Metallicities of 0.3z1.0 Galaxies in the GOODS-North Field

Metallicities of 0.3z1.0 Galaxies in the GOODS-North Field
Metallicities of 0.3z1.0 Galaxies in the GOODS-North Field

a r X i v :a s t r o -p h /0408128v 1 6 A u g 2004

Metallicities of 0.3

Henry A.Kobulnicky

Department of Physics &Astronomy

University of Wyoming

Laramie,WY 82071

Electronic Mail:chipk@https://www.360docs.net/doc/6216465418.html,

Lisa J.Kewley

Smithsonian Astrophysical Observatory,

Mail Stop 20,60Garden Street,

Cambridge,MA 02138

Electronic Mail:lkewley@https://www.360docs.net/doc/6216465418.html,

2004August 5

Accepted for Publication in The Astrophysical Journal

ABSTRACT

We measure nebular oxygen abundances for 204emission-line galaxies with

redshifts 0.3

in the universe have formed during the same interval.The slope of the L-Z re-

lation may evolve in the sense that the least luminous galaxies at each redshift

interval become increasingly metal poor compared to more luminous galaxies.

We interpret this change in slope as evidence for more rapid chemical evolu-

tion among the least luminous galaxies(M B>?20),consistent with scenarios

whereby the the formation epoch for less massive galaxies is more recent than

for massive galaxies.

Subject headings:ISM:abundances—ISM:H II regions—galaxies:abundances

—galaxies:fundamental parameters—galaxies:evolution—galaxies:starburst

1.Metallicity in Galaxy Evolution and Modeling

The chemical composition of galaxies is fundamentally important for tracing galaxy evolution and for modeling galaxy properties.Galaxy evolution prescriptions dating from Tinsley(1974,1980)to present computational codes(e.g.,STARBURST99–Leitherer et al. 1999;Bruzual&Charlot2003;P′e gase–Fioc,M.&Rocca-Volmerange1999;GRASIL–Silva et al.1998)incorporate metallicity as a primary ingredient in tracking a galaxy’s growth. Metallicity determines a galaxy’s UV and optical colors at a given age,the strength of stellar and interstellar metallic absorption lines,the shape of its interstellar extinction curve (e.g.,Pr′e vot et al.1984),its dust to gas ratio(Dwek1998;Issa,MacLaren,&Wolfendale 1990),its nucleosynthetic yields(e.g.,Woosley&Weaver1995),and perhaps even its rate of star formation(Nishi&Tashiro2000).Overall metal abundance and elemental abundance ratios trace the star formation history and nucleosynthetic history of a galaxy(reviewed by Wheeler,Sneden&Truran1989).These abundances and ratios also re?ect the importance of gas in?ow and out?ow in a galaxy’s evolution(galactic winds–Matthews&Baker1971; metal-rich galactic winds–Vader1987).Observational studies have now begun to provide detailed optical and infrared measurements of thousands of galaxies at redshifts from0.1to 6(e.g.,Rowan-Robinson et al.2001;Hippelein et al.2003;Sullivan et al.2004;van Dokkum et al.2004;Steidel et al.2004,and references therein),spanning the vast majority of cosmic time.Successful modeling of the evolutionary paths of these galaxies will require,among other parameters,secure measurements of their chemical compositions.

Direct measurements of metallicities1in distant galaxies are now becoming routine, spurred on by larger telescopes and more capable spectrographs used in deep galaxy surveys.

Kobulnicky&Zaritsky(1999)?rst applied classical nebular diagnostic techniques developed

for H II regions in local galaxies to the global spectra of14compact star-forming galaxies

at0.1M B>

?22and oxygen abundances from8.25<12+log(O/H)<9.022.Carollo&Lilly(2001) presented oxygen abundances for15luminous galaxies from the Canada-France Redshift

Survey(CFRS;Lilly et al.1995)over the range0.60M B>?22.6.

Both of these early works concluded that intermediate redshift galaxies were consistent with

the same correlation between luminosity and metallicity(i.e.,the L-Z relation)observed

in local samples(e.g.,Faber1973;Lequeux et al.1979;Skillman,Kennicutt,&Hodge

1989;Zaritsky,Kennicutt,&Huchra1994,hereafter ZKH).Meanwhile,Kobulnicky&Koo

(2000),Pettini et al.(2001),and Shapley et al.(2004)used near-infrared spectroscopy of

2.1

3.5Lyman break galaxies to measure gas-phase oxygen abundances.These authors

concluded that the high redshift objects were2-4magnitudes more luminous than z=0

galaxies with comparable8.3<12+log(O/H)<9metallicities and thus were inconsistent

with the local L-Z relation.Evidence for evolution of the L-Z relation with epoch,particularly

among sub L*galaxies with M B fainter than?20,grew with metallicity measurements of

640.26

Ke03)and66additional CFRS galaxies at0.47

LCS03;Carollo&Lilly,2001–CL01).Recently,Maier,Meisenheimer&Hippelein(2004)

measured metallicities for5sub L*galaxies at z~0.4and10sub L*galaxies at z~0.64

from the CADIS emission-line survey.These additional sub L*galaxies provide further

support for the evolution of the L-Z relation with epoch.

In this paper we present gas-phase oxygen abundance measurements for204emission-

line galaxies from0.3

(GOODS-North;Dickinson et al.2001)?eld using the publicly available spectra obtained as

part of the Team Keck Treasury Redshift Survey(TKRS;Wirth et al.2004).The new data

double the number of metallicities previously available for this redshift range and constitute

the highest quality spectra yet available for chemical analysis.In addition to providing new

constraints on the chemical enrichment of galaxies over the last8Gyr,it is our hope that

these measurements will prove useful for the community in modeling the evolution of galaxies

in this well-studied cosmological?eld.We combine these new TKRS data with existing

emission line measurements from the literature(CFRS–LCS03;DGSS–Kobulnicky et al.

2003)to assess the chemo-luminous evolution of star-forming galaxies out to z=1.Where applicable,we adopt a cosmology with H0=70km s?1Mpc?1,?m=0.3,and?Λ=0.7.

2.Data Analysis

2.1.Target Selection

The TKRS consists of Keck telescope spectroscopy with the DEIMOS(Faber et al. 2003)spectrograph over the nominal wavelength range4600?A–9800?A.Targets include both stars and galaxies,selected in an unbiased manner from all objects with R≤24.4in ground-based optical images(Wirth et al.2004).Slitmasks with1′′wide slits tilted to align with a galaxy’s major axis provided up to100spectra per1200s exposure.Spectra have typical resolutions of~3.5?A and total integration times of3600s.

We searched the publicly available TKRS spectroscopic database for galaxies with neb-ular emission lines suitable for chemical analysis.Only galaxies where it was possible to measure all of the requisite[O II]λ3727,Hβ,and[O III]λ5007lines were retained.These criteria necessarily exclude objects at redshifts of z 0.3since the requisite[O II]λ3727 line falls below the blue limit of the spectroscopic setup.Likewise,objects with redshifts z 1.0are excluded because the[O III]λ5007line falls beyond the red wavelength limit of the survey.In the2004February public release of the TKRS there were1536objects with secure redshifts.Of these,1044fell within our redshift limits.Of the1044candidates,497 were removed from consideration because no emission lines were present in the spectrum. Next,94galaxies were removed from the sample because one of the requisite strong emission lines fell o?the end of the wavelength coverage(or in between the red and blue spectral regions)due to the object’s placement on the slitmask.Following Kobulnicky et al.2003), we removed94objects in the redshift ranges0.410

strong night sky emission bands,low continuum,or poor continuum subtraction caused the extracted1-D spectrum to have continuum levels that were either negative or very close to zero.Because chemical analysis requires emission lines powered by normal stellar ionizing radiation?elds(as opposed to nonthermal sources from active galactic nuclei),we rejected9 objects exhibiting canonical AGN or LINER signatures:broad emission lines(2objects),or EW[Ne III]λ3826/EW[O II]λ3727ratios exceeding0.4(7objects)(e.g.,Osterbrock1989).The resulting usable sample contained204emission-line galaxies.

Twenty-seven of the204selected galaxies have measurable Hβbut immeasurably weak (S/N<3:1)[O III]lines.In principle,such objects should be included in the sample to avoid introducing a metallicity bias,but it is not possible to compute reliable metallicities if the oxygen lines are not detected with S/N ratio of8:1or better.We have retained these objects in the sample and measure upper limits on the[O III]line strengths which are used below to compute lower limits on oxygen abundances.

The204galaxies in our sample appear in Table1,along with their TKRS identi?ca-tions from Wirth et al.(2004)in column2,their GOODS-N3designations in column3, spectroscopic redshifts in column4,TKRS R-band magnitude in column5,and GOOD-N “i”(F775W),photometry in column6.From the observed magnitudes and and known red-shifts,we computed4restframe absolute blue magnitudes,M B,and colors,U?B,which appear in columns7and8of Table1.

In order to assess whether the204selected objects are representative of the GOODS-N TKRS galaxies with spectra in the0.3

episodes of star formation will have the bluest colors and will necessarily be the ones with nebular Hβ,[O II],and[O III]emission lines.

Figure2shows this selection in a slightly di?erent way,plotting redshift,z,versus photometric properties M B,(U?B),and i-band magnitude.Small dots indicate the whole set1044TKRS galaxies between0.3

2.2.Emission Line Measurements and Uncertainties

We utilized the boxcar extracted(not the optimally extracted)1-dimensional spectra made publicly available by the TKRS Team.We manually measured equivalent widths5of the[O II]λλ3726/29,Hβ,and[O III]λ5007emission lines present in each of the TKRS spectra with the IRAF SPLOT routine using Gaussian?ts with variable baseline,width and height.The[O II]λλ3726/29doublet,when spectrally resolved,was?t with a blend of two Gaussian pro?les.Visual inspection of this doublet showed that theλλ3726/29ratio, where su?ciently resolved,was always consistent with low electron densities less than a few hundred cm?3.We required that the?ts to the Hβand[O III]λ5007lines have the same Gaussian width.This constraint helped to make EW measurements more robust when one or the other of these nebular lines was a?ected by night sky emission lines.Table1lists the measured equivalent widths and measurement uncertainties for each line in columns9–11. The reported equivalent widths are corrected from the observed to the rest frame using

EW rest=EW observed/(1+z).(1) In all cases,we add2?A to the EW of Hβas a general correction for underlying stellar absorption(see Ke03).The EW reported for[O III]in column11includes the(unmeasured) contribution from[O III]λ4959using the assumption that I([O III]λ5007)/I([O III]λ4959)=3 so that

EW([O III])=1.3×EW([O III]λ5007)=EW([O III]λ5007)+EW([O III]λ4959).(2) In a few cases,the[O III]λ5007line was hopelessly lost in the noise from imperfectly sub-tracted night sky lines.For these objects,EW([O III])is measured using

EW([O III])=4×EW([O III]λ4959),(3) and such instances are denoted by the numeric code2in column14of Table1.For35 low-redshift objects,equivalent widths of the Hαand[N II]λ6584lines or upper limits could also be measured.The ratio EW Hα/EW[N II]λ6584is recorded in column12.Where only an upper limit on[N II]λ6584could be measured,we give the3σlower limits on the EW Hα/EW[N II]λ6584ratio.

In27of the galaxies,only upper limits on[O III]λ5007could be measured.These objects are located at the bottom of Table1and are denoted by the numeric code3in column14.To avoid a potential metallicity bias in the sample,we include these objects in our analysis and list the3σupper limits on the EW([O III])in column11of Table1.The upper limits on[O III](and[N II],where possible)are estimated by

EW=3×RMS×

1

C4σ2

C

,(5)

where L,C,σL,andσC are the line and continuum levels in photons and their associated1σuncertainties.We determineσC manually by?tting the baseline regions surrounding each emission line.We adoptσL=RMS×

due to uncertainties on the continuum placement,particularly for the[O II]λ3727line,which are di?cult to include in the error budget.

Using the observed Hβequivalent widths and calculated blue luminosities,we estimate the star formation rate for each of the TKRS galaxies in our sample.The Hβluminosity is estimated as

L Hβ(erg/s)=5.49×1031×2.5?M B×EW Hβ,(6) and then the star formation rate is calculated following Kennicutt(1998),

SF R(M⊙/yr)=2.8×L Hβ/1.26×1041(7) The resulting star formation rate estimates,in solar masses per year,appear in column13 of Table1.This is necessarily a lower limit on the star formation rate because we do not correct for extinction or stellar absorption.

2.3.Additional Galaxies

To supplement the data on204TKRS galaxies analyzed here,we have collected the 64emission line measurements from the DEEP Groth Strip Survey(Kobulnicky et al.2003;?lled symbols),and additional objects from the Canada-France Redshift Survey from LCS03 and Corollo&Lilly(2002).We use the published emission line measurements from those original works,and include only the subset of those galaxies where the EW Hβis measured with a signal-to-noise of8:1or better,in keeping with our selection criteria described above.

2.4.Chemical Analysis

Following traditional nebular diagnostic techniques(e.g.,Osterbrock1989;Pagel et al. 1979)and extensions of these techniques for distant galaxies developed by Kobulnicky,Ken-nicutt,&Pizagno(KKP;1999)and Kobulnicky&Phillips(2003),we use the equivalent width ratios of the collisionally excited[O II]λ3727and[O III]λ4959,5007lines relative to the HβBalmer series recombination lines to estimate gas-phase oxygen abundances.The ratio of emission line equivalent widths,while being one step further removed from the emis-sion line?ux ratios calibrated against photoionization models,has the advantage of being reddening-independent,to?rst order.

The principle diagnostic is the metallicity-sensitive emission line ratio,

I[O II]λ3727+I[O III]λ4959+I[O III]λ5007

log R23≡

(9)

n

where S H0is the ionizing photon?ux passing through a unit area,and n is the local number density of hydrogen atoms.This ionization parameter is divided by the speed of light to give the more commonly used ionization parameter;U≡q/c.Some R23–O/H calibrations have attempted to correct for the ionization parameter(e.g.,McGaugh1991;Kewley& Dopita2003,hereafter KD03)by using the ratio of the oxygen lines,known as O32:

log(O32)≡log I[O III]λ4959+I[O III]λ5007

and McGaugh(1991)models.A few of the TKRS galaxies have log(R23)maximum that is slightly higher by~0.1?0.2dex than the theoretical limits.The theoretical models were calculated assuming that the star-formation occurred in an instantaneous burst.This assumption may be reasonable for H II regions or for galaxy spectra dominated by one or two H II regions.However,continuous burst models may be more appropriate for modeling the emission-line spectra of active star-forming galaxies(e.g.,Kewley et al.2001).Continuous burst models would increase the theoretical upper limit by~0.2dex,making the data consistent with model limits(Kewley2005).We will compute new metallicity diagnostics utilizing continuous burst models in a future paper.

Figure3illustrates the di?culty in using R23to diagnose metallicity.Not only is R23 sensitive to ionization parameter but it is double valued in terms of the metallicity.To break the degeneracy,other metallicity-sensitive line ratios are required.For the30galaxies in our sample with measured[N II]/Hαratios,we calculate an initial metallicity using the [N II]/Hα??metallicity formulation from KD03.A further2galaxies have small[N II]/Hαupper limits that allowed us to break the R23degeneracy.For the potential ionization parameter and metallicity range of our sample(Figure3),the KD03[N II]/Hα-metallicity relation can be parameterized as

12+log(O/H)=7.04+5.28X NII+6.28X2NII+2.37X3NII

?log(q)(?2.44?2.01X NII?0.325X2NII+0.128X3NII)

+10X NII?0.2log(q)(?3.16+4.65X NII)(12)

where X NII=log EW([N II])6584/EW(Hα).The ionization parameter is calculated from the KD03[O III]/[O II]–q relation that we parameterize as

32.81?1.153y2+[12+log(O/H)](?3.396?0.025y+0.1444y2)

log(q)=

the R23ratio.Therefore,[N II]/Hαshould only be used as a crude initial estimate for a

more sensitive metallicity diagnostic such as R23.

Because the[N II]/Hαratio depends strongly on the ionization parameter and the

[O III]/[O II]ratio depends on metallicity,we iterated equations12and13until the metal-

licity varied less than the model errors(~0.1dex).Typically2-3iterations were required.

The resulting metallicity estimates indicate that29/32(91%)of the galaxies with useable

[N II]/Hαratios or upper limits lie on the upper R23branch(12+log(O/H)>8.4).In

column20of Table1we list the oxygen abundance derived from the strength of the[N II]

λ6584/Hαequivalent width ratios for the35(necessarily the lowest redshift)galaxies where

these lines can be measured.In many cases,only3σupper limits on the[N II]λ6584equiv-

alent widths are measured,so only upper limits on the oxygen abundances are given.Of the

galaxies where[N II]can be measured,only2of the least luminous galaxies(M B~?17) have[N II]λ6584/Hαratios consistent with very low metallicities that would place them on

the lower branch of the R23–O/H calibration.These objects are noted in Table1.

For the TKRS galaxies without[N II]/Hαratios,we are unable to break the R23degen-

eracy.We make the motivated assumption(see Kobulnicky&Zaritsky1999,Kobulnicky

et al.2003)that these galaxies fall on the upper-branch(12+log(O/H)>8.4)of the

double-valued R23?O/H relation,consistent with the majority of galaxies in our sample with[N II]/Hαratios.We next compute the oxygen abundances of TKRS galaxies using several di?erent,independent but related R23–O/H calibrations from the literature.

2.4.1.Zaritsky,Kennicutt,&Huchra1994,(ZKH)

Zaritsky,Kennicutt,&Huchra1994,(ZKH)provide a simple analytic relation between O/H and R23only without regard to ionization parameter:

12+log(O/H)ZKH=9.265?0.33R23?0.202R223?0.207R323?0.333R423(14) The ZKH formula is an average of three previous calibrations in the literature,namely Dopita &Evans(1986),McCall,Rybski&Shields(1985),and Edmunds&Pagel(1984).Figure6 shows these relations.The ZKH average relation was compared with a sample of disk H II regions with metallicities log(O/H)+12>8.35.As a result,the ZKH calibration is only suitable for H II regions in the metal-rich regime.

2.4.2.McGaugh1991(M91)

McGaugh(1991)calibrated the relationship between the R23ratio and gas-phase oxygen abundance using H II region models from the photoionization code CLOUDY(Ferland& Truran1981).McGaugh’s models include the e?ects of dust and variations in ionization parameter.Adopting the analytical expressions of McGaugh(1991,1998as expressed in KKP)which are based on?ts to photoionization models for the metal-rich(upper)branch of the R23–O/H relation.In terms of the reddening corrected line intensities,this relation is 12+log(O/H)M91,upper=12?2.939?0.2x?0.237x2?0.305x3?0.0283x4?

y(0.0047?0.0221x?0.102x2?0.0817x3?0.00717x4).(15)

Figure6shows graphically the relation between R23and O/H for the McGaugh(1991) and other calibrations from the literature.A circle marks the Orion Nebula value(based on data of Walter,Dufour,&Hester1992)which is in excellent agreement with the most recent solar oxygen abundance measurement of12+log(O/H)⊙=8.7(Prieto,Lambert,& Asplund2001).Oxygen abundances computed in this manner appear in Table1column17, along with a1σuncertainty computed by propagating the uncertainties on the emission line equivalent widths.This uncertainty estimate does not include the error introduced by the model uncertainties in the theoretical calibrations(typically~0.1dex).

2.4.

3.Pilyugin(2001)

Pilyugin(2001)developed an R23–O/H calibration based on a sample of H II regions with measurements of the[O III]λ4363auroral line.The[O III]λ4363auroral line provides a“direct”measurement of the electron temperature of the gas,and therefore,the metallicity. The use of the[O III]λ4363auroral line to derive metallicities is known as the T e method. Pilyugin(2001)calculated direct metallicities for a sample of H II regions spanning8.2 12+log(O/H) 8.6.His resulting R23–O/H calibration provides three curves,depending on a parameter P that accounts for the range in ionization parameter in the H II regions. Pilyugin was unable to provide a?t to H II regions for12+log(O/H) 8.6because high metallicity galaxies have weak or undetectable[O III]λ4363(see Stasinska2002for a discussion).Therefore,Pilyugin extrapolated the8.2 12+log(O/H) 8.6curves to higher metallicities.The Pilyugin curves are discussed further in Kennicutt,Bresolin,& Garnett(2003).

2.4.4.Kewley&Dopita(2003)

Kewley&Dopita(2003)provide a suite of abundance calibrations depending on the availability of particular nebular emission-lines.Their calibrations are based on a combi-nation of stellar population synthesis models(P′e gase and STARBURST99)and detailed photoionization models using the MAPPINGS code(Sutherland et al.1993).Like M91,the KD03models include the e?ects of dust.KD03include separate calibrations for ionization parameter.KD03point out that the R23curve is dependent on ionization parameter,while the common ionization parameter diagnostic(O32)depends on metallicity.They advocate the use of an iterative scheme to solve for both quantities if only[O III],[O II]and Hβare available.We provide a new parameterization of the KD03R23method(from Section 4.3in in KD03)with a similar form to the M91calibration to facilitate metallicity estima-tion and comparisons to estimates made using M91.For the potential ionization parameter and metallicity range of our sample(Figure3),the lower branch(12+log(O/H)<8.4)is parameterized by:

12+log(O/H)KD03,lower=9.40+4.65x?3.17x2?log(q)(0.272+0.547x?0.513x2)(16) where x=log(R23).The upper branch(12+log(O/H)≥8.4)is parameterized by:

12+log(O/H)KD03,upper=9.72?0.777x?0.951x2?0.072x3?0.811x4?

log(q)(0.0737?0.0713x?0.141x2+0.0373x3?0.058x4).(17)

Figure7shows graphically the relation between the metallicity and R23from equa-tion17for various values of ionization parameter q.The ionization parameter,q,is found using equation13in an iterative manner.Typically2-3iterations were required to reach convergence.This new parameterization is an improvement over the tabulated model coef-?cients of the KD03calibration because(1)the new calibration does not?x the ionization parameter or metallicity to the?nite set of model values during iteration,and(2)there is an increased sensitivity to the metallicity around the local maximum(Z=8.4)because we have introduced di?erent equations for the two branches rather than being limited to one equation.Oxygen abundances calculated via this method are given in column18of Table1. This parameterization should be regarded as an improved,implementation-friendly approach to be preferred over the tabulated R23coe?cients of KD03.

2.4.5.“Best”Adopted Oxygen Abundances

Of all the above methods for oxygen abundance computation,many arguments could be made for which are the“best”or most“accurate”.In Figure8we compare the‘P-method’, M91,ZKH and KD03R23metallicity estimates for the TKRS data.Di?erences between published calibrations shown in Figure8serve to illustrate the magnitude and severity of the possible systematic errors introduced by the di?erent calibrations.The uncertainties from any published calibration method are dominated by systematic uncertainties and/or biases in the data and/or models used to construct the calibration.The‘P-method’produces a strong systematic o?set and large scatter in the metallicity estimates compared to the three other calibrations.This o?set probably occurs because the P-method was not calibrated using any data or theoretical models for the metallicity range of our sample.We therefore do not use the P-method in our preferred metallicity estimates.

The three remaining calibrations show smaller systematic o?sets that are consistent with the error estimates of the calibrations(~0.1dex).Principle di?erences among the models include di?erent photoionizing radiation?elds from the various stellar atmospheres, stellar libraries,and stellar tracks.Di?erent photoionization models employ various atomic data and dust prescriptions.Di?erent calibration data from observations of Galactic or extragalactic H II regions over a range of metallicities and ionization parameters may also a?ect the resulting calibrations.Analyzing the nuances and resolving the di?erences between the published strong-line—metallicity calibrations is beyond the scope of this work.The exact choice of metallicity calibration is not crucial to the?rst of the two goals in this paper. Relative di?erences in metallicity between samples at di?erent redshifts will not be sensitive to the exact form of the R23–O/H calibration adopted.Section3below deals primarily with this question.Any of the three above methods which require only measurements of R23and O32will su?ce for discerning trends with redshift.

Kennicutt,Bresolin,&Garnett(2003)and Garnett,Kennicutt,&Bresolin(2004) present evidence from observations of metal-rich H II regions in M51and M101that there is a discrepancy of several tenths of a dex(factor of2or more)between the metallic-ities derived using traditional strong line methods and models compared to methods using direct measurements of[N II]electron temperatures.See these works for an extensive dis-cussion regarding possible systematic e?ects in the R23–O/H calibrations.hence,absolute metal abundances may have systematic uncertainties of0.2–0.5dex,particularly at the high metallicity end of our sample.Until these initial results are con?rmed and a more robust calibration is available,we proceed to adopt a combination of the KD03and M91strong line formulation.

For the purposes of providing the most meaningful metallicities to aid in modeling of

distant galaxies,we adopt a?nal“best estimate”oxygen abundance by averaging the KD03 and M91R23methods;12+log(O/H)avg=1/2[12+log(O/H)KD03+log(O/H)M91].We use the new KD03parameterization(equations17and13).Because the ZKH parameterization is based upon an ad-hoc average of3relatively old models and calibrations,we do not consider it for our best estimate in light of improvements to photoionization models(see for example Dopita et al.2000,Kewley et al.2001and references therein).

The resulting average abundances appear in Table1.For12+log(O/H)>8.4,the average of the KD03and M91methods can be approximated by the following simple form: 12+log(O/H)avg?upper~9.11?0.218x?0.0587x2?0.330x3?0.199x4

?y(0.00235?0.01105x?0.051x2?0.04085x3?0.003585x4)(18)

where x=log(R23)(equation8)and y=log(O32)(equation10).Note that this equation is only valid for the upper branch,and for the range of R23and O32values covered by our sample(Figure3).

https://www.360docs.net/doc/6216465418.html,parison with other properties

Figure9shows the relations between the“best estimate”oxygen abundance,Hβequiv-alent width,star formation rate,absolute B-band magnitude,U-B color,and redshift for the 204selected TKRS galaxies.This Figure illustrates that the colors and emission line equiv-alent widths of selected TKRS galaxies are distributed approximately uniformly across the full observed range of parameter space within each redshift bin.It also shows that galaxies at increasingly higher redshifts tend to be more luminous and have higher star formation rates.6 This is a type of selection e?ect due to the magnitude limited nature of the spectroscopic TKRS sample.

3.The L-Z Relation at0.3

3.1.Evolution of the L-Z Relation

The addition of204new metallicities at0.3

than before the chemo-luminous evolution of galaxies over the last8Gyr.Figure10shows

the oxygen abundance,12+log(O/H)M91versus M B for four redshift ranges.Filled symbols

show the DGSS data from Kobulnicky et al.(2003),crosses denote the CFRS data of LCS03

and CL01,and open symbols denote the TKRS data.Stars in the lower right panel are the

z>2Lyman break galaxies from Kobulnicky&Koo(2000)and Pettini et al.(2001).The dashed lines,which are the same in each panel,represent the?ts to local emission line galaxy samples as described in Kobulnicky et al.(2003).The solid lines are?ts to the DGSS data alone.The new TKRS data are in good agreement with the previous L-Z relations found

for each redshift interval.It becomes possible,for the?rst time with these data,to see the

L-Z correlation at redshifts z>0.8in the lower right panel.The most signi?cant departure

from the DGSS L-Z correlation seen in the TKRS data occurs in the0.4

in the TKRS sample which do not appear in the DGSS sample.This scatter is present to a

lesser extent in the0.6

Table1reveals these to be mostly objects with extreme values of the ionization parameter indicator,O32.Most of these galaxies have log O32

the correlation between R23and EW R23(KP03,Figure5)has a large dispersion and a systematic deviation from unity,making these points additionally uncertain.The conclusion

of Figure10is to highlight the overall good correspondence of TKRS metallicities with other surveys for similar magnitude and redshift ranges.

Figure11shows our adopted oxygen abundances,12+log(O/H)avg,for the DGSS+CFRS+TKRS galaxies versus M B in each of four redshift intervals.Lines now represent?ts to the combined

data in each redshift interval.The dotted line is a?t of M B on O/H,the dashed line is the inverse?t of O/H on M B.Since neither metallicity uncertainties nor the magnitude uncer-tainties are well characterized,we also use the method of linear bisectors described by Isobe

et al.(1990),shown by the solid line.Parameters for the linear?ts appear in Table3.The

?ts show that,regardless of the?tting method adopted,there is evidence that the slope of

the L-Z correlation changes with redshift.For the linear bisector?ts,the L-Z relation evolves monotonically from12+log(O/H)=?0.164±0.031×M B+5.57in the lowest redshift bin to

12+log(O/H)=?0.241±0.030×M B+3.73in the highest redshift bin.For the O/H on M B

?ts,the L-Z relation evolves monotonically from12+log(O/H)=?0.074±0.032×M B+7.20

in the lowest redshift bin to12+log(O/H)=?0.134±0.032×M B+5.97in the highest redshift bin.This change is driven mainly by the appearance of a population of relatively

luminous(M B~?20)but metal-poor(12+log(O/H)=8.5–8.6)galaxies in the two highest redshift bins.Ke03reported this change in slope of the L-Z relation using data out to z=0.8, and Figure11shows that this trend continues to higher redshifts.Based on plausible galaxy evolution models,optical morphologies,and their present metallicities,this population of galaxies is likely to evolve at roughly constant luminosity into comparatively metal-rich disk galaxies in the local universe rather than fade into“dwarf”galaxies(see Ke03,LCS03).

The evolution of mean galaxy metallicity with redshift can be seen more easily in Fig-ure12which shows oxygen abundance,12+log(O/H)avg,versus redshift for three di?erent luminosity bins.The upper row displays only distant galaxies,z>0.3.Lines show least squares?ts to the data with errors in O/H only.The left panel shows the lowest luminosity galaxies with?18.5>M B>?19.5.The slope of the least squares linear?t is-0.19±0.10dex per unit redshift.The linear correlation coe?cient for the60galaxies in this panel is-0.201, indicating that the probability of obtaining such a strong correlation at random is12%. For the middle panel showing100galaxies in the luminosity range?19.5>M B>?20.5, the best?t slope is-0.19±0.08dex per unit redshift.The correlation coe?cient is-0.228, indicating that the probability of exceeding this degree of correlation by chance is2%.For the right panel showing69galaxies in the luminosity range?20.5>M B>?21.5,the best ?t slope is-0.18±0.09dex per unit redshift.The correlation coe?cient is-0.197,indicating that the probability of exceeding this degree of correlation by chance is10%.Note that the relation between redshift and mean galaxy metallicity may not be(indeed,is theoretically not expected to be)a linear one if the star formation rate is not a linear function of redshift (e.g.,Somerville2001).However,the observed dispersion and limited range of luminosity and redshift of the current data do not warrant additional parameters in the?t.

The lower row of Figure12shows the same galaxies as the upper row,but with the addition of local z=0galaxies from Kennicutt(1992)and Jansen et al.(2001)used by Ke03 to de?ne the local L-Z relation.Lines show least squares?ts to the sample.The slopes are now somewhat smaller,0.14±0.05dex/z for the low-and intermediate-luminosity bins and 0.13±0.05dex/z in the high-luminosity bin.This high-luminosity bin lacks the population of low-redshift(0.3

very weak[O III]emission lines.

The mean rate of metal enrichment observed in Figure12is at least0.14dex per unit redshift for all luminosity bins.This rate of metal enrichment is signi?cantly greater(2-3σ)than the increase of0.08±0.02dex estimated by Lilly,Carollo,&Stockton(2003) over the same redshift interval.The smaller d(O/H)

Figure15,then,may all be understood as a consequence of a single underlying cause,namely the inter-relation between z,M B,and SF R among sample galaxies.

Which of the three parameters is the fundamental one driving the observed change in the L-Z relation?We argue that redshift is fundamental.Because the least squares?t in Figure14uses M B as one of the correlative variables,residuals should not depend on M B.If the star formation rate were the fundamental parameter driving the evolution of the L-Z relation(i.e.,galaxies with the highest star formation rates preferentially lie on the bright/metal-poor side of the L-Z relation),then we would also expect related parameters like the U-B color,which is sensitive to the star formation rate,to show a correlation with ?M B as well.Figure15shows that U?B is not correlated with?M B.We are left with the conclusion that the evolution of the L-Z relation is driven primarily by the redshift of the galaxies under consideration.Galaxies of a given luminosity are,on average,increasingly metal poor at higher redshifts.Said another way,galaxies of a given metallicity are,on average,more luminous at higher redshifts.This e?ect is most pronounced among the least luminous galaxies,those with M B fainter than~?20.None of the identi?ed sample selection e?ects would produce the changes in the nature of the L-Z relation seen in Figures11and 12.

https://www.360docs.net/doc/6216465418.html,parison to Metallicity Evolution in the Milky Way

One goal of studying galaxy populations at cosmological distances is to understand the evolutionary paths of individual galaxies.However,observing the evolution of the mean chemo-luminous properties of the population of star-forming galaxies over the last8Gyr (Figure11,12)is not the same thing as observing the evolution of any particular galaxy. Events shaping the evolution of any given galaxy at any particular point in its history,could, in principle,be unique to that galaxy and not be re?ected in the mean properties of galaxies at the equivalent lookback time in the distant universe.A galaxy,may,for example,spend most of its existence in a quiescent passively evolving phase after a brief initial period of star formation.Other galaxies might form stars continuously throughout their existence, while still others may not begin the star formation process until comparatively recent times. However,if the boundary conditions for galaxy evolution are determined primarily by global cosmological parameters,such as the age of the universe,the expansion rate,and the density of dark matter and gas available to form stars,then evolutionary paths of most galaxies ought to be observable in the mean evolution of galaxy properties with redshift.

Measuring ages and chemical abundances of stars in the Milky Way provides a glimpse into the history of one,presumably typical,disk galaxy.Figure13shows[O/H],the logarith-

mic oxygen abundances relative to solar,of F and G stars in the Galactic disk as a function

of age(Reddy et al.2003).We have plotted on the ordinate the redshift corresponding to

the lookback time of the measured age of the star.For our adopted cosmology of H0=70

km s?1Mpc?1,?M=0.3,?Λ=0.7,the relation between redshift,z,and the lookback time

or age,a,in Gyr is closely approximated by a polynomial,

z=1.3356?2.0847a+1.2431a2?0.33620a3+0.046737a4?0.0032106a5+0.000086938a6.(19) The solid line shows the least squares O/H on z?t to the stellar measurements.The tracks

of starred symbols show the least-squares linear?ts to distant GOODS-N+CFRS+TKRS

galaxies in the three luminosity bins from the lower row of Figure12.We use[O/H]=

12+log(O/H)-8.7in keeping with the most recent solar oxygen abundance measurements

(Allende-Prieto et al.2001).Both the overall level of metal enrichment(zero point)for

M B~20galaxies and the rate at which oxygen abundance increases with time(slope)among all GOODS-N galaxies over the redshift range z=1–0agree well with the trend observed in

Galactic stars.The slopes of the best?t relations for galaxies from the lower panel of

Figure12,0.14±0.05dex,are within the uncertainties of the slope for Milky Way stars,

0.20±0.04dex over the range0.0

LCS03presented a diagram similar to Figure13where they compare CFRS and NFGS

oxygen abundances to[Fe/H]measurements of Galactic disk stars and noted that the stars

were consistently0.2-0.3dex more metal poor than the galaxies.This o?set may be under-

stood as the signature of super-solar O/Fe ratios found in Galactic stars over most of the

history of the Milky Way re?ecting varying nucleosynthetic sources(reviewed by Wheeler,

Sneden,&Truran1989).Using oxygen measurements for Milky Way stars in Figure13

shows good agreement with the oxygen abundance measurements in distant galaxies.While

the luminosity history of the Milky Way is not known,and while the dispersion in oxygen

abundances for Galactic stars is large,the chemical enrichment process that occurred in

the disk of the Milky Way appears to be a good representation of the chemical enrichment

process in the bulk of the star-forming galaxies over the last8Gyr.Ke03observed that

the majority of the star–forming galaxies in the DEEP Groth Strip Survey over a similar

redshift range appeared to have substantial disk components based on Hubble Space Tele-

scope imaging.Thus,it appears we are able to observe directly,in ensembles of disk-like

galaxies at cosmological distances,the same chemical histories encoded in Galactic stellar

populations.In general,we expect that the luminosity-weighted nebular oxygen abundance

of the entire Milky Way would be0.1–0.3dex higher than the metallicity of the solar neigh-

borhood,given that the bulk of star formation occurs in the molecular ring at smaller radii

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关于工作的英语演讲稿

关于工作的英语演讲稿 【篇一:关于工作的英语演讲稿】 关于工作的英语演讲稿 different people have various ambitions. some want to be engineers or doctors in the future. some want to be scientists or businessmen. still some wish to be teachers or lawers when they grow up in the days to come. unlike other people, i prefer to be a farmer. however, it is not easy to be a farmer for iwill be looked upon by others. anyway,what i am trying to do is to make great contributions to agriculture. it is well known that farming is the basic of the country. above all, farming is not only a challenge but also a good opportunity for the young. we can also make a big profit by growing vegetables and food in a scientific way. besides we can apply what we have learned in school to farming. thus our countryside will become more and more properous. i believe that any man with knowledge can do whatever they can so long as this job can meet his or her interest. all the working position can provide him with a good chance to become a talent. 【篇二:关于责任感的英语演讲稿】 im grateful that ive been given this opportunity to stand here as a spokesman. facing all of you on the stage, i have the exciting feeling of participating in this speech competition. the topic today is what we cannot afford to lose. if you ask me this question, i must tell you that i think the answer is a word---- responsibility. in my elementary years, there was a little girl in the class who worked very hard, however she could never do satisfactorily in her lessons. the teacher asked me to help her, and it was obvious that she expected a lot from me. but as a young boy, i was so restless and thoughtless, i always tried to get more time to play and enjoy myself. so she was always slighted over by me. one day before the final exam, she came up to me and said, could you please explain this to me? i can not understand it. i

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