Our objectives are to determine the properties of the interstellar medium (ISM) and of star-formation in typical star-forming galaxies at high redshift. Following up on our previous multi-wavelength observations with HST, Spitzer, Herschel, and the P
lateau de Bure Interferometer (PdBI), we have studied a strongly lensed z=2.013 galaxy, the arc behind the galaxy cluster MACS J0451+0006, with ALMA to measure the [CII] 158 micron emission line, one of the main coolants of the ISM. [CII] emission from the southern part of this galaxy is detected at 10 $sigma$. Taking into account strong gravitational lensing, which provides a magnification of $mu=49$, the intrinsic lensing-corrected [CII]158 micron luminosity is $L(CII)=1.2 times 10^8 L_odot$. The observed ratio of [CII]-to-IR emission, $L(CII)/L(FIR) approx (1.2-2.4) times 10^{-3}$, is found to be similar to that in nearby galaxies. The same also holds for the observed ratio $L(CII)/L(CO)=2.3 times 10^3$, which is comparable to that of star-forming galaxies and active galaxy nuclei (AGN) at low redshift. We utilize strong gravitational lensing to extend diagnostic studies of the cold ISM to an order of magnitude lower luminosity ($L(IR) sim (1.1-1.3) times 10^{11} L_odot$) and SFR than previous work at high redshift. While larger samples are needed, our results provide evidence that the cold ISM of typical high redshift galaxies has physical characteristics similar to normal star forming galaxies in the local Universe.
We have targeted two recently discovered Lyman break galaxies (LBGs) to search for dust continuum and [CII] 158 micron line emission. The strongly lensed z~6.8 LBG A1703-zD1 behind the galaxy cluster Abell 1703, and the spectroscopically confirmed z=
7.508 LBG z8-GND-5296 in the GOODS-N field have been observed with the Plateau de Bure interferometer (PdBI) at 1.2mm. These observations have been combined with those of three z>6.5 Lya emitters (named HCM6A, Himiko, and IOK-1), for which deep measurements were recently obtained with the PdBI and ALMA. [CII] is undetected in both galaxies, providing a deep upper limit for Abell1703-zD1, comparable to recent ALMA non-detections. Dust continuum emission from Abell1703-zD1 and z8-GND-5296 is not detected with an rms of 0.12 and 0.16 mJy/beam. From these non-detections we derive upper limits on their IR luminosity and star formation rate, dust mass, and UV attenuation. Thanks to strong gravitational lensing the limit for Abell1703-zD1 is probing the sub-LIRG regime ($L_{IR} <8.1 times 10^{10}$ Lsun) and very low dust masses ($M_d<1.6 times 10^7$ Msun). We find that all five galaxies are compatible with the Calzetti IRX-$beta$ relation, their UV attenuation is compatible with several indirect estimates from other methods (the UV slope, extrapolation of the attenuation measured from the IR/UV ratio at lower redshift, and SED fits), and the dust-to-stellar mass ratio is not incompatible with that of galaxies from z=0 to 3. For their stellar mass the high-z galaxies studied here have an attenuation below the one expected from the mean relation of low redshift (z<1.5) galaxies. More and deeper (sub)-mm data are clearly needed to directly determine the UV attenuation and dust content of the dominant population of high-z star-forming galaxies and to establish more firmly their dependence on stellar mass, redshift, and other properties.
I provide an overview about star-forming galaxies at high redshift and their physical properties. Starting from the populations of Ly-$alpha$ emitters and Lyman break galaxies, I summarize their common features and distinction. Then I summarize recen
t insight onto their physical properties gained from SED models including nebular emission, and various implications from these studies on the properties of star-formation at high redshift. Finally, I present new results and an overview on the dust content and UV attenuation of $z>6$ galaxies obtained from IRAM and ALMA observations.
We examine the dependence of derived physical parameters of distant Lyman break galaxies (LBGs) on the assumed star formation histories (SFHs), their implications on the SFR-mass relation, and we propose observational tests to better constrain these
quantities. We use our SED-fitting tool including nebular emission to analyze a large sample of LBGs, assuming five different star formation histories, extending our first analysis of this sample (de Barros et al. 2012, paper I). In addition we predict the IR luminosities consistently with the SED fits. Compared to standard SED fits assuming constant SFR and neglecting nebular lines, assuming variable SFHs yield systematically lower stellar masses, higher extinction, higher SFR, higher IR luminosities, and a wider range of equivalent widths for optical emission lines. Exponentially declining and delayed SFHs yield basically identical results and generally fit best. Exponentially rising SFHs yield similar masses, but somewhat higher extinction. We find significant deviations between the derived SFR and IR luminosity from the commonly used SFR(IR) or SFR(IR+UV) calibration, due to differences in the SFHs and ages. Models with variable SFHs, favored statistically, yield generally a large scatter in the SFR-mass relation. We show that the true scatter in the SFR-mass relation can be significantly larger than inferred using SFR(UV) and/or SFR(IR). Different SFHs, and hence differences in the derived SFR-mass relation and in the specific star formation rates, can be tested/constrained observationally with future IR observations with ALMA. Measurement of emission lines, such as Halpha and [OII]3727, can also provide useful constraints on the SED models. We conclude that our findings of a large scatter in SFR-mass at high-z and an increase of the specific star formation rate above z>~3 (paper I) can be tested observationally. (abriged)
We summarize the principles and fundamental ingredients of evolutionary synthesis models, which are stellar evolution, stellar atmospheres, the IMF, star-formation histories, nebular emission, and also attenuation from the ISM and IGM. The chapter fo
cusses in particular on issues of importance for predictions of metal-poor and Population III dominated galaxies. We review recent predictions for the main physical properties and related observables of star-forming galaxies based on up-to-date inputs. The predicted metallicity dependence of these quantities and their physical causes are discussed. The predicted observables include in particular the restframe UV-to-optical domain with continuum emission from stars and the ionized ISM, as well as emission lines from H, He, and metals. Based on these predictions we summarize the main observational signatures (emission line strengths, colors etc.), which can be used to distinguish normal stellar populations from very metal-poor objects or even Pop III. Evolutionary synthesis models provide an important and fundamental tool for studies of galaxy formation and evolution, from the nearby Universe back to first galaxies. They are used in many applications to interpret existing observations, to predict and guide future missions/instruments, and to allow direct comparisons between state-of-the-art galaxy simulations and observations.
We highlight and discuss the importance of accounting for nebular emission in the SEDs of high redshift galaxies, as lines and continuum emission can contribute significantly or subtly to broad-band photometry. Physical parameters such as the galaxy
age, mass, star-formation rate, dust attenuation and others inferred from SED fits can be affected to different extent by the treatment of nebular emission. We analyse a large sample of Lyman break galaxies from z~3-6, and show some main results illustrating e.g. the importance of nebular emission for determinations of the mass-SFR relation, attenuation and age. We suggest that a fairly large scatter in such relations could be intrinsic. We find that the majority of objects (~60-70%) is better fit with SEDs accounting for nebular emission; the remaining galaxies are found to show relatively weak or no emission lines. Our modeling, and supporting empirical evidence, suggests the existence of two categories of galaxies, starbursts and post-starbursts (lower SFR and older galaxies) among the LBG population, and relatively short star-formation timescales.
Determining Lya properties of distant galaxies is of great interest for various astrophysical studies. We examine how the strength of Lya emission can be constrained from broad-band SED fits instead of relying on spectroscopy. We use our SED fitting
tool including the effects of nebular emission, considering in particular Lya emission as a free parameter, and we demonstrate our method with simulations of mock galaxies. Using this tool we analyse a large sample of U, B, V, and i dropout galaxies with multi-band photometry. We find significant trends of the fraction of galaxies with Lya emission increasing both with redshift z and towards fainter magnitude (at fixed z), and similar trends for the Lya equivalent width. Our inferred Lya properties are in good agreement with the available spectroscopic observations and other data. These results demonstrate that the strength of Lya emission in distant star-forming galaxies can be inferred quantitatively from broad-band SED fits, at least statistically for sufficiently large samples with a good photometric coverage.
Lya is a key diagnostic for numerous observations of distant star-forming galaxies. Its interpretation requires, however, detailed radiation transfer models. We provide an extensive grid of 3D radiation transfer models simulating the Lya and UV conti
nuum radiation transfer in the interstellar medium of star-forming galaxies. We have improved our Monte Carlo MCLya code, and have used it to compute a grid of 6240 radiation transfer models for homogeneous spherical shells containing HI and dust surrounding a central source. The simulations cover a wide range of parameter space. We present the detailed predictions from our models including in particular the Lya escape fraction fesc, the continuum attenuation, and detailed Lya line profiles. The Lya escape fraction is shown to depend strongly on dust content, but also on other parameters (HI column density and radial velocity). The predicted line profiles show a great diversity of morphologies ranging from broad absorption lines to emission lines with complex features. The results from our simulations are distributed in electronic format. Our models should be of use for the interpretation of observations from distant galaxies, for other simulations, and should also serve as an important base for comparison for future, more refined, radiation transfer models.
We examine various implications from a dynamical and chemical model of globular clusters (GCs), which successfully reproduces the observed abundance patterns and the multiple populations of stars in these systems assuming chemical enrichment from fas
t rotating massive stars. Using the model of Decressin et al. (2007) we determine the ratio between the observed, present-day mass of globular clusters and their initial stellar mass as a function of the stellar initial mass function (IMF). We also compute the mass of low-mass stars ejected, and the amount of hydrogen ionising photons emitted by the proto globular clusters. Typically, we find that the initial masses of GCs must be ~8-10 times (or up to 25 times, if second generation stars also escape from GCs) larger than the present-day stellar mass. The present-day Galactic GC population must then have contributed to approximately 5-8% (10-20%) of the low-mass stars in the Galactic halo. We also show that the detection of second generation stars in the Galactic halo, recently announced by different groups, provides a new constraint on the GC initial mass function (GCIMF). These observations appear to rule out a power-law GCIMF, whereas they are compatible with a log-normal one. Finally, the high initial masses also imply that GCs must have emitted a large amount of ionising photons in the early Universe. Our results reopen the question on the initial mass function of GCs, and reinforce earlier conclusions that old GCs could have represented a significant contribution to reionise the inter-galactic medium at high redshift.
