arXiv subjects

A. E. Jaskot

Publications and source records attributed to A. E. Jaskot.

11 recordsLinked to original sources

arXiv2026

The Lyman-alpha and Continuum Origins Survey II: the connection between the escape of ionizing radiation and Lyman-alpha halos in star-forming galaxies

One of the current challenges in galaxy evolution studies is to establish the mechanisms that govern the escape of ionizing radiation from galaxies. Here, we investigate the connection between Lyman Continuum (LyC) escape and the conditions of the Circumgalactic Medium (CGM), as probed by Ly$α$ halos (LAHs) in emission. We use Ly$α$ and UV continuum imaging data from the Lyman alpha and Continuum Origins Survey (LaCOS), targeting 42 nearby ($z \simeq 0.3$), star-forming galaxies with LyC observations (escape fractions of $f_{\rm esc}^{\rm LyC} \simeq 0.01-0.49$). LaCOS galaxies show extended Ly$α$ emission ubiquitously, with LyC emitters (LCEs) having more compact Ly$α$ morphologies than non-LCEs, and Ly$α$ spatial offsets that do not exceed the extent of the UV continuum. We model the diffuse LAHs using a combined Sérsic plus exponential 2D profile, and find that the characteristic scale length of the Ly$α$ halo is ten times larger than the UV, on average. We unveil a significant anti-correlation between $f_{\rm esc}^{\rm LyC}$ and the Ly$α$ Halo Fraction (HF, or contribution of the halo to the total Ly$α$ luminosity), that we propose as a new LyC indicator. Our observations show that halo scale lengths and HFs both scale positively with the optical depth of the neutral gas in the ISM, revealing a picture in which Ly$α$ and LyC photons in LCEs either emerge directly from the central starbursts or escape isotropically and, in the case of Ly$α$, minimize the number of scattering interactions in a less-extended CGM.

astro-ph.GA↗

arXiv2024

Ubiquitous broad-line emission and the relation between ionized gas outflows and Lyman continuum escape in Green Pea galaxies

We report observational evidence of highly turbulent ionized gas kinematics in a sample of 20 Lyman continuum (LyC) emitters (LCEs) at low redshift ($z\sim 0.3$). Detailed Gaussian modeling of optical emission line profiles in high-dispersion spectra consistently shows that both bright recombination and collisionally excited lines can be fitted as one or two narrow components with intrinsic velocity dispersion of $σ$ $\sim$ 40-100 km s$^{-1}$, in addition to a broader component with $σ\sim$ 100-300 km s$^{-1}$, which contributes up to $\sim$40% of the total flux and is preferentially blueshifted from the systemic velocity. We interpret the narrow emission as highly ionized gas close to the young massive star clusters and the broader emission as a signpost of unresolved ionized outflows, resulting from massive stars and supernova feedback. We find a significant correlation between the width of the broad emission and the LyC escape fraction, with strong LCEs exhibiting more complex and broader line profiles than galaxies with weaker or undetected LyC emission. We provide new observational evidence supporting predictions from models and simulations; our findings suggest that gas turbulence and outflows resulting from strong radiative and mechanical feedback play a key role in clearing channels through which LyC photons escape from galaxies. We propose that the detection of blueshifted broad emission in the nebular lines of compact extreme emission-line galaxies can provide a new indirect diagnostic of Lyman photon escape, which could be useful to identify potential LyC leakers in the epoch of reionization with the JWST.

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↗

arXiv2017

Dense CO in Mrk 71-A: Superwind Suppressed in a Young Super Star Cluster

We report the detection of CO(J=2-1) coincident with the super star cluster (SSC) Mrk 71-A in the nearby Green Pea analog galaxy, NGC 2366. Our NOEMA observations reveal a compact, ~7 pc, molecular cloud whose mass (10^5 M_sun) is similar to that of the SSC, consistent with a high star-formation efficiency, on the order of 0.5. There are two, spatially distinct components separated by 11 km/s. If expanding, these could be due to momentum-driven, stellar wind feedback. Alternatively, we may be seeing the remnant infalling, colliding clouds responsible for triggering the SSC formation. The kinematics are also consistent with a virialized system. These extreme, high-density, star-forming conditions inhibit energy-driven feedback; the co-spatial existence of a massive, molecular cloud with the SSC supports this scenario, and we quantitatively confirm that any wind-driven feedback in Mrk 71-A is momentum-driven, rather than energy-driven. Since Mrk 71-A is a candidate Lyman continuum emitter, this implies that energy-driven superwinds may not be a necessary condition for the escape of ionizing radiation. In addition, the detection of the nebular continuum emission yields an accurate astrometric position for the Mrk 71-A. We also detect four other massive, molecular clouds in this giant star-forming complex.

astro-ph.GA↗

arXiv2015

From HI to Stars: HI Depletion in Starbursts and Star-Forming Galaxies in the ALFALFA H-alpha Survey

HI in galaxies traces the fuel for future star formation and reveals the effects of feedback on neutral gas. Using a statistically uniform, HI-selected sample of 565 galaxies from the ALFALFA H-alpha survey, we explore HI properties as a function of star formation activity. ALFALFA H-alpha provides R-band and H-alpha imaging for a volume-limited subset of the 21-cm ALFALFA survey. We identify eight starbursts based on H-alpha equivalent width and six with enhanced star formation relative to the main sequence. Both starbursts and non-starbursts have similar HI to stellar mass ratios (MHI/M*), which suggests that feedback is not depleting the starbursts' HI. Consequently, the starbursts do have shorter HI depletion times (t_dep), implying more efficient HI-to-H2 conversion. While major mergers likely drive this enhanced efficiency in some starbursts, the lowest mass starbursts may experience periodic bursts, consistent with enhanced scatter in t_dep at low M*. Two starbursts appear to be pre-coalescence mergers; their elevated MHI/M* suggest that HI-to-H2 conversion is still ongoing at this stage. By comparing with the GASS sample, we find that t_dep anti-correlates with stellar surface density for disks, while spheroids show no such trend. Among early-type galaxies, t_dep does not correlate with bulge-to-disk ratio; instead, the gas distribution may determine the star formation efficiency. Finally, the weak connection between galaxies' specific star formation rates and MHI/M* contrasts with the well-known correlation between MHI/M* and color. We show that dust extinction can explain the HI-color trend, which may arise from the relationship between M*, MHI, and metallicity.

astro-ph.GA↗

arXiv2014

Linking Ly-alpha and Low-Ionization Transitions at Low Optical Depth

We suggest that low optical depth in the Lyman continuum (LyC) may relate the Ly-alpha emission, C II and Si II absorption, and C II* and Si II* emission seen in high-redshift galaxies. We base this analysis on Hubble Space Telescope COS spectra of four Green Pea (GP) galaxies, which may be analogs of z>2 Ly-alpha emitters (LAEs). In the two GPs with the strongest Ly-alpha emission, the Ly-alpha line profiles show reduced signs of resonant scattering. Instead, the Ly-alpha profiles resemble the H-alpha line profiles of evolved star ejecta, suggesting that the Ly-alpha emission originates from a low column density and similar outflow geometry. The weak C II absorption and presence of non-resonant C II* emission in these GPs support this interpretation and imply a low LyC optical depth along the line of sight. In two additional GPs, weak Ly-alpha emission and strong C II absorption suggest a higher optical depth. These two GPs differ in their Ly-alpha profile shapes and C II* emission strengths, however, indicating different inclinations of the outflows to our line of sight. With these four GPs as examples, we explain the observed trends linking Ly-alpha, C II, and C II* in stacked LAE spectra, in the context of optical depth and geometric effects. Specifically, in some galaxies with strong Ly-alpha emission, a low LyC optical depth may allow Ly-alpha to escape with reduced scattering. Furthermore, C II absorption, C II* emission, and Ly-alpha profile shape can reveal the optical depth, constrain the orientation of neutral outflows in LAEs, and identify candidate LyC emitters.

astro-ph.GA↗

arXiv2014

The Origin and Optical Depth of Ionizing Photons in the Green Pea Galaxies

Our understanding of radiative feedback and star formation in galaxies at high redshift is hindered by the rarity of similar systems at low redshift. However, the recently identified Green Pea (GP) galaxies are similar to high-redshift galaxies in their morphologies and star formation rates and are vital tools for probing the generation and transmission of ionizing photons. The GPs contain massive star clusters that emit copious amounts of high-energy radiation, as indicated by intense [OIII] 5007 emission and HeII 4686 emission. We focus on six GP galaxies with high ratios of [O III] 5007,4959/[O II] 3727 ~10 or more. Such high ratios indicate gas with a high ionization parameter or a low optical depth. The GP line ratios and ages point to chemically homogeneous massive stars, Wolf-Rayet stars, or shock ionization as the most likely sources of the He II emission. Models including shock ionization suggest that the GPs may have low optical depths, consistent with a scenario in which ionizing photons escape along passageways created by recent supernovae. The GPs and similar galaxies can shed new light on cosmic reionization by revealing how ionizing photons propagate from massive star clusters to the intergalactic medium.

astro-ph.GA↗

arXiv2014

Ionization by Massive Young Clusters as Revealed by Ionization-Parameter Mapping

Ionization-parameter mapping (IPM) is a powerful technique for tracing the optical depth of Lyman continuum radiation from massive stars. Using narrow-band line-ratio maps, we examine trends in radiative feedback from ordinary HII regions of the Magellanic Clouds and nearby starburst galaxies. We find that the aggregate escape fraction for the Lyman continuum is sufficient to ionize the diffuse, warm ionized medium in the Magellanic Clouds, and that more luminous nebulae are more likely to be optically thin. We apply ionization-parameter mapping to entire starburst galaxies, revealing ionization cones in two nearby starbursts. Within the limits of our small sample, we examine the conditions for the propagation of ionizing radiation beyond the host galaxies.

astro-ph.GA↗

arXiv2013

The Optical Depth of H II Regions in the Magellanic Clouds

We exploit ionization-parameter mapping as a powerful tool to measure the optical depth of star-forming HII regions. Our simulations using the photoionization code CLOUDY and our new, SURFBRIGHT surface brightness simulator demonstrate that this technique can directly diagnose most density-bounded, optically thin nebulae using spatially resolved emission line data. We apply this method to the Large and Small Magellanic Clouds, using the data from the Magellanic Clouds Emission Line Survey. We generate new HII region catalogs based on photoionization criteria set by the observed ionization structure in the [SII]/[OIII] ratio and Ha surface brightness. The luminosity functions from these catalogs generally agree with those from Ha-only surveys. We then use ionization-parameter mapping to crudely classify all the nebulae into optically thick vs optically thin categories, yielding fundamental new insights into Lyman continuum radiation transfer. We find that in both galaxies, the frequency of optically thin objects correlates with Ha luminosity, and that the numbers of these objects dominate above log L\geq37.0. The frequencies of optically thin objects are 40% and 33% in the LMC and SMC, respectively. Similarly, the frequency of optically thick regions correlates with H I column density, with optically thin objects dominating at the lowest N(HI). The integrated escape luminosity of ionizing radiation is dominated by the largest regions, and corresponds to luminosity-weighted, ionizing escape fractions from the H II region population of \geq0.42 and \geq0.40 in the LMC and SMC, respectively. These values correspond to global galactic escape fractions of 4% and 11%, respectively. This is sufficient to power the ionization rate of the observed diffuse ionized gas in both galaxies. Our results suggest the possibility of significant galactic escape fractions of Lyman continuum radiation.

astro-ph.CO↗

arXiv2013

The Origin and Optical Depth of Ionizing Radiation in the "Green Pea" Galaxies

Although Lyman continuum (LyC) radiation from star-forming galaxies likely drove the reionization of the Universe, observations of star-forming galaxies at low redshift generally indicate low LyC escape fractions. However, the extreme [O III]/[O II] ratios of the z=0.1-0.3 Green Pea galaxies may be due to high escape fractions of ionizing radiation. To analyze the LyC optical depths and ionizing sources of these rare, compact starbursts, we compare nebular photoionization and stellar population models with observed emission lines in the Peas' SDSS spectra. We focus on the six most extreme Green Peas, the galaxies with the highest [O III]/[O II] ratios and the best candidates for escaping ionizing radiation. The Balmer line equivalent widths and He I λ3819 emission in the extreme Peas support young ages of 3-5 Myr, and He II λ4686 emission in five extreme Peas signals the presence of hard ionizing sources. Ionization by active galactic nuclei or high-mass X-ray binaries is inconsistent with the Peas' line ratios and ages. Although stacked spectra reveal no Wolf-Rayet (WR) features, we tentatively detect WR features in the SDSS spectra of three extreme Peas. Based on the Peas' ages and line ratios, we find that WR stars, chemically homogeneous O stars, or shocks could produce the observed He II emission. If hot stars are responsible, the Peas' optical depths are ambiguous. However, accounting for emission from shocks lowers the inferred optical depth and suggests that the Peas may be optically thin. The Peas' ages likely optimize the escape of Lyman-continuum radiation; they are old enough for supernovae and stellar winds to reshape the interstellar medium, but young enough to possess large numbers of UV-luminous O or WR stars.

astro-ph.CO↗

arXiv2011

Observational Constraints on Superbubble X-ray Energy Budgets

The hot, X-ray-emitting gas in superbubbles imparts energy and enriched material to the interstellar medium (ISM) and generates the hot ionized medium, the ISM's high-temperature component. The evolution of superbubble energy budgets is not well understood, however, and the processes responsible for enhanced X-ray emission in superbubbles remain a matter of debate. We present Chandra ACIS-S observations of two X-ray-bright superbubbles in the Large Magellanic Cloud (LMC), DEM L50 (N186) and DEM L152 (N44), with an emphasis on disentangling the true superbubble X-ray emission from non-related diffuse emission and determining the spatial origin and spectral variation of the X-ray emission. An examination of the superbubble energy budgets shows that on the order of 50% of the X-ray emission comes from regions associated with supernova remnant (SNR) impacts. We find some evidence of mass-loading due to swept-up clouds and metallicity enrichment, but neither mechanism provides a significant contribution to the X-ray luminosities. We also find that one of the superbubbles, DEM L50, is likely not in collisional ionization equilibrium. We compare our observations to the predictions of the standard Weaver et al. model and to 1-D hydrodynamic simulations including cavity supernova impacts on the shell walls. Our observations show that mass-loading due to thermal evaporation from the shell walls and SNR impacts are the dominant source of enhanced X-ray luminosities in superbubbles. These two processes should affect most superbubbles, and their contribution to the X-ray luminosity must be considered when determining the energy available for transport to the ISM.

astro-ph.GA↗