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We study, from both a theoretical and observational perspective, the physical origin and spectroscopic impact of extreme nebular emission in high-redshift galaxies. The nebular continuum, which can appear during extreme starbursts, is of particular importance as it tends to redden UV slopes and has a significant contribution to the UV luminosities of galaxies. Furthermore, its shape can be used to infer the gas density and temperature of the ISM. First, we provide a theoretical background, showing how different stellar populations (SPS models, IMFs, and stellar temperatures) and nebular conditions impact observed galaxy spectra. We demonstrate that, for systems with strong nebular continuum emission, 1) UV fluxes can increase by up to 0.7~magnitudes (or more in the case of hot/massive stars) above the stellar continuum, which may help reconcile the surprising abundance of bright high-redshift galaxies and the elevated UV luminosity density at $z>10$, 2) at high gas densities, UV slopes can redden from $\beta\lesssim-2.5$ to $\beta\sim-1$, 3) observational measurements of $\xi_{ion}$ are grossly underestimated, and 4) UV downturns from two-photon emission can masquerade as DLAs. Second, we present a dataset of 58 galaxies observed with NIRSpec on JWST at $2.510$ galaxies, finding that UV slopes and UV downturns are in some cases redder or steeper than expected from SPS models, which may hint at more exotic (e.g. hotter/more massive stars or AGN) ionizing sources., Comment: 26 pages, 18 figures, submitted to The Open Journal of Astrophysics

Transit spectroscopy is one of the most commonly used techniques for exoplanet atmosphere characterisation. This technique has been used to detect ionized and neutral species in exoplanet atmospheres by comparing the observed stellar lines in and out of transit. The centre-to-limb variation (CLV) of the stellar lines across the stellar disk is an important effect for transmission spectroscopy, since it results in a change of stellar line depth when the planet transits different parts of the stellar disk. We reanalyse the transit data of HD 189733b taken with the HARPS spectrograph to study the CLV effect during transit. The transmission light curve of the Na i D line so obtained shows a clear imprint of the CLV effect. We use a one-dimensional non-LTE stellar spectral model to simulate the CLV effect. After applying the correction, the measurement of the Na i absorption in the atmosphere of HD 189733b becomes better determined. We compare the CLV effect of HD 189733b to that of HD 209458b. The CLV effects are different for these two benchmark planetary systems and this is attributed to their different stellar effective temperatures and transit impact parameters. We then explore the general CLV effect that occurs during exoplanet transits. Normally, a star with a lower effective temperature exhibits a stronger CLV effect and its CLV feature extends over a relatively broad wavelength range. The transit impact parameter (b) describes the transit trajectory on the stellar disk and thus determines the actual manifestation of the CLV effect. We introduce a b-diagram which describes the behavior of the CLV effect as the function of different impact parameters. With improving observational precision, a careful modeling and correction of the CLV effect is necessary for exoplanet atmosphere characterisation using transit spectroscopy., Comment: Accepted for publishing on A&A

Due to stellar rotation, the observed radial velocity of a star varies during the transit of a planet across its surface, a phenomenon known as the Rossiter-McLaughlin (RM) effect. The amplitude of the RM effect is related to the radius of the planet which, because of differential absorption in the planetary atmosphere, depends on wavelength. Therefore, the wavelength-dependent RM effect can be used to probe the planetary atmosphere. We measure for the first time the RM effect of the Earth transiting the Sun using a lunar eclipse observed with the ESO HARPS spectrograph. We analyze the observed RM effect at different wavelengths to obtain the transmission spectrum of the Earth's atmosphere after the correction of the solar limb-darkening and the convective blueshift. The ozone Chappuis band absorption as well as the Rayleigh scattering features are clearly detectable with this technique. Our observation demonstrates that the RM effect can be an effective technique for exoplanet atmosphere characterization. Its particular asset is that photometric reference stars are not required, circumventing the principal challenge for transmission spectroscopy studies of exoplanet atmospheres using large ground-based telescopes., Comment: 9 pages, 4 figures, 1 table, published on ApJL

The atmospheres of exoplanets are commonly studied by observing the transit of the planet passing in front of its parent star. The obscuration of part of the stellar disk during a transit will reveal aspects of its surface structure resulting from general centre-to-limb variations (CLVs). These become apparent when forming the ratio between the stellar light in and out of transit. These phenomena can be seen particularly clearly during the progress of a penumbral lunar eclipse, where the Earth transits the solar disk and masks different regions of the solar disk as the eclipse progresses. When inferring the properties of the planetary atmosphere, it is essential that this effect originating at the star is properly accounted for. Using the data observed from the 2014-April-15 lunar eclipse with the ESPaDOnS spectrograph mounted on the Canada France Hawaii Telescope (CFHT), we have obtained for the first time a time sequence of the penumbral spectra. These penumbral spectra enable us to study the centre-to-limb variations of solar Fraunhofer lines when the Earth is transiting Sun. The Na i and Ca ii absorption features reported from previous lunar eclipse observations are demonstrated to be CLV features, which dominate the corresponding line profiles and mask possible planetary signal. Detecting atmospheric species in exoplanets via transit spectroscopy must account for the CLV effect., Comment: 9 pages, 11 figures, accepted, A&A

With the rapid developments in the exoplanet field, more and more terrestrial exoplanets are being detected. Characterising their atmospheres using transit observations will become a key datum in the quest for detecting an Earth-like exoplanet. The atmospheric transmission spectrum of our Earth will be an ideal template for comparison with future exo-Earth candidates. By observing a lunar eclipse, which offers a similar configuration to that of an exoplanet transit, we have obtained a high resolution and high signal-to-noise ratio transmission spectrum of the Earth's atmosphere. This observation was performed with the High Resolution Spectrograph at Xinglong Station, China during the total lunar eclipse in December 2011. We compare the observed transmission spectrum with our atmospheric model, and determine the characteristics of the various atmospheric species in detail. In the transmission spectrum, O2, O3, O2-O2, NO2 and H2O are detected, and their column densities are measured and compared with the satellites data. The visible Chappuis band of ozone produces the most prominent absorption feature, which suggests that ozone is a promising molecule for the future exo-Earth characterization. The individual O2 lines are resolved and O2 isotopes are clearly detected. Our new observations do not confirm the absorption features of Ca II or Na I which have been reported in previous lunar eclipse observations. However, features in these and some other strong Fraunhofer line positions do occur in the observed spectrum. We propose that these are due to a Raman-scattered component in the forward-scattered sunlight appearing in the lunar umbral spectrum. Water vapour absorption is found to be rather weak in our spectrum because the atmosphere we probed is relatively dry, which prompts us to discuss the detectability of water vapour in Earth-like exoplanet atmospheres., Comment: 12 pages, 15 figures, accepted, International Journal of Astrobiology

Lyman-break galaxies are now regularly found in the high redshift Universe by searching for the break in the galaxy spectrum caused by the Lyman-limit redshifted into the optical or even near-IR. At lower redshift, this break is covered by the GALEX UV channels and small samples of z ~ 1 LBGs have been presented in the literature. Here we give results from fitting the spectral energy distributions of a small sub-set of low redshift LBGs and demonstrate the advantage of including photometric points derived from HST ACS slitless grism observations. The results show these galaxies to have very young, star forming populations, while still being massive and dusty. LBGs at low and high redshift show remarkable similarities in their properties, indicating that the LBG selection method picks similar galaxies throughout the Universe., Comment: 7 pages, 3 figures, 2 tables, accepted in A&A

[abridged] From a sample of Ly-alpha emitters in the GOODS-S field with uncontaminated photometry and optical (red) spectroscopy, we select a spatially compact object at a redshift of 5.563 (Ly-alpha) that shows a second emission line, identified as N IV] 1486 A. The SED is modelled in a way that accounts for both the N IV] line emission and the photometry in a self-consistent way. The photoionization code CLOUDY is used to calculate a range of nebular models as a function of stellar ionizing source temperature, ionization parameter, density and nebular metallicity. We compare the theoretical and observed magnitudes and search for the model parameters that also reproduce the observed N IV] luminosity and equivalent width. A nebular model with a hot blackbody ionizing source of around 100 kK and a nebular metallicity of ~5% of solar is able to fit the observed SED and, in particular, explain the large apparent Balmer break which is inferred from the pure stellar population model fitting conventionally applied to multi-band photometric observations. In our model, an apparent spectral break is produced by strong [O III] 4959, 5007 A emission falling in one of the IR bands (IRAC1 in this case). A lower limit on the total baryonic mass of a model of this type is 3.2 x 10^8 Msun . It is argued that objects with Ly-alpha emission at high redshift that show an apparent Balmer break may have their SED dominated by nebular emission and so could possibly be identified with very young starbursting galaxies rather than massive evolved stellar populations. Detailed studies of these emission nebulae with large telescopes will provide a unique insight into very early chemical evolution., Comment: 9 pages, 6 figures, A & A accepted

From optical spectropolarimetry of Cygnus A we conclude that the scattering medium in the ionization cones in Cygnus A is moving outward at a speed of 170+-34 km/s, and that the required momentum can be supplied by the radiation pressure of an average quasar. Such a process could produce a structure resembling the observed ionization cones, which are thought to result from shadowing by a circumnuclear dust torus. We detect a polarized red wing in the [O III] emission lines arising from the central kiloparsec of Cygnus A. This wing is consistent with line emission created close to the boundary of the broad-line region., Comment: 5 pages, accepted for publication in MNRAS letters

We present far-infrared and millimetric observations of a sample of 3C objects. Millimetre data were taken at 1.25 mm with the IRAM 30m antenna feeding the MPIfR bolometer array. Mid-infrared (MIR) and far-infrared (FIR) photometry were carried out with the ISOCAM and ISOPHOT cameras on the ISO Satellite. Additional FIR IRAS observations are also included. We present the entire Spectral Energy Distributions (SEDs) from the UV to radio and discuss the emitting mechanisms. Two composite spectra, one for the radio galaxies and one for the radio quasars, are built in the object rest frame. While the SEDs of the two classes differ, they are indistinguishable in the MIR and FIR range where they probably arise as thermal emission from a dusty torus and a larger-scale (cooler) dust distribution in the host galaxy., Comment: 15 pages, A&A, in press

We present the results of deep spectropolarimetry of two powerful radio galaxies at $z\sim2.5$ (4C 00.54 and 4C 23.56) obtained with the W.M. Keck II 10m telescope, aimed at studying the relative contribution of the stellar and non-stellar components to the ultraviolet continuum. Both galaxies show strong linear polarization of the continuum between rest-frame $\sim$1300-2000~\AA, and the orientation of the electric vector is perpendicular to the main axis of the UV continuum. In this sense, our objects are like most 3C radio galaxies at $z\sim1$. The total flux spectra of 4C 00.54 and 4C 23.56 do not show the strong P-Cygni absorption features or the photospheric absorption lines expected when the UV continuum is dominated by young and massive stars. The only features detected can be ascribed to interstellar absorptions by SiII, CII and OI. Our results are similar to those for 3C radio galaxies at lower $z$, suggesting that the UV continuum of powerful radio galaxies at $z\sim2.5$ is still dominated by non-stellar radiation, and that young massive stars do not contribute more than $\approx$50% to the total continuum flux at 1500~\AA., Comment: 17 pages, ApJ Letters, in press, 5 figures, 2 tables

The installation of the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) allows for the first time two-dimensional optical and ultraviolet slitless spectroscopy of faint objects from space. The STIS Parallel Survey (SPS) routinely obtains broad band images and slitless spectra of random fields in parallel with HST observations using other instruments. The SPS is designed to study a wide variety of astrophysical phenomena, including the rate of star formation in galaxies at intermediate to high redshift through the detection of emission-line galaxies. We present the first results of the SPS, which demonstrate the capability of STIS slitless spectroscopy to detect and identify high-redshift galaxies.