed at sufficient depth, such objects frequently exhibit many distinct tidal features with complex shapes. The interpretation and characterisation of such features varies significantly with image depth and object orientation, introducing significant biases in their classification. Assuming the data reduction pipeline is properly optimised, we expect the Rubin Observatory to be capable of recovering much of the flux found in the outskirts of Milky Way mass galaxies, even at intermediate redshifts (z<0.2).

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arXiv2021

A hard X-ray view of Luminous and Ultra-luminous Infrared Galaxies in GOALS: I - AGN obscuration along the merger sequence

The merger of two or more galaxies can enhance the inflow of material from galactic scales into the close environments of Active Galactic Nuclei (AGN), obscuring and feeding the supermassive black hole (SMBH). Both recent simulations and observations of AGN in mergers have confirmed that mergers are related to strong nuclear obscuration. However, it is still unclear how AGN obscuration evolves in the last phases of the merger process. We study a sample of 60 Luminous and Ultra-luminous IR galaxies (U/LIRGs) from the GOALS sample observed by NuSTAR. We find that the fraction of AGN that are Compton-thick (CT; $N_{\rm H}\geq 10^{24}\rm\,cm^{-2}$) peaks at $74_{-19}^{+14}\%$ at a late merger stage, prior to coalescence, when the nuclei have projected separations of $d_{\rm sep}\sim 0.4-6$ kpc. A similar peak is also observed in the median $N_{\rm H}$ [$(1.6\pm0.5)\times10^{24}\rm\,cm^{-2}$]. The vast majority ($85^{+7}_{-9}\%$) of the AGN in the final merger stages ($d_{\rm sep}\lesssim 10$ kpc) are heavily obscured ($N_{\rm H}\geq 10^{23}\rm\,cm^{-2}$), and the median $N_{\rm H}$ of the accreting SMBHs in our sample is systematically higher than that of local hard X-ray selected AGN, regardless of the merger stage. This implies that these objects have very obscured nuclear environments, with the $N_{\rm H}\geq 10^{23}\rm\,cm^{-2}$ gas almost completely covering the AGN in late mergers. CT AGN tend to have systematically higher absorption-corrected X-ray luminosities than less obscured sources. This could either be due to an evolutionary effect, with more obscured sources accreting more rapidly because they have more gas available in their surroundings, or to a selection bias. The latter scenario would imply that we are still missing a large fraction of heavily obscured, lower luminosity ($L_{2-10}\lesssim 10^{43}\rm\,erg\,s^{-1}$) AGN in U/LIRGs.

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arXiv2021

A Comparison between Nuclear Ring Star Formation in LIRGs and Normal Galaxies with the Very Large Array

Nuclear rings are excellent laboratories for studying intense star formation. We present results from a study of nuclear star-forming rings in five nearby normal galaxies from the Star Formation in Radio Survey (SFRS) and four local LIRGs from the Great Observatories All-sky LIRG Survey (GOALS) at sub-kpc resolutions using VLA high-frequency radio continuum observations. We find that nuclear ring star formation (NRSF) contributes 49 - 60\% of the total star formation of the LIRGs, compared to 7 - 40\% for the normal galaxies. We characterize a total of 58 individual star-forming regions in these rings, and find that with measured sizes of 10 - 200 pc, NRSF regions in the LIRGs have SFR and $Σ_\mathrm{SFR}$ up to 1.7 M$_\odot$yr$^{-1}$ and 402 M$_\odot$yr$^{-1}$kpc$^{-2}$, respectively, which are about 10 times higher than NRSF regions in the normal galaxies with similar sizes, and comparable to lensed high-$z$ star-forming regions. At $\sim 100 - 300$ pc scales, we estimate low contributions ($< 50\%$) of thermal free-free emission to total radio continuum emission at 33 GHz in the NRSF regions in the LIRGs, but large variations possibly exist at smaller physical scales. Finally, using archival sub-kpc resolution CO (J=1-0) data of nuclear rings in the normal galaxies and NGC 7469 (LIRG), we find a large scatter in gas depletion times at similar molecular gas surface densities, which tentatively points to a multi-modal star formation relation on sub-kpc scales.

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arXiv2021

Great Observatories: The Past and Future of Panchromatic Astrophysics

NASA's Great Observatories have opened up the electromagnetic spectrum from space, providing sustained access to wavelengths not accessible from the ground. Together, Hubble, Compton, Chandra, and Spitzer have provided the scientific community with an agile and powerful suite of telescopes with which to attack broad scientific questions, and react to a rapidly changing scientific landscape. As the existing Great Observatories age, or are decommissioned, community access to these wavelengths will diminish, with an accompanying loss of scientific capability. This report, commissioned by the NASA Cosmic Origins, Physics of the Cosmos and Exoplanet Exploration Program Analysis Groups (PAGs), analyzes the importance of multi-wavelength observations from space during the epoch of the Great Observatories, providing examples that span a broad range of astrophysical investigations.

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arXiv2021

The VLA Frontier Fields Survey: Deep, High-resolution Radio Imaging of the MACS Lensing Clusters at 3 and 6 GHz

The Frontier Fields project is an observational campaign targeting six galaxy clusters, with the intention of using the magnification provided by gravitational lensing to study galaxies that are extremely faint or distant. We used the Karl G. Jansky Very Large Array (VLA) at 3 and 6 GHz to observe three Frontier Fields: MACSJ0416.1$-$2403 ($z$ = 0.396), MACSJ0717.5+3745 ($z$ = 0.545), and MACSJ1149.5+2223 ($z$ = 0.543). The images reach noise levels of $\sim$1 $μ$Jy beam$^{-1}$ with sub-arcsecond resolution ($\sim$2.5 kpc at $z$ = 3), providing a high-resolution view of high-$z$ star-forming galaxies that is unbiased by dust obscuration. We generate dual-frequency continuum images at two different resolutions per band, per cluster, and derive catalogs totalling 1966 compact radio sources. Components within the areas of Hubble Space Telescope and Subaru observations are cross-matched, providing host galaxy identifications for 1296 of them. We detect 13 moderately-lensed (2.1 $<$ $μ$ $<$ 6.5) sources, one of which has a demagnified peak brightness of 0.9 $μ$Jy beam$^{-1}$, making it a candidate for the faintest radio source ever detected. There are 66 radio sources exhibiting complex morphologies, and 58 of these have host galaxy identifications. We reveal that MACSJ1149.5+2223 is not a cluster with a double relic, as the western candidate relic is resolved as a double-lobed radio galaxy associated with a foreground elliptical at $z$ = 0.24. The VLA Frontier Fields project is a public legacy survey. The image and catalog products from this work are freely available.

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arXiv2021

The VLA Frontier Field Survey: A Comparison of the Radio and UV/optical size of $0.3 \lesssim z \lesssim 3$ star-forming galaxies

To investigate the growth history of galaxies, we measure the rest-frame radio, ultraviolet (UV), and optical sizes of 98 radio-selected, star-forming galaxies (SFGs) distributed over $0.3 \lesssim z \lesssim 3$ and median stellar mass of $\log(M_\star/ \rm M_\odot)\approx10.4$. We compare the size of galaxy stellar disks, traced by rest-frame optical emission, relative to the overall extent of star formation activity that is traced by radio continuum emission. Galaxies in our sample are identified in three Hubble Frontier Fields: MACSJ0416.1$-$2403, MACSJ0717.5+3745, and MACSJ1149.5+2223. Radio continuum sizes are derived from 3 GHz and 6 GHz radio images ($\lesssim 0$''$.6$ resolution, $\approx0.9\, \rm μJy\, beam^{-1}$ noise level) from the Karl G. Jansky Very Large Array. Rest-frame UV and optical sizes are derived using observations from the Hubble Space Telescope and the ACS and WFC3 instruments. We find no clear dependence between the 3 GHz radio size and stellar mass of SFGs, which contrasts with the positive correlation between the UV/optical size and stellar mass of galaxies. Focusing on SFGs with $\log(M_\star/\rm M_\odot)>10$, we find that the radio/UV/optical emission tends to be more compact in galaxies with high star-formation rates ($\rm SFR\gtrsim 100\,M_\odot\,yr^{-1}$), suggesting that a central, compact starburst (and/or an Active Galactic Nucleus) resides in the most luminous galaxies of our sample. We also find that the physical radio/UV/optical size of radio-selected SFGs with $\log(M_\star/\rm M_\odot)>10$ increases by a factor of $1.5-2$ from $z\approx 3$ to $z\approx0.3$, yet the radio emission remains two-to-three times more compact than that from the UV/optical. These findings indicate that these massive, {radio-selected} SFGs at $0.3 \lesssim z \lesssim 3$ tend to harbor centrally enhanced star formation activity relative to their outer-disks.

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arXiv2020

A Hard X-ray Test of HCN Enhancements as a Tracer of Embedded Black Hole Growth

Enhanced emission from the dense gas tracer HCN (relative to HCO$^+ $) has been proposed as a signature of active galactic nuclei (AGN). In a previous single-dish millimeter line survey we identified galaxies with HCN/HCO$ ^+ $ (1-0) intensity ratios consistent with those of many AGN but whose mid-infrared spectral diagnostics are consistent with little to no ( $\lesssim15\% $) contribution of an AGN to the bolometric luminosity. To search for putative heavily obscured AGN, we present and analyze \nustar hard X-ray (3-79 keV) observations of four such galaxies from the Great Observatories All-sky LIRG Survey. We find no X-ray evidence for AGN in three of the systems and place strong upper limits on the energetic contribution of any heavily obscured ($N_{\rm H}>10^{24}$ cm$^{-2}$) AGN to their bolometric luminosity. The X-ray flux upper limits are presently an order of magnitude below what XDR-driven chemistry model predict are necessary to drive HCN enhancements. In a fourth system we find a hard X-ray excess consistent with the presence of an AGN, but contributing only $\sim3\%$ of the bolometric luminosity. It is also unclear if the AGN is spatially associated with the HCN enhancement. We further explore the relationship between HCN/HCO$^+$ (for several $\mathrm{J}_\mathrm{upper}$ levels) and $L_\mathrm{AGN}/L_\mathrm{IR}$ for a larger sample of systems in the literature. We find no evidence for correlations between the line ratios and the AGN fraction derived from X-rays, indicating that HCN/HCO$^+$ intensity ratios are not driven by the energetic dominance of AGN, nor are they reliable indicators of whether SMBH accretion is ongoing.

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arXiv2020

The Molecular Gas in the NGC 6240 Merging Galaxy System at the Highest Spatial Resolution

We present the highest resolution --- 15 pc (0.03'') --- ALMA $^{12}$CO(2-1) line emission and 1.3mm continuum maps, tracers of the molecular gas and dust, respectively, in the nearby merging galaxy system NGC 6240, that hosts two supermassive black holes growing simultaneously. These observations provide an excellent spatial match to existing Hubble optical and near-infrared observations of this system. A significant molecular gas mass, $\sim$9$\times$10$^9$M$_\odot$, is located in between the two nuclei, forming a clumpy stream kinematically dominated by turbulence, rather than a smooth rotating disk as previously assumed from lower resolution data. Evidence for rotation is seen in the gas surrounding the southern nucleus, but not in the northern one. Dynamical shells can be seen, likely associated with nuclear supernovae remnants. We further detect the presence of significant high velocity outflows, some of them reaching velocities $>$500 km/s, affecting a significant fraction, $\sim$11\% of the molecular gas in the nuclear region. Inside the spheres of influence of the northern and southern supermassive black holes we find molecular masses of 7.4$\times$10$^8$M$_\odot$ and 3.3$\times$10$^9$M$_\odot$, respectively. We are thus directly imaging the reservoir of gas that can accrete onto each supermassive black hole. These new ALMA maps highlight the critical need for high resolution observations of molecular gas in order to understand the feeding of supermassive black holes and its connection to galaxy evolution in the context of a major galaxy merger.

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