arXiv subjects

D. Schaerer

Publications and source records attributed to D. Schaerer.

At least 199 records · Page 11Linked to original sources

arXiv2011

The properties of the brightest Lyman alpha emitters at z~5.7

(Abridged) We use deep VLT optical and near-IR spectroscopy and deep Spitzer/IRAC imaging to examine the properties of two of the most luminous Lyman alpha emitters at z=5.7. The continuum red-ward of the Lyman alpha line is clearly detected in both objects, thus facilitating a relatively accurate measurement (10--20% uncertainties) of the observed rest-frame equivalent widths, which are around 160 Angstroms for both objects. Through detailed modelling of the profile of the Lyman alpha line with a 3-D Monte-Carlo radiative transfer code, we estimate the intrinsic rest-frame equivalent width of Lyman alpha and find values that are around 300 Angstroms, which is at the upper end of the range allowed for very young, moderately metal-poor star-forming galaxies. However, the uncertainties are large and values as high as 700 Angstroms are permitted by the data. Both Lyman alpha emitters are detected at 3.6 micron in deep images taken with the Spitzer Space Telescope. We use these measurements, the measurement of the continuum red-ward of Lyman alpha and other photometry to constrain the spectral energy distributions of these very luminous Lyman alpha emitters and to compare them to three similar Lyman alpha emitters from the literature. The contribution from nebular emission is included in our models: excluding it results in significantly higher masses. Evidence for dust is found in all objects, and emission from nebular lines often make a dominant contribution to the rest frame 3.6 micron flux. Rich in emission lines, these objects are prime targets for the next generation of extremely large telescopes, JWST and ALMA.

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arXiv2011

The spectral evolution of the first galaxies. I. James Webb Space Telescope detection limits and color criteria for 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.

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arXiv2011

Far infrared constraints on the contamination by dust obscured galaxies of high-z dropout searches

The spectral energy distributions (SED) of dusty galaxies at intermediate redshift may look similar to very high redshift galaxies in the optical/near infrared (NIR) domain. This can lead to the contamination of high redshift galaxy searches based on broad band optical/NIR photometry by lower redshift dusty galaxies as both kind of galaxies cannot be distinguished. The contamination rate could be as high as 50%. {This work shows how the far infrared (FIR) domain can help to recognize likely low-z interlopers in an optical/NIR search for high-z galaxies.} We analyse the FIR SEDs of two galaxies proposed as very high redshift ($z>7$) dropout candidates based on deep Hawk-I/VLT observations. The FIR SEDs are sampled with PACS/Herschel at 100 and 160\,$μ$m, with SPIRE/Herschel at 250, 350 and 500\,$μ$m and with LABOCA/APEX at 870\,$μ$m. We find that redshifts $>7$ would imply extreme FIR SEDs (with dust temperatures $>100$\,K and FIR luminosities $>10^{13}$\,$L_{\odot}$). At z$\sim$2, instead, the SEDs of both sources would be compatible with that of typical ULIRGs/SMGs. Considering all the data available for these sources from visible to FIR we re-estimate the redshifts and we find $z\sim$1.6--2.5. Due to the strong spectral breaks observed in these galaxies, standard templates from the literature fail to reproduce the visible-near IR part of the SEDs even when additional extinction is included. These sources resemble strongly dust obscured galaxies selected in Spitzer observations with extreme visible-to-FIR colors, and the galaxy GN10 at $z=4$. Galaxies with similar SEDs could contaminate other high redshift surveys.

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arXiv2010

Gemini GMOS spectroscopy of HeII nebulae in M33

We have carried out a narrow-band survey of the Local Group galaxy, M33, in the HeII4686 emission line, to identify HeII nebulae in this galaxy. With spectroscopic follow-up observations, we confirm three of seven candidate objects, including identification of two new HeII nebulae, BCLMP651, HBW673. We also obtain spectra of associated ionizing stars for all the HII regions, identifying two new WN stars. We demonstrate that the ionizing source for the known HeII nebula, MA 1, is consistent with being the early-type WN star MC8 (M33-WR14), by carrying out a combined stellar and nebular analysis of MC8 and MA1. We were unable to identify the helium ionizing sources for HBW 673 and BCLMP 651, which do not appear to be Wolf-Rayet stars. According to the [OIII]5007/Hbeta vs [NII]6584/Halpha diagnostic diagram, excitation mechanisms apart from hot stellar continuum are needed to account for the nebular emission in HBW 673, which appears to have no stellar source at all.

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arXiv2010

The unusual NIV]-emitter galaxy GDS J033218.92-275302.7: star formation or AGN-driven winds from a massive galaxy at z=5.56

Aims: We investigate the nature of the source GDS J033218.92-275302.7at redshift ~ 5.56. Methods: The SED of the source is well sampled by 16 bands photometry, from UV-optical, near infrared and mid-infrared (MID-IR).The detection of signal in the MID-IR Spitzer/IRAC bands 5.8, 8.0 um -- where the nebular emission contribution is less effective -- suggests the presence of a Balmer break, signature of an underlying stellar population formed at earlier epochs. The optical spectrum shows a clear Lya emission line together with semi-forbidden NIV] 1483.3-1486.5 also in emission. Results: From the SED fitting and the Lya modelling it turns out that the source seems to have an evolved component with stellar mass of ~5 x10^(10) Msolar and age ~ 0.4 Gyrs, and a young component with an age of ~ 0.01 Gyrs and SFR in the range of 30-200 Msolar yr^(-1). The limits on the effective radius derived from the ACS/z850 and VLT/Ks bands indicate that this galaxy is denser than the local ones with similar mass. A relatively high nebular gas column density is favored from the Lya line modelling (NHI>=10^(21) cm^(-2)). A vigorous outflow (~ 450 km/s) has been measured from the optical spectrum,consistent with the Lya modelling. From ACS observations it turns out that the region emitting Lya photons is spatially compact and of the same order of the effective radius estimated at the ~1400A rest-frame wavelength, whose emission is dominated by the stellar continuum and/or AGN. The gas is blown out from the central region,but given the mass of the galaxy it is uncertain whether it will pollute the IGM to large distances. We argue that a burst of star formation in a dense gas environment is active (possibly containing hot and massive stars and/or a low luminosity AGN), superimposed to an already formed fraction of stellar mass (abridged).

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arXiv2010

Improving the identification of high-z Herschel sources with position priors and optical/NIR and FIR/mm photometric redshifts

We present preliminary results about the detection of high redshift (U)LIRGs in the Bullet cluster field by the PACS and SPIRE instruments within the Herschel Lensing Survey (HLS) Program. We describe in detail a photometric procedure designed to recover robust fluxes and deblend faint Herschel sources near the confusion noise. The method is based on the use of the positions of Spitzer/MIPS 24 um sources as priors. Our catalogs are able to reliably (5 sigma) recover galaxies with fluxes above 6 and 10 mJy in the PACS 100 and 160 um channels, respectively, and 12 to 18 mJy in the SPIRE bands. We also obtain spectral energy distributions covering the optical through the far-infrared/millimeter spectral ranges of all the Herschel detected sources, and analyze them to obtain independent estimations of the photometric redshift based on either stellar population or dust emission models. We exemplify the potential of the combined use of Spitzer position priors plus independent optical and IR photometric redshifts to robustly assign optical/NIR counterparts to the sources detected by Herschel and other (sub-)mm instruments.

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arXiv2010

Deep Herschel view of obscured star formation in the Bullet cluster

We use deep, five band (100-500um) data from the Herschel Lensing Survey (HLS) to fully constrain the obscured star formation rate, SFR_FIR, of galaxies in the Bullet cluster (z=0.296), and a smaller background system (z=0.35) in the same field. Herschel detects 23 Bullet cluster members with a total SFR_FIR = 144 +/- 14 M_sun yr^-1. On average, the background system contains brighter far-infrared (FIR) galaxies, with ~50% higher SFR_FIR (21 galaxies; 207 +/- 9 M_sun yr^-1). SFRs extrapolated from 24um flux via recent templates (SFR_24) agree well with SFR_FIR for ~60% of the cluster galaxies. In the remaining ~40%, SFR_24 underestimates SFR_FIR due to a significant excess in observed S_100/S_24 (rest frame S_75/S_18) compared to templates of the same FIR luminosity.

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arXiv2010

First detection of the Sunyaev Zel'dovich effect increment at lambda < 650 um

The Sunyaev-Zel'dovich (SZ) effect is a spectral distortion of the cosmic microwave background as observed through the hot plasma in galaxy clusters. This distortion is a decrement in the CMB intensity for lambda > 1.3 mm, an increment at shorter wavelengths, and small again by lambda ~250 um. As part of the Herschel Lensing Survey (HLS) we have mapped 1E0657-56 (the Bullet cluster) with SPIRE with bands centered at 250, 350 and 500 um and have detected the SZ effect at the two longest wavelengths. The measured SZ effect increment central intensities are Delta I_{0} = 0.097 +- 0.019 MJy sr^{-1} at 350 um and Delta I_{0} = 0.268 +- 0.031 MJy sr^{-1} at 500 um, consistent with the SZ effect spectrum derived from previous measurements at 2 mm. No other diffuse emission is detected. The presence of the finite temperature SZ effect correction is preferred by the SPIRE data at a significance of 2.1 sigma, opening the possibility that the relativistic SZ effect correction can be constrained by SPIRE in a sample of clusters. The results presented here have important ramifications for both sub-mm measurements of galaxy clusters and blank field surveys with SPIRE.

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arXiv2010

The far-infrared/submillimeter properties of galaxies located behind the Bullet cluster

The Herschel Lensing Survey (HLS) takes advantage of gravitational lensing by massive galaxy clusters to sample a population of high-redshift galaxies which are too faint to be detected above the confusion limit of current far-infrared/submillimeter telescopes. Measurements from 100-500 micron bracket the peaks of the far-infrared spectral energy distributions of these galaxies, characterizing their infrared luminosities and star formation rates. We introduce initial results from our science demonstration phase observations, directed toward the Bullet cluster (1E0657-56). By combining our observations with LABOCA 870 micron and AzTEC 1.1 mm data we fully constrain the spectral energy distributions of 19 MIPS 24 micron selected galaxies which are located behind the cluster. We find that their colors are best fit using templates based on local galaxies with systematically lower infrared luminosities.This suggests that our sources are not like local ultra-luminous infrared galaxies in which vigorous star formation is contained in a compact highly dust-obscured region. Instead, they appear to be scaled up versions of lower luminosity local galaxies with star formation occurring on larger physical scales.

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arXiv2010

The Herschel Lensing Survey (HLS): overview

The Herschel Lensing Survey (HLS) will conduct deep PACS and SPIRE imaging of ~40 massive clusters of galaxies. The strong gravitational lensing power of these clusters will enable us to penetrate through the confusion noise, which sets the ultimate limit on our ability to probe the Universe with Herschel. Here, we present an overview of our survey and a summary of the major results from our Science Demonstration Phase (SDP) observations of the Bullet Cluster (z=0.297). The SDP data are rich, allowing us to study not only the background high-redshift galaxies (e.g., strongly lensed and distorted galaxies at z=2.8 and 3.2) but also the properties of cluster-member galaxies. Our preliminary analysis shows a great diversity of far-infrared/submillimeter spectral energy distributions (SEDs), indicating that we have much to learn with Herschel about the properties of galaxy SEDs. We have also detected the Sunyaev-Zel'dovich (SZ) effect increment with the SPIRE data. The success of this SDP program demonstrates the great potential of the Herschel Lensing Survey to produce exciting results in a variety of science areas.

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arXiv2009

Rest-frame ultraviolet spectrum of the gravitationally lensed galaxy `the 8 o'clock arc': stellar and interstellar medium properties

We present the first detailed analysis of the rest-frame UV spectrum of the gravitationally lensed Lyman break galaxy (LBG), the `8 o'clock arc'. The spectrum of the 8 o'clock arc is rich in stellar and interstellar medium (ISM) features, and presents several similarities to the well-known MS1512-cB58 LBG. The stellar photospheric absorption lines allowed us to constrain the systemic redshift, z_sys = 2.7350+/-0.0003, of the galaxy, and derive its stellar metallicity, Z=0.82 Z_sol. With a total stellar mass of ~4.2x10^{11} M_sol, the 8 o'clock arc agrees with the mass-metallicity relation found for z>2 star-forming galaxies. The 31 ISM absorption lines detected led to the abundance measurements of 9 elements. The metallicity of the ISM, Z=0.65 Z_sol (Si), is very comparable to the metallicity of stars and ionized gas, and suggests that the ISM of the 8 o'clock arc has been rapidly polluted and enriched by ejecta of OB stars. The ISM lines extend over ~1000 km/s and have their peak optical depth blueshifted relative to the stars, implying gas outflows of about -120 km/s. The Ly-alpha line is dominated by a damped absorption profile on top of which is superposed a weak emission, redshifted relative to the ISM lines by about +690 km/s and resulting from multiply backscattered Ly-alpha photons emitted in the HII region surrounded by the cold, expanding ISM shell. A homogeneous spherical radiation transfer shell model with a constant outflow velocity, determined by the observations, is able to reproduce the observed Ly-alpha line profile and dust content. These results fully support the scenario proposed earlier, where the diversity of Ly-alpha line profiles in LBGs and Ly-alpha emitters, from absorption to emission, is mostly due to variations of HI column density and dust content (abridged).

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arXiv2007

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.

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arXiv2007

HST's view of the youngest massive stars in the Magellanic Clouds

Accurate physical parameters of newborn massive stars are essential ingredients to shed light on their formation, which is still an unsolved problem. The rare class of compact H II regions in the Magellanic Clouds (MCs), termed ``high-excitation blobs'' (HEBs), presents a unique opportunity to acquire this information. These objects (~ 4" to 10", ~ 1 to 3 pc, in diameter) harbor the youngest massive stars of the OB association/molecular cloud complexes in the MCs accessible through high-resolution near-IR and optical techniques. We present a brief overview of the results obtained with HST mainly on two HEBs, one in the LMC (N159-5) and the other in the SMC (N81).

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arXiv2007

EROs found behind lensing clusters. II. Stellar populations and dust properties of optical dropout EROs and comparison with related objects

We determine the nature, redshift, stellar populations and dust properties of optically faint or non-detected extremely red objects (ERO) found from our survey of the lensing clusters A1835 and AC114. We perform SED fitting and use deep optical, HST, VLT, Spitzer data, and for some objects and sub-mm data. For most of the lensed EROs we find photometric redshifts showing a strong degeneracy between "low-z" (z~1-3) and high-z (z~6-7). Although formally best fits are often found at high-z, their resulting bright absolute magnitudes, the number density of these objects, and in some cases Spitzer photometry or longer wavelength observations, suggest strongly that all of these objects are at "low-z". The majority of these objects are best fitted with young (<~ 0.5-0.7 Gyr) and dusty starbursts. Indications for strong extinction, with A_V~2.4-4, are found in some objects. For dusty objects star formation rates (SFR) have been estimated from the bolometric luminosity determined after fitting of semi-empirical starburst, ERO, and ULIRG templates. Typically we find SFR~(1-18) Msun/yr. Again, SMMJ14009+0252 stands out as a LIRG with SFR 1000 Msun/yr. Concerning the comparison objects, we argue that the massive post-starburst z~6.5 galaxy candidate HUDF-J2 showing observed properties very similar to our EROs, is more likely a dusty starburst at z~2.3-2.6. This interpretation also naturally explains the observed 24 micron emission. Both empirically and from our SED fits we find that the IRAC selectec EROs from Yan et al (2004) show very similar properties to our lensed EROs. Reasonable fits are found for most of them with relatively young and dusty stellar populations.

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arXiv2007

News from z~6-10 galaxy candidates found behind gravitational lensing clusters

We summarise the current status of our project to identify and study z~6-10 galaxies thanks to strong gravitational lensing. Building on the detailed work from Richard et al. (2006), we present results from new follow-up observations (imaging) undertaken with ACS/HST and the Spitzer Space Telescope and compare our results with findings from the Hubble Ultra-Deep Field (UDF). These new observations are in agreement with the high-z nature for the vast majority of the candidates presented in Richard et al. (2006). We also discuss the properties of other optical dropout sources found in our searches and related objects (EROs, sub-mm galaxies,...) from other surveys.

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arXiv2006

Searching for the first galaxies through gravitational lenses

Observing the first galaxies formed during the reionisation epoch, i.e. approximately within the first billion years after the Big Bang, remains one of the challenges of contemporary astrophysics. Several efforts are being undertaken to search for such remote objects. Combining the near-IR imaging power of the VLT and the natural effect of strong gravitational lensing our pilot program has allowed us to identify several galaxy candidates at redshift 6 <~ z <~ 10. The properties of these objects and the resulting constraints on the star formation rate density at high redshift are discussed. Finally we present the status of follow-up observations (ISAAC spectroscopy, HST and Spitzer imaging) and discuss future developments.

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arXiv2006

VLT/GIRAFFE spectroscopic observations of the metal-poor blue compact dwarf galaxy SBS 0335-052E

We present two-dimensional spectroscopy of the extremely metal-deficient blue compact dwarf (BCD) galaxy SBS 0335-052E aiming to studyphysical conditions, element abundances and kinematical properties of the ionised gas in this galaxy. Observations were obtained in the spectral range 3620-9400A with the imaging spectrograph GIRAFFE installed on the UT2 of the Very Large Telescope (VLT). These observations are the first ones carried out so far with GIRAFFE in the ARGUS mode which allows to obtain simultaneously 308 spectra covering a 11.4"x7.3" region. We produced images of SBS 0335-052E in the continuum and in emission lines of different stages of excitation. We find that while the maximum of emission in the majority of lines, including the strong lines Hbeta 4861A, Halpha 6563A, [OIII] 4363,5007A, [OII] 3726,3729A, coincides with the youngest south-eastern star clusters 1 and 2, the emission of HeII 4686A line is offset to the more evolved north-west clusters 4,5. This suggests that hard ionising radiation responsible for the HeII 4686A emission is not related to the most massive youngest stars, but rather is connected with fast radiative shocks. This conclusion is supported by the kinematical properties of the ionised gas from the different emission lines as the velocity dispersion in the HeII 4686A line is systematically higher, by ~50%-100%, than that in other lines. The variations of the emission line profiles suggest the presence of an ionised gas outflow in the direction perpendicular to the galaxy disk. (abridged)

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arXiv2006

High spatial resolution mid-infrared spectroscopy of the starburst galaxies NGC 3256, IIZw40 and Henize 2-10

In order to show the importance of high spatial resolution observations of extra-galactic sources when compared to observations obtained with larger apertures such as ISO, we present N-band spectra (8-13 um) of some locations in three starburst galaxies. In particular, the two galactic nuclei of the spiral galaxy NGC3256, the compact IR supernebula in the dwarf galaxy IIZw40 and the two brightest IR knots in the central starburst of the WR galaxy He2-10. The spectra have been obtained with TIMMI2 on the ESO 3.6m telescope. An inventory of the spectra in terms of atomic fine-structure lines and molecular bands is presented. We show the great value of these high spatial resolution data at constraining properties such as the extinction in the mid-IR, metallicity or stellar content (age, IMF, etc.). Regarding this, we have constrained the stellar content of the IR compact knot in IIZw40 by using the mid-IR fine-structure lines and setting restrictions on the nebular geometry. Considering the PAH bands, we have constructed a new mid-/far-IR diagnostic diagram based on the 11.2 um PAH and continuum, accessible to ground-based observations. Finally, we find a dependence between the presence or non-presence of PAHs and the hardness of the radiation field as measured by the [SIV]/[NeII] ratio. In particular, sources with PAH emission have in general a [SIV]/[NeII] ratio < 0.35. We investigate possible origins for this relation and conclude that it does not necessarily imply PAH destruction, but could also be explained by the PAH-dust competition for FUV photons.

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