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Top left: 93 GHz continuum emission with sources identified; also see Table 1. Circles show apertures (diameter of 0 24) used for continuum extraction. Their colors indicate the measured in-band spectral index, as in Figure 4, where dark purple indicates synchrotron-dominated emission and yellow indicates dustdominated emission. Top right: HST Paα emission-hydrogen recombination line, n = 3, at 1.87 μm (courtesy of P. van der Werf) tracing ionized gas at ≈0 2 resolution (Marconi et al. 2000). Dust extinction of A V >36 mag obscures the Paα recombination emission at shorter wavelengths from the starburst region. Contours trace 93 GHz continuum, as described in Figure 2. Bottom left: ALMA 350 GHz continuum emission tracing dust. Bottom right: Australian LBA 2.3 GHz continuum imaging of synchrotron emission primarily from supernova remnants (Lenc & Tingay 2009).
Source publication
The nearby (3.8 Mpc) galaxy NGC 4945 hosts a nuclear starburst and Seyfert type 2 active galactic nucleus (AGN). We use the Atacama Large Millimeter/submillimeter Array (ALMA) to image the 93 GHz (3.2 mm) free–free continuum and hydrogen recombination line emission (H40 α and H42 α ) at 2.2 pc (0.″12) resolution. Our observations reveal 27 bright,...
Contexts in source publication
Context 1
... large amount of extinction present in this highinclination central region (i∼72°; Henkel et al. 2018) has previously impeded the direct observation of its star clusters. The Paschen-α (Paα) emission (Marconi et al. 2000) of the n = 3 hydrogen recombination line at 1.87 μm, shown in Figure 3, reveals faint emission above and below the starforming plane. Corrected for extinction, the clumps of ionized emission traced by Paα would give rise to free-free emission below our ALMA detection limit. ...
Context 2
... large fraction (18/29) of the 93 GHz sources coincide with peaks in dust emission at 350 GHz, as shown in Figure 3. Overall, there is a good correspondence between the two tracers. ...
Context 3
... Figure 3, the 93 GHz peaks without dust counterparts tend to be strong sources of emission at 2.3 GHz ( Lenc & Tingay 2009), a frequency where synchrotron emission typically dominates. As discussed in Lenc & Tingay (2009), the sources at this frequency are predominantly supernova remnants. ...
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Citations
... NGC 4945 is a nearby (D = 3.72 Mpc; Tully et al. 2016) edge-on (i = 78°; Ott et al. 2001) galaxy that lies in the Centaurus A/M83 group and is known to host a central starburst (Schurch et al. 2002;Emig et al. 2020) as well as a Seyfert type 2 active galactic nucleus (AGN; Iwasawa et al. 1993). The mass of the central black hole is M BH = 1.4 × 10 6 M e , and the AGN accretes at 10%-30% of the Eddington rate (Puccetti et al. 2014). ...
... Due to the complex nature of the central regions, we refrain from letting abundances vary there due to a large number of free parameters. We assume the central metallicity is solar based on previous work studying NGC 4945ʼs central super star clusters (SSCs) that power the galactic wind (Emig et al. 2020). The composite southern region has a model of CONST * PHAB-S * PHABS * (VAPEC+POWERLAW+GAUSS), and the composite northern region has a model of CONST * PHABS * PHABS * (VAPEC ...
... The similarity between the abundances of the galaxies' approaching outflows may be the result of similar outflow driving mechanisms. All three galaxies have star-formationdriven outflows powered by SSCs (Marconi et al. 2000;Melo et al. 2005;Emig et al. 2020;Mills et al. 2021;Levy et al. 2022), and thus their chemical composition reflects the supernova enrichment. ...
We analyze 330 ks of Chandra X-ray imaging and spectra of the nearby, edge-on starburst and Seyfert type 2 galaxy NGC 4945 to measure the hot gas properties along the galactic outflows. We extract and model spectra from 15 regions extending from −0.55 to +0.85 kpc above and below the galactic disk to determine the best-fit parameters and metal abundances. We find that the hot gas temperatures and number densities peak in the central regions and decrease along the outflows. These profiles are inconsistent with a spherical, adiabatically expanding wind model, suggesting the need to include mass loading and/or a nonspherical outflow geometry. We estimate the mass outflow rate of the hot wind to be 1.6 M ⊙ yr ⁻¹ . Emission from charge exchange is detected in the northern outflow, and we estimate it contributes 12% to the emitted, broadband (0.5–7 keV) X-ray flux.
... With its exquisite sensitivity and resolving power in (sub-)millimeter bands, ALMA can detect extinctionfree tracers of star formation (via free-free continuum and radio recombination lines) as well as the associated gas reservoir (via molecular lines and dust continuum) for YMCs even at extragalactic distances. Thanks to these unique capabilities, we have already identified and characterized individual forming YMCs or entire YMC populations in our Galaxy (Schmiedeke et al. 2016;Ginsburg et al. 2018) and a handful of nearby galaxies (e.g., the Large Magellanic Cloud: Nayak et al. 2019;NGC 5253: Turner et al. 2017 Emig et al. 2020; Henize 2-10: Costa et al. 2021; and the Antennae: Finn et al. 2019;He et al. 2022). ...
... Doing so requires multiwavelength observations of a sizable YMC population at matched spatial resolution, supported by rich ancillary data for the host galaxy. It is also important that the host galaxy is at a favorable viewing angle for source localization and cross-wavelength source matching, which have proven difficult in edge-on systems, including our Galaxy, NGC 253, and NGC 4945 (Stolte et al. 2014;Emig et al. 2020;Levy et al. 2022). ...
... We identify bright, compact sources in the 93 and 350 GHz continuum images as YMC candidates, following Leroy et al. (2018) and Emig et al. (2020). These sources are selected based on a minimum S/N = 5 at peak brightness. ...
We use 0.1″ observations from the Atacama Large Millimeter Array (ALMA), Hubble Space Telescope (HST), and JWST to study young massive clusters (YMCs) in their embedded “infant” phase across the central starburst ring in NGC 3351. Our new ALMA data reveal 18 bright and compact (sub-)millimeter continuum sources, of which 8 have counterparts in JWST images and only 6 have counterparts in HST images. Based on the ALMA continuum and molecular line data, as well as ancillary measurements for the HST and JWST counterparts, we identify 14 sources as infant star clusters with high stellar and/or gas masses (∼10 ⁵ M ⊙ ), small radii (≲ 5 pc), large escape velocities (6–10 km s ⁻¹ ), and short freefall times (0.5–1 Myr). Their multiwavelength properties motivate us to divide them into four categories, likely corresponding to four evolutionary stages from starless clumps to exposed H ii region–cluster complexes. Leveraging age estimates for HST-identified clusters in the same region, we infer an evolutionary timeline, ranging from ∼1–2 Myr before cluster formation as starless clumps, to ∼4–6 Myr after as exposed H ii region–cluster complexes. Finally, we show that the YMCs make up a substantial fraction of recent star formation across the ring, exhibit a nonuniform azimuthal distribution without a very coherent evolutionary trend along the ring, and are capable of driving large-scale gas outflows.
... However, it has become increasingly evident that this is not the case for GMCs typical of high interstellar medium (ISM) pressure environments (P/k B 10 8 K cm −3 ), for which both models (Fall et al. 2010;Thompson & Krumholz 2016) and numerical simulations (Grudić et al. 2018;Fukushima & Yajima 2021;Lancaster et al. 2021a;Menon et al. 2022aMenon et al. , 2023Polak et al. 2023) suggest efficiencies ò * 80% because the energy/momentum deposition rate of feedback in this regime is unable to counteract gravity. Such pressures correspond to GMCs with surface densities (Σ Σ crit = 10 3 M ☉ pc −2 )-2-3 orders of magnitude higher than typical of GMCs in the local Universe-which are the likely sites of so-called super star cluster formation (e.g., McCrady et al. 2005;Portegies Zwart et al. 2010;Turner et al. 2015;Smith et al. 2020); observational estimates seem to be consistent with a high value of ò * for these conditions (Turner et al. 2017;Emig et al. 2020;Rico-Villas et al. 2020;Smith et al. 2020;Costa et al. 2021;He et al. 2022;McKinney et al. 2023;Sun et al. 2024). The environments that host these conditions are relatively rare in the local Universelimited to scenarios such as nuclear starbursts (e.g., Leroy et al. 2018;Emig et al. 2020;Levy et al. 2021), merging luminous infrared (IR)-bright galaxies (e.g., Johnson et al. 2015;Finn et al. 2019;Inami et al. 2022), and localized starbursts in dwarf galaxies (e.g., Ochsendorf et al. 2017;Oey et al. 2017;Turner et al. 2017). ...
... Such pressures correspond to GMCs with surface densities (Σ Σ crit = 10 3 M ☉ pc −2 )-2-3 orders of magnitude higher than typical of GMCs in the local Universe-which are the likely sites of so-called super star cluster formation (e.g., McCrady et al. 2005;Portegies Zwart et al. 2010;Turner et al. 2015;Smith et al. 2020); observational estimates seem to be consistent with a high value of ò * for these conditions (Turner et al. 2017;Emig et al. 2020;Rico-Villas et al. 2020;Smith et al. 2020;Costa et al. 2021;He et al. 2022;McKinney et al. 2023;Sun et al. 2024). The environments that host these conditions are relatively rare in the local Universelimited to scenarios such as nuclear starbursts (e.g., Leroy et al. 2018;Emig et al. 2020;Levy et al. 2021), merging luminous infrared (IR)-bright galaxies (e.g., Johnson et al. 2015;Finn et al. 2019;Inami et al. 2022), and localized starbursts in dwarf galaxies (e.g., Ochsendorf et al. 2017;Oey et al. 2017;Turner et al. 2017). On the other hand, the higher densities, gas fractions, merger rates, and accretion rates of galaxies at higher redshift suggest that high-pressure conditions are more commonly realized at these epochs; indeed, conditions observed in dusty starburst galaxies (Casey et al. 2014), prequiescent massive compact galaxies (Diamond-Stanic et al. 2012;Rupke et al. 2019), and proto-globular-cluster candidates resolved via gravitational lensing (Vanzella et al. 2022a(Vanzella et al. , 2022bPascale et al. 2023) reflect these conditions. ...
The observed rest-UV luminosity function at cosmic dawn ( z ∼ 8–14) measured by JWST revealed an excess of UV-luminous galaxies relative to many prelaunch theoretical predictions. A high star formation efficiency (SFE) and a top-heavy initial mass function (IMF) are among the mechanisms proposed for explaining this excess. Although a top-heavy IMF has been proposed for its ability to increase the light-to-mass ratio (Ψ UV ), the resulting enhanced radiative pressure from young stars could decrease the SFE, potentially driving galaxy luminosities back down. In this Letter, we use idealized radiation hydrodynamic simulations of star cluster formation to explore the effects of a top-heavy IMF on the SFE of clouds typical of the high-pressure conditions found at these redshifts. We find that the SFE in star clusters with solar-neighborhood-like dust abundance decreases with increasingly top-heavy IMFs—by ∼20% for an increase of a factor of 4 in Ψ UV and by 50% for a factor of ∼10 in Ψ UV . However, we find that an expected decrease in the dust-to-gas ratio (∼0.01 × solar) at these redshifts can completely compensate for the enhanced light output. This leads to a (cloud-scale; ∼10 pc) SFE that is ≳70% even for a factor of 10 increase in Ψ UV , implying that highly efficient star formation is unavoidable for high surface density and low-metallicity conditions. Our results suggest that a top-heavy IMF, if present, likely coexists with efficient star formation in these galaxies.
... These conditions alter how star formation happens, causing the fraction of stars formed in clusters to likely increase with the surface density of star formation (e.g., Kruijssen 2012; Krumholz et al. 2019). Vigorous starbursts produce super star clusters (SSCs, e.g., Herrera et al. 2012;Whitmore et al. 2014;Turner et al. 2015;Leroy et al. 2018;Emig et al. 2020); massive (M å ∼ 10 5 M e ), and compact (R ∼ 1 pc) concentrations of stars that are the younger cousins to globular clusters (Whitmore et al. 2005;Portegies Zwart et al. 2010;Linden et al. 2021). The formation of SSCs is a particularly efficient way of converting gas into stars (Krumholz et al. 2019), likely characteristic of starburst environments. ...
We present new observations of the central 1 kpc of the M82 starburst obtained with the James Webb Space Telescope near-infrared camera instrument at a resolution θ ∼ 0.″05–0.″1 (∼1–2 pc). The data comprises images in three mostly continuum filters (F140M, F250M, and F360M), and filters that contain [Fe ii ] (F164N), H 2 v = 1 → 0 (F212N), and the 3.3 μ m polycyclic aromatic hydrocarbon (PAH) feature (F335M). We find prominent plumes of PAH emission extending outward from the central starburst region, together with a network of complex filamentary substructures and edge-brightened bubble-like features. The structure of the PAH emission closely resembles that of the ionized gas, as revealed in Paschen α and free–free radio emission. We discuss the origin of the structure, and suggest the PAHs are embedded in a combination of neutral, molecular, and photoionized gas.
... NGC 4945 differs from these other AGN in that it lacks a traditional narrow line region and [O III] is not detected (Agueero & Carranza 1986). On the other hand, it is a well known starburst galaxy (Heckman et al. 1990;Emig et al. 2020). Evidence for Fe Kα in extended regions is also found in traditional starbursts like M82 (Liu et al. 2014) and NGC 253 (Mitsuishi et al. 2011), which have many similarities with NGC 4945 (Heckman et al. 1990). ...
... The starburst itself is concentrated within the central few hundred pc of the galaxy (Heckman et al. 1990;Spoon et al. 2000) and the central AGN appears to be fully obscured in all directions (Marconi et al. 2000). Emig et al. (2020) estimate that less than 10% of the ionizing photon luminosity of the AGN is escaping into the surrounding starburst region and thus the AGN would not, at present, be creating much (if any) ionized gas in the circumnuclear ISM. ...
... The age of the actual starburst in NGC 4945 has been calculated at > 5 Myr (Spoon et al. 2000;Marconi et al. 2000) and the older population of star clusters would have formed in an instantaneous burst of star formation roughly 5 Myr ago (Emig et al. 2020). This is consistent with the lack of strong He-like Fe K with distance from the nucleus as this line is generated from young SNRs and XRBs. ...
... As instrumentation became available, work to characterize natal SSCs continued in the infrared (Vacca et al. 2002;Johnson et al. 2004;Brandl et al. 2006;Madden et al. 2006;Johnson et al. 2007;Kennicutt et al. 2007;Feigelson et al. 2013). When the Atacama Large Millimeter/submillimeter Array (ALMA) came online, sensitive, high-resolution imaging of the natal molecular clouds was finally possible, enabling numerous studies (Herrera et al. 2012;Indebetouw et al. 2013;Johnson et al. 2015;Leroy et al. 2015;Turner et al. 2017;Johnson et al. 2018;Miura et al. 2018;Finn et al. 2019;Nayak et al. 2019;Emig et al. 2020;Rivera-Soto et al. 2020;Rosolowsky et al. 2021;Finn et al. 2022;He et al. 2022). ...
As the closest major galaxy merger and home to thousands of super star clusters (SSCs), the Antennae Galaxies (NGC 4038 and NGC 4039) are an important location to study the molecular clouds at sites of vigorous star formation. We cataloged giant molecular clouds (GMCs) in the region where the two galaxies overlap using high-resolution (∼0.″1–10 pc) Atacama Large Millimeter/submillimeter Array observations of the ¹² CO(2−1) and ¹³ CO(2−1) emission lines. Of the 72 individual GMCs identified in the overlap region, 17 are within uncertainties of having the necessary mass, pressure, and size needed to form SSCs. Of those 17 GMCs, only one has significant ionizing radiation, indicating that the birth environments are likely still intact in the 16 other GMCs. We compared the physical properties calculated from ¹² CO(2−1) GMC data with observations of 10 other galaxies obtained using the same emission line and similar resolution. Compared to other sources in this sample, the GMCs from the Antennae, as well as in other starbursts and in the centers of galaxies, have the highest luminosities, surface densities, and turbulent pressures. The GMCs in starbursts and at the centers of galaxies also have large line widths, although the line widths in the Antennae are among the widest. These comparative results also indicate that the Antennae GMCs have the highest virial parameters despite their high densities.
... The 251 and 340 GHz images presented in Figure 2 showed that the compact features of region 6 align with the peaks in CO (3-2) and HCO + (3-2) shown in Figure 4. This observation is in line with the characteristics of bright and compact embedded clusters seen in other young clusters, e.g., NGC 253 (Ando et al. 2017;Leroy et al. 2018;Levy et al. 2021;Mills et al. 2021) and NGC 4945 (Emig et al. 2020). To quantify our findings, we focused on the radio continuum blob located in the western star-forming region, corresponding to region 6 as defined in Schutte & Reines (2022). ...
Henize 2–10 is a dwarf starburst galaxy hosting a ∼10 ⁶ M ⊙ black hole (BH) that is driving an ionized outflow and triggering star formation within the central ∼100 pc of the galaxy. Here, we present Atacama Large Millimeter/submillimeter Array continuum observations from 99 to 340 GHz, as well as spectral line observations of the molecules CO (1–0, 3–2), HCN (1–0, 3–2), and HCO+ (1–0, 3–2), with a focus on the BH and its vicinity. Incorporating centimeter-wave radio measurements from the literature, we show that the spectral energy distribution of the BH is dominated by synchrotron emission from 1.4 to 340 GHz, with a spectral index of α ≈ − 0.5. We analyze the spectral line data and identify an elongated molecular gas structure around the BH with a velocity distinct from the surrounding regions. The physical extent of this molecular gas structure is ≈130 pc × 30 pc and the molecular gas mass is ∼10 ⁶ M ⊙ . Despite an abundance of molecular gas in this general region, the position of the BH is significantly offset from the peak intensity, which may explain why the BH is radiating at a very low Eddington ratio. Our analysis of the spatially resolved line ratio between CO J = 3–2 and J = 1–0 implies that the CO gas in the vicinity of the BH is highly excited, particularly at the interface between the BH outflow and the regions of triggered star formation. This suggests that the cold molecular gas is being shocked by the bipolar outflow from the BH, supporting the case for positive BH feedback.
... We first consider an individual RSN as the progenitor for the radio emission. RSN are morphologically compact radio sources that span a range of radio luminosity (Weiler et al. 2002) and spectral index values (Bendo et al. 2016;Klein et al. 2018;Galvin et al. 2018;Emig et al. 2020). The radio emission associated with a RSN is powered by synchrotron processes. ...
Post-merger galaxies are unique laboratories to study the triggering and interplay of star-formation and AGN activity. Combining new, high resolution, 10 GHz Jansky Very Large Array (VLA) observations with archival radio surveys, we have examined the radio properties of 28 spheroidal post-merger galaxies. We find a general lack of extended emission at (sub-)kiloparsec scales, indicating the prevalence of compact, nuclear radio emission in these post-merger galaxies, with the majority (16/18; 89\%) being radio-quiet at 10 GHz. Using multi-wavelength data, we determine the origin of the radio emission, discovering 14 new radio AGN and 4 post-mergers dominated by emission from a population of supernova remnants. Among the radio AGN, almost all are radio-quiet (12/14; 86\%). We discover a new dual AGN (DAGN) candidate, J1511+0417, and investigate the radio properties of the DAGN candidate J0843+3549. 4 of these radio AGN are hosted by SF emission-line galaxies, suggesting that radio AGN activity may be present during periods of SF activity in post-mergers. The low jet powers and compact morphologies of these radio AGN also point to a scenario in which AGN feedback may be efficient in this sample of post-mergers. Lastly, we present simulated, multi-frequency observations of the 14 radio AGN with the Very Long Baseline Array (VLBA) and the VLBI capabilities of the Next Generation Very Large Array (ngVLA) to assess the feasibility of these instruments in searches for supermassive black hole binaries (SMBHBs).
... in Fig. 4 ). In the starburst/AGN composite galaxy, NGC 4945, Henkel et al. ( 2018 ) and Emig et al. ( 2020 ) have found that the H42 α line emission is probably blended with the c -C 3 H 2 (4 32 → 4 23 ) line, and this contamination effect may increase the flux density by a factor of between ∼ 1.1 and 1.6 based on different assumptions. If the H42 α emission is blended with c -C 3 H 2 emission in the nucleus of NGC 1808, the combined line emission could have been easier to detect than the H40 α emission. ...
... Additionally, it is unclear if the line ratios estimated from the NGC 4945 are applicable to NGC 1808. If we exclude the channels co v ering the potential c -C 3 H 2 (4 32 → 4 23 ) line between 85.63 and 85.66 GHz, corresponding to a velocity range of ∼100 km s −1 , the H42 α line flux will be reduced by ∼40 per cent, which is broadly consistent with the line flux ratios found in NGC 4945 (Emig et al. 2020 ). Having said this, the millimeter continuum emission, which is dominated by free-free emission (see Section 4 ), clearly peaks in the nucleus, which indicates the presence of the photoionized gas that would be expected to have recombination line emission associated with it. ...
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of 85.69 and 99.02 GHz continuum emission and H42α and H40α lines emission from the central 1 kpc of NGC 1808. These forms of emission are tracers of photoionizing stars but unaffected by dust obscuration that we use to test the applicability of other commonly star formation metrics. An analysis of the spectral energy distributions shows that free-free emission contributes about 60 to 90 per cent of the continuum emission in the 85-100 GHz frequency range, dependent on the region. The star formation rate (SFR) derived from the ALMA free-free emission is 3.1 ± 0.3 M⊙ yr−1. This is comparable to the SFRs measured from the infrared emission, mainly because most of the bolometric energy from the heavily obscured region is emitted as infrared emission. The radio 1.5 GHz emission yields a SFR 25 per cent lower than the ALMA value, probably because of the diffusion of the electrons producing the synchrotron emission beyond the star-forming regions. The SFRs measured from the extinction-corrected Hα line emission are about 40 to 65 per cent of the SFR derived from the ALMA data, likely because this metric was not calibrated for high extinction regions. Some SFRs based on extinction-corrected ultraviolet emission are similar to those from ALMA and infrared data, but given that the ultraviolet terms in the extinction correction equations are very small, these metrics seem inappropriate to apply to this dusty starburst.
... The much stronger 2 12 →1 01 transition of -C 3 H 2 is detected at a rest frequency of 85.34 GHz (the left column of panels in Figure 4). In the starburst/AGN composite galaxy, NGC 4945, Henkel et al. (2018) and Emig et al. (2020) have found that the H42 line emission is probably blended with the -C 3 H 2 (4 32 →4 23 ) line, and this contamination effect may increase the flux density by a fac- tor of between ∼ 1.1 to 1.6 based on different assumptions. If the H42 emission is blended with -C 3 H 2 emission in the nucleus of NGC 1808, the combined line emission could have been easier to detect than the H40 emission. ...
... Additionally, it is unclear if the line ratios estimated from the NGC 4945 are applicable to NGC 1808. If we exclude the channels covering the potential -C 3 H 2 (4 32 →4 23 ) line between 85.63 and 85.66 GHz, corresponding to a velocity range of ∼ 100 km s −1 , the H42 line flux will be reduced by ∼ 40 per cent, which is broadly consistent with the line flux ratios found in NGC 4945 (Emig et al. 2020). Having said this, the millimeter continuum emission, which is dominated by free-free emission (see in Section 4), clearly peaks in the nucleus, which indicates the presence of the photoionized gas that would be expected to have recombination line emission associated with it. ...
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of 85.69 and 99.02 GHz continuum emission and H42$\alpha$ and H40$\alpha$ lines emission from the central 1~kpc of NGC 1808. These forms of emission are tracers of photoionizing stars but unaffected by dust obscuration that we use to test the applicability of other commonly star formation metrics. An analysis of the spectral energy distributions shows that free-free emission contributes about 60 to 90 per cent of the continuum emission in the 85-100 GHz frequency range, dependent on the region. The star formation rate (SFR) derived from the ALMA free-free emission is $3.1\pm0.3$~M$_\odot$~yr$^{-1}$. This is comparable to the SFRs measured from the infrared emission, mainly because most of the bolometric energy from the heavily obscured region is emitted as infrared emission. The radio 1.5~GHz emission yields a SFR 25 per cent lower than the ALMA value, probably because of the diffusion of the electrons producing the synchrotron emission beyond the star-forming regions. The SFRs measured from the extinction-corrected H$\alpha$ line emission are about 40 to 65 per cent of the SFR derived from the ALMA data, likely because this metric was not calibrated for high extinction regions. Some SFRs based on extinction-corrected ultraviolet emission are similar to those from ALMA and infrared data, but given that the ultraviolet terms in the extinction correction equations are very small, these metrics seem inappropriate to apply to this dusty starburst.
