arXiv subjects

M. Stefanon

Publications and source records attributed to M. Stefanon.

54 records · Page 3Linked to original sources

arXiv2016

The Lyman-Continuum Photon Production Efficiency xi_{ion} of z~4-5 Galaxies from IRAC-based Halpha Measurements: Implications for the Escape Fraction and Cosmic Reionization

Galaxies represent one of the preferred candidate sources to drive the reionization of the universe. Even as gains are made in mapping the galaxy UV luminosity density to z>6, significant uncertainties remain regarding the conversion to the implied ionizing emissivity. The relevant unknowns are the Lyman-continuum (LyC) photon production efficiency xi_{ion} and the escape fraction f_{esc}. As we show here, the first of these unknowns is directly measureable in z=4-5 galaxies, based on the impact the Halpha line has on the observed IRAC fluxes. By computing a LyC photon production rate from the implied Halpha luminosities for a broad selection of z=4-5 galaxies and comparing this against the dust-corrected UV-continuum luminosities, we provide the first-ever direct estimates of the LyC photon production efficiency xi_{ion} for the z>~4 galaxy population. We find log_{10} xi_{ion}/[Hz/ergs] to have a mean value of 25.27_{-0.03}^{+0.03} and 25.34_{-0.02}^{+0.02} for sub-L* z=4-5 galaxies adopting Calzetti and SMC dust laws, respectively. Reassuringly, both values are consistent with standardly assumed xi_{ion}'s in reionization models, with a slight preference for higher xi_{ion}'s (by ~0.1 dex) adopting the SMC dust law. A modest ~0.03-dex increase in these estimates would result if the escape fraction for ionizing photons is non-zero and galaxies dominate the ionizing emissivity at z~4.4. High values of xi_{ion} (~25.5-25.8 dex) are derived for the bluest galaxies (beta<-2.3) in our samples, independent of dust law and consistent with results for a z=7.045 galaxy. Such elevated values of xi_{ion} would have important consequences, indicating that f_{esc} cannot be in excess of 13% unless the galaxy UV luminosity function does not extend down to -13 mag or the clumping factor is greater than 3. A low escape fraction would fit well with the low rate of LyC leakage observed at z~3.

astro-ph.GA↗

arXiv2016

Z > 7 galaxies with red Spitzer/IRAC [3.6]-[4.5] colors in the full CANDELS data set: the brightest-known galaxies at Z ~ 7-9 and a probable spectroscopic confirmation at Z=7.48

We identify 4 unusually bright (H < 25.5) galaxies from HST and Spitzer CANDELS data with probable redshifts z ~ 7-9. These identifications include the brightest-known galaxies to date at z > 7.5. As Y-band observations are not available over the full CANDELS program to perform a standard Lyman-break selection of z > 7 galaxies, we employ an alternate strategy using deep Spitzer/IRAC data. We identify z ~ 7.1 - 9.1 galaxies by selecting z >~ 6 galaxies from the HST CANDELS data that show quite red IRAC [3.6]-[4.5] colors, indicating strong [OIII]+Hbeta lines in the 4.5 micron band. This selection strategy was validated using a modest sample for which we have deep Y-band coverage, and subsequently used to select the brightest z > 7 sources. Applying the IRAC criteria to all HST-selected optical-dropout galaxies over the full ~900 arcmin**2 of the CANDELS survey revealed four unusually bright z ~ 7.1, 7.6, 7.9 and 8.6 candidates. The median [3.6]-[4.5] color of our selected z ~ 7.1-9.1 sample is consistent with rest-frame [OIII]+Hbeta EWs of ~1500A, in the [4.5] band. Keck/MOSFIRE spectroscopy has been independently reported for two of our selected sources, showing Ly-alpha at redshifts of 7.7302+/-0.0006 and 8.683^+0.001_-0.004, respectively. We present similar Keck/MOSFIRE spectroscopy for a third selected galaxy with a probable 4.7sigma Ly-alpha line at z_spec=7.4770+/-0.0008. All three have H-band magnitudes of ~25 mag and are ~0.5 mag more luminous (M(UV) ~ -22.0) than any previously discovered z ~ 8 galaxy, with important implications for the UV LF. Our 3 brightest, highest redshift z > 7 galaxies all lie within the CANDELS EGS field, providing a dramatic illustration of the potential impact of field-to-field variance.

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arXiv2016

The ALHAMBRA survey: evolution of galaxy spectral segregation

We study the clustering of galaxies as a function of spectral type and redshift in the range $0.35 < z < 1.1$ using data from the Advanced Large Homogeneous Area Medium Band Redshift Astronomical (ALHAMBRA) survey. The data cover 2.381 deg$^2$ in 7 fields, after applying a detailed angular selection mask, with accurate photometric redshifts [$σ_z < 0.014(1+z)$] down to $I_{AB} < 24$. From this catalog we draw five fixed number density, redshift-limited bins. We estimate the clustering evolution for two different spectral populations selected using the ALHAMBRA-based photometric templates: quiescent and star-forming galaxies. For each sample, we measure the real-space clustering using the projected correlation function. Our calculations are performed over the range $[0.03,10.0] h^{-1}$ Mpc, allowing us to find a steeper trend for $r_p \lesssim 0.2 h^{-1}$ Mpc, which is especially clear for star-forming galaxies. Our analysis also shows a clear early differentiation in the clustering properties of both populations: star-forming galaxies show weaker clustering with evolution in the correlation length over the analysed redshift range, while quiescent galaxies show stronger clustering already at high redshifts, and no appreciable evolution. We also perform the bias calculation where similar segregation is found, but now it is among the quiescent galaxies where a growing evolution with redshift is clearer. These findings clearly corroborate the well known colour-density relation, confirming that quiescent galaxies are mainly located in dark matter halos that are more massive than those typically populated by star-forming galaxies.

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arXiv2015

Ultradeep IRAC Imaging Over The HUDF And GOODS-South: Survey Design And Imaging Data Release

The IRAC ultradeep field (IUDF) and IRAC Legacy over GOODS (IGOODS) programs are two ultradeep imaging surveys at 3.6μm and 4.5μm with the Spitzer Infrared Array Camera (IRAC). The primary aim is to directly detect the infrared light of reionization epoch galaxies at z > 7 and to constrain their stellar populations. The observations cover the Hubble Ultra Deep Field (HUDF), including the two HUDF parallel fields, and the CANDELS/GOODS-South, and are combined with archival data from all previous deep programs into one ultradeep dataset. The resulting imaging reaches unprecedented coverage in IRAC 3.6μm and 4.5μm ranging from > 50 hour over 150 arcmin^2, > 100 hour over 60 sq arcmin2, to 200 hour over 5 - 10 arcmin$^2$. This paper presents the survey description, data reduction, and public release of reduced mosaics on the same astrometric system as the CANDELS/GOODS-South WFC3 data. To facilitate prior-based WFC3+IRAC photometry, we introduce a new method to create high signal-to-noise PSFs from the IRAC data and reconstruct the complex spatial variation due to survey geometry. The PSF maps are included in the release, as are registered maps of subsets of the data to enable reliability and variability studies. Simulations show that the noise in the ultradeep IRAC images decreases approximately as the square root of integration time over the range 20 - 200 hours, well below the classical confusion limit, reaching 1σ point source sensitivities as faint as of 15 nJy (28.5 AB) at 3.6μm and 18 nJy (28.3 AB) at 4.5μm. The value of such ultradeep IRAC data is illustrated by direct detections of z = 7 - 8 galaxies as faint as HAB = 28.

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arXiv2014

The ALHAMBRA survey: evolution of galaxy clustering since $z \sim 1$

We study the clustering of galaxies as function of luminosity and redshift in the range $0.35 < z < 1.25$ using data from the Advanced Large Homogeneous Area Medium Band Redshift Astronomical (ALHAMBRA) survey. The ALHAMBRA data used in this work cover $2.38 \mathrm{deg}^2$ in 7 independent fields, after applying a detailed angular selection mask, with accurate photometric redshifts, $σ_z \lesssim 0.014 (1+z)$, down to $I_{\rm AB} < 24$. Given the depth of the survey, we select samples in $B$-band luminosity down to $L^{\rm th} \simeq 0.16 L^{*}$ at $z = 0.9$. We measure the real-space clustering using the projected correlation function, accounting for photometric redshifts uncertainties. We infer the galaxy bias, and study its evolution with luminosity. We study the effect of sample variance, and confirm earlier results that the COSMOS and ELAIS-N1 fields are dominated by the presence of large structures. For the intermediate and bright samples, $L^{\rm med} \gtrsim 0.6L^{*}$, we obtain a strong dependence of bias on luminosity, in agreement with previous results at similar redshift. We are able to extend this study to fainter luminosities, where we obtain an almost flat relation, similar to that observed at low redshift. Regarding the evolution of bias with redshift, our results suggest that the different galaxy populations studied reside in haloes covering a range in mass between $\log_{10}[M_{\rm h}/(h^{-1}\mathrm{M}_{\odot})] \gtrsim 11.5$ for samples with $L^{\rm med} \simeq 0.3 L^{*}$ and $\log_{10}[M_{\rm h}/(h^{-1}\mathrm{M}_{\odot})] \gtrsim 13.0$ for samples with $L^{\rm med} \simeq 2 L^{*}$, with typical occupation numbers in the range of $\sim 1 - 3$ galaxies per halo.

astro-ph.CO↗

arXiv2010

Spectrophotometric Redshifts. A New Approach to the Reduction of Noisy Spectra and its Application to GRB090423

We have developed a new method, close in philosophy to the photometric redshift technique, which can be applied to spectral data of very low signal-to-noise ratio. Using it we intend to measure redshifts while minimising the dangers posed by the usual extraction techniques. GRB afterglows have generally very simple optical spectra over which the separate effects of absorption and reddening in the GRB host, the intergalactic medium, and our own Galaxy are superimposed. We model all these effects over a series of template afterglow spectra to produce a set of clean spectra that reproduce what would reach our telescope. We also model carefully the effects of the telescope-spectrograph combination and the properties of noise in the data, which are then applied on the template spectra. The final templates are compared to the two-dimensional spectral data, and the basic parameters (redshift, spectral index, Hydrogen absorption column) are estimated using statistical tools. We show how our method works by applying it to our data of the NIR afterglow of GRB090423. At z ~ 8.2, this was the most distant object ever observed. We use the spectrum taken by our team with the Telescopio Nazionale Galileo to derive the GRB redshift and its intrinsic neutral Hydrogen column density. Our best fit yields z=8.4^+0.05/-0.03 and N(HI)<5x10^20 cm^-2, but with a highly non-Gaussian uncertainty including the redshift range z [6.7, 8.5] at the 2-sigma confidence level. Our method will be useful to maximise the recovered information from low-quality spectra, particularly when the set of possible spectra is limited or easily parameterisable while at the same time ensuring an adequate confidence analysis.

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arXiv2010

Challenging GRB models through the broadband dataset of GRB060908

Context: Multiwavelength observations of gamma-ray burst prompt and afterglow emission are a key tool to disentangle the various possible emission processes and scenarios proposed to interpret the complex gamma-ray burst phenomenology. Aims: We collected a large dataset on GRB060908 in order to carry out a comprehensive analysis of the prompt emission as well as the early and late afterglow. Methods: Data from Swift-BAT, -XRT and -UVOT together with data from a number of different ground-based optical/NIR and millimeter telescopes allowed us to follow the afterglow evolution from about a minute from the high-energy event down to the host galaxy limit. We discuss the physical parameters required to model these emissions. Results: The prompt emission of GRB060908 was characterized by two main periods of activity, spaced by a few seconds of low intensity, with a tight correlation between activity and spectral hardness. Observations of the afterglow began less than one minute after the high-energy event, when it was already in a decaying phase, and it was characterized by a rather flat optical/NIR spectrum which can be interpreted as due to a hard energy-distribution of the emitting electrons. On the other hand, the X-ray spectrum of the afterglow could be fit by a rather soft electron distribution. Conclusions: GRB060908 is a good example of a gamma-ray burst with a rich multi-wavelength set of observations. The availability of this dataset, built thanks to the joint efforts of many different teams, allowed us to carry out stringent tests for various interpretative scenarios showing that a satisfactorily modeling of this event is challenging. In the future, similar efforts will enable us to obtain optical/NIR coverage comparable in quality and quantity to the X-ray data for more events, therefore opening new avenues to progress gamma-ray burst research.

astro-ph.HE↗

arXiv2010

The Sooner: a Large Robotic Telescope

The approach of Observational Astronomy is mainly aimed at the construction of larger aperture telescopes, more sensitive detectors and broader wavelength coverage. Certainly fruitful, this approach turns out to be not completely fulfilling the needs when phenomena related to the formation of black holes (BH), neutron stars (NS) and relativistic stars in general are concerned. Recently, mainly through the Vela, Beppo-SAX and Swift satellites, we reached a reasonable knowledge of the most violent events in the Universe and of some of the processes we believe are leading to the formation of black holes (BH). We plan to open a new window of opportunity to study the variegated physics of very fast astronomical transients, particularly the one related to extreme compact objects. The innovative approach is based on three cornerstones: 1) the design (the conceptual design has been already completed) of a 3m robotic telescope and related focal plane instrumentation characterized by the unique features: "No telescope points faster"; 2) simultaneous multi-wavelengths observations (photometry, spectroscopy o\& polarimetry); 3) high time resolution observations. The conceptual design of the telescope and related instrumentation is optimized to address the following topics: High frequency a-periodic variability, Polarization, High z GRBs, Short GRBs, GRB-Supernovae association, Multi-wavelengths simultaneous photometry and rapid low dispersion spectroscopy. This experiment will turn the "exception" (like the optical observations of GRB 080319B) to "routine".

astro-ph.IM↗

arXiv2007

The afterglow onset for GRB060418 and GRB060607A

Gamma-ray burst are thought to be produced by highly relativistic outflows. Although upper and lower limits for the outflow initial Lorentz factor $Γ_0$ are available, observational efforts to derive a direct determination of $Γ_0$ have so far failed or provided ambiguous results. As a matter of fact, the shape of the early-time afterglow light curve is strongly sensitive on $Γ_0$ which determines the time of the afterglow peak, i.e. when the outflow and the shocked circumburst material share a comparable amount of energy. We now comment early-time observations of the near-infrared afterglows of GRB 060418 and GRB 060607A performed by the REM robotic telescope. For both events, the afterglow peak was singled out and allowed us to determine the initial fireball Lorentz, $Γ_0\sim 400$.

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arXiv2007

VSOP: The Variable Star One-shot Project I. Project presentation and first data release

The Variable Star One-shot Project (VSOP) is aimed at (1) providing the variability type and spectral type of all unstudied variable stars, (2) process, publish, and make the data available as automatically as possible, and (3) generate serendipitous discoveries. This first paper describes the project itself, the acquisition of the data, the dataflow, the spectroscopic analysis and the on-line availability of the fully calibrated and reduced data. We also present the results on the 221 stars observed during the first semester of the project. We used the high-resolution echelle spectrographs HARPS and FEROS in the ESO La Silla Observatory (Chile) to survey known variable stars. Once reduced by the dedicated pipelines, the radial velocities are determined from cross correlation with synthetic template spectra, and the spectral types are determined by an automatic minimum distance matching to synthetic spectra, with traditional manual spectral typing cross-checks. The variability types are determined by manually evaluating the available light curves and the spectroscopy. In the future, a new automatic classifier, currently being developed by members of the VSOP team, based on these spectroscopic data and on the photometric classifier developed for the COROT and Gaia space missions, will be used. We confirm or revise spectral types of 221 variable stars from the GCVS. We identify 26 previously unknown multiple systems, among them several visual binaries with spectroscopic binary individual components. We present new individual results for the multiple systems V349 Vel and BC Gru, for the composite spectrum star V4385 Sgr, for the T-Tauri star V1045 Sco, and for DM Boo which we re-classify as a BY Draconis variable. The complete data release can be accessed via the VSOP web site.

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arXiv2007

The Variable Star One-shot Project, and its little child: Wikimbad

The Variable Star One-shot Project (VSOP) aimed at providing to the world-wide stellar community the necessary one-shot spectrum of unstudied variable stars, too often classified as such by an analysis of photometric data only. The VSOP has established an new kind of observational model, where all steps from observations to spectral analysis, are automatized (or are underway to be fully automatized). The project is centralized on a collaborative wiki website. The VSOP operational model is very successful, data is continously flowing and being analyszed, and VSOP is now a worldwide open collaboration of people with very different and complementary skills and expertise. The idea of a central wiki website has been extended by one of us to propose a new service to the whole astronomical community, called Wikimbad. Wikimbad is an open wiki website aimed at collecting, organizing and making publicly available all kind of reduced and published astronomical data. Its strengths and a comparison with the Virtual Observatory are discussed. See: http://vsop.sc.eso.org and http://wikimbad.org

astro-ph↗

arXiv2007

REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination

Gamma-ray burst (GRB) emission is believed to originate in highly relativistic fireballs. Currently, only lower limits were securely set to the initial fireball Lorentz factor Gamma_0. We aim to provide a direct measure of Gamma_0. The early-time afterglow light curve carries information about Gamma_0, which determines the time of the afterglow peak. We have obtained early observations of the near-infrared afterglows of GRB 060418 and GRB 060607A with the REM robotic telescope. For both events, the afterglow peak could be clearly singled out, allowing a firm determination of the fireball Lorentz of Gamma_0 ~ 400, fully confirming the highly relativistic nature of GRB fireballs. The deceleration radius was inferred to be R_dec ~ 10^17 cm. This is much larger than the internal shocks radius (believed to power the prompt emission), thus providing further evidence for a different origin of the prompt and afterglow stages of the GRB.

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arXiv2006

Evidence for a Supernova Associated with the X-ray Flash 020903

We present ground-based and Hubble Space Telescope optical observations of the X-ray flash (XRF) 020903, covering 300 days. The afterglow showed a very rapid rise in the first day, followed by a relatively slow decay in the next few days. There was a clear bump in the light curve after ~25 days, accompanied by a drastic change in the spectral energy distribution. The light curve and the spectral energy distribution are naturally interpreted as the emergence -- and subsequent decay -- of a supernova (SN), similar to SN 1998bw. At peak luminosity, the SN is estimated to be 0.8 +/- 0.1 mag fainter than SN1998bw. This argues in favor of the existence of a supernova associated with this X-ray flash. A spectrum obtained 35 days after the burst shows emission lines from the host galaxy. We use this spectrum to put an upper limit on the oxygen abundance of the host at [O/H] < -0.6 dex. We also discuss a possible trend between the softness of several bursts and the early behavior of the optical afterglow, in the sense that XRFs and X-ray rich GRBs seem to have a plateau phase or even a rising light curve. This can be naturally explained in models where XRFs are similar to GRBs but seen off the jet axis.

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arXiv2005

Multiband Photometry of the Blazar PKS 0537-441: A Major Active State in December 2004 - March 2005

Multiband VRIJHK photometry of the Blazar PKS 0537-441 obtained with the REM telescope from December 2004 to March 2005 is presented. A major period of activity is found with more than four magnitudes variability in the V filter in 50 days and of 2.5 in 10 days. In intensity and duration the activity is similar to that of 1972 reported by Eggen (1973), but it is much better documented. No clear evidence of variability on time-scale of minutes is found. The spectral energy distribution is roughly described by a power-law, with the weaker state being the softer.

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arXiv2005

GRB 021004 modelled by multiple energy injections

GRB 021004 is one of the best sampled gamma-ray bursts (GRB) to date, although the nature of its light curve is still being debated. Here we present a large amount (107) of new optical, near-infrared (NIR) and millimetre observations, ranging from 2 hours to more than a year after the burst. Fitting the multiband data to a model based on multiple energy injections suggests that at least 7 refreshed shocks took place during the evolution of the afterglow, implying a total energy release (collimated within an angle of 1.8 deg) of ~ 8x10^51 erg. Analysis of the late photometry reveals that the GRB 021004 host is a low extinction (Av ~ 0.1) starburst galaxy with M_B ~ -22.0.

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arXiv2003

Optical and NIR Observations of the Afterglow of GRB 020813

We present optical and near-infrared (NIR) photometry of the bright afterglow of GRB 020813. Our data span from 3 hours to 4 days after the GRB event. A rather sharp achromatic break is present in the light curve, 14 hours after the trigger. In the framework of jetted fireballs, this break corresponds to a jet half opening angle of (1.9 +- 0.2) deg, the smallest value ever inferred for a GRB. We discuss our results in the framework of currently available models, and find that they have problems in explaining the joint temporal and spectral properties, and in particular the slow decay before the break.

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arXiv2002

Polarization Evolution of the GRB 020405 Afterglow

Polarization measurements for the optical counterpart to GRB 020405 are presented and discussed. Our observations were performed with the VLT-UT3 (Melipal) during the second and third night after the gamma-ray burst discovery. The polarization degree (and the position angle) appears to be constant between our two observations at a level around (1.5 - 2)%. The polarization can be intrinsic but it is not possible to unambiguously exclude that a substantial fraction of it is induced by dust in the host galaxy.

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arXiv2002

Variable stars in nearby galaxies. V. Search for Cepheids in Field A of NGC 6822

The results of a CCD survey for variability of stars in the nearby galaxy NGC 6822 are presented. The goal of the survey was to obtain good light curves of Cepheids for Fourier decomposition and to detect shorter period Cepheids. Since the program was carried out with a relatively small telescope, the Dutch 0.9 m at ESO-La Silla, the observations were unfiltered (white light, or Wh-band). The analysis revealed the presence of more than 130 variable stars. 21 population I Cepheids are detected; 6 of them were already known from previous works (Kayser, 1967). For at least three Cepheids, however, the previous identification or period was wrong. Some probable population II (W Vir) stars are also identified. The dispersion of the fundamental mode Cepheid PL relation appears to be small.

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