The multiwavelength properties of red QSOs: Evidence for dusty winds as the origin of QSO reddening

Fundamental differences in the radio properties of red quasars (QSOs), as compared to blue QSOs, have been recently discovered, positioning them as a potential key population in the evolution of galaxies and black holes across cosmic time. To elucidate the nature of these objects, we exploited a rich compilation of broad-band photometry and spectroscopic data to model their spectral energy distributions (SEDs) from the ultraviolet to the far-infrared and characterise their emission-line properties. Following a systematic comparison approach, we characterise the properties of the QSO accretion, obscuration, and host galaxies in a sample of ∼1800 QSOs at 0.2 z 1000 km s−1) in red QSOs as compared to the control sample. We find that red QSOs that exhibit evidence for high-velocity wind components present a stronger signature of the infrared excess, suggesting a causal connection between QSO reddening and the presence of hot dust distributions in QSO winds. We propose that dusty winds at nuclear scales are potentially the physical ingredient responsible for the optical colours in red QSOs, as well as a key parameter for the regulation of accretion material in the nucleus.</p

MIGHTEE: Are giant radio galaxies more common than we thought?

We report the discovery of two new giant radio galaxies (GRGs) using the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) survey. Both GRGs were found within a ∼1 deg2 region inside the COSMOS field. They have redshifts of z = 0.1656 and z = 0.3363 and physical sizes of 2.4 Mpc and 2.0 Mpc, respectively. Only the cores of these GRGs were clearly visible in previous high resolution VLA observations, since the diffuse emission of the lobes was resolved out. However, the excellent sensitivity and uv coverage of the new MeerKAT telescope allowed this diffuse emission to be detected. The GRGs occupy an unpopulated region of radio power – size parameter space. Based on a recent estimate of the GRG number density, the probability of finding two or more GRGs with such large sizes at z < 0.4 in a ∼1 deg2 field is only 2.7 × 10−6, assuming Poisson statistics. This supports the hypothesis that the prevalence of GRGs has been significantly underestimated in the past due to limited sensitivity to low surface brightness emission. The two GRGs presented here may be the first of a new population to be revealed through surveys like MIGHTEE which provide exquisite sensitivity to diffuse, extended emission

The REBELS ALMA Survey: efficient Ly α transmission of UV-bright z ≃ 7 galaxies from large velocity offsets and broad line widths

Recent work has shown that UV-luminous reionization-era galaxies often exhibit strong Lyman-alpha emission despite being situated at redshifts where the IGM is thought to be substantially neutral. It has been argued that this enhanced Ly α transmission reflects the presence of massive galaxies in overdense regions which power large ionized bubbles. An alternative explanation is that massive galaxies shift more of their Ly α profile to large velocities (relative to the systemic redshift) where the IGM damping wing absorption is reduced. Such a mass-dependent trend is seen at lower redshifts, but whether one exists at z ∼7 remains unclear owing to the small number of existing systemic redshift measurements in the reionization era. This is now changing with the emergence of [C ii]-based redshifts from ALMA. Here, we report MMT/Binospec Ly α spectroscopy of eight UV-bright (MUV ∼-22) galaxies at z ≃ 7 selected from the ALMA REBELS survey. We detect Ly α in four of eight galaxies and use the [C ii] systemic redshifts to investigate the Ly α velocity profiles. The Ly α lines are significantly redshifted from systemic (average velocity offset = 223 km s-1) and broad (FWHM ≈ 300-650 km s-1), with two sources showing emission extending to ≈750 km s-1. We find that the broadest Ly α profiles are associated with the largest [C ii] line widths, suggesting a potential link between the Ly α FWHM and the dynamical mass. Since Ly α photons at high velocities transmit efficiently through the z = 7 IGM, our data suggest that velocity profiles play a significant role in boosting the Ly α visibility of the most UV-luminous reionization-era galaxies

The ALMA REBELS Survey: specific star formation rates in the reionization era

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+−57 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

The ALMA REBELS survey: the dust content of z ∼7 Lyman break galaxies

We include a fully coupled treatment of metal and dust enrichment into the DELPHI semi-analytic model of galaxy formation to explain the dust content of 13 Lyman Break Galaxies (LBGs) detected by the Atacama Large millimetre Array (ALMA) REBELS Large Program at z ≃ 7. We find that the galaxy dust mass, Md, is regulated by the combination of SNII dust production, astration, shock destruction, and ejection in outflows; grain growth (with a standard timescale τ0 = 30 Myr) plays a negligible role. The model predicts a dust-to-stellar mass ratio of ∼0.07−0.1 per cent and a UV-to-total star formation rate relation such that log(ψUV) = −0.05 [log(ψ)]2 + 0.86 log(ψ) − 0.05 (implying that 55-80 per cent of the star formation is obscured) for REBELS galaxies with stellar mass M∗=109−10M⊙⁠. This relation reconciles the intrinsic UV luminosity of LBGs with their observed luminosity function at z = 7. However, 2 out of the 13 systems show dust-to-stellar mass ratios (⁠∼0.94−1.1 per cent⁠) that are up to 18 × larger than expected from the fiducial relation. Due to the physical coupling between dust and metal enrichment, even decreasing τ0 to very low values (0.3 Myr) only increases the dust-to-stellar mass ratio by a factor ∼2. Given that grain growth is not a viable explanation for such high observed ratios of the dust-to-stellar mass, we propose alternative solutions

The ALMA REBELS survey: obscured star formation in massive Lyman-break galaxies at z = 4-8 revealed by the IRX-β and M*relations

We investigate the degree of dust obscured star formation in 49 massive (log 10 ( M*/M ⊙) > 9) Lyman-break galaxies (LBGs) at z = 6.5-8 observed as part of the Atacama Large Millimeter/submillimeter Array (ALMA) Reionization Era Bright Emission Line Surv e y (REBELS) large program. By creating deep stacks of the photometric data and the REBELS ALMA measurements we determine the average rest-frame ultraviolet (UV), optical, and far-infrared (FIR) properties which reveal a significant fraction ( f obs = 0.4-0.7) of obscured star formation, consistent with previous studies. From measurements of the rest-frame UV slope, we find that the brightest LBGs at these redshifts show bluer ( β≃ -2.2) colours than expected from an extrapolation of the colour-magnitude relation found at fainter magnitudes. Assuming a modified blackbody spectral energy distribution (SED) in the FIR (with dust temperature of T d = 46 K and βd = 2.0), we find that the REBELS sources are in agreement with the local 'Calzetti-like' starburst Infrared-excess (IRX)-βrelation. By re-analysing the data available for 108 galaxies at z ≃ 4-6 from the ALMA Large Program to Investigate C + at Early Times (ALPINE) using a consistent methodology and assumed FIR SED, we show that from z ≃ 4-8, massive galaxies selected in the rest-frame UV have no appreciable evolution in their derived IRX-βrelation. When comparing the IRX-M*relation derived from the combined ALPINE and REBELS sample to relations established at z 4 the proportion of obscured star formation is lower by a factor of ≳3 at a given a M*. Our IRX-βresults are in good agreement with the high-redshift predictions of simulations and semi-analytic models for z ≃ 7 galaxies with similar stellar masses and star formation rates

The ALMA REBELS Survey: dust continuum detections at z > 6.5

We report 18 dust continuum detections (≥3.3σ) at ∼88 and 158 $\mu{\rm m}$ out of 49 ultraviolet (UV)-bright galaxies (MUV 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

Normal, dust-obscured galaxies in the epoch of reionization.

Over the past decades, rest-frame ultraviolet (UV) observations have provided large samples of UV luminous galaxies at redshift (z) greater than 6 (refs. 1-3), during the so-called epoch of reionization. While a few of these UV-identified galaxies revealed substantial dust reservoirs4-7, very heavily dust-obscured sources at these early times have remained elusive. They are limited to a rare population of extreme starburst galaxies8-12 and companions of rare quasars13,14. These studies conclude that the contribution of dust-obscured galaxies to the cosmic star formation rate density at z > 6 is sub-dominant. Recent ALMA and Spitzer observations have identified a more abundant, less extreme population of obscured galaxies at z = 3-6 (refs. 15,16). However, this population has not been confirmed in the reionization epoch so far. Here, we report the discovery of two dust-obscured star-forming galaxies at z = 6.6813 ± 0.0005 and z = 7.3521 ± 0.0005. These objects are not detected in existing rest-frame UV data and were discovered only through their far-infrared [C II] lines and dust continuum emission as companions to typical UV-luminous galaxies at the same redshift. The two galaxies exhibit lower infrared luminosities and star-formation rates than extreme starbursts, in line with typical star-forming galaxies at z ≈ 7. This population of heavily dust-obscured galaxies appears to contribute 10-25% to the z > 6 cosmic star formation rate density

The ALMA REBELS Survey. Epoch of Reionization giants: Properties of dusty galaxies at z ≈ 7

We analyse FIR dust continuum measurements for 14 galaxies (redshift z ≈ 7) in the ALMA REBELS Large Program to derive their physical properties. Our model uses three input data, i.e. (a) the UV spectral slope, β, (b) the observed UV continuum flux at 1500 Å, F1500, (c) the observed continuum flux at ≈158μm, F158, and considers Milky Way (MW) and SMC extinction curves, along with different dust geometries. We find that REBELS galaxies have 28 − 90.5 per cent of their star formation obscured; the total (UV+IR) star formation rates are in the range 31.5 1 M⊙, which is likely inconsistent with pure SN production, and might require dust growth via accretion of heavy elements from the interstellar medium. With the SFR predicted by the model and a MW extinction curve, REBELS galaxies detected in [C II] nicely follow the local LCII −SFR relation, and are approximately located on the Kennicutt-Schmidt relation. The sample-averaged gas depletion time is of 0.11y−2P Gyr, where yP is the ratio of the gas-to-stellar distribution radius. For some systems a solution simultaneously matching the observed (β, F1500, F158) values cannot be found. This occurs when the index Im = (F158/F1500)/(β − βint), where βint is the intrinsic UV slope, exceeds I∗m≈1120 for a MW curve. For these objects we argue that the FIR and UV emitting regions are not co-spatial, questioning the use of the IRX-β relation

The ALMA REBELS Survey: discovery of a massive, highly star-forming, and morphologically complex ULIRG at z = 7.31

We present Atacama Large Millimeter/Submillimeter Array (ALMA) [C ii] and ∼158 continuum observations of REBELS-25, a massive, morphologically complex ultra-luminous infrared galaxy (ULIRG; LIR = L⊙) at z = 7.31, spectroscopically confirmed by the Reionization Era Bright Emission Line Survey (REBELS) ALMA Large Programme. REBELS-25 has a significant stellar mass of. From dust-continuum and ultraviolet observations, we determine a total obscured + unobscured star formation rate of SFR. This is about four times the SFR estimated from an extrapolated main sequence. We also infer a [C ii]-based molecular gas mass of, implying a molecular gas depletion time of Gyr. We observe a [C ii] velocity gradient consistent with disc rotation, but given the current resolution we cannot rule out a more complex velocity structure such as a merger. The spectrum exhibits excess [C ii] emission at large positive velocities (∼500 km s-1), which we interpret as either a merging companion or an outflow. In the outflow scenario, we derive a lower limit of the mass outflow rate of 200, which is consistent with expectations for a star-formation-driven outflow. Given its large stellar mass, SFR, and molecular gas reservoir ∼700 Myr after the big bang, we explore the future evolution of REBELS-25. Considering a simple, conservative model assuming an exponentially declining star formation history, constant star formation efficiency, and no additional gas inflow, we find that REBELS-25 has the potential to evolve into a galaxy consistent with the properties of high-mass quiescent galaxies recently observed at z ∼4