Star formation occurs on physical scales corresponding to individual star forming regions, typically of order ~100 parsecs in size, but current observational facilities cannot resolve these scales within field galaxies beyond the local universe. Howe
ver, the magnification from strong gravitational lensing allows us to measure the properties of these discrete star forming regions within galaxies in the distant universe. New results from multi-wavelength spectroscopic studies of a sample of extremely bright, highly magnified lensed galaxies are revealing the complexity of star formation on sub-galaxy scales during the era of peak star formation in the universe. We find a wide range of properties in the rest-frame UV spectra of individual galaxies, as well as in spectra that originate from different star forming regions within the same galaxy. Large variations in the strengths and velocity structure of Lyman-alpha and strong P Cygni lines such as C IV, and MgII provide new insights into the astrophysical relationships between extremely massive stars, the elemental abundances and physical properties of the nebular gas those stars ionize, and the galactic-scale outflows they power.
We present a detailed analysis of multi-wavelength HST/WFC3 imaging and Keck/OSIRIS near-IR AO-assisted integral field spectroscopy for a highly magnified lensed galaxy at z=1.70. This young starburst is representative of UV-selected star-forming gal
axies (SFG) at z~2 and contains multiple individual star-forming regions. Due to the lensing magnification, we can resolve spatial scales down to 100pc in the source plane of the galaxy. The velocity field shows disturbed kinematics suggestive of an ongoing interaction, and there is a clear signature of a tidal tail. We constrain the age, reddening, SFR and stellar mass of the star-forming clumps from SED modelling of the WFC3 photometry and measure their H-alpha luminosity, metallicity and outflow properties from the OSIRIS data. With strong star formation driven outflows in four clumps, RCSGA0327 is the first high redshift SFG at stellar mass <10^10 M_sun with spatially resolved stellar winds. We compare the H-alpha luminosities, sizes and dispersions of the star-forming regions to other high-z clumps as well as local giant HII regions and find no evidence for increased clump star formation surface densities in interacting systems, unlike in the local Universe. Spatially resolved SED modelling unveils an established stellar population at the location of the largest clump and a second mass concentration near the edge of the system which is not detected in H-alpha emission. This suggests a picture of an equal-mass mixed major merger, which has not triggered a new burst of star formation or caused a tidal tail in the gas-poor component.
We present optical and near-IR imaging and spectroscopy of SGAS J105039.6$+$001730, a strongly lensed galaxy at z $=$ 3.6252 magnified by $>$30$times$, and derive its physical properties. We measure a stellar mass of log(M$_{*}$/M$_{odot}$) $=$ 9.5 $
pm$ 0.35, star formation rates from [O II]$lambda$$lambda$3727 and H-$beta$ of 55 $pm$ 20 and 84 $pm$ 17 M$_{odot}$ yr$^{-1}$, respectively, an electron density of n$_{e} leq$ 10$^{3}$ cm$^{-2}$, an electron temperature of T$_{e} leq$ 14000 K, and a metallicity of 12+log(O/H) $=$ 8.3 $pm$ 0.1. The strong C III]$lambda$$lambda$1907,1909 emission and abundance ratios of C, N, O and Si are consistent with well-studied starbursts at z $sim$ 0 with similar metallicities. Strong P Cygni lines and He II$lambda$1640 emission indicate a significant population of Wolf-Rayet stars, but synthetic spectra of individual populations of young, hot stars do not reproduce the observed integrated P Cygni absorption features. The rest-frame UV spectral features are indicative of a young starburst with high ionization, implying either 1) an ionization parameter significantly higher than suggest by rest-frame optical nebular lines, or 2) differences in one or both of the initial mass function and the properties of ionizing spectra of massive stars. We argue that the observed features are likely the result of a superposition of star forming regions with different physical properties. These results demonstrate the complexity of star formation on scales smaller than individual galaxies, and highlight the importance of systematic effects that result from smearing together the signatures of individual star forming regions within galaxies.
We present the discovery and a detailed multi-wavelength study of a strongly-lensed luminous infrared galaxy at z=0.816. Unlike most known lensed galaxies discovered at optical or near-infrared wavelengths this lensed source is red, r-Ks = 3.9 [AB],
which the data presented here demonstrate is due to ongoing dusty star formation. The overall lensing magnification (a factor of 17) facilitates observations from the blue optical through to 500micron, fully capturing both the stellar photospheric emission as well as the re-processed thermal dust emission. We also present optical and near-IR spectroscopy. These extensive data show that this lensed galaxy is in many ways typical of IR-detected sources at z~1, with both a total luminosity and size in accordance with other (albeit much less detailed) measurements in samples of galaxies observed in deep fields with the Spitzer telescope. Its far-infrared spectral energy distribution is well-fit by local templates that are an order of magnitude less luminous than the lensed galaxy; local templates of comparable luminosity are too hot to fit. Its size (D~7kpc) is much larger than local luminous infrared galaxies, but in line with sizes observed for such galaxies at z~1. The star formation appears uniform across this spatial scale. In this source, the luminosity of which is typical of sources that dominate the cosmic infrared background, we find that star formation is spatially extended and well organised, quite unlike the compact merger-driven starbursts which are typical for sources of this luminosity at z~0.
We present a comprehensive analysis of the rest-frame UV to near-IR spectral energy distributions and rest-frame optical spectra of four of the brightest gravitationally lensed galaxies in the literature: RCSGA 032727-132609 at z=1.70, MS1512-cB58 at
z=2.73, SGAS J152745.1+065219 at z=2.76 and SGAS J122651.3+215220 at z=2.92. This includes new Spitzer imaging for RCSGA0327 as well as new spectra, near-IR imaging and Spitzer imaging for SGAS1527 and SGAS1226. Lensing magnifications of 3-4 magnitudes allow a detailed study of the stellar populations and physical conditions. We compare star formation rates as measured from the SED fit, the H-alpha and [OII] emission lines, and the UV+IR bolometric luminosity where 24 micron photometry is available. The SFR estimate from the SED fit is consistently higher than the other indicators, which suggests that the Calzetti dust extinction law used in the SED fitting is too flat for young star-forming galaxies at z~2. Our analysis finds similar stellar population parameters for all four lensed galaxies: stellar masses 3-7*10^9 M_sun, young ages ~ 100 Myr, little dust content E(B-V)=0.10-0.25, and star formation rates around 20-100 M_sun/yr. Compared to typical values for the galaxy population at z~2, this suggests we are looking at newly formed, starbursting systems that have only recently started the build-up of stellar mass. These results constitute the first detailed, uniform analysis of a sample of the growing number of strongly lensed galaxies known at z~2.
The [O IV] 25.89 micron line has been shown to be an accurate indicator of active galactic nucleus (AGN) intrinsic luminosity in that it correlates well with hard (10-200 keV) X-ray emission. We present measurements of [O IV] for 89 Seyfert galaxies
from the unbiased Revised Shapley-Ames (RSA) sample. The [O IV] luminosity distributions of obscured and unobscured Seyferts are indistinguishable, indicating that their intrinsic AGN luminosities are quite similar and that the RSA sample is well suited for tests of the unified model. In addition, we analyze several commonly used proxies for AGN luminosity, including [O III] 5007 A, 6 cm radio, and 2-10 keV X-ray emission. We find that the radio luminosity distributions of obscured and unobscured AGNs show no significant difference, indicating that radio luminosity is a useful isotropic luminosity indicator. However, the observed [O III] and 2-10 keV luminosities are systematically smaller for obscured Seyferts, indicating that they are not emitted isotropically.
