arXiv subjects

Charlotte Simmonds

Publications and source records attributed to Charlotte Simmonds.

At least 55 records · Page 3Linked to original sources

arXiv2024

JADES Data Release 3 -- NIRSpec/MSA spectroscopy for 4,000 galaxies in the GOODS fields

We present the third data release of JADES, the JWST Advanced Deep Extragalactic Survey, providing both imaging and spectroscopy in the two GOODS fields. Spectroscopy consists of medium-depth and deep NIRSpec/MSA spectra of 4,000 targets, covering the spectral range 0.6-5.3 $μ$m and observed with both the low-dispersion prism (R=30-300) and all three medium-resolution gratings (R=500-1,500). We describe the observations, data reduction, sample selection, and target allocation. We measured 2,375 redshifts (2,053 from multiple emission lines); our targets span the range from z=0.5 up to z=13, including 404 at z>5. The data release includes 2-d and 1-d fully reduced spectra, with slit-loss corrections and background subtraction optimized for point sources. We also provide redshifts and S/N>5 emission-line flux catalogs for the prism and grating spectra, and concise guidelines on how to use these data products. Alongside spectroscopy, we are also publishing fully calibrated NIRCam imaging, which enables studying the JADES sample with the combined power of imaging and spectroscopy. Together, these data provide the largest statistical sample to date to characterize the properties of galaxy populations in the first billion years after the Big Bang.

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arXiv2024

JADES: The incidence rate and properties of galactic outflows in low-mass galaxies across 3 < z < 9

We investigate the incidence and properties of ionized gas outflows in a sample of 52 galaxies with stellar mass between $10^7$ M$_{\odot}$ and $10^9$ M$_{\odot}$ observed with ultra-deep JWST/NIRSpec MSA spectroscopy as part of the JWST Advanced Deep Extragalactic Survey (JADES). The high-spectral resolution (R2700) NIRSpec observations allowed us to identify for the first time the signature of outflows in the rest-frame optical nebular lines in low-mass galaxies at $z>3$. The incidence fraction of ionized outflows, traced by broad components, is about 25-40$\%$ depending on the intensity of the emission lines. The low incidence fraction might be due to both the sensitivity limit and the fact that outflows are not isotropic but have a limited opening angle which results in a detection only when this is directed toward our line of sight. Evidence for outflows increases slightly with stellar mass and star-formation rate. The median velocity and mass loading factor (i.e., the ratio between mass outflow rate and star formation rate) of the outflowing ionized gas are 350 km s$^{-1}$ and $η=2.0^{+1.6}_{-1.5}$, respectively. These are 1.5 and 100 times higher, respectively than the typical values observed in local dwarf galaxies. These outflows are able to escape the gravitational potential of the galaxy and enrich the circum-galactic medium and, potentially, the inter-galactic medium. Our results indicate that outflows can significantly impact the star formation activity in low-mass galaxies within the first 2 Gyr of the Universe.

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arXiv2024

GN-z11: The environment of an AGN at $z=$10.603

Recent observations with the \textit{James Webb} Space Telescope (JWST) have further refined the spectroscopic redshift of GN-z11, one of the most distant galaxies identified with the \textit{Hubble} Space Telescope (HST) at $z=10.603$. The presence of extremely dense gas ($>10^{10}$ cm$^{-3}$), the detection of high-ionisation lines and of CII*1335 emission, as well as the presence of an ionisation cone, indicate that GN-z11 also hosts an Active Galactic Nucleus (AGN). Further photometric and spectroscopic follow-up demonstrates that it lies in a large-scale, overdense structure with possible signatures of Population III (PopIII) stars in its halo. Surprisingly, Ly$α$ has also been detected despite the expected largely neutral inter-galactic medium at such a redshift. We exploit recent JWST/NIRSpec IFU observations to demonstrate that the Ly$α$ emission in GN-z11 is part of an extended halo with a minimum size of 0.8--3.2 kpc, depending on the definition used to derive the halo size. The surface brightness of the Ly$α$ halo around GN-z11 appears consistent with Ly$α$ halos observed around $z\sim6$ quasars. At the wavelength of Ly$α$ at $z\sim$10.6, we identify three other emission line candidates within the IFU Field-of-View with no UV rest-frame counterpart visible in deep images from the JWST/NIRCam. If confirmed, this could be the first evidence that the local region of GN-z11 represents a candidate protocluster core, forming just 400 Myr after the Big Bang. We give a first estimate of the dark matter halo mass of this structure ($M_h$=2.96$^{+0.44}_{-0.39} \times$10$^{10}$ M$_{\odot}$), consistent with a Coma-like cluster progenitor.

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arXiv2024

JADES: The production and escape of ionizing photons from faint Lyman-alpha emitters in the epoch of reionization

We present the properties of 17 faint Ly$α$ emitting galaxies (LAEs) at $z>5.8$ from the JWST Advanced Deep Extragalactic Survey (JADES) in the Hubble Ultra Deep Field/GOODS-S. These LAEs span a redshift range $z\approx5.8-8.0$ and a UV magnitude range $M_{UV}\approx-17$ to $-20.6$, with the Ly$α$ equivalent width (EW) in the range $\approx 25-350$ Å. The detection of other rest-optical emission lines in the spectra of these LAEs enables the determination of accurate systemic redshifts and Lyα velocity offsets, as well as the physical and chemical composition of their stars and interstellar media. These faint LAEs are consistent with metal-poor systems with high ionization parameters, similar to the general galaxy population at $z>6$. We measured an average ionizing photon production efficiency, log($ξ_\rm{ion}$/erg$^{-1}$ Hz) $\approx25.57$ across our LAEs, which does not evolve strongly with redshift. We report an anti-correlation between the Ly$α$ escape fraction (f_\rm{esc}) and the velocity offset from systemic redshift, consistent with model expectations. We further find that the strength and velocity offset of Ly$α$ are neither correlated with galaxy spectroscopic properties nor with $ξ_\rm{ion}$. We find a decrease in $f_\rm{esc}$(Ly$α$) with redshift, indicative of decreasing sizes of ionized bubbles around LAEs at high redshifts. We used a range of galaxy properties to predict Lyman continuum $f_\rm{esc}$ for our LAEs, finding that the ionizing photon output into the intergalactic medium remains roughly constant across the observed Ly$α$ EW, showing a mild increase at fainter M$_{UV}$ and at higher redshifts. We derived correlations between the ionizing photon output from LAEs and $M_{UV}$, Ly$α$ EW and redshifts, which can be used to constrain the ionizing photon contribution of LAEs at $z > 6$ towards cosmic reionization.

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arXiv2024

Deciphering Lyman-$α$ Emission Deep into the Epoch of Reionisation

During the epoch of reionisation the first galaxies were enshrouded in pristine neutral gas, with one of the brightest emission lines in star-forming galaxies, Lyman-$α$ (Ly$α$), expected to remain undetected until the Universe became ionised. Providing an explanation for the surprising detection of Ly$α$ in these early galaxies is a major challenge for extra-galactic studies. Recent JWST observations have reignited the debate on whether residence in an overdensity of galaxies is a it sufficient and necessary condition for Ly$α$ to escape. Here, we take unique advantage of both high-resolution and high-sensitivity images from the JWST instrument NIRCam to reveal that all galaxies in a sample of z>7 Ly$α$ emitters have close companions. We exploit novel on-the-fly radiative transfer magnetohydrodynamical simulations with cosmic ray feedback to show that galaxies with frequent mergers have very bursty star formation which drives episodes of high intrinsic Ly$α$ emission and facilitates the escape of Ly$α$ photons along channels cleared of neutral gas. We conclude that the rapid build up of stellar mass through mergers presents a compelling solution to the long-standing puzzle of the detection of Ly$α$ emission deep into the epoch of reionisation.

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arXiv2024

The Cosmos in its Infancy: JADES Galaxy Candidates at z > 8 in GOODS-S and GOODS-N

We present a catalog of 717 candidate galaxies at $z > 8$ selected from 125 square arcminutes of NIRCam imaging as part of the JWST Advanced Deep Extragalactic Survey (JADES). We combine the full JADES imaging dataset with data from the JEMS and FRESCO JWST surveys along with extremely deep existing observations from HST/ACS for a final filter set that includes fifteen JWST/NIRCam filters and five HST/ACS filters. The high-redshift galaxy candidates were selected from their estimated photometric redshifts calculated using a template fitting approach, followed by visual inspection from seven independent reviewers. We explore these candidates in detail, highlighting interesting resolved or extended sources, sources with very red long-wavelength slopes, and our highest redshift candidates, which extend to $z_{phot} = 18$. We also investigate potential contamination by stellar objects, and do not find strong evidence from SED fitting that these faint high-redshift galaxy candidates are low-mass stars. Over 93\% of the sources are newly identified from our deep JADES imaging, including 31 new galaxy candidates at $z_{phot} > 12$. Using 42 sources in our sample with measured spectroscopic redshifts from NIRSpec and FRESCO, we find excellent agreement to our photometric redshift estimates, with no catastrophic outliers and an average difference of $\langle Δz = z_{phot}- z_{spec} \rangle= 0.26$. These sources comprise one of the most robust samples for probing the early buildup of galaxies within the first few hundred million years of the Universe's history.

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arXiv2024

Inside the bubble: exploring the environments of reionisation-era Lyman-$α$ emitting galaxies with JADES and FRESCO

We present a study of the environments of 17 Lyman-$α$ (Ly$α$) emitting galaxies (LAEs) in the reionisation era ($5.8 < z < 8$) identified by JWST/NIRSpec as part of the JWST Advanced Deep Extragalactic Survey (JADES). Unless situated in sufficiently (re)ionised regions, Ly$α$ emission from these galaxies would be strongly absorbed by neutral gas in the intergalactic medium (IGM). We conservatively estimate sizes of the ionised regions required to reconcile the relatively low Ly$α$ velocity offsets ($Δv_\text{Ly$α$}<300\,\mathrm{km\,s^{-1}}$) with moderately high Ly$α$ escape fractions ($f_\mathrm{esc,\,Lyα}>5\%$) observed in our sample of LAEs, suggesting the presence of ionised hydrogen along the line of sight towards at least eight out of 17 LAEs. We find minimum physical `bubble' sizes of the order of $R_\text{ion}\sim0.1$-$1\,\mathrm{pMpc}$ are required in a patchy reionisation scenario where ionised bubbles containing the LAEs are embedded in a fully neutral IGM. Around half of the LAEs in our sample are found to coincide with large-scale galaxy overdensities seen in FRESCO at $z \sim 5.8$-$5.9$ and $z\sim7.3$, suggesting Ly$α$ transmission is strongly enhanced in such overdense regions, and underlining the importance of LAEs as tracers of the first large-scale ionised bubbles. Considering only spectroscopically confirmed galaxies, we find our sample of UV-faint LAEs ($M_\text{UV}\gtrsim-20\,\mathrm{mag}$) and their direct neighbours are generally not able to produce the required ionised regions based on the Ly$α$ transmission properties, suggesting lower-luminosity sources likely play an important role in carving out these bubbles. These observations demonstrate the combined power of JWST multi-object and slitless spectroscopy in acquiring a unique view of the early Universe during cosmic reionisation via the most distant LAEs.

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arXiv2023

JADES: Discovery of extremely high equivalent width Lyman-alpha emission from a faint galaxy within an ionized bubble at z=7.3

We report the discovery of a remarkable Ly$α$ emitting galaxy at z = 7.2782, JADES-GS-z7-LA, with EW$_0$(Ly$α$) $= 388.0 \pm 88.8$Åand UV magnitude -17.0. The spectroscopic redshift is confirmed via rest-frame optical lines [O II], H$β$ and [O III] in its JWST/NIRSpec Micro-Shutter Assembly (MSA) spectrum. The Ly$α$ line is detected in both lower resolution PRISM as well as medium resolution G140M grating spectra. The LSF-deconvolved Ly$α$ FWHM in the grating is $383.9 \pm 56.2$ km/s and the Ly$α$ velocity offset compared to the systemic redshift is $113.3 \pm 80.0$ km/s, indicative of very little neutral gas or dust within the galaxy. We estimate the Ly$α$ escape fraction to be >70%. JADES-GS-z7-LA has a O32 ratio of $11.1 \pm 2.2$ and a R23 ratio of $11.2 \pm 2.6$, consistent with low metallicity and high ionization parameters. Deep NIRCam imaging also revealed a close companion source (separated by 0.23"), which exhibits similar photometry to that of JADES-GS-z7-LA, with a photometric excess in the F410M NIRCam image consistent with [O III]+H$β$ emission at the same redshift. The spectral energy distribution of JADES-GS-z7-LA indicates a "bursty" star formation history, with a low stellar mass of $\approx 10^7$ $M_\odot$. Assuming that the Ly$α$ transmission through the intergalactic medium is the same as its measured escape fraction, an ionized region of size > 1.5 pMpc is needed to explain the high Ly$α$ EW and low velocity offset compared to systemic seen in JADES-GS-z7-LA. Owing to its UV-faintness, we show that it is incapable of single-handedly ionizing a region large enough to explain its Ly$α$ emission. Therefore, we suggest that JADES-GS-z7-LA (and possibly the companion source) may be a part of a larger overdensity, presenting direct evidence of overlapping ionized bubbles at $z>7$.

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arXiv2023

JWST/NIRSpec Measurements of Extremely Low Metallicities in High Equivalent Width Lyman-$α$ Emitters

Deep VLT/MUSE optical integral field spectroscopy has recently revealed an abundant population of ultra-faint galaxies ($M_{UV} \approx -15$; 0.01 $L_{\star}$) at $z=$2.9$-$6.7 due to their strong Lyman-$α$ emission with no detectable continuum. The implied Lyman-$α$ equivalent widths can be in excess of 100-200 Angstrom, challenging existing models of normal star formation and indicating extremely young ages, small stellar masses, and a very low amount of metal enrichment. We use JWST/NIRSpec's microshutter array to follow-up 45 of these galaxies (11h in G235M/F170LP and 7h in G395M/F290LP), as well as 45 lower-equivalent width Lyman-$α$ emitters. Our spectroscopy covers the range 1.7$-$5.1 micron in order to target strong optical emission lines: H$α$, [N II], [O III], and H$β$. Individual measurements as well as stacks reveal line ratios consistent with a metal poor nature (2$-$40% $Z_{\odot}$, depending on the calibration). The galaxies with the highest equivalent widths of Lyman-$α$, in excess of 90 Angstrom, have lower [N II]/H$α$ (1.9-$σ$) and [O III]/H$β$ (2.2-$σ$) ratios than those with lower equivalent widths, implying lower gas-phase metallicities at a combined significance of 2.4-$σ$. This implies a selection based on Lyman-$α$ equivalent width is an efficient technique for identifying younger, less chemically enriched systems.

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arXiv2023

First Light And Reionisation Epoch Simulations (FLARES) XIII: The Lyman-continuum emission of high-redshift galaxies

The history of reionisation is highly dependent on the ionising properties of high-redshift galaxies. It is therefore important to have a solid understanding of how the ionising properties of galaxies are linked to physical and observable quantities. In this paper, we use the First Light and Reionisation Epoch Simulations (FLARES) to study the Lyman-continuum (LyC, i.e. hydrogen-ionising) emission of massive ($M_*>10^8\,\mathrm{M_\odot}$) galaxies at redshifts $z=5-10$. We find that the specific ionising emissivity (i.e. intrinsic ionising emissivity per unit stellar mass) decreases as stellar mass increases, due to the combined effects of increasing age and metallicity. FLARES predicts a median ionising photon production efficiency (i.e. intrinsic ionising emissivity per unit intrinsic far-UV luminosity) of $\log_{10}(ξ_{\rm ion}\rm{/erg^{-1}Hz})=25.40^{+0.16}_{-0.17}$, with values spanning the range $\log_{10}(ξ_{\rm ion}\rm{/erg^{-1}Hz})=25-25.75$. This is within the range of many observational estimates, but below some of the extremes observed. We compare the production efficiency with observable properties, and find a weak negative correlation with the UV-continuum slope, and a positive correlation with the OIII equivalent width. We also consider the dust-attenuated production efficiency (i.e. intrinsic ionising emissivity per unit dust-attenuated far-UV luminosity), and find a median of $\log_{10}(ξ_{\rm ion}\rm{/erg^{-1}Hz})\sim25.5$. Within our sample of $M_*>10^8\,\mathrm{M_\odot}$ galaxies, it is the stellar populations in low mass galaxies that contribute the most to the total ionising emissivity. Active galactic nuclei (AGN) emission accounts for $10-20$ % of the total emissivity at a given redshift, and extends the LyC luminosity function by $\sim0.5$ dex.

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arXiv2023

JWST NIRCam+NIRSpec: Interstellar medium and stellar populations of young galaxies with rising star formation and evolving gas reservoirs

We present an interstellar medium and stellar population analysis of three spectroscopically confirmed $z>7$ galaxies in the ERO JWST NIRCam and JWST NIRSpec data of the SMACS J0723.3-7327 cluster. We use the Bayesian spectral energy distribution (SED) fitting code \texttt{Prospector} with a flexible star-formation history (SFH), a variable dust attenuation law, and a self-consistent model of nebular emission (continuum and emission lines). Importantly, we self-consistently fit both the emission line fluxes from JWST NIRSpec and the broad-band photometry from JWST NIRCam, taking into account slit-loss effects. We find that these three $z=7.6-8.5$ galaxies ($M_{\star}\approx10^{8}~M_{\odot}$) are young with rising SFHs and mass-weighted ages of $3-4$ Myr, though we find indications for underlying older stellar populations. The inferred gas-phase metallicities broadly agree with the direct metallicity estimates from the auroral lines. The galaxy with the lowest gas-phase metallicity ($\mathrm{Z}_{\rm gas}=0.06~\mathrm{Z}_{\odot}$) has a steeply rising SFH, is very compact ($<0.2~\mathrm{kpc}$) and has a high star-formation rate surface density ($Σ_{\rm SFR}\approx22~\mathrm{M}_{\odot}~\mathrm{yr}^{-1}~\mathrm{kpc}^{-2}$), consistent with rapid gas accretion. The two other objects with higher gas-phase metallicity show more complex multi-component morphologies on kpc scales, indicating that their recent increase in star-formation rate is driven by mergers or internal, gravitational instabilities. We discuss effects of assuming different SFH priors or only fitting the photometric data. Our analysis highlights the strength and importance of combining JWST imaging and spectroscopy for fully assessing the nature of galaxies at the earliest epochs.

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arXiv2023

The ionising photon production efficiency at z~6 for Lyman-alpha emitters using JEMS and MUSE

We study the ionising photon production efficiency at the end of the Epoch of Reionisation ($z \sim 5.4 - 6.6$) for a sample of 30 Lyman-$α$ emitters. This is a crucial quantity to infer the ionising photon budget of the Universe. These objects were selected to have reliable spectroscopic redshifts, assigned based on the profile of their Lyman-$α$ emission line, detected in the MUSE deep fields. We exploit medium-band observations from the JWST extragalactic medium band survey (JEMS) to find the flux excess corresponding to the redshifted \ha\ emission line. We estimate the ultra-violet (UV) luminosity by fitting the full JEMS photometry, along with several HST photometric points, with \texttt{Prospector}. We find a median UV continuum slope of $β= -2.09^{+0.23}_{-0.21}$ for the sample, indicating young stellar populations with little-to-no dust attenuation. Supported by this, we derive $ξ_{ion,0}$ with no dust attenuation and find a median value of log$\frac{ξ_{ion,0}}{\text{Hz erg}^{-1}} = 25.44^{+0.21}_{-0.15}$. If we perform dust attenuation corrections and assume a Calzetti attenuation law, our values are lowered by $\sim 0.1$ dex. Our results suggest Lyman-$α$ emitters at the Epoch of Reionisation have enhanced $ξ_{ion,0}$ compared to previous estimations from literature, in particular, when compared to the non-Lyman-$α$ emitting population. This initial study provides a promising outlook on the characterisation of ionising photon production in the early Universe. In the future, a more extensive study will be performed on the entire dataset provided by the JWST Advanced Deep Extragalactic Survey (JADES). Thus, for the first time, allowing us to place constraints on the wider galaxy populations driving reionisation.

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arXiv2023

JEMS: A deep medium-band imaging survey in the Hubble Ultra-Deep Field with JWST NIRCam & NIRISS

We present JEMS (JWST Extragalactic Medium-band Survey), the first public medium-band imaging survey carried out using JWST/NIRCam and NIRISS. These observations use $\sim2μ$m and $\sim4μ$m medium-band filters (NIRCam F182M, F210M, F430M, F460M, F480M; and NIRISS F430M & F480M in parallel) over 15.6 square arcminutes in the Hubble Ultra Deep Field (UDF), thereby building on the deepest multi-wavelength public datasets available anywhere on the sky. We describe our science goals, survey design, NIRCam and NIRISS image reduction methods, and describe our first data release of the science-ready mosaics. Our chosen filters create a JWST imaging survey in the UDF that enables novel analysis of a range of spectral features potentially across the redshift range of $0.3 1$ magnitude) across redshifts $1.5<z<9.3$, most prominently H$α$+[NII] and [OIII]+H$β$. We present our first data release including science-ready mosaics of each medium-band image available to the community, adding to the legacy value of past and future surveys in the UDF. We also describe future data releases. This survey demonstrates the power of medium-band imaging with JWST, informing future extragalactic survey strategies using JWST observations.

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arXiv2022

Impact of Population III homogeneous stellar evolution on early cosmic reionisation

Context: Population III (Pop III) stars may be fast rotating. An expected consequence of fast rotation is strong internal mixing that deeply affects their evolutionary tracks in the Hertzsprung-Russell diagram and hence their ionising power. aims: We investigate the impact on the ionising power of Pop III stars in an extreme case of internal mixing, the one leading to chemically homogeneous evolution (CHE). In that situation, during the main sequence phase, the star keeps the same chemical composition from its center to its surface. Homogeneous stars have larger effective temperatures and luminosities than stars evolving non-homogeneously and thus are stronger ionising sources. Methods: The stellar evolution models are based on $n=3$ polytropes with a time varying hydrogen mass fraction. The ionisation model employs the self-similar champagne flow solution from Shu et al. (2002), as well as numerical simulations for the stochastic treatment of star clusters over a grid of redshifts and halo masses. Results: We find that haloes containing chemically homogeneous stars have an escape fraction of ionising photons about twice that of haloes containing classical Pop III stars. By extrapolating the high-$z$ ionisation history powered by Pop III stars (at $z\gtrsim 15$) to the post-reionisation epoch, we derive the Thomson scattering optical depth $τ$, which is compared with the value measured by $\textit{Planck}$. We find that $τ$ is overproduced by $\sim1.5- 5σ$, when all Pop III stars evolve homogeneously. This indicates that CHE is unlikely to be realised in the majority of Pop III stars, although the present study cannot exclude that a fraction of them undergoes CHE. Conclusions: Fast rotation might have a significant impact on the ionising budget of Pop III stars, and thus on early cosmic reionisation.

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arXiv2022

The MUSE Extremely Deep Field: a first panoramic view of an Mg II emitting intragroup medium

Using the exquisite MUSE eXtremely Deep Field data, we report the discovery of an MgII emission nebula with an area above a 2$σ$ significance level of 1000 proper kpc$^2$, providing the first panoramic view of the spatial distribution of magnesium in the intragroup medium of a low mass group of five star-forming galaxies at z=1.31. The galaxy group members are separated by less than 50 physical kpc in projection and $\approx$120 km/s in velocity space. The most massive galaxy has a stellar mass of 10$^{9.35}$ M$_\odot$ and shows an MgII P-Cygni line profile indicating the presence of an outflow, which is consistent with the spatially resolved spectral analysis showing $\approx+$120 km/s shift of the MgII emission lines with respect to the systemic redshift. The other galaxies are less massive and only show MgII in emission. The detected MgII nebula has a maximal projected extent of $\approx$70 kpc including a low surface brightness (2 $\times$ 10$^{-19}$ erg/s/cm$^{2}$/arcsec$^{2}$) gaseous bridge between two subgroups of galaxies. The presence of absorption features in the spectrum of a background galaxy located at an impact parameter of 19 kpc from the closest galaxy of the group indicates the presence of gas enriched in magnesium even beyond the detected nebula seen in emission, suggesting that we are observing the tip of a larger intragroup medium. The observed MgII velocity gradient suggests an overall rotation of the structure along the major axis of the most massive galaxy. Our MUSE data also reveal extended Fe II* emission in the vicinity of the most massive galaxy, aligned with its minor axis. Extended [OII] emission is found around the galaxy group members and at the location of the MgII bridge. Our results suggest that both tidal stripping effects from galaxy interactions and outflows are enriching the intragroup medium of this system.

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arXiv2021

Can nebular HeII emission be explained by ultra-luminous X-ray sources?

The shape of the ionising spectra of galaxies is a key ingredient to reveal their physical properties and to our understanding of the ionising background radiation. A long-standing unsolved problem is the presence of HeII nebular emission in many low-metallicity star-forming galaxies. This emission requires ionising photons with energy >54 eV, which are not produced in sufficient amounts by normal stellar populations. To examine if high mass X-ray binaries and ultra-luminous X-ray sources (HMXB/ULX) can explain the observed HeII nebular emission and how their presence alters other emission lines, we compute photoionisation models of galaxies including such sources. We combine spectral energy distributions (SEDs) of integrated stellar populations with constrained SEDs of ULXs to obtain composite spectra with varying amounts of X-ray luminosity, parameterised by Lx/SFR. With these we compute photoionisation models to predict the emission line fluxes of the optical recombination lines of H and He+, and the main metal lines of OIII, OII, OI, and NII. The predictions are then compared to a large sample of low-metallicity galaxies. We find that it is possible to reproduce the nebular HeII and other line observations with our spectra and with amounts of Lx/SFR compatible with the observations. Our work suggests that HMBX/ULX could be responsible for the observed nebular HeII emission. However, the strengths of the high and low ionisation lines, such as HeII and OI, depend strongly on the X-ray contribution and on the assumed SEDs of the high energy source(s); the latter are poorly known.

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arXiv2020

AstroSat detection of Lyman continuum emission from a z=1.42 galaxy

One of the outstanding problems of current observational cosmology is to understand the nature of sources that produced the bulk of the ionizing radiation after the Cosmic Dark Age. Direct detection of these reionization sources is practically infeasible at high redshift due to the steep decline of intergalactic medium transmission. However, a number of low-redshift analogs emitting Lyman continuum at~900 Angstrom ~restframe are now detected at $z< 0.4$ and there are detections in the range $ 2.5< z< 3.5$ also. Here, we report the detection of Lyman continuum emission with a high escape fraction (>20%) from a low-mass clumpy galaxy at z=1.42, in the middle of the redshift range where no detection has been made before and near the peak of the Cosmic Star-formation history. The observation was made in the Hubble Extreme Deep field by the wide-field Ultra-Violet-Imaging Telescope onboard AstroSat. This is the first detection of Extreme Ultraviolet radiation from a distant galaxy at a rest-frame wavelength of 600 Angstrom, and it opens up a new window to constrain the shape of the ionization spectrum. Further observations with AstroSat should significantly increase the sample of Lyman continuum leaking galaxies at Cosmic Noon.

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arXiv2018

XZ: Deriving redshifts from X-ray spectra of obscured AGN

Context: Redshifts are fundamental for our understanding of extragalactic X-ray sources. Ambiguous counterpart associations, expensive optical spectroscopy and/or multimission multiwavelength coverage to resolve degeneracies make estimation often difficult in practice. Aims: We attempt to constrain redshifts of obscured Active Galactic Nuclei (AGN) using only low-resolution X-ray spectra. Methods: Our XZ method fits AGN X-ray spectra with a moderately complex spectral model incorporating a corona, torus obscurer and warm mirror. Using the Bayesian X-ray Astronomy (BXA) package, we constrain redshift, column density, photon index and luminosity simultaneously. The redshift information primarily comes from absorption edges in Compton-thin AGN, and from the Fe K$α$ fluorescent line in heavily obscured AGN. A new generic background fitting method allows us to extract more information from limited numbers of source counts. Results: We derive redshift constraints for 74/321 hard-band detected sources in the Chandra deep field South. Comparing with spectroscopic redshifts, we find an outlier fraction of 8%, indicating that our model assumptions are valid. For three Chandra deep fields, we release our XZ redshift estimates. Conclusions: The independent XZ estimate is easy to apply and effective for a large fraction of obscured AGN in todays deep surveys without the need for any additional data. Comparing to different redshift estimation methods, XZ can resolve degeneracies in photometric redshifts, help to detect potential association problems and confirm uncertain single-line spectroscopic redshifts. With high spectral resolution and large collecting area, this technique will be highly effective for Athena/WFI observations.

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