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Laporte et al. (2021), MNRAS, 505, 3336
"Probing cosmic dawn: Ages and star formation histories of candidate z ≥ 9 galaxies"

We discuss the SEDs and physical properties of six galaxies whose photo-zs suggest they lie beyond a redshift z ≃ 9. Each was selected on account of a prominent excess seen in the Spitzer/IRAC 4.5 μm band which, for a redshift above z = 9.0, likely indicates the presence of a rest-frame Balmer break and a stellar component that formed earlier than a redshift z ≃ 10. In addition to constraining the earlier star formation activity on the basis of fits using stellar population models with BAGPIPES, we have undertaken the necessary, but challenging, follow-up spectroscopy for each candidate using various combinations of Keck/MOSFIRE, VLT/X-shooter, Gemini/FLAMINGOS2, and ALMA. Together with earlier spectroscopic data for MACS1149-JD1, our analysis of this enlarged sample provides further support for a cosmic star formation history extending beyond redshifts z ≃ 10. We use our best-fitting stellar population models to reconstruct the past rest-frame UV luminosities of our sources and discuss the implications for tracing earlier progenitors of such systems with the JWST.

Roberts-Borsani et al. (2021), ApJ, 910, 86
"Improving z~7-11 galaxy property estimates with JWST/NIRCam medium-band photometry"

The past decade has seen impressive progress in the detection of z>7 galaxies with HST, however little is known about their properties. JWST will revolutionise the high-z field by providing NIR (i.e., rest-frame optical) data of unprecedented depth and spatial resolution. Measuring galaxy quantities such as resolved stellar ages or gas metallicity gradients traditionally requires spectroscopy, as broad-band imaging filters are generally too coarse to fully isolate diagnostics such as the 4000 Å (rest-frame) break, continuum emission from aged stars, and key emission lines (e.g., [OII], [OIII], Hβ). However, in this paper, we show that adding NIRCam images through a strategically chosen medium-band filter to common wide-band filters sets adopted by ERS and GTO programs delivers tighter constraints on these galactic properties. To constrain the choice of filter, we perform a systematic investigation of which combinations of wide-band filters from ERS and GTO programs and single medium-band filters offer the tightest constraints on several galaxy properties at redshifts z∼7−11.

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Morishita et al. (2020), ApJ, 904, 50

"SuperBoRG: Exploration of point sources at z∼8 in HST parallel fields"

We explore point source candidates at redshift z∼8 in SuperBoRG, a compilation of ∼0.4 deg^2 archival medium-deep (m_F160W∼26.5 AB mag, 5σ) parallel IR images taken with the Hubble Space Telescope. Initial candidates are selected by using the Lyman-break technique. We then carefully analyze source morphology, and robustly identify 3 point sources at z∼8. Photometric redshift analysis reveals that they are preferentially fit by extra-galactic templates, and we conclude that they are unlikely to be low-z interlopers, including brown dwarfs. A clear IRAC ch2 flux excess is seen in one of the point sources, which is expected if the source has strong Hβ+[O III] emission with rest-frame equivalent width of ∼3000 A. Deep spectroscopic data taken with Keck/MOSFIRE, however, do not reveal Lyα emission from the object. In combination with the estimated Hβ+[O III] equivalent width, we place an upper limit on its Lyα escape fraction f_{esc,Lyα}<2%. We estimate the number density of these point sources ∼1×10^−6 Mpc^−3 mag^−1 at MUV∼−23 mag. The final interpretation of our results remains inconclusive: extrapolation from low-z studies of faint quasars suggests that >100× survey volume may be required to find one of this luminosity. 

Roberts-Borsani et al. (2020), MNRAS, 497, 3440

"Interpreting the Spitzer/IRAC colours of 7 ≤ z ≤ 9 galaxies: distinguishing between line emission and starlight using ALMA"

Prior to the launch of JWST, Spitzer/IRAC photometry offers the only means of studying the rest-frame optical properties of z>7 galaxies. Many such high redshift galaxies display a red [3.6]-[4.5] micron colour, often referred to as the "IRAC excess", which has conventionally been interpreted as arising from intense [OIII]+Hbeta emission within the [4.5] micron bandpass. In this paper we demonstrate that, given the photometric uncertainties, established stellar populations with Balmer breaks, such as those inferred at z>9 where line emission does not contaminate the IRAC bands, can equally well explain the redshift-dependent behaviour of the IRAC colours in 7 < z < 9 galaxies. We discuss possible ways of distinguishing between the two hypotheses using ALMA measures of [OIII] 88 micron and dust continuum fluxes. Prior to further studies with JWST, we show that the distinction is important in determining the assembly history of galaxies in the first 500 Myr.


Jones et al. (2020), ApJ, 903, 150
"The Mass-Metallicity Relation at z=8: Direct-Method Metallicity Constraints and Near-Future Prospects"

We use reported measurements of [O III] 88 micron emission and SFR to estimate gas-phase oxygen abundances in five galaxies at z=7.1-9.1 using the direct T_e method. We find typical abundances 12+log(O/H) = 7.9 (~0.2 times the solar value) and an evolution of 0.9+/-0.5 dex in oxygen abundance at fixed stellar mass from z=8 to 0. These results are compatible with theoretical predictions, albeit with large (conservative) uncertainties in both mass and metallicity. We assess both statistical and systematic uncertainties to identify promising means of improvement with the ALMA and JWST. In particular we highlight [O III] 52 micron as a valuable feature for robust metallicity measurements. Precision of 0.1-0.2 dex in T_e-based O/H abundance can be reasonably achieved for galaxies at z=5-8 by combining [O III] 52 micron with rest-frame optical strong lines.

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Roberts-Borsani (2020), MNRAS, 494, 4266 

"Observational Constraints on the Multiphase Nature of Outflows Using Large Spectroscopic Surveys at z∼0"

Mass outflow rates and loading factors are typically used to infer the quenching potential of galactic-scale outflows. However, these generally rely on observations of a single gas phase which can severely underestimate the total ejected gas mass. To address this, we use observations of high mass (>1010M☉), normal star-forming galaxies at z∼0 from the MaNGA, xCOLD GASS, xGASS and ALFALFA surveys and a stacking of NaD, Hα, CO(1-0) and HI 21cm tracers with the aim of placing constraints on an average, total mass outflow rate and loading factor. We find detections of outflows in both neutral and ionised gas tracers, with no detections in stacks of molecular or atomic gas emission. Combining the detections and upper limits, we find average total outflow rates of <27 M☉/yr and a loading factor of η<6.39, with molecular gas likely contributing <72% of the total mass outflow rate, and neutral and ionised gas contributing ~28% and <1%, respectively. Our results suggest that, to first order, a degree of quenching via ejective feedback could occur in normal galaxies when considering all gas phases, even in the absence of an AGN.


Roberts-Borsani et al. (2020), MNRAS, 493, 3081
"Outflows in Star-forming Galaxies: Stacking Analyses of Resolved Winds and the Relation to Their Hosts' Properties"

We present an analysis of 405 high mass (log M*/M☉ > 10), star-forming galaxies (excluding AGN) with low inclinations at z~0, using stacking techniques of the NaD neutral gas tracer in IFU observations from the MaNGA DR15 survey. We detect outflows in the central regions of 78/405 galaxies and determine their extent and power through the construction of stacked annuli, finding them to be most powerful in central regions. The stacking of spaxels over key galaxy quantities reveals outflow detections in regions of high ΣSFR and ΣM*along the resolved main sequence. Clear correlations with ΣSFR suggest it is the main regulator of outflows. Furthermore, measurements of the Hδ and Dn4000 indices reveal virtually identical star formation histories between galaxies with outflows and those without. Finally, through stacking of HI 21 cm observations for a subset of our sample, we find outflow galaxies show reduced HI gas fractions at central velocities compared to their non-detection control counterparts, suggestive of some removal of HI gas.

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