arXiv subjects

Simon J. Lilly

Publications and source records attributed to Simon J. Lilly.

At least 73 records · Page 4Linked to original sources

arXiv2012

Shocked Superwinds from the z~2 Clumpy Star-forming Galaxy, ZC406690

We have obtained high-resolution data of the z 2 ring-like, clumpy star-forming galaxy (SFG) ZC406690 using the VLT/SINFONI with AO (in K-band) and in seeing-limited mode (in H- and J-band). Our data includes all of the main strong optical emission lines: [OII], [OIII], Ha, Hb, [NII] and [SII]. We find broad, blueshifted Ha and [OIII] emission line wings in the spectra of the galaxy's massive, star-forming clumps (sigma \sim 85 km s^-1) and even broader wings (up to 70% of the total Ha flux, with sigma \sim 290 km s^-1) in regions spatially offset from the clumps by \sim 2 kpc. The broad emission likely originates from large-scale outflows with mass outflow rates from individual clumps that are 1-8x the SFR of the clumps. Based on emission line ratio diagnostics ([NII]/Ha and [SII]/Ha) and photoionization and shock models, we find that the emission from the clumps is due to a combination of photoionization from the star-forming regions and shocks generated in the outflowing component, with 5-30% of the emission deriving from shocks. In terms of the ionization parameter (6x10^7-10^8 cm/s, based on both the SFR and the O32 ratio), density (local electron densities of 300-1800 cm^-3 in and around the clumps, and ionized gas column densities of 1200-8000 Msol/pc^2), and SFR (10-40 Msol/yr), these clumps more closely resemble nuclear starburst regions of local ULIRGs and dwarf irregulars than HII regions in local galaxies. However, the star-forming clumps are not located in the nucleus as in local starburst galaxies but instead are situated in a ring several kpc from the center of their high-redshift host galaxy, and have an overall disk-like morphology. The two brightest clumps are quite different in terms of their internal properties, energetics and relative ages, and thus we are given a glimpse at two different stages in the formation and evolution of rapidly star-forming giant clumps at high-z.

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arXiv2010

The bulk of the black hole growth since z~1 occurs in a secular universe: No major merger-AGN connection

What is the relevance of major mergers and interactions as triggering mechanisms for active galactic nuclei (AGN) activity? To answer this longstanding question, we analyze 140 XMM-selected AGN host galaxies and a matched control sample of 1264 inactive galaxies over z~0.3-1.0 and log(M_*/M_sun)<11.7 with high-resolution HST/ACS imaging from the COSMOS field. The visual analysis of their morphologies by 10 independent human classifiers yields a measure of the fraction of distorted morphologies in the AGN and control samples, i.e. quantifying the signature of recent mergers which might potentially be responsible for fueling/triggering the AGN. We find that (1) the vast majority (>85%) of the AGN host galaxies do not show strong distortions, and (2) there is no significant difference in the distortion fractions between active and inactive galaxies. Our findings provide the best direct evidence that, since z~1, the bulk of black hole accretion has not been triggered by major galaxy mergers, therefore arguing that the alternative mechanisms, i.e., secular processes and minor interactions, are the leading triggers for the episodes of major black hole growth. We also exclude an alternative interpretation of our results: a significant time lag between merging and the observability of the AGN phase could wash out the most significant merging signatures, explaining the lack of enhancement of strong distortions on the AGN hosts. We show that this alternative scenario is unlikely due to: (1) recent major mergers being ruled out for the majority of sources due to the high fraction of disk-hosted AGN, (2) the lack of a significant X-ray signal in merging inactive galaxies as a signature of a potential buried AGN, and (3) the low levels of soft X-ray obscuration for AGN hosted by interacting galaxies, in contrast to model predictions.

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arXiv2009

Photo-z Performance for Precision Cosmology

Current and future weak lensing surveys will rely on photometrically estimated redshifts of very large numbers of galaxies. In this paper, we address several different aspects of the demanding photo-z performance that will be required for future experiments, such as the proposed ESA Euclid mission. It is first shown that the proposed all-sky near-infrared photometry from Euclid, in combination with anticipated ground-based photometry (e.g. PanStarrs-2 or DES) should yield the required precision in individual photo-z of sigma(z) < 0.05(1+z) at I_AB < 24.5. Simple a priori rejection schemes based on the photometry alone can be tuned to recognise objects with wildly discrepant photo-z and to reduce the outlier fraction to < 0.25% with only modest loss of otherwise usable objects. Turning to the more challenging problem of determining the mean redshift of a set of galaxies to a precision of 0.002(1+z) we argue that, for many different reasons, this is best accomplished by relying on the photo-z themselves rather than on the direct measurement of from spectroscopic redshifts of a representative subset of the galaxies. A simple adaptive scheme based on the statistical properties of the photo-z likelihood functions is shown to meet this stringent systematic requirement. We also examine the effect of an imprecise correction for Galactic extinction and the effects of contamination by fainter over-lapping objects in photo-z determination. The overall conclusion of this work is that the acquisition of photometrically estimated redshifts with the precision required for Euclid, or other similar experiments, will be challenging but possible. (abridged)

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arXiv2009

The SINS Survey: Broad Emission Lines in High-Redshift Star-Forming Galaxies

High signal-to-noise, representative spectra of star-forming galaxies at z~2, obtained via stacking, reveal a high-velocity component underneath the narrow H-alpha and [NII] emission lines. When modeled as a single Gaussian, this broad component has FWHM > 1500 km/s; when modeled as broad wings on the H-alpha and [NII] features, it has FWHM > 500 km/s. This feature is preferentially found in the more massive and more rapidly star-forming systems, which also tend to be older and larger galaxies. We interpret this emission as evidence of either powerful starburst-driven galactic winds or active supermassive black holes. If galactic winds are responsible for the broad emission, the observed luminosity and velocity of this gas imply mass outflow rates comparable to the star formation rate. On the other hand, if the broad line regions of active black holes account for the broad feature, the corresponding black holes masses are estimated to be an order of magnitude lower than those predicted by local scaling relations, suggesting a delayed assembly of supermassive black holes with respect to their host bulges.

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arXiv2008

The COSMOS AGN Spectroscopic Survey I: XMM Counterparts

We present optical spectroscopy for an X-ray and optical flux-limited sample of 677 XMM-Newton selected targets covering the 2 deg^2 COSMOS field, with a yield of 485 high-confidence redshifts. The majority of the spectra were obtained over three seasons (2005-2007) with the IMACS instrument on the Magellan (Baade) telescope. We also include in the sample previously published Sloan Digital Sky Survey spectra and supplemental observations with MMT/Hectospec. We detail the observations and classification analyses. The survey is 90% complete to flux limits of f_{0.5-10 keV}>8 x 10^-16 erg cm^-2 s^-1 and i_AB+<22, where over 90% of targets have high-confidence redshifts. Making simple corrections for incompleteness due to redshift and spectral type allows for a description of the complete population to $i_AB+<23. The corrected sample includes 57% broad emission line (Type 1, unobscured) AGN at 0.13 3 x 10^42 erg s^-1) to z<1, of both optically obscured and unobscured types. We find statistically significant evidence that the obscured to unobscured AGN ratio at z<1 increases with redshift and decreases with luminosity.

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arXiv2008

Strong magnetic fields in normal galaxies at high redshifts

The origin and growth of magnetic fields in galaxies is still something of an enigma. It is generally assumed that seed fields are amplified over time through the dynamo effect, but there are few constraints on the timescale. It has recently been demonstrated that field strengths as traced by rotation measures of distant quasars are comparable to those seen today, but it was unclear whether the high fields were in the exotic environments of the quasars themselves or distributed along the line of sight. Here we demonstrate that the quasars with strong MgII absorption lines are unambiguously associated with larger rotation measures. Since MgII absorption occurs in the haloes of normal galaxies along the sightline to the quasars, this association requires that organized fields of surprisingly high strength are associated with normal galaxies when the Universe was only about one-third of its present age.

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arXiv2007

Mapping Neutral Hydrogen During Reionization with the Ly-alpha Emission from Quasar Ionization Fronts

We present a new method to directly map the neutral-hydrogen distribution during the reionization epoch and to constrain the emission properties of the highest-redshift quasars (QSOs). As a tracer of HI, we propose to use the Ly-alpha radiation produced by quasar ionization fronts (I-fronts) that expand in the partially ionized intergalactic medium (IGM) before reionization is complete. These Ly-alpha photons are mainly generated by collisional excitations of hydrogen atoms in the boundary of the rapidly expanding HII region. The observable signal is produced by the part of the I-front that lies behind the QSO with respect to the observer. Combining two radiative transfer models (one for the QSO ionizing radiation and one for the Ly-alpha photons), we estimate the expected Ly-alpha spectral shape and surface brightness (SB) for a large number of configurations where we varied both the properties of the ionizing QSO and of the surrounding medium. We find that the expected signal is observable as a single (broad) emission line with a characteristic width of 100-200 km/s. The expected SB produced at redshift z~6.5 within a fully neutral region (at mean density) by a typical QSO I-front lies in the range $10^{-21}-10^{-20}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ and decreases proportionally to $(1+z)^2$ for a given QSO age. QSOs with harder spectra may produce a significantly brighter emission at early phases. The signal may cover up to a few hundred square arcmin on the sky and should be already detectable with current facilities by means of moderate/high resolution spectroscopy. The detection of this Ly-alpha emission can shed new light on the reionization history, the age and the emission properties of the highest-redshift QSOs. (abridged)

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arXiv2007

Plausible fluorescent Ly-alpha emitters around the z=3.1 QSO0420-388

We report the results of a survey for fluorescent Ly-alpha emission carried out in the field surrounding the z=3.1 quasar QSO0420-388 using the FORS2 instrument on the VLT. We first review the properties expected for fluorescent Ly-alpha emitters, compared with those of other non-fluorescent Ly-alpha emitters. Our observational search detected 13 Ly-alpha sources sparsely sampling a volume of ~14000 comoving Mpc^3 around the quasar. The properties of these in terms of i) the line equivalent width, ii) the line profile and iii) the value of the surface brightness related to the distance from the quasar, all suggest that several of these may be plausibly fluorescent. Moreover, their number is in good agreement with the expectation from theoretical models. One of the best candidates for fluorescence is sufficiently far behind QSO0420-388 that it would imply that the quasar has been active for (at least) ~60 Myrs. Further studies on such objects will give information about proto-galactic clouds and on the radiative history (and beaming) of the high-redshift quasars.

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arXiv2007

Strong MgII systems in quasar and gamma-ray burst spectra

The incidence of strong MgII systems in gamma-ray burst (GRB) spectra is a few times higher than in quasar (QSO) spectra. We investigate several possible explanations for this effect, including: dust obscuration bias, clustering of the absorbers, different beam sizes of the sources, multiband magnification bias of GRBs, association of the absorbers with the GRB event or the circumburst environment. We find that: i) the incidence rate of MgII systems in QSO spectra could be underestimated by a factor 1.3-2 due to dust obscuration; ii) the equivalent-width distribution of the MgII absorbers along GRBs is consistent with that observed along QSOs thus suggesting that the absorbers are more extended than the beam sizes of the sources; iii) on average, GRB afterglows showing more than one MgII system are a factor of 1.7 brighter than the others, suggesting a lensing origin of the observed discrepancy; iv) gravitational lensing (in different forms, from galaxy lensing to microlensing) can bias high the counts of MgII systems along GRBs if the luminosity functions of the prompt gamma-ray emission and of the optical afterglows have a mean faint-end slope approaching -5/3 -- -2; v) some of the absorbers can be associated with the circumburst environment or produced by supernova remnants unrelated to the GRB event itself but lying in the same star-forming region. With the possible exception of magnification bias, it is unlikely that one of these effects on its own can fully account for the observed counts. However, the combined action of some of them can substantially reduce the statistical significance of the discrepancy.

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arXiv2006

Report by the ESA-ESO Working Group on Fundamental Cosmology

ESO and ESA agreed to establish a number of Working Groups to explore possible synergies between these two major European astronomical institutions. This Working Group's mandate was to concentrate on fundamental questions in cosmology, and the scope for tackling these in Europe over the next ~15 years. One major resulting recommendation concerns the provision of new generations of imaging survey, where the image quality and near-IR sensitivity that can be attained only in space are naturally matched by ground-based imaging and spectroscopy to yield massive datasets with well-understood photometric redshifts (photo-z's). Such information is essential for a range of new cosmological tests using gravitational lensing, large-scale structure, clusters of galaxies, and supernovae. Great scope in future cosmology also exists for ELT studies of the intergalactic medium and space-based studies of the CMB and gravitational waves; here the synergy is less direct, but these areas will remain of the highest mutual interest to the agencies. All these recommended facilities will produce vast datasets of general applicability, which will have a tremendous impact on broad areas of astronomy.

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arXiv2006

The James Webb Space Telescope

The James Webb Space Telescope (JWST) is a large (6.6m), cold (50K), infrared-optimized space observatory that will be launched early in the next decade. The observatory will have four instruments: a near-infrared camera, a near-infrared multi-object spectrograph, and a tunable filter imager will cover the wavelength range, 0.6 to 5.0 microns, while the mid-infrared instrument will do both imaging and spectroscopy from 5.0 to 29 microns. The JWST science goals are divided into four themes. The End of the Dark Ages: First Light and Reionization theme seeks to identify the first luminous sources to form and to determine the ionization history of the early universe. The Assembly of Galaxies theme seeks to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and active nuclei within them evolved from the epoch of reionization to the present day. The Birth of Stars and Protoplanetary Systems theme seeks to unravel the birth and early evolution of stars, from infall on to dust-enshrouded protostars to the genesis of planetary systems. The Planetary Systems and the Origins of Life theme seeks to determine the physical and chemical properties of planetary systems including our own, and investigate the potential for the origins of life in those systems. To enable these observations, JWST consists of a telescope, an instrument package, a spacecraft and a sunshield. The telescope consists of 18 beryllium segments, some of which are deployed. The segments will be brought into optical alignment on-orbit through a process of periodic wavefront sensing and control. The JWST operations plan is based on that used for previous space observatories, and the majority of JWST observing time will be allocated to the international astronomical community through annual peer-reviewed proposal opportunities.

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arXiv2006

Magellan Spectroscopy of AGN Candidates in the COSMOS Field

We present spectroscopic redshifts for the first 466 X-ray and radio-selected AGN targets in the 2 deg^2 COSMOS field. Spectra were obtained with the IMACS instrument on the Magellan (Baade) telescope, using the nod-and-shuffle technique. We identify a variety of Type 1 and Type 2 AGN, as well as red galaxies with no emission lines. Our redshift yield is 72% down to i_AB=24, although the yield is >90% for i_AB<22. We expect the completeness to increase as the survey continues. When our survey is complete and additional redshifts from the zCOSMOS project are included, we anticipate ~1100 AGN with redshifts over the entire COSMOS field. Our redshift survey is consistent with an obscured AGN population that peaks at z~0.7, although further work is necessary to disentangle the selection effects.

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arXiv2005

Fluorescent Ly-alpha emission from the high-redshift intergalactic medium

We combine a high-resolution hydro-simulation of the LambdaCDM cosmology with two radiative transfer schemes (for continuum and line radiation) to predict the properties, spectra and spatial distribution of fluorescent Ly-alpha emission at z~3. We focus on line radiation produced by recombinations in the dense intergalactic medium ionized by UV photons. In particular, we consider both a uniform background and the case where gas clouds are illuminated by a nearby quasar. We find that the emission from optically thick regions is substantially less than predicted from the widely used static, plane-parallel model. The effects induced by a realistic velocity field and by the complex geometric structure of the emitting regions are discussed in detail. We make predictions for the expected brightness and size distributions of the fluorescent sources.Our results account for recent null detections and can be used to plan new observational campaigns both in the field (to measure the intensity of the diffuse UV background) and in the proximity of bright quasars (to understand the origin of high colum-density absorbers).

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arXiv2004

The Number and Observability of Population III Supernovae at High Redshifts

We consider the feasibility of detecting Population III pair-instability supernovae (PISN) at very high redshifts with the James Webb Space Telescope (JWST). Four published estimates for the PISNe rate show a rather wide dispersion, between 50-2200 deg-2 yr-1. Correcting problems with several of these, we conclude that even a fairly optimistic estimate is probably a further order of magnitude lower than this range, at a rate of order 4 deg-2 yr-1 at z \~ 15 and 0.2 deg-2 yr-1 at z ~ 25, both with substantial uncertainty. Although such supernovae would be bright enough to be readily detectable with the JWST at any relevant redshift, the lower number densities derived here will likely require either a dedicated wide-angle search strategy or a serendipitous search. We expect that typically about 1 deg2 (or 500 JWST NIRCam images) per detected supernova at 4.5 mm must be imaged to detect one PISN at z ~ 15 and about 35 deg2 to detect one at z ~ 25. If some Population III star-formation persists to lower redshifts z ~ 5, then PISNe may also be detectable in wide-angle ground-based Z-band imaging surveys at ZAB ~ 23, at a density of order 1 deg-2 of surveyed area. In an Appendix, we consider the possible effects of intergalactic dust in obscuring high redshift supernovae or other high redshift sources. We show that the obscuration at a given rest-wavelength will peak at some maximum redshift and thereafter decline. While it may be a significant effect in observations of the very high redshift Universe, it is unlikely, even under rather pessimistic assumptions, to completely obscure primordial objects.

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arXiv2004

A VLT/FORS2 Multi-Slit Search for Lyman-alpha Emitting Galaxies at z~6.5

We present results from a deep spectroscopic search in the 9150A atmospheric window for z~6.5 Lyman-alpha emitting galaxies using the VLT/FORS2. Our multi-slit+narrow-band filter survey covers a total spatial area of 17.6 sq. arcmin in four different fields and reaches fluxes down to 5x10^(-18) erg/s/cm^2 (7 sigma detection). Our detection limit is significantly fainter than narrow-band searches at this redshift and fainter also than the unlensed brightness of Hu et al.'s HCM6A at z=6.56, and thus provides better overlap with surveys at much lower redshifts. Eighty secure emission line galaxies are detected. However, based on their clear continuum emission shortward of the line or the presence of multiple lines, none of these can be Ly-alpha emission at z~6.5. Our null result of finding no z~6.5 Ly-alpha emitters suggests that the number density of Ly-alpha emitters with L>2x10^(42) erg/s declines by ~2 between z~3 and z~6.5.

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arXiv2003

The matallicities of star-forming galaxies at intermediate redshifts 0.47 < z < 0.92

Estimates of the [O/H] metallicity of the star-forming gas in a sample of 66 CFRS galaxies between 0.47 < z < 0.92 have been made based on the flux ratios of bright emission lines. Most galaxies (> 75%) have the [O/H] ~ 8.9 metallicities that are seen locally in galaxies of similar luminosities. However, a minority (< 25%) appear to have significantly lower metallicities [O/H] < 8.6 as indicated by high values of the Pagel R23 parameter. The high metallicities of the majority of the galaxies suggest that they do not fade to be low metallicity dwarf galaxies today. Only one of the 66 galaxies has an optical emission line spectrum similar to the few Lyman break galaxies recently observed at z ~ 3, emphasizing the differences with that population. The inferred emission line gas [O/H] metallicity broadly correlates with luminosity in both the rest B- and J-bands but with considerable scatter introduced. The metallicity does not appear to correlate well with galaxy size, H-beta strength or, with the very limited data available, the kinematics. The metallicity does correlate well with the continuum optical-infrared colors in a way that could be explained as a combination of effects, none of which should produce large variations in the M/L ratio. These results support a "down-sizing" picture of galaxy evolution rather than a "fading dwarf" picture in which the luminous active galaxies at high redshift are highly brightened dwarf galaxies. The overall change in metallicity of star-forming galaxies over the last half of the age of the Universe appears to have been modest, Delta[O/H] = 0.08 +/- 0.06. This is consistent with the age-metallicity relation in the Galactic disk and is broadly consistent with models for the chemical evolution of the Universe, especially those that consider different environments.

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arXiv2002

The metallicity of star-forming gas over cosmic time

A brief review concludes that there is now good overall agreement between theoretical estimates of the energy associated with the production of the observed metal content of the Universe and the observed extragalactic background light. In addition the overall form of the star-formation history over 0 < z < 5 is reasonably well constrained. The study of emission line gas in galaxies as a function of redshift provides a complementary view of chemical evolution to that obtained from studies of absorption line systems in quasar spectra. Emission line gas is more relevant for some questions, including the confrontation with models for the chemical evolution of our own and other galaxies, and Origins-related questions about the formation of stars and planets. Using relatively crude diagnostic parameters such as Pagel's R23, the increase of the metallicity of star-forming gas with cosmic epoch can be tracked. Observations of a large sample of CFRS galaxies at z ~ 0.8 (a look-back time of 0.5 t0 show the appearance of a significant number of low metallicity systems amongst luminous L* galaxies. However, overall there is only a modest change in mean metallicity compared with the present, D log Z = -0.08 +- 0.06. These data do not support a fading-dwarf scenario for the ``faint blue galaxies''. At z ~ 3 it is likely that all star-forming galaxies have sub-solar metallicities. The overall increase in the metallicity of star-forming gas with cosmic epoch matches rather well the age-metallicity relation in the solar neighbourhood and some recent models for the evolution of the global metallicity of gas in the Universe.

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arXiv2001

The Metallicity of Redshift 0.5<z<1 Field Galaxies

We are undertaking a program to measure emission line ratios in a selected sample of $0.5 < z < 1.0$ CFRS starforming galaxies in order to compute their Interstellar Medium metallicities. The latter are derived by means of the empirically-calibrated $R_{23}$ estimator introduced by Pagel et al. (1979). Here we focus on a subsample of 15 galaxies with $L_{Hβ} > 1.2 \times 10^{41}$ erg s$^{-1}$. In addition to the optical CFHT spectra discussed in Carollo & Lilly (2001), where a preliminary account of this work can be found, in this paper we also refer to new $J$-band Keck spectroscopy of the $Hα$ and [NII]6583 lines for several objects. These lines allow us to break the degeneracy between low ($\lta 0.1Z_\odot$) and high ($\gta 0.5Z_\odot$) metallicity of the $R_{23}$ estimator, and put on solid ground the finding that the $0.5 < z < 1.0$ blue galaxies in the high-$L_{Hβ}$ sample are significantly metal-enriched ($Z \gta 0.3Z_\odot$ up to $Z_\odot$). Therefore, our results do not support previous suggestions in which the $0.5 < z < 1.0$ starforming galaxies are dwarf galaxies brightened by large bursts of star-formation, but support instead a picture where at least a significant fraction of them evolves to become today's massive metal-rich systems.

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