arXiv subjects

J. Chisholm

Publications and source records attributed to J. Chisholm.

At least 37 records · Page 2Linked to original sources

arXiv2023

Abundances of CNO elements in z ~ 0.3-0.4 LyC leaking galaxies

We present observations with the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope of eleven Lyman continuum (LyC) leaking galaxies at redshifts, z, in the range 0.29-0.43, with oxygen abundances 12+log(O/H)=7.64-8.16, stellar masses Mstar~10^7.8-10^9.8 Msun and O32=[OIII]5007/[OII]3727 of ~5-20, aiming to detect CIII]1908 emission line. We combine these observations with the optical Sloan Digital Sky Survey (SDSS) spectra for the determination of carbon, nitrogen and oxygen abundances. Our sample was supplemented by thirty one galaxies from the literature, for which carbon, nitrogen and oxygen abundances can be derived from the HST and SDSS spectra. These additional galaxies, however, do not have LyC observations. We find that log(C/O) for the entire sample at 12+log(O/H)<8.1 does not depend on metallicity, with a small dispersion of ~0.13 dex around the average value of ~ -0.75 dex. On the other hand, the log(N/O) in galaxies at z>0.1, including LyC leakers, is systematically higher compared to the rest of the sample with lower metallicity. We find that log(C/O) slightly decreases with increasing Mstar from ~ -0.65 at Mstar=10^6 Msun to ~ -0.80 at Mstar=10^9-10^10 Msun, whereas log(N/O) is considerably enhanced at Mstar>10^8 Msun. The origin of these trends remains basically unknown. One of the possible solutions is to assume that the upper mass limit of the stellar initial mass function (IMF) in more massive galaxies is higher. This would result in higher production of oxygen and larger fraction of massive stars with stellar wind polluting interstellar medium with nitrogen.

astro-ph.GA↗

arXiv2022

An extreme blue nugget, UV-bright starburst at z=3.613 with ninety per cent of Lyman continuum photon escape

We present the discovery and analysis of J1316+2614 at z=3.6130, a UV-bright star-forming galaxy ($M_{\rm UV} \simeq -24.7$) with large escape of Lyman continuum (LyC) radiation. J1316+2614 is a young ($\simeq 10$ Myr) star-forming galaxy with $SFR \simeq 500 M_{\odot}$ yr$^{-1}$ and a starburst mass of log($M_{\star}/M_{\odot}) \simeq 9.7$. It shows a very steep UV continuum, $β_{\rm UV} \simeq -2.59 \pm 0.05$, consistent with residual dust obscuration, $E(B-V)\simeq 0$. LyC emission is detected with high significance ($\simeq 17 σ$) down to $830$Å, for which a very high relative (absolute) LyC escape fraction $f_{\rm esc} \rm (LyC) \simeq 0.92$ ($\simeq 0.87$) is inferred. The contribution of a foreground or AGN contamination to the LyC signal is discussed but is unlikely. J1316$+$2614 is the most powerful ionizing source known among the star-forming galaxy population, both in terms of production ($Q_{\rm H} \approx 10^{56}$ s$^{-1}$) and escape of ionizing photons ($f_{\rm esc} \rm (LyC) \approx 0.9$). Nebular emission in Ly$α$, H$β$, and other rest-frame optical lines are detected, but these are weak ($EW_{0} \rm [Hβ] \simeq 35$Å), with their strengths reduced roughly by $\simeq 90\%$. J1316+2614 is the first case known where the effect of large escape of ionizing photons on the strength of nebular lines and continuum emission is clearly observed. Gas inflows are detected in J1316+2614 from the blue-dominated peak Ly$α$ emission (with a blue-to-red peak line ratio $I_{\rm blue}/I_{\rm red} \simeq 3.7$) and redshifted ISM absorption ($\simeq 100$ km s$^{-1}$). Our results suggest that J1316+2614 is undergoing a gas compaction event, possibly representing a short-lived phase in the evolution of massive and compact galaxies, where strong gas inflows have triggered an extreme star formation episode and nearly $100\%$ LyC photons are escaping.

astro-ph.GA↗

arXiv2022

CLusters in the Uv as EngineS (CLUES): I. Survey presentation \& FUV spectral analysis of the stellar light

The CLusters in the Uv as EngineS (CLUES) survey is a Cosmic Origins Spectrograph (COS) campaign aimed at acquiring the 1130 to 1770 Å, restframe spectroscopy of very young (<20 Myr) and massive (>10^4 solar masses) star clusters in galaxies that are part of the Hubble treasury program Legacy ExtraGalactic Uv Survey (LEGUS). In this first paper of a series, we describe the CLUES sample consisting of 20 young star clusters and report their physical properties as derived by both multi-wavelength photometry and far-UV (FUV) spectroscopy with Hubble Space Telescope (HST). Thanks to the synergy of the two different datasets we build a coherent picture of the diverse stellar populations found in each region (with sizes of 40 to 160 pc). We associate the FUV-brightest stellar population to the central targeted star cluster and the other modeled population to the diffuse stars that are included in the COS aperture. We observe better agreement between photometric and spectroscopic ages for star clusters younger than 5 Myr. For clusters older than 5 Myr, photometry and spectroscopy measurements deviate, with the latter producing older ages, due to the degeneracy of photometric models. FUV spectroscopy enables us to better constrain the stellar metallicities, a parameter that optical colors are insensitive to. Finally, the derived E(B-V) are quite similar, with a tendency for FUV spectroscopy to favor solutions with higher extinctions. The recovered masses are in agreement within a factor of 2 for all the clusters.

astro-ph.GA↗

arXiv2022

The Far-Ultraviolet Continuum Slope as a Lyman Continuum Escape Estimator at High-redshift

Most of the hydrogen in the intergalactic medium (IGM) was rapidly ionized at high-redshifts. While observations have established that reionization occurred, observational constraints on the emissivity of ionizing photons at high-redshift remains elusive. Here, we present a new analysis of the Low-redshift Lyman Continuum Survey (LzLCS) and archival observations, a combined sample of 89 star-forming galaxies at z~0.3 with Hubble Space Telescope observations of their ionizing continua (or Lyman Continuum, LyC). We find a strong (6$σ$ significant) inverse correlation between the continuum slope at 1550Å (defined as F$_λ\proptoλ^β$) and both the LyC escape fraction (f$_{esc}$) and f$_{esc}$ times the ionizing photon production efficiency ($ξ_{ ion}$). On average, galaxies with redder continuum slopes have smaller f$_{esc}$ than galaxies with bluer slopes due to higher dust attenuation. More than 5% (20%) of the LyC emission escapes galaxies with $β$<-2.1 (-2.6). We find strong correlations between $β$ and the gas-phase ionization ([OIII]/[OII] flux ratio; at 7.5$σ$ significance), galaxy stellar mass (at 5.9$σ$), the gas-phase metallicity (at 4.6$σ$), and the observed FUV absolute magnitude (M$_{UV}$ at 3.4$σ$). Using previous observations of $β$ at high-redshift, we estimate the evolution of f$_{esc}$ with both $z$ and M$_{UV}$. The LzLCS suggest that fainter and lower mass galaxies dominate the ionizing photon budget at higher redshift, possibly due to their rapidly evolving metal and dust content. Finally, we use our correlation between $β$ and f$_{ esc}\timesξ_{ion}$ to predict the ionizing emissivity of galaxies during the epoch of reionization. Our estimated emissivities match IGM observations, and suggest that star-forming galaxies emit sufficient LyC photons into the IGM to exceed recombinations near redshifts of 7-8.

astro-ph.GA↗

arXiv2022

Lyman alpha and Lyman continuum emission of MgII-selected star-forming galaxies

We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of seven compact low-mass star-forming galaxies at redshifts, z,in the range 0.3161-0.4276, with various O3Mg2=[OIII]5007/MgII 2796+2803 and Mg2=MgII 2796/MgII 2803 emission-line ratios. We aim to study the dependence of leaking Lyman continuum (LyC) emission on the characteristics of MgII emission together with the dependences on other indirect indicators of escaping ionizing radiation. LyC emission with escape fractions fesc(LyC)=3.1-4.6 per cent is detected in four galaxies, whereas only 1sigma upper limits of fesc(LyC) in the remaining three galaxies were derived. A strong narrow Ly-alpha emission line with two peaks separated by Vsep~298-592 km/s was observed in four galaxies with detected LyC emission and very weak Ly-alpha emission is observed in galaxies with LyC non-detections. Our new data confirm the tight anti-correlation between fesc(LyC) and Vsep found for previous low-redshift galaxy samples. Vsep remains the best indirect indicator of LyC leakage among all considered indicators. It is found that escaping LyC emission is detected predominantly in galaxies with Mg2>1.3. A tendency of an increase of fesc(LyC) with increasing of both the O3Mg2 and Mg2 is possibly present. However, there is substantial scatter in these relations not allowing their use for reliable prediction of fesc(LyC).

astro-ph.GA↗

arXiv2022

No correlation of the Lyman continuum escape fraction with spectral hardness

The properties that govern the production and escape of hydrogen ionizing photons (Lyman continuum, LyC; with energies >13.6 eV) in star-forming galaxies are still poorly understood, but they are key to identifying and characterizing the sources that reionized the Universe. Here we empirically explore the relationship between the hardness of ionizing radiation and the LyC leakage in a large sample of low-$z$ star-forming galaxies from the recent Hubble Space Telescope Low-$z$ Lyman Continuum Survey. Using Sloan Digital Sky Survey stacks and deep XShooter observations, we investigate the hardness of the ionizing spectra ($Q_{\rm He^+}/Q_{\rm H}$) between 54.4 eV (He$^{+}$) and 13.6 eV (H) from the optical recombination lines HeII 4686A and H$β$ 4861A for galaxies with LyC escape fractions spanning a wide range, $f_{\rm esc} \rm (LyC) \simeq 0 - 90\%$. We find that the observed intensity of HeII/H$β$ is primarily driven by variations in the metallicity, but is not correlated with LyC leakage. Both very strong ($ \simeq 0.5$) and nonleakers ($ < f_{\rm esc} \rm (LyC) > \simeq 0$) present similar observed intensities of HeII/H$β$ at comparable metallicity, between $\simeq 0.01$ and $\simeq 0.02$ for $12 + \log({\rm O/H}) > 8.0$ and $<8.0$, respectively. Our results demonstrate that $Q_{\rm He^+}/Q_{\rm H}$ does not correlate with $f_{\rm esc} \rm (LyC)$, which implies that strong LyC emitters do not show harder ionizing spectra than nonleakers at similar metallicity.

astro-ph.GA↗

arXiv2022

Strong Lyman continuum emitting galaxies show intense CIV 1550 emission

Using the Space Telescope Imaging Spectrograph, we have obtained ultraviolet (UV) spectra from $\sim 1200$ to 2000 Å of known Lyman continuum (LyC) emitting galaxies at low redshift ($z \sim 0.3-0.4$) with varying absolute LyC escape fractions (fesc $\sim 0.01 - 0.72$). Our observations include in particular the galaxy J1243+4646, which has the highest known LyC escape fraction at low redshift. While all galaxies are known Lyman alpha emitters, we consistently detect an inventory of additional emission lines, including CIV 1550, HeII 1640, OIII] 1666, and CIII] 1909, whose origin is presumably essentially nebular. CIV 1550 emission is detected above 4 $σ$ in six out of eight galaxies, with equivalent widths of EW(CIV)$=12-15$ Ang for two galaxies, which exceeds the previously reported maximum emission in low-$z$ star-forming galaxies. We detect CIV 1550 emission in all LyC emitters with escape fractions fesc $> 0.1$ and find a tentative increase in the flux ratio CIV 1550/ CIII] 1909 with fesc. Based on the data, we propose a new criterion to select and classify strong leakers (galaxies with fesc $> 0.1$): CIV 1550/ CIII] 1909 $> 0.75$. Finally, we also find HeII 1640 emission in all the strong leakers with equivalent widths from 3 to 8 Ang rest frame. These are among the highest values observed in star-forming galaxies and are primarily due to a high rate of ionizing photon production. The nebular HeII 1640 emission of the strong LyC emitters does not require harder ionizing spectra at $>54$ eV compared to those of typical star-forming galaxies at similarly low metallicity.

astro-ph.GA↗

arXiv2021

Probing Cosmic Dawn : Ages and Star Formation Histories of Candidate $z\geq$9 Galaxies

We discuss the spectral energy distributions and physical properties of six galaxies whose photometric redshifts suggest they lie beyond a redshift $z\simeq$9. Each was selected on account of a prominent excess seen in the Spitzer/IRAC 4.5$μ$m 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 $z\simeq10$. 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. Based on either Lyman-$α$ or [OIII] 88 $μ$m emission, we determine a convincing redshift of $z$=8.78 for GN-z-10-3 and a likely redshift of $z$=9.28 for the lensed galaxy MACS0416-JD. For GN-z9-1, we conclude the case remains promising for a source beyond $z\simeq$9. 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\simeq$10. We use our best-fit 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 James Webb Space Telescope.

astro-ph.GA↗

arXiv2021

Charged bosons made of fermions in a solid state system without Cooper pairing

We report experimental evidence for charged boson states in a solid without Cooper pairing, based on attaching two free carriers to an exciton in a semiconducting system. Theoretical calculations show that this type of complex is stable in bilayer systems next to a parallel metal layer. Our experimental measurements on structures made using two different materials show a new spectral line at the predicted energy, if and only if all the required conditions for this complex are fulfilled, including a parallel metal layer that significantly screens the repulsive interaction between the like-charge carriers, and with the predicted dependence on the distance to the metal layer. This suggests a new path for pursuing room temperature superconductivity without Cooper pairing.

cond-mat.supr-con↗

arXiv2021

Lyman continuum leakage from low-mass galaxies with Mstar < 1.E8 Msun

We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of nine low-mass star-forming galaxies at redshifts, z, in the range 0.3179-0.4524, with stellar masses Mstar < 10^8 M and veruny high specific star-formation rates sSFR~150-630 Gyr^{-1}, aiming to study the dependence of leaking Lyman continuum (LyC) emission on stellar mass and some other characteristics of the galaxy. We detect LyC emission in four out of nine galaxies with escape fractions, fesc(LyC), in the range of 11-35 per cent, and establish upper limits for fesc(LyC) in the remaining five galaxies. We observe a narrow Ly-alpha emission line with two peaks in seven galaxies and likely more complex Ly-alpha profiles in the two remaining galaxies. The velocity separation between the peaks Vsep varies in the range from ~229 km/s to ~512 km/s. Our additional data on low-mass galaxies confirm and strengthen the tight anti-correlation between fesc(LyC) and Vsep found for previous low-redshift galaxy samples with higher stellar masses. Vsep remains the best indirect indicator of LyC leakage. It is better than O32 on which fesc(LyC) depends weakly, with a large scatter. Finally, contrary to expectations, we find no increase of fesc(LyC) with decreasing galaxy stellar mass Mstar.

astro-ph.GA↗

arXiv2020

Optically-Thin Spatially-Resolved Mg II Emission Maps the Escape of Ionizing Photons

Early star-forming galaxies produced copious ionizing photons. A fraction of these photons escaped gas within galaxies to reionize the entire Universe. This escape fraction is crucial for determining how the Universe became reionized, but the neutral intergalactic medium precludes direct measurement of the escape fraction at high-redshifts. Indirect estimates of the escape fraction must describe how the Universe was reionized. Here, we present new Keck Cosmic Web Imager spatially-resolved spectroscopy of the resonant Mg II 2800 doublet from a redshift 0.36 galaxy, J1503+3644, with a previously observed escape fraction of 6%. The Mg II emission has a similar spatial extent as the stellar continuum, and each of the Mg II doublet lines are well-fit by single Gaussians. The Mg II is optically thin. The intrinsic flux ratio of the red and blue Mg II emission line doublet, $R = F_{2796}/F_{2803}$, is set by atomic physics to be two, but Mg$^+$ gas along the line of sight decreases $R$ proportional to the Mg II optical depth. Combined with the metallicity, $R$ estimates the neutral gas column density. The observed $R$ ranges across the galaxy from 0.8-2.7, implying a factor of 2 spatial variation of the relative escape fraction. All of the ionizing photons that escape J1503+3644 pass through regions of high $R$. We combine the Mg II emission and dust attenuation to accurately estimate the absolute escape fractions for ten local Lyman Continuum emitting galaxies and suggest that Mg II can predict escape fraction within the Epoch of Reionization.

astro-ph.GA↗

arXiv2020

The origin of the escape of Lyman alpha and ionizing photons in Lyman Continuum Emitters

Identifying the mechanisms driving the escape of Lyman Continuum (LyC) photons is crucial to find Lyman Continuum Emitter (LCE) candidates. To understand the physical properties involved in the leakage of LyC photons, we investigate the connection between the HI covering fraction, HI velocity width, the Lyman alpha (LyA) properties and escape of LyC photons in a sample of 22 star-forming galaxies including 13 LCEs. We fit the stellar continua, dust attenuation, and absorption lines between 920 and 1300 A to extract the HI covering fractions and dust attenuation. Additionally, we measure the HI velocity widths of the optically thick Lyman series and derive the LyA equivalent widths (EW), escape fractions (fesc), peak velocities and fluxes at the minimum of the LyA profiles. Overall, we highlight strong correlations between the presence of low HI covering fractions and (1) low LyA peak velocities; (2) more flux at the profile minimum; and (3) larger EW(LyA), fesc(LyA), and fesc(LyC). Hence, low column density channels are crucial ISM ingredients for the leakage of LyC and LyA photons. Additionally, galaxies with narrower HI absorption velocity widths have higher LyA equivalent widths, larger LyA escape fractions, and lower LyA peak velocity separations. This suggests that these galaxies have low HI column density. Finally, we find that dust regulates the amount of LyA and LyC radiation that actually escapes the ISM. Overall, the ISM porosity is one origin of strong LyA emission and enables the escape of ionizing photons in low-z leakers. However, this is not enough to explain the largest fesc(LyC) observed, which indicates that the most extreme LCEs are likely density-bounded along all lines of sight to the observer. Overall, the neutral gas porosity constrains a lower limit to the escape fraction of LyC and LyA photons, providing a key estimator of the leakage of ionizing photons.

astro-ph.GA↗

arXiv2019

An X-ray Detection of Star Formation In a Highly Magnified Giant Arc

In the past decade, our understanding of how stars and galaxies formed during the first 5 billion years after the Big Bang has been revolutionized by observations that leverage gravitational lensing by intervening masses, which act as natural cosmic telescopes to magnify background sources. Previous studies have harnessed this effect to probe the distant universe at ultraviolet, optical, infrared and millimeter wavelengths. However, strong lensing studies of young, star-forming galaxies have never extended into X-ray wavelengths, which uniquely trace high-energy phenomena. Here we report an X-ray detection of star formation in a highly magnified, strongly lensed galaxy. This lensed galaxy, seen during the first third of the history of the Universe, is a low--mass, low--metallicity starburst with elevated X-ray emission, and is a likely analog to the first generation of galaxies. Our measurements yield insight into the role that X-ray emission from stellar populations in the first generation of galaxies may play in re-ionizing the Universe. This observation paves the way for future strong lensing-assisted X-ray studies of distant galaxies reaching orders of magnitude below the detection limits of current deep fields, and previews the depths that will be attainable with future X-ray observatories.

astro-ph.HE↗

arXiv2019

Towards sub-kpc scale kinematics of molecular and ionized gas of star-forming galaxies at $z\sim1$

We compare the molecular and ionized gas kinematics of two strongly lensed galaxies at $z\sim1$ based on observations from ALMA and MUSE. We derive the CO and [OII] rotation curves and dispersion profiles of these two galaxies. We find a difference between the observed molecular and ionized gas rotation curves for one of the galaxies, the Cosmic Snake, for which we obtain a spatial resolution of few hundred parsecs along the major axis. The rotation curve of the molecular gas is steeper than the rotation curve of the ionized gas. In the second galaxy, A521, the molecular and ionized gas rotation curves are consistent, but the spatial resolution is only of few kpc on the major axis. Using simulations, we investigate the effect of the thickness of the gas disk and effective radius on the observed rotation curves and find that a more extended and thicker disk smooths the curve. We also find that the presence of a strongly inclined thick disk (>1 kpc) can smooth the rotation curve because it degrades the spatial resolution along the line of sight. By building a model using a stellar disk and two gas disks, we reproduce the rotation curves of the Cosmic Snake with a molecular gas disk that is more massive and more radially and vertically concentrated than the ionized gas disk. Finally, we also obtain an intrinsic velocity dispersion in the Cosmic Snake of 18.5+-7 km/s and 19.5+-6 km/s for the molecular and ionized gas, respectively, which is consistent with a molecular disk with a smaller and thinner disk. For A521, the intrinsic velocity dispersion values are 11+-8 km/s and 54+-11 km/s, with a higher value for the ionized gas. This could indicate that the ionized gas disk is thicker and more turbulent in this galaxy. These results highlight the different spatial distribution of the molecular and ionized gas disks at $z\sim1$ and suggest the presence of thick ionized gas disks at this epoch.

astro-ph.GA↗

arXiv2019

Constraining the metallicities, ages, star formation histories, and ionizing continua of extragalactic massive star populations

We infer the properties of massive star populations using the far-ultraviolet stellar continua of 61 star-forming galaxies: 42 at low-z observed with HST and 19 at z~2 from the Megasaura sample. We fit each stellar continuum with a linear combination of up to 50 single age and single metallicity Starburst99 models. From these fits, we derive light-weighted ages and metallicities, which agree with stellar wind and photospheric spectral features, and infer the spectral shapes and strengths of the ionizing continua. Inferred light-weighted stellar metallicities span 0.05-1.5 Z$_\odot$ and are similar to the measured nebular metallicities. We quantify the ionizing continua using the ratio of the ionizing flux at 900Å to the non-ionizing flux at 1500Å and demonstrate the evolution of this ratio with stellar age and metallicity using theoretical single burst models. These single burst models only match the inferred ionizing continua of half of the sample, while the other half are described by a mixture of stellar ages. Mixed age populations produce stronger and harder ionizing spectra than continuous star formation histories, but, contrary to previous studies that assume constant star formation, have similar stellar and nebular metallicities. Stellar population age and metallicity affect the far-UV continua in different and distinguishable ways; assuming a constant star formation history diminishes the diagnostic power. Finally, we provide simple prescriptions to determine the ionizing photon production efficiency ($ξ_{ion}$) from the stellar population properties. $ξ_{ion}$ has a range of log($ξ_{ion})=24.4-25.7$ Hz erg$^{-1}$ that depends on stellar age, metallicity, star formation history, and contributions from binary star evolution. These stellar population properties must be observationally determined to determine the number of ionizing photons generated by massive stars.

astro-ph.GA↗

arXiv2018

Metal-enriched galactic outflows shape the mass-metallicity relationship

The gas-phase metallicity of low-mass galaxies increases with increasing stellar mass ($M_\ast$) and is nearly constant for high-mass galaxies. Theory suggests that this tight mass-metallicity relationship is shaped by galactic outflows removing metal-enriched gas from galaxies. Here, we observationally model the outflow metallicities of the warm outflowing phase from a sample of seven local star-forming galaxies with stellar masses between 10$^{7}$-10$^{11}$ M$_\odot$. We estimate the outflow metallicities using four weak rest-frame ultraviolet absorption lines, the observed stellar continua, and photoionization models. The outflow metallicity is flat with $M_\ast$, with a median metallicity of $1.0\pm0.6$ Z$_\odot$. The observed outflows are metal-enriched: low and high-mass galaxies have outflow metallicities 10-50 and 2.6 times larger than their ISM metallicities, respectively. The observed outflows are mainly composed of entrained ISM gas with at most 22% of the metals directly coming from recent supernovae enrichment. The metal outflow rate shallowly increases with $M_\ast$, as $M_\ast^{0.2 \pm 0.1}$, because the mass outflow rate shallow increases with $M_\ast$. Finally, we normalize the metal outflow rate by the rate at which star formation retains metals to calculate the metal-loading factor. The metal-loading factor inversely scales with $M_\ast$. The normalization and scaling of the metal-loading factor agree with analytic expressions that reproduce observed mass-metallicity relations. Galactic outflows fundamentally shape the observed mass-metallicity relationship.

astro-ph.GA↗

arXiv2018

Intense CIII] 1907,1909 emission from a strong Lyman continuum emitting galaxy

We have obtained the first complete ultraviolet (UV) spectrum of a strong Lyman continuum(LyC) emitter at low redshift -- the compact, low-metallicity, star-forming galaxy J1154+2443 -- with a Lyman continuum escape fraction of 46% discovered recently. The Space Telescope Imaging Spectrograph spectrum shows strong Lya and CIII] 1909 emission, as well as OIII] 1666. Our observations show that strong LyC emitters can have UV emission lines with a high equivalent width (e.g. EW(CIII])$=11.7 \pm 2.9 Å$ rest-frame), although their equivalent widths should be reduced due to the loss of ionizing photons. The intrinsic ionizing photon production efficiency of J1154+2443 is high, $\log(ξ_{\rm ion}^0)=25.56$ erg$^{-1}$ Hz, comparable to that of other recently discovered $z \sim 0.3-0.4$ LyC emitters. Combining our measurements and earlier determinations from the literature, we find a trend of increasing $ξ_{\rm ion}^0$ with increasing CIII] 1909 equivalent width, which can be understood by a combination of decreasing stellar population age and metallicity. Simple ionization and density-bounded photoionization models can explain the main observational features including the UV spectrum of J1154+2443.

astro-ph.GA↗

arXiv2018

Accurately predicting the escape fraction of ionizing photons using restframe ultraviolet absorption lines

The fraction of ionizing photons that escape high-redshift galaxies sensitively determines whether galaxies reionized the early universe. However, this escape fraction cannot be measured from high-redshift galaxies because the opacity of the intergalactic medium is large at high redshifts. Without methods to indirectly measure the escape fraction of high-redshift galaxies, it is unlikely that we will know what reionized the universe. Here, we analyze the far-ultraviolet (UV) H I (Lyman series) and low-ionization metal absorption lines of nine low-redshift, confirmed Lyman continuum emitting galaxies. We use the H I covering fractions, column densities, and dust attenuations measured in a companion paper to predict the escape fraction of ionizing photons. We find good agreement between the predicted and observed Lyman continuum escape fractions (within $1.4σ$) using both the H I and ISM absorption lines. The ionizing photons escape through holes in the H I, but we show that dust attenuation reduces the fraction of photons that escape galaxies. This means that the average high-redshift galaxy likely emits more ionizing photons than low-redshift galaxies. Two other indirect methods accurately predict the escape fractions: the Ly$α$ escape fraction and the optical [O III]/[O II] flux ratio. We use these indirect methods to predict the escape fraction of a sample of 21 galaxies with rest-frame UV spectra but without Lyman continuum observations. Many of these galaxies have low escape fractions ($f_{\rm esc} \le 1$\%), but 11 have escape fractions $>1$\%. The methods presented here will measure the escape fractions of high-redshift galaxies, enabling future telescopes to determine whether star-forming galaxies reionized the early universe.

astro-ph.GA↗