ABSTRACT
We report 18 dust continuum detections (≥3.3σ) at ∼88 and 158 $\mu{\rm m}$ out of 49 ultraviolet (UV)-bright galaxies (MUV < −21.3 mag) at $z$ > 6.5, observed by the Cycle-7 Atacama Large ...Millimeter/submillimeter Array (ALMA) Large Program, Reionization-Era Bright Emission Line Survey (REBELS) and its pilot programs. This has more than tripled the number of dust continuum detections known at $z$ > 6.5. Out of these 18 detections, 12 are reported for the first time as part of REBELS. In addition, 15 of the dust continuum detected galaxies also show a C ii$_{\rm 158\,{\rm \mu m}}$ emission line, providing us with accurate redshifts. We anticipate more line emission detections from six targets (including three continuum detected targets) where observations are still ongoing. We estimate that all of the sources have an infrared (IR) luminosity (LIR) in a range of $3\!-\!8 \times 10^{11}\, {\rm L_\odot }$, except for one with $L_{\rm IR} = 1.5^{+0.8}_{-0.5} \times 10^{12}\, \, {\rm L_{\odot }}$. Their fraction of obscured star formation is significant at ${\gtrsim} 50{{\ \rm per\ cent}}$, despite being UV-selected galaxies. Some of the dust continuum detected galaxies show spatial offsets (∼0.5–1.5 arcsec) between the rest-UV and far-IR emission peaks. These separations could imply spatially decoupled phases of obscured and unobscured star formation, but a higher spatial resolution observation is required to confirm this. REBELS offers the best available statistical constraints on obscured star formation in UV-luminous galaxies at $z$ > 6.5.
ABSTRACT
We present specific star formation rates (sSFRs) for 40 ultraviolet (UV)-bright galaxies at z ∼ 7–8 observed as part of the Reionization Era Bright Emission Line Survey (REBELS) Atacama ...Large Millimeter/submillimeter Array (ALMA) large programme. The sSFRs are derived using improved star formation rate (SFR) calibrations and spectral energy distribution (SED)-based stellar masses, made possible by measurements of far-infrared (FIR) continuum emission and C ii-based spectroscopic redshifts. The median sSFR of the sample is $18_{-5}^{+7}$ Gyr−1, significantly larger than literature measurements lacking constraints in the FIR, reflecting the larger obscured SFRs derived from the dust continuum relative to that implied by the UV+optical SED. We suggest that such differences may reflect spatial variations in dust across these luminous galaxies, with the component dominating the FIR distinct from that dominating the UV. We demonstrate that the inferred stellar masses (and hence sSFRs) are strongly dependent on the assumed star formation history in reionization-era galaxies. When large sSFR galaxies (a population that is common at z > 6) are modelled with non-parametric star formation histories, the derived stellar masses can increase by an order of magnitude relative to constant star formation models, owing to the presence of a significant old stellar population that is outshined by the recent burst. The C ii line widths in the largest sSFR systems are often very broad, suggesting dynamical masses capable of accommodating an old stellar population suggested by non-parametric models. Regardless of these systematic uncertainties among derived parameters, we find that sSFRs increase rapidly toward higher redshifts for massive galaxies (9.6 < log (M*/M⊙) < 9.8), evolving as (1 + z)1.7 ± 0.3, broadly consistent with expectations from the evolving baryon accretion rates.
ABSTRACT
Cosmic dust is an essential component shaping both the evolution of galaxies and their observational signatures. How quickly dust builds up in the early Universe remains an open question ...that requires deep observations at (sub-)millimetre wavelengths to resolve. Here, we use Atacama Large Millimeter Array observations of 45 galaxies from the Reionization Era Bright Emission Line Survey (REBELS) and its pilot programs, designed to target C ii and dust emission in UV-selected galaxies at z ∼ 7, to investigate the dust content of high-redshift galaxies through a stacking analysis. We find that the typical fraction of obscured star formation fobs = SFRIR/SFRUV+IR depends on stellar mass, similar to what is observed at lower redshift, and ranges from fobs ≈ 0.3 − 0.6 for galaxies with log10(M⋆/M⊙) = 9.4–10.4. We further adopt the z ∼ 7 stellar mass function from the literature to extract the obscured cosmic star formation rate density (SFRD) from the REBELS survey. Our results suggest only a modest decrease in the SFRD between 3 ≲ z ≲ 7, with dust-obscured star formation still contributing ${\sim}30{{\ \rm per\ cent}}$ at z ∼ 7. While we extensively discuss potential caveats, our analysis highlights the continued importance of dust-obscured star formation even well into the epoch of reionization.
ABSTRACT
In this work, we characterize the contributions from both ongoing star formation and the ambient radiation field in Local Group galaxy M33, as well as estimate the scale of the local ...dust-energy balance (i.e. the scale at which the dust is re-emitting starlight generated in that same region) in this galaxy through high-resolution radiative transfer (RT) modelling, with defined stellar and dust geometries. We have characterized the spectral energy distribution (SED) of M33 from UV to sub-mm wavelengths, at a spatial scale of 100 pc. We constructed input maps of the various stellar and dust geometries for use in the RT modelling. By modifying our dust mix (fewer very small carbon grains and a lower silicate-to-carbon ratio as compared to the Milky Way), we can much better fit the sub-mm dust continuum. Using this new dust composition, we find that we are able to well reproduce the observed SED of M33 using our adopted model. In terms of stellar attenuation by dust, we find a reasonably strong, broad UV bump, as well as significant systematic differences in the amount of dust attenuation when compared to standard SED modelling. We also find discrepancies in the residuals of the spiral arms versus the diffuse interstellar medium (ISM), indicating a difference in properties between these two regimes. The dust emission is dominated by heating due to the young stellar populations at all wavelengths (∼80 per cent at 10 $\mu$m to ∼50 per cent at 1 mm). We find that the local dust-energy balance is restored at spatial scales greater than around 1.5 kpc.
ABSTRACT
We combine observations from Atacama Large Millimeter/submillimeter Array (ALMA), Australia Telescope Compact Array, Multi Unit Spectroscopic Explorer (MUSE), and Herschel to study ...gas-to-dust ratios in 15 Fornax cluster galaxies detected in the FIR/sub-mm by Herschel and observed by ALMA as part of the ALMA Fornax Cluster Survey. The sample spans a stellar mass range of 8.3 ≤ log(M⋆/M⊙) ≤ 11.16, and a variety of morphological types. We use gas-phase metallicities derived from MUSE observations (from the Fornax3D survey) to study these ratios as a function of metallicity, and to study dust-to-metal ratios, in a sub-sample of nine galaxies. We find that gas-to-dust ratios in Fornax galaxies are systematically lower than those in field galaxies at fixed stellar mass/metallicity. This implies that a relatively large fraction of the metals in these Fornax systems is locked up in dust, which is possibly due to altered chemical evolution as a result of the dense environment. The low ratios are not only driven by H i deficiencies, but H2-to-dust ratios are also significantly decreased. This is different in the Virgo cluster, where low gas-to-dust ratios inside the virial radius are driven by low H i-to-dust ratios, while H2-to-dust ratios are increased. Resolved observations of NGC 1436 show a radial increase in H2-to-dust ratio, and show that low ratios are present throughout the disc. We propose various explanations for the low H2-to-dust ratios in the Fornax cluster, including the more efficient stripping of H2 compared to dust, more efficient enrichment of dust in the star formation process, and altered interstellar medium physics in the cluster environment.
ABSTRACT
Star formation histories (SFHs) of galaxies are affected by a variety of factors, both external (field versus cluster/group) and internal presence of a bar and active galactic nucleus (AGN), ...morphological type. In this work, we extend our previous study and apply the $\langle \mathrm{SFR}_{\textnormal {5}} \rangle \big / \langle \mathrm{SFR}_{\textnormal {200}} \rangle$ metric to a sample of 11 nearby galaxies with Multi-Unit Spectroscopic Explorer observations. Based on a combination of H α and ultraviolet photometry, $\langle \mathrm{SFR}_{\textnormal {5}} \rangle \big / \langle \mathrm{SFR}_{\textnormal {200}} \rangle$ is sensitive to star formation time-scales of ∼5–200 Myr and therefore measures the present-day rate of change in the star formation rate (SFR), dSFR/dt. Within this limited galaxy sample, we do not observe systematic variations between the global value of $\langle \mathrm{SFR}_{\textnormal {5}} \rangle \big / \langle \mathrm{SFR}_{\textnormal {200}} \rangle$ and the presence of an AGN, stellar bar, or group or cluster membership. Within some of the individual galaxies, we, however, observe significant differences in $\langle \mathrm{SFR}_{\textnormal {5}} \rangle \big / \langle \mathrm{SFR}_{\textnormal {200}} \rangle$ between the arm and interarm regions. In half of the galaxies, the recent SFH of both arm and interarm regions has been very similar. However, in the galaxies with higher bulge-to-total light ratios and earlier morphological type, the SFR is declining more rapidly in the interarm regions. This decline in SFR is not a result of low molecular gas surface density or a decrease in the star formation efficiency, implying that other factors are responsible for this SFR decrease.
ABSTRACT
Dust grains form in the clumpy ejecta of core-collapse supernovae where they are subject to the reverse shock, which is able to disrupt the clumps and destroy the grains. Important dust ...destruction processes include thermal and kinetic sputtering as well as fragmentation and grain vaporization. In the present study, we focus on the effect of magnetic fields on the destruction processes. We have performed magnetohydrodynamical simulations using AstroBEAR to model a shock wave interacting with an ejecta clump. The dust transport and destruction fractions are computed using our post-processing code Paperboats, in which the acceleration of grains due to the magnetic field and a procedure that allows partial grain vaporization have been newly implemented. For the oxygen-rich supernova remnant Cassiopeia A, we found a significantly lower dust survival rate when magnetic fields are aligned perpendicular to the shock direction compared to the non-magnetic case. For a parallel field alignment, the destruction is also enhanced but at a lower level. The survival fractions depend sensitively on the gas density contrast between the clump and the ambient medium and on the grain sizes. For a low-density contrast of 100, e.g. 5 nm silicate grains are completely destroyed while the survival fraction of $1\,\mu{\rm m}$ grains is 86 per cent. For a high-density contrast of 1000, 95 per cent of the 5 nm grains survive while the survival fraction of $1\,\mu{\rm m}$ grains is 26 per cent. Alternative clump sizes or dust materials (carbon) have non-negligible effects on the survival rate but have a lower impact compared to density contrast, magnetic field strength, and grain size.
ABSTRACT
Star formation histories (SFHs) are integral to our understanding of galaxy evolution. We can study recent SFHs by comparing the star formation rate (SFR) calculated using different tracers, ...as each probes a different time-scale. We aim to calibrate a proxy for the present-day rate of change in SFR, dSFR/dt, which does not require full spectral energy distribution (SED) modelling and depends on as few observables as possible, to guarantee its broad applicability. To achieve this, we create a set of models in cigale and define an SFR change diagnostic as the ratio of the SFR averaged over the past 5 and 200 Myr, $\langle SFR_{\rm {5}} \rangle \big / \langle SFR_{\rm {200}} \rangle$, probed by the H α–FUV colour. We apply $\langle SFR_{\rm {5}} \rangle \big / \langle SFR_{\rm {200}} \rangle$ to the nearby spiral NGC 628 and find that its star formation activity has overall been declining in the recent past, with the spiral arms, however, maintaining a higher level of activity. The impact of the spiral arm structure is observed to be stronger on $\langle SFR_{\rm {5}} \rangle \big / \langle SFR_{\rm {200}} \rangle$ than on the star formation efficiency. In addition, increasing disc pressure tends to increase recent star formation, and consequently $\langle SFR_{\rm {5}} \rangle \big / \langle SFR_{\rm {200}} \rangle$. We conclude that $\langle SFR_{\rm {5}} \rangle \big / \langle SFR_{\rm {200}} \rangle$ is sensitive to the molecular gas content, spiral arm structure, and disc pressure. The $\langle SFR_{\rm {5}} \rangle \big / \langle SFR_{\rm {200}} \rangle$ indicator is general and can be used to reconstruct the recent SFH of any star-forming galaxy for which H α, FUV, and either mid- or far-IR photometry is available, without the need of detailed modelling.
ABSTRACT
Understanding how dust attenuation laws vary between and within galaxies is a key question if we want to reliably measure the physical properties of galaxies at both global and local scales. ...To shed new light on this question, we present a detailed study of the slope and bump strength of the attenuation law in the nearby spiral galaxy NGC 628 at the resolved spatial scale of 325 pc. To do so, we have modelled a broad multiwavelength data set from the ultraviolet (UV) to the infrared (IR) with the state-of-the-art SED fitting code cigale, including SWIFT UVOT data for which we have developed a new optimized reduction pipeline. We find that the median dust attenuation curve of NGC 628 is fairly steep, but not as steep as the SMC curve, and has a sub-MW-type UV bump. We observe intriguing variations within the galaxy, with regions of high AV exhibiting a shallower attenuation curve. We argue that the flattening of the curve is due to a dominance of absorption over scattering events at higher AV. No trend between the bump strength and the IRAC 8.0 $\mu$m emission was found. However, this does not necessarily rule out polycyclic aromatic hydrocarbons as the main contributors to the UV bump.
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