Current data indicate that the reionization of the Universe was complete by redshift z~6-7, and while the sources responsible for this process have yet to be identified, star-forming galaxies are often considered the most likely candidates. However,
the contribution from z>6 galaxies to cosmic reionization critically depends on the fraction of ionizing (Lyman continuum, LyC) photons escaping from these objects and into the intergalactic medium. At z<4, the escaping LyC flux can be measured directly, but the opacity of the neutral intergalactic medium precludes such measurements at higher redshifts. In a recent paper, we argue that since the LyC escape fraction regulates the contribution of nebular emission to the rest-frame optical/UV spectra of galaxies, the James Webb Space Telescope should be able to indirectly assess the LyC escape fraction for galaxies at z~6-9. JWST can, on the other hand, not constrain the fraction of LyC photons directly absorbed by dust, and this is where SPICA comes in. The dust continuum emission from gravitationally lensed LyC-leakers at z=6 may in principle be detectable with SPICA, thereby constraining the level of LyC extinction in these objects.
The fraction of ionizing photons (fesc) that escape from z>6 galaxies is an important parameter when assessing the role of these objects in the reionization of the Universe, but the opacity of the intergalactic medium precludes a direct measurement o
f fesc for individual galaxies at these epochs. We argue, that since fesc regulates the impact of nebular emission on the spectra of galaxies, it should nonetheless be possible to indirectly probe fesc well into the reionization epoch. As a first step, we demonstrate that by combining measurements of the rest-frame UV slope beta with the equivalent width of the Hb emission line, galaxies with very high Lyman continuum escape fractions (fesc>0.5) should be identifiable up to z~9 through spectroscopy with the upcoming James Webb Space Telescope (JWST). By targeting strongly lensed galaxies behind low-redshift galaxy clusters, JWST spectra of sufficiently good quality can be obtained for M(1500)<-16.0 galaxies at z~7 and for M(1500)<-17.5 galaxies at z~9. Dust-obscured star formation may complicate the analysis, but supporting observations with ALMA or the planned SPICA mission may provide useful constraints on the dust properties of these galaxies.
Small galaxies consisting entirely of population III (pop III) stars may form at high redshifts, and could constitute one of the best probes of such stars. Here, we explore the prospects of detecting gravitationally lensed pop III galaxies behind the
galaxy cluster J0717.5+3745 (J0717) with both the Hubble Space Telescope (HST) and the upcoming James Webb Space Telescope (JWST). By projecting simulated catalogs of pop III galaxies at z~7-15 through the J0717 magnification maps, we estimate the lensed number counts as a function of flux detection threshold. We find that the ongoing HST survey CLASH, targeting a total of 25 galaxy clusters including J0717, potentially could detect a small number of pop III galaxies if ~1% of the baryons in these systems have been converted into pop III stars. Using JWST exposures of J0717, this limit can be pushed to ~0.1% of the baryons. Ultra-deep JWST observations of unlensed fields are predicted to do somewhat worse, but will be able to probe pop III galaxies with luminosities intermediate between those detectable in HST/CLASH and in JWST observations of J0717. We also explain how current measurements of the galaxy luminosity function at z=7-10 can be used to constrain pop III galaxy models with very high star formation efficiencies (~10% of the baryons converted into pop III stars).
Population III galaxies, made partly or exclusively of metal-free stars, are predicted to exist at high redshifts and may produce very strong Lya emission. A substantial fraction of these Lya photons are likely absorbed in the intergalactic medium at
z>6, but recent simulations suggest that significant Lya emission may be detectable up to z~8.5, i.e. well into the reionization epoch. Here, we argue that high-redshift population III galaxies with strong Lya emission can be identified in Hubble Space Telescope imaging data because of their unusual colours. We quantify this effect in some of the filters used in Y-band dropout searches for galaxies at z~8 and find that population III galaxies with high Lya fluxes may exhibit much bluer J-H colours at z=8-10 than any normal type of galaxy at these redshifts. This colour signature can arise even if pop III stars account for as little as ~1e-3 to ~1e-2 of the stellar mass in these galaxies. Some of the anomalously blue objects reported in current Y-band dropout samples do in fact meet the colour criteria for Lya-emitting population III galaxies.
The James Webb Space Telescope (JWST) is expected to revolutionize our understanding of the high-redshift Universe, and may be able to test the prediction that the first, chemically pristine (population III) stars formed with very high characteristic
masses. Since isolated population III stars are likely to be beyond the reach of JWST, small population III galaxies may offer the best prospects of directly probing the properties of metal-free stars. Here, we present Yggdrasil, a new spectral synthesis code geared towards the first galaxies. Using this model, we explore the JWST imaging detection limits for population III galaxies and investigate to what extent such objects may be identified based on their JWST colours. We predict that JWST should be able to detect population III galaxies with stellar population masses as low as ~10^5 Msolar at z=10 in ultra deep exposures. Over limited redshift intervals, it may also be possible to use colour criteria to select population III galaxy candidates for follow-up spectroscopy. The colours of young population III galaxies dominated by direct star light can be used to probe the stellar initial mass function (IMF), but this requires almost complete leakage of ionizing photons into the intergalactic medium. The colours of objects dominated by nebular emission show no corresponding IMF sensitivity. We also note that a clean selection of population III galaxies at z~7-8 can be achieved by adding two JWST/MIRI filters to the JWST/NIRCam filter sets usually discussed in the context of JWST ultra deep fields.
Red halos are faint, extended and extremely red structures that have been reported around various types of galaxies since the mid-1990s. The colours of these halos are too red to be reconciled with any hitherto known type of stellar population, and i
nstead indicative of a very bottom-heavy stellar initial mass function (IMF). Due to the large mass-to-light ratios of such stellar halos, they could contribute substantially to the baryonic masses of galaxies while adding very little to their overall luminosities. The red halos of galaxies therefore constitute potential reservoirs for some of the baryons still missing from inventories in the low-redshift Universe. While most studies of red halos have focused on disk galaxies, a red excess has also been reported in the faint outskirts of blue compact galaxies (BCGs). A bottom-heavy IMF can explain the colours of these structures as well, but due to model degeneracies, stellar populations with standard IMFs and abnormally high metallicities have also been demonstrated to fit the data. Here, we show that due to recent developments in the field of spectral synthesis, the metallicities required in this alternative scenario may be less extreme than previously thought. This suggests that the red excess seen in the outskirts of BCGs may stem from a normal, intermediate-metallicity host galaxy rather than a red halo of the type seen around disk galaxies. The inferred host metallicity does, however, still require the host to be more metal-rich than the gas in the central starburst of BCGs, in contradiction with current simulations of how BCGs form.
The cold dark matter scenario predicts that a large number of dark subhalos should be located within the halo of each Milky-way sized galaxy. One tell-tale signature of such dark subhalos could be additional milliarcsecond-scale image splitting of qu
asars previously known to be multiply-imaged on arcsecond scales. Here, we estimate the image separations for the subhalo density profiles favoured by recent N-body simulations, and compare these to the angular resolution of both existing and upcoming observational facilities. We find, that the image separations produced are very sensitive to the exact subhalo density profile assumed, but in all cases considerably smaller than previous estimates based on the premise that subhalos can be approximated by singular isothermal spheres. Only the most optimistic subhalo models produce image separations that would be detectable with current technology, and many models produce image separations that will remain unresolved with all telescopes expected to become available in the foreseeable future. Detections of dark subhalos through image-splitting effects will therefore be far more challenging than currently believed, albeit not necessarily impossible.
A substantial fraction of the light emitted from young or star-forming galaxies at ultraviolet to near-infrared wavelengths comes from the ionized interstellar medium in the form of emission lines and a nebular continuum. At high redshifts, star form
ation rates are on average higher and stellar populations younger than in the local Universe. Both of these effects act to boost the impact of nebular emission on the overall spectrum of galaxies. Even so, the broadband fluxes and colours of high-redshift galaxies are routinely analyzed under the assumption that the light observed originates directly from stars. Here, we assess the impact of nebular emission on broadband fluxes in Johnson/Cousins BVRIJHK, Sloan Digital Sky Survey griz and Spitzer IRAC/MIPS filters as a function of observed redshift (up to z=15) for galaxies with different star formation histories. We find that nebular emission may account for a non-negligible fraction of the light received from high-redshift galaxies. The ages and masses inferred for such objects through the use of spectral evolutionary models that omit the nebular contribution are therefore likely to contain systematic errors. We argue that a careful treatment of the nebular component will be essential for the interpretation of the rest-frame ultraviolet-to-infrared properties of the first galaxies formed, like the ones expected to be detected with the James Webb Space Telescope.
Deep optical/near-IR surface photometry of galaxies outside the Local Group have revealed faint and very red halos around objects as diverse as disk galaxies and starbursting dwarf galaxies. The colours of these structures are too extreme to be recon
ciled with stellar populations similar to those seen in the stellar halos of the Milky Way or M31, and alternative explanations like dust reddening, high metallicities or nebular emission are also disfavoured. A stellar population obeying an extremely bottom-heavy initial mass function (IMF), is on the other hand consistent with all available data. Because of its high mass-to-light ratio, such a population would effectively behave as baryonic dark matter and could account for some of the baryons still missing in the low-redshift Universe. Here, we give an overview of current red halo detections, alternative explanations for the origin of the red colours and ongoing searches for red halos around types of galaxies for which this phenomenon has not yet been reported. A number of potential tests of the bottom-heavy IMF hypothesis are also discussed.
