Dr Aayush Saxena

Abstract:

We report observations of a powerful ionized gas outflow in the z = 4.1 luminous radio galaxy TNJ1338-1942 hosting an obscured quasar using the Near Infrared Spectrograph (NIRSpec) on board JWST. We spatially resolve a large-scale (∼15 kpc) outflow and measure outflow rates. The outflowing gas shows velocities exceeding 900 km s−1 and broad line profiles with widths exceeding 1200 km s−1 located at an ∼10 kpc projected distance from the central nucleus. The outflowing nebula spatially overlaps with the brightest radio lobe, indicating that the powerful radio jets are responsible for the outflow kinematics. The gas is possibly ionized by the obscured quasar with a contribution from shocks induced by the jets. The mass outflow rate map shows that the region with the broadest line profiles exhibits the strongest outflow rates. The total mass outflow rate is ∼500 M ⊙ yr−1, and the mass loading factor is ∼1, indicating that a significant part of the gas is displaced outwards by the outflow. Our hypothesis is that the overpressured shocked jet fluid expands laterally to create an expanding ellipsoidal “cocoon” that causes the surrounding gas to accelerate outwards. The total kinetic energy injected by the radio jet is about 3 orders of magnitude larger than the energy in the outflowing ionized gas. This implies that kinetic energy must be transferred inefficiently from the jets to the gas. The bulk of the deposited energy possibly lies in the form of hot X-ray-emitting gas.

Abstract:

Constraining the escape fraction of Lyman Continuum (LyC) photons from high-redshift galaxies is crucial to understanding reionization. Recent observations have demonstrated that various characteristics of the Ly α emission line correlate with the inferred LyC escape fraction (f LyC esc ) of low-redshift galaxies. Using a data set of 9600 mock Ly α spectra of star-forming galaxies at 4.64 ≤ z ≤ 6 from the SPHINX20 cosmological radiation hydrodynamical simulation, we study the physics controlling the escape of Ly α and LyC photons. We find that our mock Ly α observations are representative of high-redshift observations and that typical observational methods tend to overpredict the Ly α escape fraction (f Ly α esc ) by as much as 2 dex. We investigate the correlations between f LyC esc and f Ly α esc , Ly α equivalent width (Wλ(Ly α)), peak separation (vsep), central escape fraction (fcen), and red peak asymmetry (Ared f ). We find that f Ly α esc and fcen are good diagnostics for LyC leakage, selecting for galaxies with lower neutral gas densities and less UV attenuation that have recently experienced supernova feedback. In contrast, Wλ(Ly α) and vsep are found to be necessary but insufficient diagnostics, while Ared f carries little information. Finally, we use stacks of Ly α, H α, and F150W mock surface brightness profiles to find that galaxies with high f LyC esc tend to have less extended Ly α and F150W haloes but larger H α haloes than their non-leaking counterparts. This confirms that Ly α spectral profiles and surface brightness morphology can be used to better understand the escape of LyC photons from galaxies during the epoch of reionization.

Abstract:

We identify a low-metallicity (12 + log(O/H) = 7.59) Ly𝛼-emitting galaxy at 𝑧 = 5.943 with evidence of a strong Balmer jump, arising from nebular continuum. While Balmer jumps are sometimes observed in low-redshift star-forming galaxies, this galaxy also exhibits a steep turnover in the UV continuum. Such turnovers are typically attributed to absorption by a damped Ly𝛼 system (DLA); however, the shape of the turnover and the high observed Ly𝛼 escape fraction ( 𝑓esc,Ly𝛼 ∼ 27%) is also consistent with strong nebular two-photon continuum emission. Modelling the UV turnover with a DLA requires extreme column densities (𝑁HI > 1023 cm−2 ), and simultaneously explaining the high 𝑓esc,Ly𝛼 requires a fine-tuned geometry. In contrast, modelling the spectrum as primarily nebular provides a good fit to both the continuum and emission lines, motivating scenarios in which (a) we are observing only nebular emission or (b) the ionizing source is powering extreme nebular emission that outshines the stellar emission. The nebular-only scenario could arise if the ionising source has ‘turned off’ more recently than the recombination timescale (∼1,000 yr), hence we may be catching the object at a very specific time. Alternatively, hot stars with 𝑇eff ≳ 105 K (e.g. Wolf-Rayet or low-metallicity massive stars) produce enough ionizing photons such that the two-photon emission becomes visible. While several stellar SEDs from the literature fit the observed spectrum well, the hot-star scenario requires that the number of ≳ 50 M⊙ stars relative to ∼ 5 − 50 M⊙ stars is significantly higher than predicted by typical stellar initial mass functions (IMFs). The identification of more galaxies with similar spectra may provide evidence for a top-heavy IMF at high redshift.

Abstract:

Using deep JWST imaging from JADES, JEMS, and SMILES, we characterize optically faint and extremely red galaxies at z > 3 that were previously missing from galaxy census estimates. The data indicate the existence of abundant, dusty, and poststarburst-like galaxies down to 108 M ⊙, below the sensitivity limit of Spitzer and the Atacama Large Millimeter/submillimeter Array (ALMA). Modeling the NIRCam and Hubble Space Telescope (HST) photometry of these red sources can result in extremely high values for both stellar mass and star formation rate (SFR); however, including seven MIRI filters out to 21 μm results in decreased masses (median 0.6 dex for log10(M∗/M⊙) > 10) and SFRs (median 10× for SFR > 100 M ⊙ yr−1). At z > 6, our sample includes a high fraction of “little red dots” (LRDs; NIRCam-selected dust-reddened active galactic nucleus (AGN) candidates). We significantly measure older stellar populations in the LRDs out to rest-frame 3 μm (the stellar bump) and rule out a dominant contribution from hot dust emission, a signature of AGN contamination to stellar population measurements. This allows us to measure their contribution to the cosmic census at z > 3, below the typical detection limits of ALMA (L IR < 1012 L ⊙). We find that these sources, which are overwhelmingly missed by HST and ALMA, could effectively double the obscured fraction of the star formation rate density at 4 < z < 6 compared to some estimates, showing that prior to JWST, the obscured contribution from fainter sources could be underestimated. Finally, we identify five sources with evidence for Balmer breaks and high stellar masses at 5.5 < z < 7.7. While spectroscopy is required to determine their nature, we discuss possible measurement systematics to explore with future data.