Stellar populations and Lyα emission in two lensed z ≳ 6 galaxies

We present an analysis of two strongly lensed galaxies at z= 6.56 and z∼ 7 for which multi-band photometric and spectroscopic observations are available. For one source, the data include recent Hubble Space Telescope (HST) and Spitzer observations. Using a spectral energy distribution (SED) fitting technique considering a number of parameters (various libraries of empirical and theoretical template spectra, variable extinction and extinction laws), we attempt to constrain the properties of their stellar populations (age, star formation (SF) history and mass) and their intrinsic Lyman α (Lyα) emission. The following main results are obtained for the individual galaxies. Triple arc in Abell 2218, probable z∼ 7 galaxy. The most likely redshift of this source is z∼ 6.0-7.2, taking into account both our photometric determination and lensing considerations. SED fits indicate generally a low extinction [E(B−V) ≲ 0.05], but do not strongly constrain the SF history. Best fits have typical ages of ∼3-400 Myr. A reasonable maximum age of 250-650 Myr (1s interval) can be estimated. However, the apparent 4000-Å break observed recently from the combination of (Infrared Array Camera) IRAC/Spitzer and HST observations can also be reproduced equally well with the template of a young (∼3-5 Myr) burst where strong rest-frame optical emission lines enhance the 3.6- and 4.5-μm fluxes. The estimated star formation rate (SFR) is typically ∼1 M⊙ yr−1 for a Salpeter initial mass function (IMF) from 1 to 100 M⊙, in agreement with previous estimates. The unknowns on the age and SF history could easily explain the apparent absence of Lyα in this galaxy. Abell 370 HCM 6A. The available Lyα and continuum observations indicate basically two possible solutions: (1) a young burst or ongoing constant SF with non-negligible extinction or (2) a composite young +‘old' stellar population. In the first case, one obtains a best fit, E(B−V) ∼ 0.25, or AV∼ 0.5-1.8 at a 1s level. In consequence, we obtain SFR ∼11-41 M⊙ yr−1, higher than earlier estimates, and we estimate a fairly high total luminosity [L∼ (1 − 4) × 1011 L⊙] for this galaxy, in the range of luminous infrared galaxies. A Lyα transmission of ∼23-90 per cent is estimated from our best-fitting models. Other properties (age and SF history) remain largely unconstrained. In case of composite stellar populations, the SFR, mass and luminosity estimate are lower. The two scenarios may be distinguishable with IRAC/Spitzer observations at 3.6 and 4.5 μ

Stellar Population in Extremely Red Galaxies

We describe our results on multi-colour observations of extremely red galaxies found in two gravitational lensing clusters (Abell 1835 and AC114). Applying a colour criteria of R-K≥ 5.6 we have selected 12(10) EROs, of which 10(4) have no R-band detection. 5 of these objects have exceptional red colours (R-K>7.5). We compare their colours with other galaxy populations, like distant red galaxies and infrared detected extremely red galaxies, in order to differentiate between the

EROs found behind lensing clusters: II.Empirical properties, classification, and SED modelling based on multi-wavelength observations

We study the properties and nature of extremely red galaxies (ERO, R-K>5.6) found behind two lensing clusters and compare them with other known galaxy populations. New HST/ACS observations, Spitzer IRAC and MIPS, and Chandra/ACIS observations of the two lensing clusters Abell 1835 and AC114 contemplate our earlier optical and near-IR observations and have been used to study extremely red objects (EROs) in these deep fields. We have found 6 and 9 EROs in Abell 1835 and AC114. Several (7) of these objects are undetected up to the I and/or z band, and are hence ``optical'' drop-out sources. The photometric redshifts of most of our sources (80%) are z~0.7-1.5. According to simple colour-colour diagrams the majority of our objects would be classified as hosting old stellar populations. However, there are clear signs of dusty starbursts for several among them. These objects correspond to the most extreme ones in R-K colour. We estimate a surface density of (0.97+-0.31) arcmin-2 for EROs with (R-K>5.6) at K<20.5. Among our 15 EROs 6 (40 %) also classify as distant red galaxies (DRGs). 11 of 13 EROs with available IRAC photometry also fulfil the selection criteria for IRAC selected EROs (IEROs) of Yan et al. (2004). SED modelling shows that ~ 36 % of the IEROs in our sample are luminous or ultra-luminous infrared galaxies ((U)LIRG). Some very red DRGs are found to be very dusty starbursts, even (U)LIRGs, as also supported by their mid-IR photometry. No indication for AGNs is found, although faint activity cannot be excluded for all objects. From mid-IR and X-ray data 5 objects are clearly classified as starbursts. The derived properties are quite similar to those of DRGs and IEROs, except for 5 extreme objects in terms of colours, for which a very high extinction (Av>3) is found.Comment: 20 pages, 10 figures, accepted for publication in A&

News from z6-10 galaxy candidates found behind gravitational lensing clusters

We summarise the current status of our project to identify and classify ~ 6-10 galaxies thanks to strong gravitational lensing. Building on the detailed work by Richard et al. (2006), we present results from new follow-up observations undertaken with the ACS/HST and the Spitzer space telescope and compare our results with findings from the Hubble Ultra-Deep Field (UDF

Dust in the reionization era: ALMA observations of a zz=8.38 Galaxy

We report on the detailed analysis of a gravitationally-lensed Y-band dropout, A2744_YD4, selected from deep Hubble Space Telescope imaging in the Frontier Field cluster Abell 2744. Band 7 observations with the Atacama Large Millimeter Array (ALMA) indicate the proximate detection of a significant 1mm continuum flux suggesting the presence of dust for a star-forming galaxy with a photometric redshift of $z\simeq8$. Deep X-SHOOTER spectra confirms the high redshift identity of A2744_YD4 via the detection of Lyman $\alpha$ emission at a redshift $z$=8.38. The association with the ALMA detection is confirmed by the presence of [OIII] 88$\mu$m emission at the same redshift. Although both emission features are only significant at the 4 $\sigma$ level, we argue their joint detection and the positional coincidence with a high redshift dropout in the HST images confirms the physical association. Analysis of the available photometric data and the modest gravitational magnification ($\mu\simeq2$) indicates A2744_YD4 has a stellar mass of $\sim$ 2$\times$10$^9$ M$_{\odot}$, a star formation rate of $\sim20$ M$_{\odot}$/yr and a dust mass of $\sim$6$\times$10$^{6}$ M$_{\odot}$. We discuss the implications of the formation of such a dust mass only $\simeq$200 Myr after the onset of cosmic reionisation.Comment: Accepted for publication in ApJ

Extremely Red Galaxies in Abell 1835

We use multi-colour data of the gravitational lensing cluster Abell 1835 to search for high-z candidates (Richard et al. 2006) and extremely red galaxies. Applying a colour criteria of R-K ≥ 5.6 we have selected 12 EROs, of which 10(9) have no R(I)-band detection. Two of these sources posses multiple detections in the ACS z850 band, possibly gravitationally bound objects. Using the method of Pozzetti and Mannucci (2000) to classify an ERO either as evolved elliptical or dusty starburst, we find that two thirds of all objects have colours assigned to evolved ellipticals. One of the remaining 4 sources is the known sub-mm source SMMJ14009+0252. The spectral energy distributions of sources with no optical detection show strong similarities to the SED of the high-z object of Mobasher et al. (2005), i.e. a degeneracy between a low- and high-z solution. However, for objects we accept the low-z solution as the better fit. A detailed analyses of the colour properties and spectral energy distribution will be published in two forthcoming papers (Schaerer et al., Hempel et al.

Comparing the relation between star formation and galaxy mass in different environments

Analyzing 24 mu m MIPS/Spitzer data and the [O II]3727 line of a sample of galaxies at 0.4 < z < 0.8 from the ESO Distant Cluster Survey (EDisCS), we investigate the ongoing star formation rate (SFR) and the specific star formation rate (SSFR) as a function of stellar mass in galaxy clusters and groups, and compare with field studies. As for the field, we find a decline in SFR with time, indicating that star formation (SF) was more active in the past, and a decline in SSFR as galaxy stellar mass increases, showing that the current SF contributes more to the fractional growth of low-mass galaxies than high-mass galaxies. However, we find a lower median SFR (by a factor of ~1.5) in cluster star-forming galaxies than in the field. The difference is highly significant when all Spitzer and emission-line galaxies are considered, regardless of color. It remains significant at z>0.6 after removing red emission-line (REL) galaxies, to avoid possible AGN contamination. While there is overlap between the cluster and field SFR-Mass relations, we find a population of cluster galaxies (10-25%) with reduced SFR for their mass. These are likely to be in transition from star-forming to passive. Comparing separately clusters and groups at z>0.6, only cluster trends are significantly different from the field, and the average cluster SFR at a given mass is ~2 times lower than the field. We conclude that the average SFR in star-forming galaxies varies with galaxy environment at a fixed galaxy mass.Comment: ApJL in pres

Properties of high-z galaxies as seen through lensing clusters

We discuss the first results obtained on the study of a sample of high-z galaxies (2 < z < 7), using the gravitational amplification effect in the core of lensing clusters. Sources are located close to the critical lines in clusters with well constrained mass distributions, and selected through photometric redshifts, computed on a large wavelength domain, and lens inversion techniques.Comment: 5 pages, 3 figures, Conference Proceedings of the "Clustering at High Redshift" Conference, June 29 to July 2, 1999, Marseille (France

Biermann Mechanism in Primordial Supernova Remnant and Seed Fields

We have studied the generation of magnetic fields by the Biermann mechanism in the pair-instability supernovae explosions of the first stars. The Biermann mechanism produces magnetic fields in the shocked region between the bubble and interstellar medium (ISM), even if magnetic fields are absent initially. We have performed a series of two-dimensional magnetohydrodynamic simulations with the Biermann term and estimate the amplitude and total energy of the produced magnetic fields. We find that magnetic fields with amplitude $10^{-14}-10^{-17}$ G are generated inside the bubble, though the amount of magnetic fields generated depend on specific values of initial conditions. This corresponds to magnetic fields of $10^{28}-10^{31}$ ergs per each supernova remnant, which is strong enough to be the seed magnetic field for a galactic and/or interstellar dynamo.Comment: 4pages, 2figures, to appear in the Proc. of the Int. Conference on "The Origin and Evolution of Cosmic Magnetism", Bologna, 29 August - 2 September 2005, eds. R. Beck, G. Brunetti, L. Feretti and B. Gaensler (Atronomische Nachrichten, Wiley, 2005