arXiv subjects

Max Pettini

Publications and source records attributed to Max Pettini.

At least 127 records · Page 7Linked to original sources

arXiv2006

A Spectroscopic Survey of Redshift 1.4<z<3.0 Galaxies in the Goods-North Field: Survey Description, Catalogs, and Properties

We present the results of a spectroscopic survey with the Keck I telescope of more than 280 star-forming galaxies and AGN at redshifts 1.4 1.4 in GOODS-N. Extensive multi-wavelength data allow us to investigate the stellar populations, stellar masses, bolometric luminosities (L_bol), and extinction of z~2 galaxies. Deep Chandra and Spitzer data indicate that the sample includes galaxies with a wide range in L_bol, from 10^10 L_sun to >10^12 L_sun, and covering 4 orders of magnitude in dust obscuration (L_bol/L_UV). The sample includes galaxies with a large dynamic range in evolutionary state, from very young galaxies (ages <50 Myr) with small stellar masses (M* ~ 10^9 M_sun) to evolved galaxies (ages >2 Gyr) with stellar masses comparable to the most massive galaxies at these redshifts (M* > 10^11 M_sun). Spitzer data indicate that the optical sample includes some fraction of the obscured AGN population at high redshifts: at least 3 of 11 AGN in the z>1.4 sample are undetected in the deep X-ray data but exhibit power-law SEDs longward of ~2 micron (rest-frame) indicative of obscured AGN. The results of our survey indicate that rest-frame UV selection and spectroscopy presently constitute the most time-wise efficient method of culling large samples of high redshift galaxies with a wide range in intrinsic properties, and the data presented here will add significantly to the multi-wavelength legacy of the GOODS survey. [Abridged]

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arXiv2006

CaII and DLA absorption line systems: dust, metals and star formation at 0.4<z<1.3

Absorption line studies of galaxies along the line-of-sight to distant quasars allow a direct observational link between the properties of the extended gaseous disk/halo and of the star forming region of galaxies. In these proceedings we review recent work on CaII absorbers detected in the SDSS at 0.4<z<1.3 which, because of their dust content and chemical properties, may lie spatially closer to the central host galaxy than most DLAs. We present direct evidence for the presence of star formation, through observation of the [OII]3727,3730 emission line, in both CaII absorbers and MgII-selected Damped Lyman-alpha (DLA) systems. The measured star formation rate (SFR) from light falling within the SDSS fibre apertures (corresponding to physical radii of 6-9 h^{-1}kpc) is 0.11-0.48 Msol/yr for the CaII-absorbers and 0.11-0.14 Msol/yr for the MgII-selected DLAs. The contribution of both CaII absorbers and DLAs to the total observed SFR density, in the redshift range 0.4 < z < 1.3, is small, <10%. Our result contrasts with recent conclusions, based on the Schmidt law, that DLA absorbers can account for the majority of the total observed SFR density in the same redshift range.

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arXiv2006

Intergalactic CIV absorption at redshifts 5.4 to 6

We report the discovery of a strong CIV 1548,1550 absorption system at z_abs = 5.7238 in the near-infrared spectrum (J-band) of the z_em = 6.28 QSO SDSS J1030+0524. These observations, obtained with the Infrared Spectrometer And Array Camera (ISAAC) on the European Southern Observatory Very Large Telescope (ESO VLT), demonstrate that, with modern instrumentation, QSO absorption line spectroscopy can be successfully extended to near-infrared wavelengths to probe the intergalactic medium near the end of the reionization epoch. Although the statistics of this pilot study are limited, the mass density of triply ionized carbon implied by our data is comparable to the values of Omega_CIV reported at lower redshifts. Neither the column density distribution of CIV absorbers nor its integral show significant redshift evolution over a period of time which stretches from 1 to 4.5 Gyr after the big bang, suggesting that a large fraction of intergalactic metals may already have been in place at redshifts above 6. Alternatively, the strong CIV system we have detected may be associated with outflowing, highly-ionized, gas from a foreground massive galaxy; deep imaging and spectroscopy of galaxies near the QSO sightline should be able to distinguish between these two possibilities.

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arXiv2006

The Direct Detection of Lyman-Continuum Emission from Star-forming Galaxies at z~3

We present the results of rest-frame ultraviolet spectroscopic observations of a sample of 14 z~3 star-forming galaxies in the SSA22a field. These spectra are characterized by unprecedented depth in the Lyman-continuum region. For the first time, we have detected escaping ionizing radiation from individual galaxies at high redshift, with two of the 14 objects showing significant emission below the Lyman limit. We also measured the ratio of emergent flux density at 1500 AA to that in the Lyman-continuum region, for the individual detections (C49 and D3) and the sample average. If a correction for the average IGM opacity is applied to the spectra of the objects C49 and D3, we find f_1500/f_900,corr(C49)=4.5 and f_1500/f_900,corr(D3)=2.9. The average emergent flux-density ratio in our sample is =22, implying an escape fraction ~4.5 times lower than inferred from the composite spectrum in Steidel et al. (2001). If this new estimate is representative of LBGs, their contribution to the metagalactic ionizing radiation field is J_nu(900)~2.6x10^{-22} erg/s/cm^2/Hz/sr, comparable to the contribution of optically-selected quasars at the same redshift. The sum of the contributions from galaxies and quasars is consistent with recent estimates of the level of the ionizing background at z~3, inferred from the HI Ly-alpha forest optical depth. There is significant variance among the emergent far-UV spectra in our sample, yet the factors controlling the detection or non-detection of Lyman-continuum emission from galaxies are not well-determined. Because we do not yet understand the source of this variance, significantly larger samples will be required to obtain robust constraints on the galaxy contribution to the ionizing background at z~3 and beyond (abridged).

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arXiv2006

H-alpha Observations of a Large Sample of Galaxies at z~2: Implications for Star Formation in High Redshift Galaxies

Using H-alpha spectra of 114 rest-frame UV-selected galaxies at z~2, we compare inferred star formation rates (SFRs) with those determined from the UV continuum luminosity. After correcting for extinction using standard techniques based on the UV continuum slope, we find excellent agreement between the indicators, with = 31 Msun/yr and = 29 Msun/yr. The agreement between the indicators suggests that the UV luminosity is attenuated by an typical factor of ~4.5 (with a range from no attenuation to a factor of ~100 for the most obscured object in the sample), in good agreement with estimates of obscuration from X-ray, radio and mid-IR data. The H-alpha luminosity is attenuated by a factor of ~1.7 on average, and the maximum H-alpha attenuation is a factor of ~5. In agreement with X-ray and mid-IR studies, we find that the SFR increases with increasing stellar mass and at brighter K magnitudes, to ~ 60 Msun/yr for galaxies with K<20; the correlation between K magnitude and SFR is much stronger than the correlation between stellar mass and SFR. All galaxies in the sample have SFRs per unit area Sigma_SFR in the range observed in local starbursts. We compare the instantaneous SFRs and the past average SFRs as inferred from the ages and stellar masses, finding that for most of the sample, the current SFR is an adequate representation of the past average. There is some evidence that the most massive galaxies (M_star >10^11 Msun) have had higher SFRs in the past.

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arXiv2006

The Stellar, Gas and Dynamical Masses of Star-Forming Galaxies at z~2

We present analysis of the near-infrared spectra of 114 rest-frame UV-selected star-forming galaxies at z~2. By combining the H-alpha spectra with photometric measurements from observed 0.3--8 micron, we assess the relationships between kinematics, dynamical masses, inferred gas fractions, and stellar masses and ages. The H-alpha line widths give a mean dynamical mass M_dyn=6.9 +/- 0.6 x 10^10 Msun within a typical radius of ~6 kpc, after excluding AGN. The average dynamical mass is ~2 times larger than the average stellar mass, and the two agree to within a factor of several for most objects. However, ~15% of the sample has M_dyn >> M_star. These objects are best fit by young stellar populations and tend to have high H-alpha equivalent widths, suggesting that they are young starbursts with large gas masses. Rest-frame optical luminosity and velocity dispersion are correlated with 4 sigma significance. Using the local empirical correlation between star formation rate per unit area and gas surface density, we estimate the mass of the gas associated with star formation, and find a mean gas fraction of ~50% and a strong decrease in gas fraction with increasing stellar mass. The masses of gas and stars combined are considerably better correlated with the dynamical masses than are the stellar masses alone, and agree to within a factor of three for 85% of the sample. The combination of kinematic measurements, estimates of gas masses, and stellar population properties suggest that the factor of ~500 range in stellar mass across the sample cannot be fully explained by intrinsic differences in the total masses of the galaxies, which vary by a factor of ~40; the remaining variation is due to the evolution of the stellar population and the conversion of gas into stars. [Abridged]

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arXiv2006

Chemical Elements at High and Low Redshifts

The past few years have seen a steady progress in the determination of element abundances at high redshifts, with new and more accurate measures of metallicities in star-forming galaxies, in QSO absorbers, and in the intergalactic medium. We have also become more aware of the limitations of the tools at our disposal in such endeavours. I summarise these recent developments and--in tune with the theme of this meeting--consider the clues which chemical abundance studies offer to the links between the high redshift galaxy populations and today's galaxies. The new data are `fleshing out' the overall picture of element abundances at redshifts z = 2 - 3 which has been gradually coming into focus over the last decade. In particular, we can now account for at least 40% of the metals produced by the global star formation activity in the universe from the Big Bang to z = 2.5, and we have strong indications of where the remainder are likely to be found.

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arXiv2006

Star Formation and Extinction in Redshift z~2 Galaxies: Inferences from Spitzer MIPS Observations

Using very deep Spitzer/MIPS 24 micron observations, we present an analysis of the bolometric luminosities (L[bol]) and UV extinction properties of more than 200 spectroscopically identified, optically selected (UGR) z~2 galaxies in the GOODS-N field. The large spectroscopic sample is supplemented with near-IR selected (BzK/DRG) galaxies and submm sources at similar redshifts in the same field, providing a representative collection of relatively massive (M*>1e10 Msun) galaxies at high redshifts. We focus on the redshift range 1.5-2.6, where MIPS is sensitive to the strength of the mid-IR PAH features in the galaxy spectra (rest-frame 5-8.5 micron). We demonstrate, using stacked X-ray data and a subset of galaxies with H-alpha measurements, that L(5-8.5) provides a reliable estimate of L(IR) for most star forming galaxies at z~2. The range of L(IR) in the samples considered extends from ~1e10 to >1e12 Lsun, with a mean of 2e11 Lsun. Using 24 micron observations to infer dust extinction in high redshift galaxies, we find that, as in the local universe, the obscuration (L[IR]/L[1600]) is strongly dependent on L(bol), and ranges in value from <1 to \~1000. However, the obscuration is ~10 times smaller at a given L(bol) at z~2 than at z=0. We show that the values of L(IR) and obscuration inferred from the UV spectral slope beta generally agree well with the values inferred from L(5-8.5) for L(bol)<1e12 Lsun. For galaxies with L(bol)>1e12 Lsun, it is common for UV-based estimates to underpredict L(IR) by a factor of ~10-100. Using the specific SFR as a proxy for cold gas fraction, we find a wide range in the evolutionary state of galaxies at z~2, from those which have just begun to form stars to those which have already accumulated most of their stellar mass and are about to become, or already are, passively-evolving. [Abridged]

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arXiv2006

The Mass-Metallicity Relation at z~2

We use a sample of 87 rest-frame UV-selected star-forming galaxies with mean spectroscopic redshift z=2.26 to study the correlation between metallicity and stellar mass at high redshift. Using stellar masses determined from SED fitting to 0.3-8 micron photometry, we divide the sample into six bins in stellar mass, and construct six composite H-alpha+[NII] spectra from all of the objects in each bin. We estimate the mean oxygen abundance in each bin from the [NII]/H-alpha ratio, and find a monotonic increase in metallicity with increasing stellar mass, from 12+log(O/H) < 8.2 for galaxies with = 2.7e9 Msun to 12+log(O/H) = 8.6 for galaxies with = 1e11 Msun. We use the empirical relation between star formation rate density and gas density to estimate the gas fractions of the galaxies, finding an increase in gas fraction with decreasing stellar mass. These gas fractions combined with the observed metallicities allow the estimation of the effective yield y_eff as a function of stellar mass; in constrast to observations in the local universe which show a decrease in y_eff with decreasing baryonic mass, we find a slight increase. Such a variation of metallicity with gas fraction is best fit by a model with supersolar yield and an outflow rate ~4 times higher than the star formation rate. We conclude that the mass-metallicity relation at high redshift is driven by the increase in metallicity as the gas fraction decreases through star formation, and is likely modulated by metal loss from strong outflows in galaxies of all masses. There is no evidence for preferential loss of metals from low mass galaxies as has been suggested in the local universe. [Abridged]

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arXiv2006

Deuterium at High Redshifts: Recent Advances and Open Issues

Among the light elements created in the Big Bang, deuterium is one of the most difficult to detect but is also the one whose abundance depends most sensitively on the density of baryons. Thus, although we still have only a few positive identifications of D at high redshifts--when the D/H ratio was close to its primordial value--they give us the most reliable determination of the baryon density, in excellent agreement with measures obtained from entirely different probes, such as the anisotropy of the cosmic microwave background temperature and the average absorption of the UV light of quasars by the intergalactic medium. In this review, I relate observations of D/H in distant gas clouds to the large body of data on the local abundance of D obtained in the last few years with the FUSE satellite. I also discuss some of the outstanding problems in light element abundances and consider future prospects for advances in this area.

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arXiv2005

Selecting damped Lyman-alpha systems through CaII absorption. I: Dust depletions and reddening at z~1

We use the average E(B-V) and ZnII column densities of a sample of z~1 CaII (3935, 3970) absorption line systems selected from the Sloan Digital Sky Survey (SDSS DR4) to show that on average, with conservative assumptions regarding metallicities and dust-to-gas ratios, they contain column densities of neutral hydrogen greater than the damped Lyman-alpha (DLA) limit. We propose that selection by CaII absorption is an effective way of identifying high column densities of neutral hydrogen, and thus large samples of DLAs at z<1.3 from the SDSS. The number density of strong CaII absorbers (with rest-frame equivalent width W_3935>0.5AA), is ~20-30% that of DLAs, after correcting for the significant bias against their detection due to obscuration of the background quasars by dust. On average these absorbers have E(B-V)>0.1mag; the dustiest absorbers show depletions of refractory elements at a level of the largest depletions seen in DLAs. For the first time we can measure the dust-to-metals ratio in a sample of absorption selected galaxies, and find values close to, or even larger than, those observed locally. All of these properties suggest that a substantial fraction of the CaII absorbers are more chemically evolved than typical DLAs. From consideration of the E(B-V) distribution in our sample, and assuming CaII absorbers represent a subset of DLAs, we calculate that dust obscuration causes an underestimation in the number density of DLAs by at least 8-12% at these redshifts. Finally, the removal of Broad Absorption Line (BAL) quasars from the SDSS quasar sample increases the sensitivity of the detection of reddening by intervening absorbers. To this end, we describe a new, automated, principal component analysis (PCA) method for identifying BAL quasars. [abridged]

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arXiv2005

A Comparison of Absorption and Emission Line Abundances in the Nearby Damped Lyman-alpha Galaxy SBS 1543+593

We have used the Space Telescope Imaging Spectrograph (STIS) aboard HST to measure a sulfur abundance of [S/H] = -0.41 +/-0.06 in the interstellar medium (ISM) of the nearby damped Lyman-alpha (DLA) absorbing galaxy SBS 1543+593. A direct comparison between this QSO absorption line abundance on the one hand, and abundances measured from HII region emission line diagnostics on the other, yield the same result: the abundance of sulfur in the neutral ISM is in good agreement with that of oxygen measured in an HII region 3 kpc away. Our result contrasts with those of other recent studies which have claimed order-of-magnitude differences between HI (absorption) and HII (emission) region abundances. We also derive a nickel abundance of [Ni/H] < -0.81, some three times less than that of sulfur, and suggest that the depletion is due to dust, although we cannot rule out an over-abundance of alpha-elements as the cause of the lower metallicity. It is possible that our measure of [S/H] is over-estimated if some SII arises in ionized gas; adopting a plausible star formation rate for the galaxy along the line of sight, and a measurement of the CII* 1335.7 absorption line detected from SBS 1543+593, we determine that the metallicity is unlikely to be smaller than we derive by more than 0.25 dex. We estimate that the cooling rate of the cool neutral medium is log [l_c (ergs s^{-1} H atom^{-1})] = -27.0, the same value as that seen in the high redshift DLA population.

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arXiv2005

A Census of Optical and Near-Infrared Selected Star-Forming and Passively Evolving Galaxies at Redshift Z~2

Using the extensive multi-wavelength data in the GOODS-North field, we construct and draw comparisons between samples of optical and near-IR selected star-forming and passively evolving galaxies at redshifts 1.4 2.3 galaxies (Distant Red Galaxies; DRGs) are very similar as a function of K, with K<20 galaxies having ~120 Msun/yr, a factor of two to three higher than those with K>20.5. The absence of X-ray emission from the reddest DRGs and BzK galaxies with z-K>3 indicates they must have declining star formation histories to explain their red colors and low SFRs. While the M/L ratio of passively-evolving galaxies may be larger on average, the Spitzer/IRAC data indicate that their inferred stellar masses do not exceed the range spanned by optically selected galaxies, suggesting that the disparity in current SFR may not indicate a fundamental difference between optical and near-IR selected massive galaxies (M* > 10^11 Msun). We consider the contribution of UGR, BzK, DRG, and submillimeter galaxies (SMGs) to the SFRD at z~2, taking into account sample overlap. The total SFRD in the interval 1.4 80% could be selected by the UGR, BzK, and/or DRG criteria.

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arXiv2005

Zn and Cr abundances in damped Lyman alpha systems from the CORALS survey

We present metal abundances in 15 damped Lyman alpha systems (DLAs) from the Complete Optical and Radio Absorption Line System (CORALS) survey, designed to be free from any biasing effects due to extinction of QSOs by dust in intervening absorbers. It has long been suggested that such biasing may explain differences in metallicity between DLAs and coeval luminous galaxies, and between model predictions and observations. We use our measured zinc and chromium abundances (combined with those for five more CORALS DLAs from the literature, giving us a very nearly complete sample) to test whether the metallicity and degree of dust depletion in CORALS DLAs are significantly different from those of existing, larger, samples of DLAs drawn from magnitude limited, optical surveys. We find that the column density weighted metallicity of CORALS DLAs, [<(Zn/H)_DLA>] = -0.88 +/- 0.21 in the redshift interval 1.86 < z(abs) < 3.45, is only marginally higher than that of a control sample from the recent compilation by Kulkarni et al., [<(Zn/H)_DLA>] = -1.09 +/- 0.10. With the present limited statistics this difference is not highly significant. Furthermore, we find no evidence for increased dust depletions in CORALS DLAs--their [Cr/Zn] ratios conform to the known trend of increasing depletion (decreasing [Cr/Zn]) with increasing metallicity, and we have encountered no cases where Cr is as depleted as in local cold interstellar clouds. These results, when combined with the earlier findings of the CORALS survey reported by Ellison et al. in 2001, make it difficult to invoke a dust-induced bias to explain the generally low level of chemical evolution exhibited by most DLAs. Rather, they indicate that large scale optical QSO surveys give a fair census of the population of high redshift absorbers.

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arXiv2005

Spectroscopic Identification of a Proto-Cluster at z=2.300: Environmental Dependence of Galaxy Properties at High Redshift

We have discovered a highly significant over-density of galaxies at z=2.300+/-0.015 in the course of a redshift survey designed to select star-forming galaxies in the redshift range z=2.3+/-0.4 in the field of the bright z=2.72 QSO HS1700+643. The structure has a redshift-space galaxy over-density of delta_g,z ~= 7 and an estimated matter over-density in real space of delta_m ~= 1.8, indicating that it will virialize by z~0 with a mass scale of ~= 1.4x10^15 M_sun, that of a rich galaxy cluster. Detailed modeling of the spectral energy distribution -- from the rest-far-UV to the rest-near-IR -- of the 72 spectroscopically confirmed galaxies in this field for which we have obtained K_s and Spitzer/IRAC photometry, allows for a first direct comparison of galaxy properties as a function of large-scale environment at high redshift. We find that galaxies in the proto-cluster environment have mean stellar masses and inferred ages that are ~2 times larger (at z=2.30) than identically UV-selected galaxies outside of the structure, and show that this is consistent with simple theoretical expectations for the acceleration of structure formation in a region that is over-dense on large scales by the observed amount. The proto-cluster environment contains a significant number of galaxies that already appear old, with large stellar masses (>10^11 M_sun), by z=2.3.

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arXiv2005

UV to Mid-IR Observations of Star-forming Galaxies at z~2: Stellar Masses and Stellar Populations

We present the broad-band UV through mid-infrared spectral energy distributions (SEDs) of a sample of 72 spectroscopically-confirmed star-forming galaxies at z=2.30+/-0.3. Located in a 72 arcmin-squared field centered on the bright background QSO, HS1700+643, these galaxies were pre-selected to lie at z~2 based solely on their rest-frame UV colors and luminosities, and should be representative of UV-selected samples at high redshift. In addition to deep ground-based photometry spanning from 0.35-2.15 microns, we make use of Spitzer/IRAC data, which probes the rest-frame near-IR at z~2. The range of stellar populations present in the sample is investigated with simple, single-component stellar population synthesis models. Emphasizing stellar mass estimates, which are less subject to systematic uncertainties than other parameters, we find =10.32+/-0.51 for the sample. Allowing for the possibility of episodic star formation, we find that typical galaxies in our sample could contain up to three times more stellar mass in an old underlying burst than what was inferred from single-component modeling. In contrast, mass estimates for the most massive galaxies in the sample (M*>10^11 Msun) are fairly insensitive to the method used to model the stellar population. Galaxies in this massive tail, which are also the oldest objects in the sample, could plausibly evolve into the passive galaxies discovered at z~1.7 with near-IR selection techniques. In the general framework of hierarchical galaxy formation and mergers, which implies episodic star-formation histories, galaxies at high redshift may pass in and out of UV-selected and near-IR color-selected samples as they evolve from phases of active star formation to quiescence and back again (Abridged).

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arXiv2005

Strong Spatial Clustering of UV-selected Galaxies with Magnitude Ks<20.5 and Redshift Z~2

We obtained deep 8.5'x8.5' near-infrared images within four high-redshift survey fields, measured the Ks magnitudes of 300 optically selected galaxies with spectroscopic redshift 1.8 20.5. We found at greater than 95% confidence that the brighter galaxies cluster more strongly. The best-fit correlation lengths for the bright and faint samples are 10+-3 and 4+-0.8 comoving Mpc/h, respectively (1sigma), although the unusual density of bright QSOs in one of our survey fields may imply that the result is not representative of the universe as a whole. Neglecting this possibility, the correlation length for the optically selected sample with Ks<20.5 agrees well with that reported for comparably bright near-IR-selected samples. The differences in correlation length between optically selected and near-IR-selected samples have been presented as evidence that the two techniques find orthogonal populations of high-redshift galaxies. Our results favor a more nuanced view.

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arXiv2004

Spectral Modelling of Star-Forming Regions in the Ultraviolet: Stellar Metallicity Diagnostics for High Redshift Galaxies

The chemical composition of high redshift galaxies is an important property which gives clues to their past history and future evolution and yet is difficult to measure with current techniques. In this paper we investigate new metallicity indicators, based upon the strengths of stellar photospheric features at rest-frame ultraviolet wavelengths. By combining the evolutionary spectral synthesis code Starburst99 with the output from the non-LTE model atmosphere code WM-basic, we have developed a code that can model the integrated ultraviolet stellar spectra of star-forming regions at metallicities between 1/20 and twice solar. We use our models to explore a number of spectral regions that are sensitive to metallicity and clean of other spectral features. The most promising metallicity indicator is an absorption feature between 1935 A and 2020 A, which arises from the blending of numerous Fe III transitions. We compare our model spectra to observations of two well studied high redshift star-forming galaxies, MS1512-cB58 (a Lyman break galaxy at z = 2.7276), and Q1307-BM1163 (a UV-bright galaxy at z = 1.411). The profiles of the photospheric absorption features observed in these galaxies are well reproduced by the models. In addition, the metallicities inferred from their equivalent widths are in good agreement with previous determinations based on interstellar absorption and nebular emission lines. Our new technique appears to be a promising alternative, or complement, to established methods which have only a limited applicability at high redshifts.

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