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Register a new userA$^3$COSMOS: The Dust Attenuation of Star-Forming Galaxies at $z=2.5-4.0$ from the COSMOS-ALMA Archive
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We present an analysis of the dust attenuation of star forming galaxies at $z=2.5-4.0$ through the relationship between the UV spectral slope ($beta$), stellar mass ($M_{ast}$) and the infrared excess (IRX$=L_{rm{IR}}/L_{rm{UV}}$) based on far-infrared continuum observations from the Atacama Large Millimeter/sub-millimeter Array (ALMA). Our study exploits the full ALMA archive over the COSMOS field processed by the A$^3$COSMOS team, which includes an unprecedented sample of $sim1500$ galaxies at $zsim3$ as primary or secondary targets in ALMA band 6 or 7 observations with a median continuum sensitivity of 126 $rm{mu Jy/beam}$ (1$sigma$). The detection rate is highly mass dependent, decreasing drastically below $log (M_{ast}/M_{odot})=10.5$. The detected galaxies show that the IRX-$beta$ relationship of massive ($log M_{ast}/M_{odot} > 10$) main sequence galaxies at $z=2.5-4.0$ is consistent with that of local galaxies, while starbursts are generally offset by $sim0.5,{rm dex}$ to larger IRX values. At the low mass end, we derive upper limits on the infrared luminosities through stacking of the ALMA data. The combined IRX-$M_{ast}$ relation at $rm{log,(M_{ast}/M_{odot})>9}$ exhibits a significantly steeper slope than reported in previous studies at similar redshifts, implying little dust obscuration at $log M_{ast}/M_{odot}<10$. However, our results are consistent with early measurements at $zsim5.5$, indicating a potential redshift evolution between $zsim2$ and $zsim6$. Deeper observations targeting low mass galaxies will be required to confirm this finding.
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We present the results of a new study of dust attenuation at redshifts $3 < z < 4$ based on a sample of $236$ star-forming galaxies from the VANDELS spectroscopic survey. Motivated by results from the First Billion Years (FiBY) simulation project, we argue that the intrinsic spectral energy distributions (SEDs) of star-forming galaxies at these redshifts have a self-similar shape across the mass range $8.2 leq$ log$(M_{star}/M_{odot}) leq 10.6$ probed by our sample. Using FiBY data, we construct a set of intrinsic SED templates which incorporate both detailed star formation and chemical abundance histories, and a variety of stellar population synthesis (SPS) model assumptions. With this set of intrinsic SEDs, we present a novel approach for directly recovering the shape and normalization of the dust attenuation curve. We find, across all of the intrinsic templates considered, that the average attenuation curve for star-forming galaxies at $zsimeq3.5$ is similar in shape to the commonly-adopted Calzetti starburst law, with an average total-to-selective attenuation ratio of $R_{V}=4.18pm0.29$. We show that the optical attenuation ($A_V$) versus stellar mass ($M_{star}$) relation predicted using our method is consistent with recent ALMA observations of galaxies at $2<z<3$ in the emph{Hubble} emph{Ultra} emph{Deep} emph{Field} (HUDF), as well as empirical $A_V - M_{star}$ relations predicted by a Calzetti-like law. Our results, combined with other literature data, suggest that the $A_V - M_{star}$ relation does not evolve over the redshift range $0<z<5$, at least for galaxies with log$(M_{star}/M_{odot}) gtrsim 9.5$. Finally, we present tentative evidence which suggests that the attenuation curve may become steeper at log$(M_{star}/M_{odot}) lesssim 9.0$.
We present the results of a new study of the relationship between infrared excess (IRX), UV spectral slope (beta) and stellar mass at redshifts 2<z<3, based on a deep Atacama Large Millimeter Array (ALMA) 1.3-mm continuum mosaic of the Hubble Ultra Deep Field (HUDF). Excluding the most heavily-obscured sources, we use a stacking analysis to show that z~2.5 star-forming galaxies in the mass range 9.25 <= log(M/Msun) <= 10.75 are fully consistent with the IRX-beta relation expected for a relatively grey attenuation curve, similar to the commonly adopted Calzetti law. Based on a large, mass complete, sample of 2 <= z <= 3 star-forming galaxies drawn from multiple surveys, we proceed to derive a new empirical relationship between beta and stellar mass, making it possible to predict UV attenuation (A_1600) and IRX as a function of stellar mass, for any assumed attenuation law. Once again, we find that z~2.5 star-forming galaxies follow A_1600-mass and IRX-mass relations consistent with a relatively grey attenuation law, and find no compelling evidence that star-forming galaxies at this epoch follow a reddening law as steep as the Small Magellanic Cloud (SMC) extinction curve. In fact, we use a simple simulation to demonstrate that previous determinations of the IRX-beta relation may have been biased toward low values of IRX at red values of beta, mimicking the signature expected for an SMC-like dust law. We show that this provides a plausible mechanism for reconciling apparently contradictory results in the literature and that, based on typical measurement uncertainties, stellar mass provides a cleaner prediction of UV attenuation than beta. Although the situation at lower stellar masses remains uncertain, we conclude that for 2<z<3 star-forming galaxies with log(M/Msun) >= 9.75, both the IRX-beta and IRX-mass relations are well described by a Calzetti-like attenuation law.
Dust attenuation in galaxies has been extensively studied nearby, however, there are still many unknowns regarding attenuation in distant galaxies. We contribute to this effort using observations of star-forming galaxies in the redshift range z = 0.05-0.15 from the DYNAMO survey. Highly star-forming DYNAMO galaxies share many similar attributes to clumpy, star-forming galaxies at high redshift. Considering integrated Sloan Digital Sky Survey observations, trends between attenuation and other galaxy properties for DYNAMO galaxies are well matched to star-forming galaxies at high redshift. Integrated gas attenuations of DYNAMO galaxies are 0.2-2.0 mags in the V-band, and the ratio of stellar E(B-V) and gas E(B-V) is 0.78-0.08 (compared to 0.44 at low redshift). Four highly star-forming DYNAMO galaxies were observed at H-alpha using the Hubble Space Telescope and at Pa-alpha using integral field spectroscopy at Keck. The latter achieve similar resolution (~0.8-1 kpc) to our HST imaging using adaptive optics, providing resolved observations of gas attenuations of these galaxies on sub-kpc scales. We find < 1.0 mag of variation in attenuation (at H-alpha) from clump to clump, with no evidence of highly attenuated star formation. Attenuations are in the range 0.3-2.2 mags in the V band, consistent with attenuations of low redshift star-forming galaxies. The small spatial variation on attenuation suggests that a majority of the star-formation activity in these four galaxies occurs in relatively unobscured regions and, thus, star-formation is well characterised by our H-alpha observations.
We make use of SHARDS, an ultra-deep (<26.5AB) galaxy survey that provides optical photo-spectra at resolution R~50, via medium band filters (FWHM~150A). This dataset is combined with ancillary optical and NIR fluxes to constrain the dust attenuation law in the rest-frame NUV region of star-forming galaxies within the redshift window 1.5<z<3. We focus on the NUV bump strength (B) and the total-to-selective extinction ratio (Rv), targeting a sample of 1,753 galaxies. By comparing the data with a set of population synthesis models coupled to a parametric dust attenuation law, we constrain Rv and B, as well as the colour excess, E(B-V). We find a correlation between Rv and B, that can be interpreted either as a result of the grain size distribution, or a variation of the dust geometry among galaxies. According to the former, small dust grains are associated with a stronger NUV bump. The latter would lead to a range of clumpiness in the distribution of dust within the interstellar medium of star-forming galaxies. The observed wide range of NUV bump strengths can lead to a systematic in the interpretation of the UV slope ($beta$) typically used to characterize the dust content. In this study we quantify these variations, concluding that the effects are $Deltabeta$~0.4.
The infrared spectral energy distributions (SEDs) of main-sequence galaxies in the early universe (z > 4) is currently unconstrained as infrared continuum observations are time consuming and not feasible for large samples. We present Atacama Large Millimetre Array (ALMA) Band 8 observations of four main-sequence galaxies at z ~ 5.5 to study their infrared SED shape in detail. Our continuum data (rest-frame 110$rm mu m$, close to the peak of infrared emission) allows us to constrain luminosity weighted dust temperatures and total infrared luminosities. With data at longer wavelengths, we measure for the first time the emissivity index at these redshifts to provide more robust estimates of molecular gas masses based on dust continuum. The Band 8 observations of three out of four galaxies can only be reconciled with optically thin emission redward of rest-frame 100$rm mu m$. The derived dust peak temperatures at z ~ 5.5 (38$pm$8K) are elevated compared to average local galaxies, however, 5-10K below what would be predicted from an extrapolation of the trend at $z<4$. This behaviour can be explained by decreasing dust abundance (or density) towards high redshifts, which would cause the infrared SED at the peak to be more optically thin, making hot dust more visible to the external observer. From the 850$rm mu m$ dust continuum, we derive molecular gas masses between $10^{10}$ and $10^{11},{rm M_{odot}}$ and gas fractions (gas over total mass) of 30-80% (gas depletion times of 100-220Myrs). All in all, our results provide a first measured benchmark SED to interpret future millimetre observations of normal, main-sequence galaxies in the early Universe.
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