arXiv subjects

Matthew Hayes

Publications and source records attributed to Matthew Hayes.

120 records · Page 7Linked to original sources

arXiv2010

On the origin of the red excess in very young super star clusters: the case of SBS 0335-052E

The spectral energy distribution analysis of very young unresolved star clusters challenges our understanding of the cluster formation process. Studies of resolved massive clusters in the Milky Way and in the nearby Magellanic Clouds show us that the contribution from photoionized gas is very important during the first Myr of cluster evolution. We present our models which include both a self-consistent treatment of the photoionized gas and the stellar continuum and quantify the impact of such nebular component on the total flux of young unresolved star clusters. A comparison with other available models is considered. The very young star clusters in the SBS 0335-052E dwarf starburst galaxy are used as a test for our models. Due to the low metallicity of the galactic medium our models predict a longer lasted nebular phase which contributes between 10-40% of the total near infrared (NIR) fluxes at around 10 Myr. We propose thus a possible solution for the observed flux excess in the 6 bright super star clusters of SBS 0335-052E. Reines et al. showed that the observed cluster fluxes, in the red-optical and NIR range, sit irreconcilably above the provided stellar continuum models. We find that in the age range estimated from the H_alpha emission we can explain the red excess in all the 6 super star clusters as due to nebular emission, which at cluster ages around 10 Myr still affects the NIR wavebands substantially.

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arXiv2010

Escape of about five per cent of Lyman-alpha photons from high-redshift star-forming galaxies

The Lyman-alpha (Lya) emission line is the primary observational signature of star-forming galaxies at the highest redshifts, and has enabled the compilation of large samples of galaxies with which to study cosmic evolution. The resonant nature of the line, however, means that Lya photons scatter in the neutral interstellar medium of their host galaxies, and their sensitivity to absorption by interstellar dust may therefore be enhanced greatly. This implies that the Lya luminosity may be significantly reduced, or even completely suppressed. Hitherto, no unbiased empirical test of the escaping fraction (f_esc) of Lya photons has been performed at high redshifts. Here we report that the average fesc from star-forming galaxies at redshift z = 2.2 is just 5 per cent by performing a blind narrowband survey in Lya and Ha. This implies that numerous conclusions based on Lya-selected samples will require upwards revision by an order of magnitude and we provide a benchmark for this revision. We demonstrate that almost 90 per cent of star-forming galaxies emit insufficient Lya to be detected by standard selection criteria. Both samples show an anti-correlation of fesc with dust content, and we show that Lya- and Ha-selection recovers populations that differ substantially in dust content and fesc.

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arXiv2009

The H-alpha luminosity function at redshift 2.2: A new determination using VLT/HAWK-I

We aim to place new, strengthened constraints on the luminosity function (LF) of H-alpha emitting galaxies at redshift z=2.2, and to further constrain the instantaneous star-formation rate density of the universe (rho*). We have used the new HAWK-I instrument at ESO-VLT to obtain extremely deep narrow-band (line; NB2090) and broad-band (continuum; Ks) imaging observations. The target field is in the GOODS-South, providing us with a rich multi-wavelength auxiliary data set, which we utilise for redshift confirmation and to estimate dust content. We use this new data to measure the faint-end slope (alpha) of LF(H-alpha) with unprecedented precision. The data are well fit by a Schechter function and also a single power-law, yielding alpha=(-1.72 +/- 0.20) and (-1.77 +/- 0.21), respectively. Thus we are able to confirm the steepening of alpha from low- to high-z predicted by a number of authors and observed at other wavelengths. We combine our LF data-points with those from a much shallower but wider survey at z=2.2 (Geach et al. 2008), constructing a LF spanning a factor of 50 in luminosity. Re-fitting the Schechter parameters, we obtain log L*=(43.07+/-0.22)erg s^-1 ; log phi*=(-3.45+/-0.52)Mpc^-3 ; alpha=(-1.60+/-0.15). We integrate over LF(Halpha) and apply a correction for dust attenuation to determine the instantaneous cosmic star-formation rate density at z=2 without assuming alpha or extrapolating it from lower-z. Our measurement of rho* is (0.215+/-0.090) Msun yr^-1 Mpc^-3, integrated over a range of 37 <log(LHhalpha / erg s^-1) < 47.

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arXiv2009

Empirical Estimate of Lyman-alpha Escape Fraction in a Statistical Sample of Lyman-alpha Emitters

The Lyman-alpha (Lya) recombination line is a fundamental tool for galaxy evolution studies and modern observational cosmology. However, subsequent interpretations are still prone to a number of uncertainties. Besides numerical efforts, empirical data are urgently needed for a better understanding of Lya escape process. We empirically estimate the Lyman-alpha escape fraction fesc(Lya) in a statistically significant sample of z ~ 0 - 0.3 galaxies in order to calibrate high-redshift Lyman-alpha observations. An optical spectroscopic follow-up of a sub-sample of 24 Lyman-alpha emitters (LAEs) detected by GALEX at z ~ 0.2-0.3, combined with a UV-optical sample of local starbursts, both with matched apertures, allow us to quantify the dust extinction through Balmer lines, and to estimate the Lyman-alpha escape fraction from the Halpha flux corrected for extinction in the framework of the recombination theory. The global escape fraction of Lyman-alpha radiation spans nearly the entire range of values, from 0.5 to 100 %, and fesc(Lya) clearly decreases with increasing nebular dust extinction E(B-V). Several objects show fesc(Lya) greater than fesc(continuum) which may be an observational evidence for clumpy ISM geometry or for an aspherical ISM. Selection biases and aperture size effects may still prevail between z ~ 0.2-0.3 LAEs and local starbursts, which may explain the difference observed for fesc(Lya).

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arXiv2009

The Extremely Young Star Cluster Population In Haro 11

We have performed a deep multi-band photometric analysis of the star cluster population of Haro 11. This starburst galaxy (log L_FUV = 10.3 L_sun) is considered a nearby analogue of Lyman break galaxies (LBGs) at high redshift. The study of the numerous star clusters in the systems is an effective way to investigate the formation and evolution of the starburst phase. In fact, the SED fitting models have revealed a surprisingly young star cluster population, with ages between 0.5 and 40 Myr, and estimated masses between 10^3 and 10^7 solar masses. An independent age estimation has been done with the EW(Halpha) analysis of each cluster. This last analysis has confirmed the young ages of the clusters. We noticed that the clusters with ages between 1 and 10 Myr show a flux excess in H (NIC3/F160W) and/or I (WFPC2/F814W) bands with respect to the evolutionary models. Once more Haro 11 represents a challenge to our understanding.

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arXiv2009

Continuum subtracting Lyman-alpha images: Low redshift studies using the Solar Blind Channel of HST/ACS

[ABRIDGED] We are undertaking an imaging study of local star-forming galaxies in the Lyman-alpha (Lya) emission line using the Solar Blind Channel (SBC) of the ACS onboard the HST. Observations have been obtained in Lya and H-alpha (Ha) and 6 continuum filters between ~1500AA and the I-band. Previously (Hayes et al 2005) we demonstrated that the production of Lya line-only images in the SBC-only data-set is non-trivial and that supporting data is a requirement. We here develop various methods of continuum subtraction and assess their relative performance using a variety of spectral energy distributions (SED) as input. We conclude that simple assumptions about the behavior of the ultraviolet continuum consistently lead to results that are wildly erroneous, and determine that an SED fitting approach is essential. Moreover, a single component stellar or stellar+nebular spectrum is not always sufficient for realistic template SEDs and, in order to successfully recover the input observables, care must be taken to control contributions of nebular gas and underlying stellar populations. Independent measurements of the metallicity must first be obtained, but details of the initial mass function play only a small role. We identify the need to bin together pixels in our data to obtain signal-to-noise (S/N) of ~10 in each band before processing. At S/N=10 we can recover Lya fluxes accurate to ~30% for Lya lines with intrinsic equivalent width [W(Lya)] of 10AA. This accuracy improves to ~10% for W(Lya)=100AA. We describe the image processing applied to the observations presented in Östlin et al. (2009) and the associated data-release. We also present simulations for an observing strategy for an alternative low-redshift SBC Lya imaging campaign using adjacent combinations of long-pass filters at slightly higher redshift.

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arXiv2009

The Lyman alpha morphology of local starburst galaxies: release of calibrated images

We present reduced and calibrated high resolution Lyman-alpha (Lya) images for a sample of 6 local star forming galaxies. Targets were selected to represent a range in luminosity and metallicity and to include both known Lya emitters and non-emitters. Far ultraviolet imaging was carried out with the Solar Blind Channel of the ACS on HST in the F122M (Lya on-line) and F140LP (continuum) filters. The resulting Lya images are the product of careful modeling of both the stellar and nebular continua, facilitated by supporting HST imaging in Ha and 5 continuum bands, combined with Starburst99 evolutionary synthesis models, and prescriptions for dust extinction on the continuum. In all, the resulting morphologies in Lya, Ha, and UV-continuum are qualitatively very different and we show that the bulk of Lya emerges in a diffuse component resulting from resonant scattering events. Lya escape fractions, computed from integrated Ha luminosities and recombination theory, are found never to exceed 14%. Internal dust extinction is estimated in each pixel and used to correct Lya fluxes. However, the extinction corrections are far too small (factors from 2.6 to infinity) to reconcile the global Lya luminosities with recombination theory. Surprisingly, when comparing the global equivalent widths of Lya and Ha, the two quantities appear anti-correlated, which may be due to the evolution of mechanical feedback. This calls for caution in the interpretation of Lya observations. The images presented have a physical resolution 3 orders of magnitude better than attainable at high-z from the ground with current instrumentation and our images may therefore serve as useful templates for comparing with observations and modeling of primeval galaxy formation. We therefore provide the reduced Lya, Ha, and continuum images to the community.

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arXiv2009

Tracing the star formation history of three Blue Compact galaxies through the analysis of their star clusters

We present preliminary results from a study of the compact star cluster populations in three local luminous blue compact galaxies: ESO 185-IG 013, ESO 350-IG 038 (a.k.a. Haro 11), and MRK 930. These systems show peculiar morphologies and the presence of hundreds of SCs that have been produced by the past, recent, and/or current starburst phases. We use a complete set of HST images ranging from the UV to IR for each galaxy. Deep images in V (WFPC2/f606w) and I (WFPC2/f814w) are used to capture most of the star cluster candidates up to the old ones (fainter) which have had, in the past, less possibility to be detected. The other bands are used in the SED fitting technique for constraining ages and masses. Our goals are to investigate the evolution of these three blue compact galaxies and the star cluster formation impact on their star formation history.

astro-ph.CO↗

arXiv2008

On the Detectability of Lyman-alpha Emission in Star-forming Galaxies: The Role of Dust

Lyman-alpha is now widely used to investigate the galaxy formation and evolution in the high redshift universe. However, without a rigorous understanding of the processes which regulate the Lya escape fraction, physical interpretations of high-z observations remain questionable. We examine six nearby star-forming galaxies to disentangle the role of the dust from other parameters such as gas kinematics, geometry and ISM morphology in the obscuration of Ly-alpha. Thereby we aim to understand the Ly-a escape physics and infer the implications for high-redshift studies. We use HST/ACS to produce continuum-subtracted Lya maps, and ground-based observations (ESO/NTT and NOT) to map the Halpha emission and the extinction E(B-V) in the gas phase derived from the Balmer decrement Halpha/Hbeta. When large outflows are present, the Lya emission appears not to correlate with the dust content, confirming the role of the HI kinematics in the escape of Lya photons. In the case of a dense, static HI covering, we observe a damped absorption with a declining relationship between Lya and E(B-V). We found that the Lya escape fraction does not exceed 10% in all our galaxies and is mostly about 3% or below. Finally, because of the radiative transfer complexity of the Lya line, star formation rate based on Lya luminosity is underestimated with respect to that derived from UV luminosity. The failure of simple dust correction to recover the intrinsic Lya/Ha ratio or the total star formation rate should prompt us to be more cautious when interpreting high-z observations and related properties, such as SFRs based on Lya alone. To this end we propose a more realistic calibration for SFR(Lya) which accounts for dust attenuation and resonant scattering effects via the Lya escape fraction.

astro-ph↗

arXiv2007

The escape of Lyman photons from a young starburst: the case of Haro 11

(Abridged) Lyman-alpha (Lya) is a dominant probe of the galaxy population at high-z. However, interpretation of data drawn from Lya alone hinges on the Lya escape fraction which, due to the complex radiative transport, may vary greatly. Here we map the Lya emission from local starburst Haro 11, a Lya emitter and the only known candidate for low-z Lyman continuum emission (LyC). To aid in the interpretation we perform a detailed multi-wavelength analysis and model the stellar population, dust distribution, ionising photon budget, and star-cluster population. We use archival X-ray observations to further constrain properties of the starburst and estimate the HI column density. The Lya morphology is found to be strongly decoupled from stellar and nebular (H-alpha) morphologies. General surface photometry finds only very slight correlation between Lya and H-halpha, E(B-V), and stellar age. Only around the central Lya-bright cluster do we find the Lya/Ha ratio at values predicted by recombination theory. The total Lya escape fraction is found to be just 3%. We compute that ~90% of the Lya photons that escape do so after undergoing multiple resonance scattering events, masking their point of origin. This leads to a largely symmetric distribution and, by increasing the distance that photons must travel to escape, decreases the escape probability significantly. While dust must ultimately be responsible for the destruction of Lya, it plays little role in governing the observed morphology, which is regulated more by ISM kinematics and geometry. We find tentative evidence for local Lya equivalent width in the immediate vicinity of star-clusters being a function of cluster age, consistent with hydrodynamic studies. We estimate the ionising photon production and further constrain the escape fraction at 900 AA to <~9% .

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arXiv2006

On the narrowband detection properties of high-redshift Lyman-alpha emitters

Numerous surveys are underway or planned that aim to exploit the Lyman-alpha emission line as a probe of the distant universe. We investigate the reliability of such high-z Lya studies and the validity of the conclusions that are based upon them. We examine whether reliable Lya fluxes (F_Lya) and equivalent widths (W_Lya) can be estimated from narrowband imaging surveys and whether any observational biases may be present. We have developed software to simulate the observed line and continuum properties of synthetic LAEs in the distant universe by adopting various typical observational techniques. This was used to investigate how detected F_Lya and W_Lya vary with properties of the host or IGM: internal dust reddening; intervening Lya absorption systems; the presence of underlying stellar populations. None of the techniques studied are greatly susceptible to underlying stellar populations or the contribution of nebular gas. We find that techniques that use one off-line filter on the red side of Lya result in highly inaccurate measurements of W_Lya. Adopting two off-line filters to estimate continuum at Lya is an improvement but is still unreliable when dust extinction is considered. Techniques that employ single narrow- and broad-band filters with the same central wavelength are not susceptible to internal dust, but IGM Lya absorption can cause W_Lya to be overestimated by factors of up to 2: at z=6, the median W_Lya is overestimated by ~25%. The most robust approach is a SED fitting technique that fits E(B-V) and burst-age from synthetic models - broadband observations are needed that sample the UV continuum slope, 2175AA dust feature, and the 4000AA discontinuity. We also notice a redshift-dependent incompleteness that results from DLA systems close to the target LAEs, amounting to ~10% at z=6.

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arXiv2006

VADER - A Satellite Mission Concept For High Precision Dark Energy Studies

We present a satellite mission concept to measure the dark energy equation of state parameter w with percent-level precision. The Very Ambitious Dark Energy Research satellite (VADER) is a multi-wavelength survey mission joining X-ray, optical, and IR instruments for a simultaneous spectral coverage from 4microns (0.3eV) to 10keV over a field of view (FoV) of 1 square degree. VADER combines several clean methods for dark energy studies, the baryonic acoustic oscillations in the galaxy and galaxy cluster power spectrum and weak lensing, for a joint analysis over an unrivalled survey volume. The payload consists of two XMM-like X-ray telescopes with an effective area of 2,800cm^2 at 1.5keV and state-of-the-art wide field DEPFET pixel detectors (0.1-10keV) in a curved focal plane configuration to extend the FoV. The X-ray telescopes are complemented by a 1.5m optical/IR telescope with 8 instruments for simultaneous coverage of the same FoV from 0.3 to 4 microns. The 8 dichroic-separated bands (u,g,r,z,J,H,K,L) provide accurate photometric galaxy redshifts, whereas the diffraction-limited resolution of the central z-band allows precise shape measurements for cosmic shear analysis. The 5 year VADER survey will cover a contiguous sky area of 3,500 square degrees to a depth of z~2 and will yield accurate photometric redshifts and multi-wavelength object parameters for about 175,000 galaxy clusters, one billion galaxies, and 5 million AGN. VADER will not only provide unprecedented constraints on the nature of dark energy, but will additionally extend and trigger a multitude of cosmic evolution studies to very large (>10 Gyrs) look-back times.

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