arXiv subjects

Max Pettini

Publications and source records attributed to Max Pettini.

At least 181 records · Page 10Linked to original sources

arXiv2000

Mining for Metals in the Lyman Alpha Forest

In order to ascertain the extent of metal enrichment in the Ly alpha forest, we have analysed a very high S/N spectrum of the z = 3.625 QSO Q1422+231. We find that in high column density Ly alpha clouds, the power law column density distribution function of C IV continues down to log N(C IV) = 11.7. In addition, by analysing pixel-by-pixel optical depths we show that there are considerably more metals in the Ly alpha forest than are currently directly detectable.

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arXiv2000

The First Galaxies: Clues from Element Abundances

It has recently become possible to measure directly the abundances of several chemical elements in a variety of environments at redshifts up to z = 5. In this review I summarise the latest observations of Lyman break galaxies, damped Lyman alpha systems and the Lyman alpha forest with a view to uncovering any clues which these data may offer to the first episodes of star formation. The picture which is emerging is one where the universe at z = 3 already included many of the components of today's galaxies--even at these early times we see evidence for Populations I and II stars, while the `smoking gun' for Population III objects may be hidden in the chemical composition of the lowest density regions of the IGM, yet to be deciphered.

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arXiv1999

Si and Mn Abundances in Damped Lya Systems with Low Dust Content

We have measured the abundances of Zn, Si, Mn, Cr, Fe, and Ni in three damped Lyman alpha systems at redshifts z < 1 from high resolution echelle spectra of QSOs recorded with the Keck I telescope. In all three cases the abundances of Cr, Fe, and Ni relative to Zn indicate low levels of dust depletions. We propose that when the proportion of refractory elements locked up in dust grains is less than about 50 percent, it is plausible to assume an approximately uniform level of depletion for all grain constituents and, by applying a small dust correction, recover the intrisic abundances of Si and Mn. We use this approach on a small sample of damped systems for which it is appropriate, with the aim of comparing the metallicity dependence of the ratios [Si/Fe] and [Mn/Fe] with analogous measurements in Milky Way stars. The main conclusion is that the relative abundances of both elements in distant galaxies are broadly in line with expectations based on Galactic data. Si displays a mild enhancement at low metallicities, as expected for an alpha-capture element, but there are also examples of near-solar [Si/Fe] at [Fe/H] < -1. The underabundance of Mn at low metallicities is possibly even more pronounced than that in metal-poor stars, and no absorption system has yet been found where [Mn/Fe] is solar. The heterogeneous chemical properties of damped Lyman alpha systems, evident even from this limited set of measurements, provide further support for the conclusion from imaging studies that a varied population of galaxies gives rise to this class of QSO absorbers.

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arXiv1999

The Ultraviolet Spectrum of MS 1512-cB58: An Insight into Lyman Break Galaxies

We present an intermediate resolution, high S/N spectrum of the z = 2.7268 galaxy MS 1512-cB58, obtained with LRIS on the Keck I telescope and covering the rest frame far-UV from 1150 to 1930 A. Gravitational lensing by a foreground cluster boosts the flux from cB58 by a factor of about 30 and provides the opportunity for a first quantitative study of the physical properties of star forming galaxies at high redshift. Our main results are as follows. The ultraviolet spectral properties of cB58 are remarkably similar to those of nearby star forming galaxies. The P-Cygni profiles of CIV and NV are best matched by continuous star formation with a Salpeter IMF extending beyond 50 solar masses---we find no evidence for either a flatter IMF (at the high mass end), or an IMF deficient in the most massive stars. There are clues in our data that the metallicity of both the stars and the gas is a few times below solar. Our best estimate, approximately equal to 1/4 solar, is 3 times higher than the typical metallicity of damped Lyman alpha systems at the same redshift, consistent with the proposal that the galaxies which dominate the H I absorption cross-section are generally forming stars at a slower rate than L* Lyman break galaxies like cB58. The relative velocities of the stellar lines, interstellar absorption, and H II emission indicate the existence of large-scale outflows in the interstellar medium of cB58, with a bulk outward motion of 200 km/s and a mass loss rate of approximately 60 solar masses per year, roughly comparable to the star formation rate. Such galactic winds may well be the mechanism which self-regulates star formation, distributes metals over large volumes and allows the escape of ionizing photons into the IGM.

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arXiv1999

KECK HIRES Spectroscopy of APM 08279+5255

With an optical R-band magnitude of 15.2, the recently discovered z=3.911 BAL quasar APM 08279+5255 is an exceptionally bright high redshift source. Its brightness has allowed us to acquire a high signal-to-noise ratio (~100), high resolution (~6 km/s) spectrum using the HIRES echelle spectrograph on the 10-m Keck I telescope. Given the quality of the data, these observations provide an unprecedented view of associated and intervening absorption systems. Here we announce the availability of this spectrum to the general astronomical community and present a brief analysis of some of its main features.

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arXiv1999

Metal Abundances in the Lyman Alpha Forest

We present high S/N echelle spectra of the recently discovered ultraluminous QSO APM 08279+5255 and use these data to re-examine the abundance of Carbon in Ly alpha forest clouds. In agreement with previous work, we find that approximately half of the Ly alpha clouds whose hydrogen column densities log N(HI) > 14.5 have associated weak CIV absorption with log N(CIV) > 12, and we derive a median N(CIV)/N(H I) = 1.4 x 10^-3. We then investigate whether there is any CIV absorption in weaker Ly alpha clouds with 13.5 < log N(HI) < 14.0. Two contrasting techniques are compared - firstly by stacking individual CIV lines to produce a high S/N (=580) composite spectrum and secondly by constructing pixel-by-pixel optical depth distributions, a technique recently developed by Cowie & Songaila (1998). By simulating stacks of synthetic absorption lines, the composite spectrum technique is found to be severely hampered by random redshift offsets between CIV lines and their corresponding HI. Whilst the optical depth method showed evidence for CIV asborption even at HI optical depths as small as 0.5 - 2, simulations showed that even higher S/N ratios are required in order to determine whether the CIV/HI ratio remains constant at these low column densities.

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arXiv1999

Element Abundances at High Redshifts

I review measurements of element abundances in different components of the high redshift universe, including the Lyman alpha forest, damped Lyman alpha systems, and Lyman break galaxies. Although progress is being made in all three areas, recent work has also produced some surprises and shown that established ideas about the nature of the damped Lyman alpha systems in particular may be too simplistic. Overall, our knowledge of metal abundances at high z is still very sketchy. Most significantly, there seems to be an order of magnitude shortfall in the comoving density of metals which have been measured up to now compared with those produced by the star formation activity seen in Lyman break galaxies. At least some of the missing metals are likely to be in hot gas in galactic halos and proto-clusters.

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arXiv1998

Cosmology with the Lyman-alpha Forest

We outline the physical picture of the high-redshift Ly-alpha forest that has emerged from cosmological simulations, describe statistical characteristics of the forest that can be used to test theories of structure formation, present a preliminary comparison between simulation results and measurements from Keck HIRES spectra, and discuss a recent determination of the slope and amplitude of the linear mass power spectrum P(k) at z=2.5 from moderate resolution spectra. The physical picture is simple if each QSO spectrum is viewed as a continuous non-linear map of the line-of-sight density field rather than a collection of discrete absorption lines. The distribution of flux decrements depends mainly on the amplitude and PDF (Gaussian vs. non-Gaussian) of the primordial density fluctuations. The threshold crossing frequency, analogous to the 3-d genus curve, responds to the shape and amplitude of P(k) and to the values of Omega and Lambda. Open and Lambda-CDM models agree well with the measured flux decrement distribution at smoothing lengths of 25 km/s and 100 km/s and with the threshold crossing frequency at 100 km/s. Discrepancy with the observed threshold crossing frequency at 25 km/s may reflect the combined effects of noise in the data and limited mass resolution of the simulations. The slope of the measured P(k) agrees with the predictions of inflation+CDM models. Combining the amplitude with COBE normalization imposes a constraint on these models of the form Omega h^x n^y Omega_b^z = constant. Assuming Gaussian primordial fluctuations and a power spectrum shape parameter Gamma~0.2, consistency of the measured P(k) with the observed cluster mass function at z=0 requires Omega=0.46^{+0.12}_{-0.10} for an open universe and Omega=0.34^{+0.13}_{-0.09} for a flat universe (1-sigma errors). (Shortened)

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arXiv1998

The Power Spectrum of Mass Fluctuations Measured from the Lyman-alpha Forest at Redshift z=2.5

We measure the linear power spectrum of mass density fluctuations at redshift z=2.5 from the \lya forest absorption in a sample of 19 QSO spectra, using the method introduced by Croft et al. (1998). The P(k) measurement covers the range 2π/k ~ 450-2350 km/s (2-12 comoving \hmpc for Ω=1). We examine a number of possible sources of systematic error and find none that are significant on these scales. In particular, we show that spatial variations in the UV background caused by the discreteness of the source population should have negligible effect on our P(k) measurement. We obtain consistent results from the high and low redshift halves of the data set and from an entirely independent sample of nine QSO spectra with mean redshift z=2.1. A power law fit to our measured P(k) yields a logarithmic slope n=-2.25 +/- 0.18 and an amplitude Δ^2(k_p) = 0.57^{+0.26}_{-0.18}, where $Δ^2$ is the contribution to the density variance from a unit interval of lnk and k_p=0.008 (km/s)^{-1}. Direct comparison of our mass P(k) to the measured clustering of Lyman Break Galaxies shows that they are a highly biased population, with a bias factor b~2-5. The slope of the linear P(k), never previously measured on these scales, is close to that predicted by models based on inflation and Cold Dark Matter (CDM). The P(k) amplitude is consistent with some scale-invariant, COBE-normalized CDM models (e.g., an open model with Ω_0=0.4) and inconsistent with others (e.g., Ω=1). Even with limited dynamic range and substantial statistical uncertainty, a measurement of P(k) that has no unknown ``bias factors'' offers many opportunities for testing theories of structure formation and constraining cosmological parameters. (Shortened)

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arXiv1998

Metal Abundances at z < 1.5: Fresh Clues to the Chemical Enrichment History of Damped Lyman alpha Systems

We explore the redshift evolution of the metal content of damped Lyman alpha systems (DLAs) with new observations of four absorbers at z < 1.5 . The main conclusion is that the column density--weighted mean metallicity, [ ] = -1.03 +/- 0.23 (on a logarithmic scale), is not significantly higher at z < 1.5 than at earlier epochs, despite the fact that the comoving star formation rate density of the universe was near its maximum value at this redshift. For three of the four DLAs our observations include absorption lines of Si, Mn, Cr, Fe, and Ni, as well as Zn. We argue that the relative abundances of these elements are consistent with a moderate degree of dust depletion which, once accounted for, leaves no room for the enhancement of the alpha-elements over iron seen in metal poor stars in the Milky Way. This is contrary to previous assertions that DLAs have been enriched solely by Type II supernovae, but can be understood if the rate of star formation in the systems studied proceeded more slowly than in the early history of our Galaxy. These results add to a growing body of data all pointing to the conclusion that known DLAs do not trace the galaxy population responsible for the bulk of star formation. Possible reasons are that sight-lines through metal rich gas are systematically underrepresented because the background QSOs are reddened, and that the most actively star forming galaxies are also the most compact, presenting too small a cross-section to have been probed yet with the limited statistics of current samples.

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arXiv1998

Infared Observations of Nebular Emission Lines from Galaxies at z = 3

We present the first results from a program of near-infrared spectroscopy aimed at studying the familiar rest-frame optical emission lines from the H II regions of Lyman break galaxies at z = 3. By targeting redshifts which bring the lines of interest into gaps between the strong OH sky emission, we have been successful in detecting Balmer and [O III] emission lines in all five galaxies observed so far with CGS4 on UKIRT. For a Salpeter IMF and a H_0 = 70 km/s/Mpc, q_0 = 0.1 cosmology, the Hbeta luminosities uncorrected for dust extinction imply star formation rates of 20 - 270 solar masses per year. On the basis of the present limited sample it appears that an extinction of 1 - 2 magnitudes at 1500 A may be typical of Lyman break galaxies. This value is consistent with recent estimates of dust obscuration in star forming galaxies at z < 1, and does not require a substantial revision of the broad picture of star formation over the Hubble time proposed by Madau et al. (1996). In four out of five cases the velocity dispersion of the emission line gas is sigma = 70 km/s, while in the fifth the line widths are nearly three times larger. Virial masses in the range from 1 x 10^{10} to 5 x 10^{10} solar masses are suggested, but both velocities and masses could be higher because our observations are only sensitive to the brightest cores of these systems where the line widths may not sample the full gravitational potential. The relative redshifts of interstellar absorption, nebular emission, and Lyman alpha emission lines differ by several hundred km/s and suggest that large-scale outflows may be a common characteristic of Lyman break galaxies.

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arXiv1998

The Angular Clustering of Lyman-Break Galaxies at Redshift z=3

We have measured the angular correlation function w(theta) for a sample of 871 Lyman-break galaxies (LBGs) at z=3. Fitting a power-law to a weighted average of w(theta) from 5 fields, we find the amplitude to be A_w=2 arcsec^β and the slope β=0.9. The slope is the same as in the local and moderate redshift universe. A slope β=0.25 or shallower is ruled out by the data at the 99.9% confidence level. Because N(z) of LBGs is well determined from 376 spectroscopic redshifts, the real-space correlation function can be derived from the angular one through the Limber transform. This inversion is rather insensitive to the still relatively large errors on A_w and β, and the spatial correlation length is much more tightly constrained than either of these parameters. We estimate r_0=3.3 -0.6 +0.7 (2.1 -0.5 +0.4) h^{-1} Mpc (comoving) for q_0=0.1 (0.5) at z=3.04 (h is in units of 100 km/s/Mpc). The correlation length of LBGs at z=3 is comparable to that of present-day spiral galaxies and is only about 50% smaller than that of the ellipticals; it is as large or larger than any measured in galaxy samples at 0.3<z<1. By comparing the observed galaxy correlation length to that of the mass predicted from CDM theory, we estimate a bias for LBGs of b\sim 1.5 (4.5) for q_0=0.1 (0.5), in agreement with our previous estimates based on preliminary spectroscopy. The strong clustering and the large bias of the LBGs are consistent with biased galaxy formation theories and provide additional evidence that these systems are associated with massive dark matter halos. The clustering of LBGs at z=3 emphasizes that apparent evolution of galaxy clustering may be due as much to variations in effective bias parameter among different samples as to evolution in the mass distribution through gravitational instability.

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arXiv1997

Lyman alpha absorption in the nearby universe: the sightline to Q1821+643

We present the first results of a survey designed to understand the origin of the Lyman alpha-forest absorption systems at low redshift. Using the WYFFOS and HYDRA multi-fibre spectrographs on the WHT and WIYN telescopes, we have identified 51 galaxies brighter than b_j=18.5 within 30 arcmins of the sightline of the QSO 1821+643. We find three galaxies within 500 h-1 kpc of the QSO sightline; the nearest galaxy is 104 h-1 kpc away from the line of sight, and is at the same redshift as a strong Lyman alpha absorption line. The remaining two galaxies have no corresponding absorption to extremely low equivalent width limits (< 0.05 Å). Beyond 500 h-1 kpc Lyman-alpha absorption lines are found at similar redshifts as several galaxies, but we show that these coincidences are likely to be accidental. Half of the Lyman alpha systems for which we could have found at least an L^* galaxy have no galaxies at the absorbers' redshifts. For the majority of the remainder, we show that any apparent association with galaxies is probably coincidental. These \lya systems are characterised by their weak equivalent widths (W_r < 0.2 Å), and we conclude that this population of absorbers is uncorrelated, or at best, weakly correlated, with galaxies.

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arXiv1997

Estimating the mass density of neutral gas at $z < 1$

We use the relationships between galactic HI mass and B-band luminosity determined by Rao & Briggs to recalculate the mass density of neutral gas at the present epoch based on more recent measures of the galaxy luminosity function than were available to those authors. We find $Ω_{gas}(z=0) \simeq 5 \times 10^{-4}$ in good agreement with the original Rao & Briggs value, suggesting that this quantity is now reasonably secure. We then show that, if the scaling between H I mass and B-band luminosity has remained approximately constant since $z = 1$, the evolution of the luminosity function found by the Canada-France redshift survey translates to an increase of $Ω_{gas}$ by a factor of $\approx 3$ at $z = 0.5 - 1$ . A similar value is obtained quite independently from consideration of the luminosity function of Mg II absorbers at $z = 0.65$. By combining these new estimates with data from damped \lya systems at higher redshift, it is possible to assemble a rough sketch of the evolution of $Ω_{gas}$ over the last 90% of the age of the universe. The consumption of H I gas with time is in broad agreement with models of chemical evolution which include the effects of dust, although more extensive samples of damped \lya systems at low and intermediate redshift are required for a quantitative assessment of the dust bias.

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arXiv1997

The Discovery of Primeval Galaxies and the Epoch of Galaxy Formation

We review the steps which have led to the discovery of a widespread population of objects at z = 3 with many of the characteristics which we expect for primeval galaxies, and emphasize in particular the advantages of a colour selection technique which targets the Lyman discontinuity at 912 Angstroms. Star forming galaxies at z = 3 resemble local starbursts, although they are typically more luminous by more than one order of magnitude. The ultraviolet continuum is dominated by the integrated light of O and early B type stars and shows prominent interstellar absorption lines which are often blueshifted relative to the systemic velocity of the galaxy, indicating highly energetic outflows in the interstellar medium. Lyman alpha emission is generally weak, probably as a result of resonant scattering. The spectral slope of the ultraviolet continuum and the strength of the Hbeta emission line, which we have detected in a few cases with pilot observations in the infrared K band, suggest that some interstellar dust is already present in these young galaxies and that it attenuates their UV luminosities by a factor of about 3. The efficiency of our photometric selection technique has allowed us to establish that large scale concentrations of galaxies were already in place at z = 3, possibly the precursors of galaxy clusters as they as they were beginning to decouple from the Hubble expansion. In the context of Cold Dark Matter models of structure formation, the observed clustering of the z~3 galaxies suggests that they are associated with dark matter halos of mass greater than 10^12 solar masses. We conclude by pointing out the need for infrared space observatories, such as the proposed Next Generation Space Telescope, for pushing the quest for the origin of galaxies beyond z=5.

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arXiv1997

The Spectra of Star Forming Galaxies at High Redshift

We review the spectral properties of the population of star forming galaxies at z = 3 discovered using a colour selection technique which targets the Lyman discontinuity at 912 Angstroms. The UV luminosity of the typical z = 3 galaxy exceeds by more than one order of magnitude that of the most luminous starbursts in the nearby universe, although the maximum star formation intensity (SFR per unit area) is within the limits found in local surveys. We consider in detail the likely magnitude of dust extinction and conclude that published estimates of the volume-averaged star formation rate at high z need to be revised to higher values by a factor of about 3. This correction improves the agreement between the observations and recent theoretical predictions. Lyman alpha emission is generally weak, most likely as a result of resonant scattering. The large equivalent widths of the strongest interstellar lines and their systematic blueshift (by up to several hundred km/s) relative to the Lyman alpha emission line are indicative of highly energetic outflows in the interstellar medium. Pilot observations have detected the redshifted Hbeta and [O III] emission lines in the K band. The widths of these features imply dynamical masses of approximately 10^{10} solar masses for the innermost star forming regions; the total masses involved are likely to exceed 10^{12} solar masses.

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arXiv1997

Zinc and Chromium Abundances in a Third Damped Lyman alpha System at Intermediate Redshift

We have determined the metallicity of the $z_{abs} = 1.0093$ damped Lyman alpha system in the bright QSO EX 0302-223; this is only the third such measurement at redshifts $z \simlt 1$. Unlike the previous two cases, we find that the abundance of Zn is only a factor of $\sim 2$ lower than in the Galactic interstellar medium today and is entirely compatible with the typical metallicity of stars in the Milky Way disk at a look-back time of 9.5 Gyrs. Although the galaxy responsible for producing the absorption system has yet to be positively identified, our observations show that galaxies on a chemical evolution path similar to that of the Milky Way do contribute to the damped Lyman alpha population at intermediate redshifts. Cr is 2.5 times less abundant than Zn, presumably because of depletion onto dust; however, the degree of depletion is less severe than in diffuse interstellar clouds in the disk of our Galaxy and in the Magellanic Clouds. Evidently, the interstellar environment in damped Lyman alpha galaxies is less conducive to the formation and survival of dust grains (and molecular hydrogen), but the physical processes at the root of this effect have yet to be clarified.

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arXiv1997

The Metallicity of High Redshift Galaxies: The Abundance of Zinc in 34 Damped Lyman Alpha Systems from z = 0.7 to 3.4

We report new observations of ZnII and CrII absorption lines in 10 damped \lya systems (DLAs), mostly at redshift $z_{abs} \simgt 2.5$ . By combining these results with those from our earlier survey (Pettini et al. 1994) and other recent data, we construct a sample of 34 measurements (or upper limits) of the Zn abundance relative to hydrogen [Zn/H]; the sample includes more than one third of the total number of DLAs known. The plot of the abundance of Zn as a function of redshift reinforces the two main findings of our previous study. (1) Damped \lya systems are mostly metal-poor, at all redshifts sampled; the column density weighted mean for the whole data set is [Zn/H] $= -1.13 \pm 0.38$ (on a logarithmic scale), or approximately 1/13 of solar. (2) There is a large spread, by up to two orders of magnitude, in the metallicities we measure at essentially the same redshifts. We propose that damped \lya systems are drawn from a varied population of galaxies of different morphological types and at different stages of chemical evolution, supporting the idea of a protracted epoch of galaxy formation. At redshifts $z \simgt 2$ the typical metallicity of the damped \lya systems is in agreement with expectations based on the consumption of HI gas implied by the recent measurements of $Ω_{DLA}$ by Storrie-Lombardi et al. (1996a), and with the metal ejection rates in the universe at these epochs deduced by Madau (1996) from the ultraviolet luminosities of high redshift galaxies revealed by deep imaging surveys. There are indications in our data for an increase in the mean metallicity of the damped \lya systems from $z > 3$ to $\approx 2$, consistent with the rise in the comoving star formation rate indicated by the relative numbers of $U$ and $B$ drop-outs in the Hubble Deep Field. Although such comparisons are still tentative, it appears that these different avenues for exploring the early evolution of galaxies give a broadly consistent picture.

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