arXiv subjects

Daniel Schaerer

Publications and source records attributed to Daniel Schaerer.

At least 181 records · Page 10Linked to original sources

arXiv2006

Constraining the population of 6 < z < 10 star-forming galaxies with deep near-IR images of lensing clusters

(abridged) We present the first results of our deep survey of lensing clusters aimed at constraining the abundance of star-forming galaxies at z~6-10. Deep near-IR photometry of two lensing clusters (A1835 and AC114) was obtained with ISAAC/VLT. These images, combined with existing data in the optical bands, including HST images, were used to select very high redshift candidates at z>~6 among the optical-dropouts. We have identified 18(8) first and second-category optical dropouts in A1835 (AC114), detected in more than one filter up to H(Vega)~23.8 (AB~25.2,uncorrected for lensing). Among them, 8(5) exhibit homogeneous SEDs compatible with star-forming galaxies at z>~6, and 5(1) are more likely intermediate-redshift EROs based on luminosity considerations. We have also identified a number of fainter sources in these fields fulfilling our photometric selection and located around the critical lines. We use all these data to make a first attempt at constraining the density of star-forming galaxies present at 6 6 candidates appears compatible with that of LBGs at z~3. The turnover observed by Bouwens et al. (2005) towards the bright end relative to the z~3 LF is not observed in this sample. Also the upper limit for the UV SFR density at z~6-10 is compatible with the usual values derived at z~5-6, but higher than the estimates obtained in the NICMOS Ultra Deep Field (UDF). Increasing the number of blank and lensing fields with ultra-deep near-IR photometry is essential to get more accurate constraints on the abundance of z>6 galaxies.

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arXiv2006

Subarcsecond Resolution Mid-Infrared Observations of Super Star Clusters in the Antennae (NGC4038/4039)

In this letter we present ground-based subarcsecond mid-infrared imaging and spectroscopy of young super star clusters in the overlap region of the merging galaxies NGC4038/4039 (the Antennae) obtained with the VLT Imager and Spectrometer for mid-Infrared (VISIR). With its unprecedented spatial resolution VISIR begins to resolve the HII/PDR complexes around the star-forming regions for the first time. In the N-band spectra of two young star clusters unexpectedly low polycyclic aromatic hydrocarbon (PAH) emission is observed, compared to what is seen with the Infrared Space Observatory (ISO) and with the Spitzer Space Telescope. We conclude that a large fraction of the PAH emission cannot directly be associated with the super star clusters, but originate from an extended region of at least 215 pc radius around the clusters. In the distribution of [NeII] 12.81 micron emission a highly obscured cluster is discovered that does not have an optical or near-infrared counterpart.

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arXiv2005

O stars with weak winds: the Galactic case

We study the stellar and wind properties of a sample of Galactic O dwarfs to track the conditions under which weak winds (i.e mass loss rates lower than ~ 1e-8 Msol/yr) appear. The sample is composed of low and high luminosity dwarfs including Vz stars and stars known to display qualitatively weak winds. Atmosphere models including non-LTE treatment, spherical expansion and line blanketing are computed with the code CMFGEN. Both UV and Ha lines are used to derive wind properties while optical H and He lines give the stellar parameters. Mass loss rates of all stars are found to be lower than expected from the hydrodynamical predictions of Vink et al. (2001). For stars with log L/Lsol > 5.2, the reduction is by less than a factor 5 and is mainly due to the inclusion of clumping in the models. For stars with log L/Lsol < 5.2 the reduction can be as high as a factor 100. The inclusion of X-ray emission in models with low density is crucial to derive accurate mass loss rates from UV lines. The modified wind momentum - luminosity relation shows a significant change of slope around this transition luminosity. Terminal velocities of low luminosity stars are also found to be low. The physical reason for such weak winds is still not clear although the finding of weak winds in Galactic stars excludes the role of a reduced metallicity. X-rays, through the change in the ionisation structure they imply, may be at the origin of a reduction of the radiative acceleration, leading to lower mass loss rates. A better understanding of the origin of X-rays is of crucial importance for the study of the physics of weak winds.

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arXiv2005

Stellar populations and Lya 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 HST and Spitzer observations. Using an SED fitting technique considering a number of parameters we attempt to constrain the properties of their stellar populations and their intrinsic Lya emission. The following main results are obtained for the individual galaxies: * Triple arc in Abell 2218: The most likely redshift of this source is z ~ 6.0-7.2. SED fits indicate generally a low extinction but do not strongly constrain the star formation (SF) history. Best fits have typical ages of ~3 to 400 Myr. The apparent 4000Ang break observed recently from combination of IRAC/Spitzer and HST observations, can also equally well be reproduced with the template of a young (~3-5 Myr). The estimated SFR is typically ~1 Msun/yr, in agreement with previous estimates. The unknowns on the age and star formation history could easily explain the apparent absence of Lya in this galaxy. * Abell 370 HCM 6A: The available Lya and continuum observations indicate 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 A_V ~ 0.5-1.8 at a 1 sigma level. In consequence we obtain SFR ~ 11-41 Msun/yr, higher than earlier estimates, and we estimate a fairly high total luminosity (L ~ (1-4)*10^{11} Lsun) for this galaxy, in the range LIRG. A Lya transmission of ~23-90% is estimated from our best fit models. Other properties remain largely unconstrained. In case of composite stellar populations the SFR, mass, and luminosity estimate is lower. The two scenarios may be distinguishable with IRAC/Spitzer observations at 3.6 and 4.5 micron.

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arXiv2005

A VLT study of metal-rich extragalactic H II regions. I. Observations and empirical abundances

We have obtained spectroscopic observations from 3600 Angstrom to 9200 Angstrom with FORS at the Very Large Telescope for approximately 70 H II regions located in the spiral galaxies NGC 1232, NGC 1365, NGC 2903, NGC 2997 and NGC 5236. These data are part of a project aiming at measuring the chemical abundances and characterizing the massive stellar content of metal-rich extragalactic H II regions. In this paper we describe our dataset, and present emission line fluxes for the whole sample. In 32 H II regions we measure at least one of the following auroral lines: [S II]4072, [N II]5755, [S III]6312 and [O II]7325. From these we derive electron temperatures, as well as oxygen, nitrogen and sulphur abundances, using classical empirical methods (both so-called "Te-based methods" and "strong line methods"). Under the assumption that the temperature gradient does not introduce severe biases, we find that the most metal-rich nebulae with detected auroral lines are found at 12+log(O/H)~8.9, i.e. about 60% larger than the adopted solar value. However, classical abundance determinations in metal-rich H II regions may be severely biased and must be tested with realistic photoionization models. The spectroscopic observations presented in this paper will serve as a homogeneous and high-quality database for such purpose.

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arXiv2004

Starbursts at z >~6: from the present to Herschel and ALMA

The SED of the lensed z=6.56 galaxy HCM6A behind the cluster Abell 370 has been analysed. We find clear indications for the presence of dust in this galaxy, and we estimate the properties of its stellar populations (SFR, age, etc.). From its estimated luminosity, L~(1-4)e.+11 Lsun, this galaxy ranks as a luminous infrared galaxy. This case is then used to examine the detectability of high-z galaxies with Herschel and ALMA. It is evident that with the use of strong gravitational lensing SPIRE/Herschel observations could provide very interesting information on z > 5 galaxies. Strong synergies between ground-based near-IR instruments on 8-10m class telescopes and ELTs, Herschel, the JWST, and ALMA can be expected for the exploration of the first galaxies in the Universe.

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arXiv2004

Stellar populations and Lyman-alpha emission from lensed z > 6 galaxies

We present results from an SED analysis of two lensed high-z objects, the z=6.56 galaxy HCM6A behind the cluster Abell 370 discovered by Hu et al. (2002) and the triple arc at z~7 behind Abell 2218 found by Kneib et al. (2004). For HCM 6A we find indications for the presence of dust in this galaxy, and we estimate the properties of its stellar populations (SFR, age, etc.), and the intrinsic Lyman-alpha emission. From the "best fit" reddening (E(B-V)~0.25) its estimated luminosity is L~(1-4)e.+11 Lsun, in the range of luminous infrared galaxies. For the arc behind Abell 2218 we find a most likely redshift of z~6.0-7.2 taking into account both our photometric determination and lensing considerations. SED fits indicate generally a low extinction but do not strongly constrain the SF history. Best fits have typical ages of ~3 to 400 Myr. The apparent 4000 Ang break observed recently by Egami et al. (2004) from combination of IRAC/Spitzer and HST observations can also well be reproduced with templates of young populations (~15 Myr or even younger) and does not necessarily imply old ages. Finally, we briefly examine the detectability of dusty lensed high-z galaxies with Herschel and ALMA.

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arXiv2004

Stellar Population in LLAGN.I: Ground-based observations

(Abridge): We present a spectroscopic study of the stellar populations of Low Luminosity AGN (LLAGN). Our main goal is to determine whether the stars who live in the innermost (100 pc-scale) regions of these galaxies are in some way related to the emission line properties, which would imply a link between the stellar population and the ionization mechanism. High signal to noise, ground based long-slit spectra in the 3500--5500 A interval were collected for 60 galaxies.Our main findings are: (1) Few LLAGN have a detectable young (< 10 Myr) starburst component, indicating that very massive stars do not contribute significantly to the optical continuum. In particular, no features due to Wolf-Rayet stars were convincingly detected. (2) High Order Balmer absorption lines of HI (HOBLs), on the other hand, are detected in 40% of LLAGN. These features, which are strongest in 100 Myr--1 Gyr intermediate age stellar populations, are accompanied by diluted metal absorption lines and bluer colors than other objects in the sample. (3) These intermediate age populations are very common (50 %) in LLAGN with relatively weak [OI] emission, but rare (10 %) in LLAGN with stronger [OI]. This is intriguing since LLAGN with weak [OI] have been previously hypothesized to be ``transition objects'' in which both an AGN and young stars contribute to the emission-line excitation. Massive stars, if present, are completely outshone by intermediate age and old stars in the optical. This happens in at least a couple of objects where independent UV spectroscopy detects young starbursts not seen in the optical. (4) Objects with predominantly old stars span the whole range of [OI]/Halpha values, but (5) sources with significant young and/or intermediate age populations are nearly all (90%) weak [OI] emitters.

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arXiv2003

Expected properties of primeval galaxies and confrontation with existing observations

We discuss the expected spectral properties of starbursts of zero and very low metallicity covering their restframe EUV and UV to optical spectra. The main criteria to establish such a "primeval" nature are summarised. Finally we compare these expected properties, including continuum and emission lines, to existing observations of high-z galaxies and briefly discuss future observational prospects.

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arXiv2002

The transition from Population III to normal galaxies: Ly-a and HeII 1640 emission and the ionising properties of high redshift starburst galaxies

We examine spectral properties of the ionising continua, the Lyman-break, and the Ly-a and HeII 1640 recombination lines in starbursts. The transition from primordial galaxies to currently observed metallicities, is examined. For the average properties of starbursts, the main findings are: 1) The Lyman continuum flux increases with decreasing metallicity. For a universal Salpeter IMF from 1-100 Msun the enhancement reaches typically a factor of ~3 between solar metallicity and Pop III objects. 2) While for metallicities Z >~ 1/50 Zsun the amplitude of the Lyman-break depends little on Z, a reduction by a factor ~2 is found at lower Z, due to the strong increase of the average stellar temperature. 3) Using theoretical models and empirical constraints we discuss the expected evolution of the hardness of He+ to H ionising photons with metallicity and possible uncertainties. We also provide a simple estimate of the possible impact of hot WR like stars on Q(He+)/Q(H) at very low metallicities. 4) Calibrations for star formation rate determinations from various recombination lines at all metallicities and for various IMFs are derived. For young bursts the maximum Ly-a equivalent width is shown to increase strongly with decreasing metallicity for the same IMF. However, for well known reasons, the Ly-a emission predicted likely represents an upper limit. Non-negligible HeII 1640 emission due to stellar photoionisation appears to be limited to very low metallicities and Population III objects.(abridged abstract)

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arXiv2002

The massive star IMF

We review our current knowledge on the IMF in nearby environments, massive star forming regions, super star clusters, starbursts and alike objects from studies of integrated light, and discuss the various techniques used to constrain the IMF. In most cases, including UV-optical studies of stellar features and optical-IR analysis of nebular emission, the data is found to be compatible with a "universal" Salpeter-like IMF with a high upper mass cut-off over a large metallicity range. In contrast, near-IR observations of nuclear starbursts and LIRG show indications of a lower M_up and/or a steeper IMF slope, for which no alternate explanation has yet been found. Also, dynamical mass measurements of seven super star clusters provide so far no simple picture of the IMF. Finally we present recent results of a direct stellar probe of the upper end of the IMF in metal-rich HII regions, showing no deficiency of massive stars at high metallicity, and determining a lower limit on M_up of >~ 60--90 Msun.

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arXiv2002

VLT observations of metal-rich extra galactic HII regions. I. Massive star populations and the upper end of the IMF

We have obtained high quality FORS1/VLT optical spectra of 85 disk HII regions several nearby spiral galaxies. Our sample of metal-rich HII regions with metallicities close to solar and higher reveal the presence of Wolf-Rayet (WR) stars in 27 objects from the blue WR bump and 15 additional candidate WR regions. This provides for the first time a large set of metal-rich WR regions. Approximately half of the WR regions also show broad CIV emission attributed to WR stars of the WC subtype (...). The WR regions show quite clear trends between their observed WR features and the Hbeta emission line. Detailed synthesis models are presented to understand/interpret these observations. (...) The availability of a fairly large sample of metal-rich WR regions allows us to improve existing estimates of the upper mass cut-off of the IMF in a robust way and independently of detailed modeling: from the observed maximum Hbeta equivalent width of the WR regions we derive a LOWER LIMIT for M_up of 60-90 Msun in the case of a Salpeter slope and larger values for steeper IMF slopes. From our direct probe of the massive star content we conclude that there is at present no evidence for systematic variations of the upper mass cut-off of the IMF in metal-rich environments, in contrast to some claims based on indirect nebular diagnostics. (abridged abstract)

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arXiv2002

Looking for the First Galaxies with EMIR/GTC

The new Pop III models by Schaerer (2002) have been used to derive the observed properties of the first galaxies in terms of the expected magnitudes and colors. The dependence of their properties on the IMF and upper mass limit for star formation are studied. The emerging synthetic spectra are used to discuss the implications on different observational features. Strong emission lines, such as Lyman-alpha and HeII 1640, could easily be detected with a good S/N with near-IR medium resolution spectrographs, such as EMIR at GTC. Our simulations aim at exploring possible observational constraints on the formation epoch of the first stars in galaxies.

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arXiv2001

On the properties of massive Population III stars and metal-free stellar populations

We present realistic models for massive Population III stars and stellar populations based on non-LTE model atmospheres, recent stellar evolution tracks and up-to-date evolutionary synthesis models, to study their spectral properties, including their dependence on age, star formation history, and IMF. (..) The main results regarding integrated stellar populations are: * For young bursts and the case of a constant SFR, nebular continuous emission - neglected in previous studies - dominates the spectrum redward of Lyman-alpha (...). Therefore predicted emission line equivalent widths are considerably smaller than found in earlier studies, whereas the detection of the continuum is eased. Nebular line and continuous emission strongly affect the broad band photometric properties of Pop III objects. * Due to stellar evolution, the hardness of the ionising spectrum decreases rapidly, leading to the disappearance of the characteristic HeII 1640 recombination lines after ~ 3 Myr in instantaneous bursts. * The relative efficiency of ionising photon energy to heavy element rest mass production, eta, of metal-poor and metal-free populations is increased by factors of ~ 4 to 18 with respect to solar metallicity and for ``standard'' IMFs. * The lowest values of eta ~ 1.6 - 2.2 % are obtained for IMFs exclusively populated with high mass stars (M_low >~ 50 Msun). If correct, the yields dominated by pair creation SNae then predict large overabundances of O/C and Si/C compared to solar abundance ratios. Detailed results are given in tabular form and as fit formulae for implementation in other calculations. (abridged abstract)

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arXiv2001

Observing the first galaxies - a case for an intermediate resolution multi-object IR spectrograph

We present a special science case for an intermediate resolution multi-object IR spectrograph for the VLT. We have constructed new models for massive Population III stars and metal-free stellar populations (see Schaerer 2001). The properties of individual stars and integrated populations, including their ionising fluxes and SEDs are discussed. We also study their dependence on the poorly known IMF and the star formation history. The synthetic spectra are used to simulate spectroscopic observations of the expected emission lines of Pop III galaxies. We show that such an instrument should allow the detection and efficient observations of the first galaxies with the VLT.

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arXiv2001

The evolution of emission lines in HII galaxies

We constructed diagnostic diagrams using emission line ratios and equivalent widths observed in several samples of HII galaxies. The diagrams are compared to predictions from new photoionization models for evolving starbursts. We find that HII galaxies from objective-prism surveys are not reproduced by models of instantaneous starbursts surrounded by constant density, ionization bounded HII regions. The observed relations between emission line ratios and Hb equivalent width (W(Hb)) can be understood if older stellar populations are not negligible in HII galaxies. Also, different dust obscuration for stars and gas and leakage of Lyman continuum photons from the observed HII regions can be important. As a result, HII galaxies selected from objective-prism surveys are unlikely to contain many objects in which the most recent starburst is older than about 5~Myr. The observed increase of [OI]/Hb with decreasing W(Hb) can result from the dynamical effects of winds and supernovae. This interpretation provides also a natural explanation of the small range of ionization parameters in giant HII regions. The [OIII]/Hb vs [OII]/Hb diagnostic diagram cannot be fully understood in terms of pure photoionization models and indicate the need for additional heating sources. The [NII]/[OII] ratio is shown to increase as W(Hb) decreases. A possible explanation is an N/O increase due to gradual enrichment by winds from Wolf-Rayet stars on a time scale of 5 Myr. (abridged abstract)

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arXiv2001

On ionization effects and abundance ratios in damped Lyman alpha systems

The similarity between observed velocity structures of Al III and singly ionized species in damped Lyman-alpha systems (DLAs) suggests the presence of ionized gas in the regions where most metal absorption lines are formed. A simplified model consisting of Region 1) a plane-parallel ionization bounded region illuminated by an internal radiation field, and Region 2) a neutral region with a negligible metal content is considered. We calculate photoionization equilibrium models for region 1, and constrain the ionization parameter by the observed N(Al III)/N(Si II) column density ratio. Under these conditions we find that ionization effects are important. If these effects are taken into account, the element abundance ratios in DLAs are quite consistent with those observed in Milky Way stars and in H II regions of local low-metallicity blue compact dwarf galaxies. In particular we cannot exclude the same primary N origin in both DLAs and metal-poor galaxies. No depletion of heavy elements on dust grains needs to be invoked, although our models do not exclude the presence of little depletion. Although highly simplified and relying on the strong assumption of a significantly lower metal content in region 2, our model appears to be supported by recent data on a local DLA and it is not in contradiction with the current knowledge on high redshift DLAs. If correct, it offers a clear simplification in the understanding of heavy element abundance ratios in DLAs and their comparison with the local Universe.

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arXiv2000

Stellar features in integrated starburst spectra as stellar population diagnostics

We review the main stellar features observed in starburst spectra from the UV to the near-IR and their use as fundamental tools to determine the properties of stellar populations from integrated spectra. The origin and dependence of the features on stellar properties are discussed, and we summarise existing modeling techniques used for quantitative analysis. Recent results from studies based on UV, optical and near-IR observations of starbursts and active galaxies are summarised. Finally, we briefly discuss combined starburst + photoionisation models including also observations from nebular emission lines. The present review is complementary to the recent summary by Schaerer (2000) (http://xxx.lpthe.jussieu.fr/abs/astro-ph/0007307) discussing more extensively nebular analysis of starbursts and related objects.

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