Using Lyman-alpha to detect galaxies that leak Lyman continuum

We propose to infer ionising continuum leaking properties of galaxies by looking at their Lyman-alpha line profiles. We carry out Lyman-alpha radiation transfer calculations in two models of HII regions which are porous to ionising continuum escape: 1) the so-called "density bounded" media, in which massive stars produce enough ionising photons to keep the surrounding interstellar medium transparent to the ionising continuum, i.e almost totally ionised, and 2) "riddled ionisation-bounded" media, surrounded by neutral interstellar medium, but with holes, i.e. with a covering factor lower than unity. The Lyman-alpha spectra emergent from these configurations have distinctive features: 1) a "classical" asymmetric redshifted profile in the first case, but with a small shift of the maximum of the profile compare to the systemic redshift (Vpeak < 150 km/s); 2) a main peak at the systemic redshift in the second case (Vpeak = 0 km/s), with, as a consequence, a non-zero Lyman-alpha flux bluewards the systemic redshift. Assuming that in a galaxy leaking ionising photons, the Lyman-alpha component emerging from the leaking star cluster(s) dominates the total Lyman-alpha spectrum, the Lyman-alpha shape may be used as a pre-selection tool to detect Lyman continuum (LyC) leaking galaxies, in objects with well determined systemic redshift, and high spectral resolution Lyman-alpha spectra (R >= 4000). The examination of a sample of 10 local starbursts with high resolution HST-COS Lyman-alpha spectra and known in the literature as LyC leakers or leaking candidates, corroborates our predictions. Observations of Lyman-alpha profiles at high resolution should show definite signatures revealing the escape of Lyman continuum photons from star-forming galaxies.Comment: A&A in pres

Detection of high Lyman continuum leakage from four low-redshift compact star-forming galaxies

Following our first detection reported in Izotov et al. (2016), we present the detection of Lyman continuum (LyC) radiation of four other compact star-forming galaxies observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST). These galaxies, at redshifts of z~0.3, are characterized by high emission-line flux ratios [OIII]5007/[OII]3727 > 5. The escape fractions of the LyC radiation fesc(LyC) in these galaxies are in the range of ~6%-13%, the highest values found so far in low-redshift star-forming galaxies. Narrow double-peaked Lyalpha emission lines are detected in the spectra of all four galaxies, compatible with predictions for Lyman continuum leakers. We find escape fractions of Lyalpha, fesc(Lyalpha) ~20%-40%, among the highest known for Lyalpha emitters (LAEs). Surface brightness profiles produced from the COS acquisition images reveal bright star-forming regions in the center and exponential discs in the outskirts with disc scale lengths alpha in the range ~0.6-1.4 kpc. Our galaxies are characterized by low metallicity, ~1/8-1/5 solar, low stellar mass ~(0.2 - 4)e9 Msun, high star formation rates SFR~14-36 Msun/yr, and high SFR densities Sigma~2-35 Msun/yr/kpc^2. These properties are comparable to those of high-redshift star-forming galaxies. Finally, our observations, combined with our first detection reported in Izotov et al. (2016), reveal that a selection for compact star-forming galaxies showing high [OIII]5007/[OII]3727 ratios appears to pick up very efficiently sources with escaping Lyman continuum radiation: all five of our selected galaxies are LyC leakers.Comment: 21 pages, 14 figures, accepted for publication in MNRAS; corrected Lyalpha escape fraction

Lyman-alpha spectral properties of five newly discovered Lyman continuum emitters

We have recently reported the discovery of five low redshift Lyman continuum (LyC) emitters (LCEs, hereafter) with absolute escape fractions fesc(LyC) ranging from 6 to 13%, higher than previously found, and which more than doubles the number of low redshift LCEs.We use these observations to test theoretical predictions about a link between the characteristics of the Lyman-alpha (Lya) line from galaxies and the escape of ionising photons. We analyse the Lya spectra of eight LCEs of the local Universe observed with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope (our five leakers and three galaxies from the litterature), and compare their strengths and shapes to the theoretical criteria and comparison samples of local galaxies: the Lyman Alpha Reference Survey, Lyman Break Analogs, Green Peas, and the high-redshift strong LyC leaker Ion2. Our LCEs are found to be strong Lya emitters, with high equivalent widths, EW(Lya)> 70 {\AA}, and large Lya escape fractions, fesc(Lya) > 20%. The Lya profiles are all double-peaked with a small peak separation, in agreement with our theoretical expectations. They also have no underlying absorption at the Lya position. All these characteristics are very different from the Lya properties of typical star-forming galaxies of the local Universe. A subset of the comparison samples (2-3 Green Pea galaxies) share these extreme values, indicating that they could also be leaking. We also find a strong correlation between the star formation rate surface density and the escape fraction of ionising photons, indicating that the compactness of star-forming regions plays a role in shaping low column density paths in the interstellar medium of LCEs. The Lya properties of LCEs are peculiar: Lya can be used as a reliable tracer of LyC escape from galaxies, in complement to other indirect diagnostics proposed in the literature.Comment: 11 pages, 10 figures, accepted for publication in A&

J1154+2443: a low-redshift compact star-forming galaxy with a 46 per cent leakage of Lyman continuum photons

We report the detection of the Lyman continuum (LyC) radiation of the compact star-forming galaxy (SFG) J1154+2443 observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope. This galaxy, at a redshift of z=0.3690, is characterized by a high emission-line flux ratio O32=[OIII]5007/[OII]3727=11.5. The escape fraction of the LyC radiation fesc(LyC) in this galaxy is 46 per cent, the highest value found so far in low-redshift SFGs and one of the highest values found in galaxies at any redshift. The narrow double-peaked Lya emission line is detected in the spectrum of J1154+2443 with a separation between the peaks Vsep of 199 km/s, one of the lowest known for Lya-emitting galaxies, implying a high fesc(Lya). Comparing the extinction-corrected Lya/Hb flux ratio with the case B value we find fesc(Lya) = 98 per cent. Our observations, combined with previous detections in the literature, reveal an increase of O32 with increasing fesc(LyC). We also find a tight anticorrelation between fesc(LyC) and Vsep. The surface brightness profile derived from the COS acquisition image reveals a bright star-forming region in the centre and an exponential disc in the outskirts with a disc scale length alpha=1.09 kpc. J1154+2443, compared to other known low-redshift LyC leakers, is characterized by the lowest metallicity, 12+logO/H=7.65+/-0.01, the lowest stellar mass M*=10^8.20 Msun, a similar star formation rate SFR=18.9 Msun/yr and a high specific SFR of 1.2x10^-7 yr^-1.Comment: 16 pages, 8 figures, accepted for publication in MNRAS. arXiv admin note: text overlap with arXiv:1605.0516

Urban Sprawl in Europe. Joint EEA-FOEN report. No 11/2016.

Executive summary Urban sprawl is associated with a number of ecological, economic and social effects. Some of these relate to people's desires, for example, to live in single-family homes with gardens. However, urban sprawl has detrimental and long-lasting effects. For example, urban sprawl contributes significantly to the loss of fertile farmland, to soil sealing and to the loss of ecological soil functions. The increase in built-up areas reduces the size of wildlife habitats and increases landscape fragmentation and the spread of invasive species. Urban sprawl leads to higher greenhouse gas emissions, higher infrastructure costs for transport, water and electrical power, the loss of open landscapes, and the degradation of various ecosystem services. Despite various efforts to address this problem, urban sprawl has increased rapidly in Europe in recent decades. Thus, urban sprawl presents a major challenge with regard to sustainable land use, as the International Year of Soils 2015 highlighted. Sprawl is a result not only of population growth but also of lifestyles that take up more space. Accordingly, urban sprawl has increased even in regions with a declining human population. Many more urban development and transport infrastructure projects are planned for the future, in particular in the European Union (EU) Member States which joined after 2004. Consequently, further increases in urban sprawl in the future will be significant. Therefore, consistent data on the degree of urban sprawl are needed, particularly data that are suitable for the comparison of regions across Europe. This report investigates the degree of urban sprawl in 32 countries in Europe by considering two points in time (2006 and 2009) at three levels. The three levels include the country level, the NUTS-2 region level (based on the Nomenclature of Territorial Units for Statistics (NUTS)) and the 1-km2 cell level (based on the Land and Ecosystem Accounting (LEAC) grid). The comparison of two points in time allowed an assessment of temporal changes in urban sprawl. This report applies the method of 'weighted urban proliferation' (WUP), which quantifies the degree of urban sprawl for any given landscape through a combination of three components: (1) the size of the built-up areas; (2) the spatial configuration (dispersion) of the built-up areas in the landscape; and (3) the uptake of built-up area per inhabitant or job. The report provides, for the first time, an assessment of urban sprawl in all EU and European Free Trade Association (EFTA) countries using the WUP method. The urban sprawl values obtained cover a large range, from low values for large parts of Scandinavia ( 4 UPU/m2) and very high values for large parts of western and central Europe (> 6 UPU/m2). The two largest clusters of high-sprawl values in Europe are located in (1) north-eastern France, Belgium, the Netherlands and part of western Germany; and (2) in the United Kingdom between London and the Midlands. The analysis of sprawl at the 1-km2-grid level shows that sprawl is most pronounced in wide rings around city centres, along large transport corridors, and along many coastlines (particularly in the Mediterranean countries). The lowest levels of sprawl are mainly associated with mountain ranges or remote areas. The level of sprawl, as measured by WUP, increased in all European countries between 2006 and 2009. The overall WUP value for Europe (all 32 countries combined) increased from 1.56 urban permeation units (UPU)/m2 in 2006 to 1.64 UPU/m2 in 2009, that is by 5 % in 3 years or by 1.7 % per year. In most countries, the increase was higher than 1 % per year, and in many countries WUP increased by more than 2 % per year. This was also the case for most NUTS-2 regions. Future studies using additional time-points will allow more detailed temporal comparisons. Base data for 2012 will be available in 2016 and these could be analysed in a follow-up project. Driving forces and predictive models of urban sprawl The level of urban sprawl is largely a function of socio-economic and demographic drivers, and the geophysical context. Current levels of urban sprawl need to be interpreted within the context of regional socio-economic and geophysical conditions. Therefore, the second part of this study investigated the potential factors that may contribute to an increase or decrease in the degree of urban sprawl, and determined their relative importance. The report applied a set of statistical models to determine which of these factors drive the process of urban sprawl in Europe. We analysed the statistical relationships between urban sprawl and a range of explanatory variables (14 variables at the country level and 12 at the NUTS-2 level). We also applied these relationships to predict the expected sprawl values for all regions in our study area and compared actual values with predicted values. Most of our hypotheses about the likely driving forces of urban sprawl were confirmed by the statistical analyses. The relevant variables identified as affecting urban sprawl are population density, road density, railway density, household size, governmental effectiveness, the number of cars per 1 000 inhabitants and two environmental factors (i.e. net primary production and relief energy). This result was consistent for both of the years (2006 and 2009) considered in the analysis. The results indicate that economic development has, largely, not been decoupled from increases in urban sprawl. A high amount of variation in the level of urban sprawl, as measured by WUP, was explained by the predictor variables: 72–80 % at the country level and 80–81 % at the NUTS-2 level. The variation explained for the three components of WUP ranged between 67 % and 94 % at the NUTS-2 level. Efforts to control urban sprawl should take these driving forces into account. Relevance for monitoring and policymaking The results provided by this study are intended to contribute to more sustainable political decision- making and planning throughout Europe. In the last 15 years (2000–2015), several projects and programmes at the European level have proposed a suite of concepts and measures to address urban sprawl and promote more sustainable land use. The most recent (2014), and perhaps most important, of these is the Seventh Environment Action Programme (7EAP), which calls for indicators of resource efficiency to be established in order to guide public and private decision-makers. Although the urgent challenge presented by urban sprawl has been recognised, there is still no monitoring in place for European urban sprawl. This report aims to help close this gap. The results confirm the conclusion of earlier reports (e.g. EEA, 2006a; EEA, 2006b) namely that there is an increasingly urgent need for action. Large discrepancies between the predicted and observed levels of urban sprawl provide a basis for identifying areas for prioritising management action. Our data also provide a basis for scenarios regarding the future development of urban sprawl in Europe. There is an increasing need and interest in including indicators of urban sprawl in systems for monitoring sustainable development, the state of the environment, biodiversity and landscape quality. The results presented in this report are intended for this purpose and can be updated on a regular basis in order to detect trends in urban sprawl. This report also demonstrates the usefulness of the WUP method as a tool for urban and regional planning and for performance review based on benchmarks, targets and limits. This study provides a comparable measurement of urban sprawl for most of the European continent using a consistent data set across Europe. The results will support managers and policymakers with the allocation of resources for the better protection of agricultural soils and landscape quality, and more sustainable political decision-making related to land use. The report also identifies the most immediate priorities and future research needs

Annexes 1-5: Urban Sprawl in Europe. Joint EEA-FOEN report. No 11/2016.

See main report "Urban Sprawl in Europe. Joint EEA-FOEN report"

Zersiedelung in Europa: Ländervergleich und treibende Kräfte. (Urban sprawl in Europe: Comparison of countries and driving forces.)

Steigende Zersiedelung steht im Widerspruch zu den Prinzipien und dem Geist von Nachhaltigkeit. Daten zur Zersiedelung werden benötigt, um die Wirksamkeit von Maßnahmen und Verordnungen zu überprüfen, welche die Zersiedelung begrenzen sollen. Die Autoren haben die Methode der „gewichteten Zersiedelung“ (Weighted Urban Proliferation, WUP) auf 32 Länder in Europa angewendet, um die Zersiedelung zu messen. Dazu wurden die europäischen HRL-Imperviousness-Daten von 2006 und 2009 verwendet. Die Ergebnisse zeigen, dass große Teile Europas von Zersiedelung betroffen sind. Der Gesamtwert von WUP für Europa beträgt 1,64 Durchsiedlungseinheiten/m2 (2009). Er ist zwischen 2006 und 2009 deutlich angestiegen, aber die Werte der einzel- nen Länder unterscheiden sich stark voneinander. Außerdem wurde der Zusammenhang mit zwölf potenziellen Treibern der Zersiedelung statistisch untersucht. Dieses Projekt ist die erste Analyse der Zersiedelung und ihrer zeitlichen Veränderung für einen gesamten Kontinent mit WUP. Der Beitrag stellt erste Ergebnisse vor und weist auf die Veröffentlichungen hin, die derzeit in Vorbereitung sind

The VIMOS Ultra Deep Survey: The role of HI kinematics and HI column density on the escape of Lyα photons in star-forming galaxies at 2 < z < 4

© 2017 ESO. Aims. We wish to assess the role of kinematics and neutral hydrogen column density in the escape and distribution of Lya photons. Methods. We selected a sample of 76 Lya emitting galaxies from the VIMOS Ultra Deep Survey (VUDS) at 2 = z = 4. We estimated the velocity of the neutral gas flowing out of the interstellar medium as the velocity offset, δv, between the systemic redshift (zsys) and the center of low-ionization absorption line systems (LIS). To increase the S/N of VUDS spectra, we stacked subsamples defined based on median values of their photometric and spectroscopic properties. We measured the systemic redshift from the rest-frame UV spectroscopic data using the CIII]1908 nebular emission line, and we considered SiII1526 as the highest signal-to-noise LIS line. We calculated the Lya peak shift with respect to the zsys, the EW(Lya), and the Lya spatial extension, Ext(Lya-C), from the in the 2D stacked spectra. Results. The galaxies that are faint in the rest-frame UV continuum, strong in Lya and CIII] , with compact UV morphology, and localized in an underdense environment are characterized by outflow velocities of the order of a few hundreds of km s -1 . The subsamples with smaller δv are characterized by larger Lya peak shifts, larger Ext(Lya-C), and smaller EW(Lya). In general we find that EW(Lya) anti-correlates with Ext(Lya-C) and Lya peak shift. Conclusions. We interpret these trends using a radiative-transfer shell model. The model predicts that an HI gas with a column density larger than 1020 cm -2 is able to produce Lya peak shifts larger than > 300 km s -1 . An ISM with this value of NHI would favour a large amount of scattering events, especially when the medium is static, so it can explain large values of Ext(Lya-C) and small EW(Lya). On the contrary, an ISM with a lower NHI, but large velocity outflows would lead to a Lya spatial profile peaked at the galaxy center (i.e. low values of Ext(Lya-C)) and to a large EW(Lya), as we see in our data. Our results and their interpretation via radiative-transfer models tell us that it is possible to use Lya to study the properties of the HI gas. Also, the fact that Lya emitters are characterized by large δv could give hints about their stage of evolution in the sense that they could be experiencing short bursts of star formation that push strong outflows.This work is supported by funding from the European Research Council Advanced Grant ERC–2010–AdG–268107–EARLY and by INAF Grants PRIN 2010, PRIN 2012, and PICS 2013. This work is based on data products made available at the CESAM data center, Laboratoire d’Astrophysique de Marseille, France. R.A. acknowledges support from the ERC Advanced Grant 695671 “QUENCH” and I.O. from the Czech Science Foundation grant 17-06217Y