Nebular and stellar properties of a metal poor HII galaxy at z=3.36

a r X i v :a s t r o -p h /0409352v 1 14 S e p 2004

Mon.Not.R.Astron.Soc.000,1–??(2001)Printed 2February 2008

(MN L A T E X style ?le v1.4)

Nebular and stellar properties of a metal poor HII galaxy

at z =3.36.

M.Villar-Mart′?n 1?,M.Cervi?n o 1&R.M.Gonz′a lez Delgado 1

1Instituto

de Astrof′?sica de Andaluc′?a (CSIC),Aptdo.3004,18080,Granada,Spain

ABSTRACT

We have characterized the physical properties (electron temperature,density,

metallicity)of the ionized gas and the ionizing population (age,metallicity,pres-ence of WR stars)in the Lynx arc,a HII galaxy at z =3.36.The UV doublets (CIII],SiIII]and NIV)imply the existence of a density gradient in this object,with a high density region (0.1-1.0×105cm ?3)and a lower density region (<3200cm ?3).The temperature sensitive ratio [OIII]λλ1661,1666/λ5007implies an electron temperature T e =17300+500?700K,in agreement within the errors with photoionization model predic-tions.Nebular abundance determination using standard techniques and the results from photoionization models imply a nebular metallicity of O/H ～10±3%(O/H)⊙,in good agreement with Fosbury et al (2003).Both methods suggest that nitrogen is overabundant relative to other elements,with [N/O]～2.0-3.0×[N/O]⊙.We do not ?nd evidence for Si overabundance,as Fosbury et al.(2003).

Photoionization models imply that the ionizing stellar population in the Lynx arc

has an age of <～5Myr.If He +

is ionized by WR stars,then the ionizing stars in the Lynx arc have metallicities Z star >5%Z ⊙and ages ～2.8-3.4Myr (depending on Z star ),when WR stars appear and are responsible for the He +2emission.However,alternative excitation mechanisms for this species are not discarded.Since the emission lines trace the properties of the present burst only,nothing can be said about the possible presence of an underlying old stellar population.

The Lynx arc is a low metallicity HII galaxy that is undergoing a burst of star formation of <～5Myr age.One possible scenario that explains the emission line spec-trum of the Lynx arc,the large strength of the nitrogen lines and the He +2emission is that the object has experienced a merger event that has triggered a burst of star formation.WR stars have formed that contribute to a fast enrichment of the ISM.

As Fosbury et al.(2003),we ?nd a factor of >～10discrepancy between the mass of the instantaneous burst required to power the luminosity of the H βline and the mass implied by the continuum level measured for the Lynx arc.We discuss several possible solutions to this problem.The most likely explanation is that gas and stars have di?erent spatial distributions so that the emission lines and the stellar continuum su?er di?erent gravitational ampli?cation by the intervening cluster.

Key words:cosmology:observations –galaxies:abundances –galaxies:high redshift –HII regions –stars:formation

1INTRODUCTION

H II galaxies are dwarf emission-line galaxies undergoing a burst of star formation.They are characterized by strong and narrow emission lines originated in a giant star-forming region which dominate their observable properties at opti-

?e-mail:montse@iaa.es cal wavelengths (e.g.(Terlevich et al.2004)).Most are blue

compact galaxies (BCGs).They have very low metallicities,high rates of star formation and a very young stellar con-tent.Many are compact and isolated.One of the reasons why these objects have attracted signi?cant attention is the possibility that they are very young galaxies in the process of formation.This possibility,however,has been challenged since evidence for an old (several Gyr)stellar population has

c

2001RAS

2Villar-Mart′?n,Cervi?n o&Gonz′a lez Delgado

been found in numerous BCGs(see(Kunth&¨Ostlin2000) for a review).Therefore,a model with a single,instanta-

neous burst of star formation does not seem appropriate to

describe these galaxies and a succession of short starbursts separated by quiescent periods seems more likely((Terlevich

et al.2004)).

Even in the case if HII galaxies are not primeval,little chemical evolution has happened and they provide impor-

tant information about how galaxies form and evolve,as well

as about the process of star formation in low metallicity en-

vironments.

The most distant HII galaxy in the catalogue compiled

by Terlevich et al.(1991)is at redshift z～0.31,while the

vast majority are at z<0.1.The proximity of these galaxies allows studies of their structure,metal content and stellar

population with high sensitivity and precision,di?cult to

achieve for high redshift star forming galaxies.Still,?nding high redshift HII galaxies is of great interest to investigate

their abundance patterns and stellar properties at earlier

epochs,compare them with the nearby counterparts and ob-tain further information about the formation and evolution

of galaxies using objects that have undergone little chemical

evolution.

The Lynx arc is a star forming object at z=3.36.It was discovered during spectroscopic follow-up of the cluster RXJ

0848+4456(z=0.57)from the ROSAT Deep Cluster Survey

((Holden et al.2001)).The arc shows a very red R-K color and very strong optical and UV(rest frame)narrow emission

lines.From the analysis of HST WFPC2images and Keck

optical and near infrared spectroscopy,Fosbury et al.(2003, FOSB03hereafter)concluded that the arc is an HII galaxy

magni?ed by a factor of～10by a complex intervening clus-

ter environment.The authors concluded that the continuum

is mostly nebular.

By means of photoionization modeling,(FOSB03)

showed that the spectroscopic properties of the Lynx arc are

all consistent with a simple HII region model of a cluster of ～106massive stars,characterized by a very high T ef f～80 000K,a high ionizing parameter(U～0.1)and low nebu-

lar metallicity Z～5%Z⊙.Indirect arguments suggest that the ionizing stellar population could have much lower https://www.sodocs.net/doc/0d16225337.html,ing simple photoionizaton models,the authors conclude that the spectroscopic properties of the Lynx arc are consistent with those of a metal-poor nebula ionized by a cluster of primordial(i.e.metal-free)stars.In this scenario, the overabundance of Si implied by the models is explained as due to enrichment by past pair-instability supernovae, requiring stars more massive than120M⊙.

We investigate in this paper the possibility that normal

(i.e.non primordial)stars are responsible for the excitation of the gas in the Lynx arc.Our?rst goal is to constrain the physical properties of the gas:electron temperature,density, metallicity.We will?rst use standard spectroscopic tech-niques of nebular analysis(e.g.(Aller1984)).These results will be compared with those from detailed photoionization modeling and those obtained by(FOSB03).

The second goal is to characterize the ionizing stellar population:age,metallicity,stellar mass of the burst,possi-ble presence of WR stars.This will be achieved by means of detailed photoionization models,whose objective is to?nd an ionizing stellar population able to explain all the spec-troscopic properties of the Lynx arc(line ratios,equivalent widths,intensity of the continuum).In addition,such mod-els set tight constrains on the nebular properties,which must be consistent with the results from the standard techniques mentioned above.

We present in§2the results of the nebular analysis and the photoionization models.These results will be discussed in§3.Summary and conclusions are presented in§4.

2ANALYSIS

2.1Nebular properties

In this section we will set constrains on the electron tem-perature,density and the nebular abundances of the ionized gas in the Lynx arc.

All the analysis and discussion presented below are based on spectra obtained with di?erent slit widths and orientations(see(FOSB03)for a detailed description of the data set and reduction techniques).For this reason, (FOSB03)estimated carefully scaling factors that were ap-plied in order to derive a consistent calibration.We will therefore assume that the errors introduced by calibration uncertainties are negligible.

The results are also dependent on whether reddening is present or not.We have ignored this e?ect.The very strong UV lines and the lack of evidence for dust reddening from the photoionization models and the?t to the continuum shape((FOSB03))makes as con?dent that such assumption is reasonable and our results are not seriously a?ected by it.Our own work presented in this paper will show that the line ratios are consistent with no dust reddening.

2.1.1Electron temperature

The[OIII]4363/[OIII]5007ratio has been traditionally used to measure electron temperatures in ionized nebulae.Unfor-tunately,the[OIII]λ4363line is outside the observed spec-tral range.We have used the UV[OIII]doublet instead,since the[OIII]λλ1661,1666/[OIII]λ5007ratio is mostly sensitive to electron temperature.

We show in Fig.1 the dependence of the[OIII]λλ1661,1666/[OIII]λ5007ratio with electron temperature The position of the Lynx arc(the value of the ratio is0.074±0.005,see Table1)is also shown as a?lled circle.It implies T e～17300+500

?700

K.This value is somewhat higher than that predicted by the photoioniza-tion models presented in§2.2which give T e=16200±500K, although taking errors into account,the discrepancy is not important.This further supports that dust reddening e?ects are negligible,otherwise,the temperature derived from Fig. 1would be a lower limit and large discrepancies with the photoionization model predictions would arise.This tem-perature is much lower than that derived by(FOSB03)and (Binette et al.(2003))by means of photoionization model-ing(～20000K).The discrepancy is due to a combination of the very hard ionizing continuum(very energetic electrons are therefore released in the ionization processes)and high U(ionization parameter)value used by these authors.

c 2001RAS,MNRAS000,1–??

A metal poor HII galaxy at z =3.36

3

Figure 1.Dependence of the [OIII]λλ1661,1666/[OIII]λ5007ra-tio with electron temperature T e .The position of the Lynx arc is indicated with a solid circle.This diagram implies T e ～17300+500?700K for this object.

CIII]1907

1892

1.5±0.1<2000

NIV 1483

Table 1.Electron densities measured for the Lynx arc using the CIII],SiIII]and NIV]UV doublets.

2.1.2Electron density

There are three UV line doublets we can use to estimate

the electron density in the Lynx arc:CIII]λλ1907,1909,SiIII]λλ1883,1892and NIV λλ1483,1487(see (Keenan,Feibleman &Berrington 1992),(Keenan et al.1995)).The results shown in Table 1correspond to T e ～10000-20000K range,where the ratios are quite insensitive to temperature.The upper limits take the errors into account (i.e.the densi-ties shown correspond to the minimum possible value of the line ratio).

The SiIII]doublet density determination has to be con-sidered carefully.We have noticed that the predictions by Keenan,Feibleman &Berrington (1992,see Fig.2)are ac-tually inconsistent with the results of our photoionization models (§2.3).As an example,for some of the n e =1000cm ?3models presented in §2.3,SiIII]1883/1892=1.8,which is above the maximum possible value (～1.5)predicted by those authors for this ratio.For models with n e =10cm ?3,we ?nd that the ratio reaches a value of 2in some cases.We also ?nd that the ratio varies for a ?xed density (be-tween 1.45and 1.8for n e =1000cm ?3),depending on other parameters such as the spectral energy distribution (SED)properties.

The range of densities implied by the di?erent doublets suggests that there is a gradient in the Lynx arc.The highly ionized region responsible for the emission of the N +3lines has n e ～[0.1-1.0]×105cm ?3.The C +3lines (1549,1551?A )

are also emitted within this region.There is a zone of lower ionization level,where the C +2,Si +2and N +2lines are emit-ted and which has n e <3200cm ?3.

Usually,the optical estimates of electron densities of H II regions,based on forbidden emission line ratios,are in the range 100to 1500cm ?3.However,radio techniques have revealed the existence in the Galaxy of very small and dense (n 4>104-105cm ?3)objects,the so-called compact or ultracompact HII regions (e.g.(Habing &Israel 1979)).

Density gradients in several giant and galactic HII re-gions have been observed (e.g.(Copetti et al.2000)).In most of these cases,the spatial variation of density may be interpreted as a radial gradient with the density decreasing from the centre to the edges.A good example is the Orion nebula.Based on measurements of the ratio [O II]3729/3726,Osterbrock &Flather (1959)showed the existence of a steep radial density gradient in the Orion Nebula,with the elec-tron density decreasing from 1.8×104cm ?3in the centre to 2.6×102cm ?3near the edge.2.1.3

Nebular abundances

Photoionization models by (FOSB03)imply that the nebu-lar abundances in the Lynx arc are ～5%solar within a fac-tor of 2.We set here tighter constraints by means of using standard techniques of nebular abundance determination.

The following expressions were used to calculate the ionic abundances of several elements (Aller 1984),assuming the low density regime.These expressions will then be used to calculate total element abundances.O +2

t E 04,2×10

1.25/t [OIII ]5007,4959

H +=7.29×10?7

√

Hβ

Ne +2

t E 04,2×10

1.59/t

[NeIII ]3869

H +=2.04×10?8

√

HβC +2

t E 04,2×10

3.28/t CIII ]1906,1909

H +

=2.17×10?8

√

Hβ

N +3

t E 04,2×10

4.20/t

NIV ]1487

H +

=2.99×10?7

√

Hβ

where t =T e /104and E 04,2=1.387t

?0.983

×10?0.042/t For silicon we have used the expression:

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2001RAS,MNRAS 000,1–??

4Villar-Mart′?n,Cervi?n o&Gonz′a lez Delgado Si+2

CIII]1906,1909

(from(Garnett et al.1995))

Garnett et al.(1995)showed that this expression is a good approximations within the temperature range10000-20000K and for the n e values expected for the Lynx arc, well below the critical densities of all the lines considered above.

The total element abundances are then calculated as follows:

O

H+Ne

H+

In both cases,we assume that most oxygen and neon atoms are twice ionized and therefore the ion abundance ra-tio is a good representation of the element abundance ratio. This assumption is supported by the high level of ionization of the gas implied by the high[OIII]/Hβand[OIII]/[OII] ratios of the Lynx arc.It is also con?rmed by our photoion-ization models(see§2.2)which show that～99%of O and Ne are in the O+2and Ne+2forms.

For the other elements we use:

C

H+

N

H+～

N+3+N+2

C =

Si+2

X(C+2)

]?1=

Si+2

Ion Lynx Lynx

t=1.7t=1.6

Element Lynx Lynx

t=1.7t=1.6

Table2.Abundances of some ions and elements in the Lynx arc

for two di?erent t values1.6and1.7.The range given for Si cor-

responds to the extreme values of the ionization correction factor

ICF implied by our photoionization models(see text).The sec-

ond value corresponds to models with WR stars.The two values

given for oxygen correspond to the calculations performed using

the[OIII]λλ4959,5007and the[OIII]λλ1661,1666doublets.The

errors on the abundances relative to the Sun are less than few per

cent.This only considers the errors on the ionic abundances and

do not account for uncertainties on the Solar values.

solar abundance values come from photospheric abundances

of Allende-Prieto,Lambert&Asplun(2002,2001),while N,

Ne,Mg,Si and Fe are from(Holweger(2001)).Uncertainties

on these values were not considered in the error calculation.

Calculations were done for two temperature values

T e=16000and17000K(t=1.6and1.7)to account for the

most likely T e range implied by the photoionization models

(§2.2)and the[OIII]lines(§2.1.1).It is important to note

that the electron temperature is di?erent in high and low

ionization zones of HII regions.To perform a more accurate

abundance determination,this temperature variation should

be taken into account.Most of the gas is ionized in the Lynx

arc(～99%of oxygen in the O+2form)and there is not a

low ionization zone(i.e.a region where species such as[OI],

[OII],[SII],etc exist).We expect therefore that the electron

temperatures measured using the[OIII]lines can be safely

assumed to be representative of a large fraction of the neb-

ula.Somewhat higher temperatures might be present in the

most highly ionized region.In such case,the abundances

presented in Table2,are upper limits?.

We conclude that the nebular metallicity in the Lynx

this value is determined by the uncertainties on the reconstruction

model of the line pro?le,rather than the line?ux measurements

((FOSB03))

?The small discrepancy between the O/H values calculated with

the[OIII]λλ5007,4959and OIII]λλ1661,1666values(see Table

2)could be due the fact that the excitation of the UV doublet

requires somewhat higher electron temperatures than the optical

doublet.

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A metal poor HII galaxy at z=3.365

arc(de?ned as O/H)is in the range～10-<20%Z⊙.This is somewhat higher than the value derived by(FOSB03)of ～5%Z⊙.

In addition,we conclude that nitrogen is overabundant relative to oxygen by a factor of～2.0-3.0compared with the solar abundance.

Departures from the constant density case may produce variations in the derived ICF values and therefore,the el-ement abundances(Luridiana&Cervi?n o2003,Luridiana et al.2004,in prep.).This does not a?ect oxygen seriously. However,the derived abundance of nitrogen can be o?by up to a factor of1.75for their models with the steepest power law density gradient(index>1.5).It is therefore possible, that N is not overabundant and the large derived N/O is a consequence of assuming constant density.

(FOSB03)concluded that silicon has to be well over-abundant relative to other elements,to be able to explain the strong SiIII]lines detected in the Lynx arc.The reason for this discrepancy will be discussed in§2.2.

Photoionization models in§2.2will set further con-strains on the nebular abundances of all elements.

2.2Age and metallicity of the ionizing stellar

population

We have characterized the age and metallicity of the ioniz-ing stellar population in the Lynx arc by means of produc-ing photoionization models that best reproduce the emission line properties.Further constraints on the nebular physical properties and chemical abundances will also be set.

The most outstanding spectroscopic properties of the Lynx arc that models should reproduce are(see(FOSB03)):?The large strength of the UV collisionally excited lines (NIVλ1486,CIVλ1550,OIII]λ1663,NIII]λ1750,etc)relative to recombination lines such as Hβ

?The large[OIII]/Hβ(=7.5)and the weakness of [OII]λ3727relative to other lines([OIII]/[OII]≥30, [OII]/Hβ≤0.25and[NeIII]λ3869/[OII]≥2.8)).I.e.,the high ionization level of the gas.

?The strength of the He+2emission (HeIIλ1640/Hβ=0.11)

?The extremely weak UV and optical continuum

All the SEDs used in this paper were computed by Cervi?n o,Mas-Hesse&Kunth(2004,hereafter CMK04) available at https://www.sodocs.net/doc/0d16225337.html,eff.esa.es/users/mcs/SED. The isocrones were computed following the prescriptions quoted in(Cervi?n o,G′o mez-Flechoso&Castander(2001)) and the models assume a power law Initial Mass Function (IMF)with a Salpeter slope,α=2.35((Salpeter1955))in the mass range2–120M⊙.We have adopted the evolution-ary tracks with standard mass loss rates by(Schaller et al.(1992);Schaerer et al.(1993a);Schaerer et al.(1993b); Charbonnel et al.(1993))and the following atmosphere models:(Schaerer&de Koter(1997))CoStar for main-sequence hot stars more massive than20M⊙,(Schmutz, Leitherer&Gruenwald(1992))for WR stars and(Kurucz (1991))for the remaining stars.An instantaneous star for-mation law was assumed.We have also considered the in-?uence of the X-ray emission of the starburst in the SED by varying the e?ciency of conversion of kinetic energy into X-rays.However,we have found that this e?ect does not produce signi?cant di?erences in the predicted emission line spectrum at the young ages considered here.Therefore,we have ignored it and we present only the results for SED models without X-ray emission.

We used the numerical code Cloudy((Ferland et al. 1998))to predict the line ratios in di?erent circumstances.

Given the complete lack of information on the geometry of the gas and the relative distribution between stars and gas in the arc,we have assumed the standard spherical geometry often used in photoionization models of other extragalactic HII regions and HII galaxies.The distance between the ion-izing source and the gas r0was assumed to be10pc and we used constant density n=1000cm?3.The?lling factor ff was varied until?nding the model that best reproduces the Lynx arc line ratios.Conclusions on the size and geometry of the ionized region cannot be extracted from our modeling since di?erent values of ff,n and r0might produce the same output spectrum.In addition,the assumption of spherical geometry is likely to be too simple,since the ionizing stars might be distributed in several clusters.

We adopt log Q(H)=55.20,as calculated by(FOSB03) from the Hβ?ux and corrected for magni?cation.Q(H)is the ionizing luminosity in erg s?1.Since the authors assumed covering factor CC=1in these calculations,we have also adopted this value.

We show in Tables3to6the results of the models that best reproduce the Lynx spectrum,together with the Lynx measurements.Although He+2is a problem for some of these models,they are also shown because of the very good agreement with the rest of the line ratios.The He+2 problem will be discussed in more detail in§3.1.We?nd that the nebular metallicity Z neb that produces the best?t to the Lynx spectrum is～10%Z⊙.This is in very good agreement with the results obtained in§2.1.3.

Z neb=5%Z⊙models are rejected.The models that can produce[OIII]5007/Hβ>～7,require very high?lling factor ff>～0.32(i.e.high ionization level).The predicted electron temperature is～20000K,much higher than the value im-plied by the[OIII]lines(§2.1.1).In addition,due to the high T e and ionization level of the nebula,the high ionization UV lines such as CIVλ1550and[OIII]λ1665become too strong, with CIV/Hβ>7.

The opposite e?ect is found for models with Z neb≥15% Z⊙,which can also be rejected with con?dence.Due to the higher metallicity,lower?lling factors(i.e.ionization level) are required to produce the same[OIII]λ5007/Hβ.For those models with[OIII]5007/Hβ>～7-8,the predicted T e is～14 000K,too low compared with the expected value(§2.1.1). In addition,because of the low ionization level and the low electron temperature,the high ionization UV collisionally excited lines become too weak,with CIV/Hβ<～1.

Therefore,all the models presented here assume Z neb=10%Z⊙for all elements(except nitrogen,see below). The most discrepant line ratios will always be shown in bold characters.

Z stars=5%Z⊙models

The results of these models are shown in Table3.The 2Myr(a)model assumes that all elements have an abun-dance of10%the solar value.Notice that the NIII]and NIV lines are severely underestimated.This is the case for all models considered below,as long as N abundance is not en-

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6Villar-Mart′?n,Cervi?n o&Gonz′a lez Delgado

hanced.The discrepancy disappears if we assume that this element is overabundant by a factor of～3.This result was

also suggested by the nebular abundance determination in

§2.1.3.Hereafter,this overabundance will be assumed in all models.

Models with ages≤4Myr reproduce very well the Lynx

line properties,except the He+2emission.Ages of5Myr and specially older present strong discrepancies with the mea-sured line ratios,due to the drastic reduction of the ionizing photons as stars get older.We only show5and a6Myr models in Table3for illustration.

The He+2lines are always predicted too faint.The rea-

son is that according to the stellar evolution models no WR stars are formed in such low metallicity environment(but see§3.1).

Z stars=20%Z⊙models

Models with ages<～3Myr are in very good agreement with the Lynx spectrum(Table4),except for the He+2lines which are predicted too faint.At～3.4Myr Wolf Rayet stars make an important contribution to the the hard ionizing radiation and the He+2problem is solved.NV should be detected according to this model.However,given the uncer-tainties on the estimation of the upper limit for this line, the predicted ratio is likely to be well within the errors ((FOSB03))of the measured value.Ages of4Myr and older produce strong discrepancies with the measured spectrum. We show4and5Myr models in Table4for illustration.

Z stars=40%Z⊙models

Models with ages<3Myr are in good agreement with

the Lynx emission line spectrum,except for the He+2lines

which are predicted too weak(Table5).Between3.0and 4Myr,WR stars appear and the He+2problem is solved. Ages of4Myr and older produce strong discrepancies with the measured spectrum.We show4and5Myr models in Table5for illustration.

Z stars=Z⊙models

Models with ages<～5Myr can explain the Lynx emission line spectrum remarkably well,except the He+2lines,which are predicted too weak.The appearance of WR stars at2.8 Myr solves the He+2problem and produces a good?t to the observed spectrum.Ages of6Myr and older produce strong discrepancies with the measured spectrum.We only show a 6Myr model in Table6for illustration.

To summarize,the best photoionization models imply:?The nebular abundances in the Lynx arc are～10±3% Z⊙.This is in very good agreement with the measured abundances in§2.1.3and with(FOSB03)within the errors. Lower(<7%Z⊙)and higher(≥15%Z⊙)metallicities pro-duce strong discrepancies with the measured line ratios.?Nitrogen is overabundant.The models suggest N/O～3×(N/O)⊙.This is in good agreement with the results obtained in§2.1.3.

?Our models do not imply overabundance of Si relative to O,contrary to the results obtained by(FOSB03).The reason to claim Si overabundance by this authors is that their model predicts SiIII]/Hβratios a factor of～100below the measured value.Si+2is the species found in the Lynx arc with the lowest ionization potential(IP).All other lines detected are emitted by ions with higher IP(N+2and C+2 are next,with47.4eV and47.9eV respectively).A hot black body(80000K in(FOSB03)models)produces a large supply of ionizing photons that e?ciently remove Si+2while still keeping a considerable amount of other species such as N+2and C+2.The same can be said about(Binette et al. (2003))models.The ionization of the gas in our models is dominated by much colder stars that allow the survival of Si+2in the nebula.

?The electron temperature is T e～16200±500K,in good agreement within the errors with the value determined from the[OIII]1663/5007ratio(see§2.1.1).

?Ages<～5Myr produce good?ts to the Lynx spectrum, except for the He+2lines,which are predicted too faint in most cases.Only when Wolf Rayet stars appear,the He+2 lines are properly?t.If Wolf Rayet stars are responsible for the ionization of He+(but see§3.1),this implies:

–Z stars must be>5%Z⊙,since no Wolf Rayet stars are formed at lower metallicities,according to the models (but see§3.1).

–the Lynx arc is undergoing a WR phase and the age of the burst is in the range2.8-3.4Myr,depending on the stellar metallicity.

3DISCUSSION

3.1The He+2problem

All models in Tables3to6with ages of～5Myr or younger (depending on the stellar metallicity)reproduce remarkably well most line ratios of the Lynx spectrum.Some of those models,however(those with no contribution from WR stars) cannot explain the He+2nebular emission.

HeIIλ4686nebular emission has been detected in the optical spectrum of numerous extragalactic HII regions and star forming galaxies(e.g.(Garnett et al.1991)).If the excitation of this line is due to photoionization by hot stars, these must have T ef f>～55000K.Based on photoionization modeling,(FOSB03)concluded that T ef f～80000K in the Lynx arc,in the range of values typical of Wolf Rayet stars.

We have shown that Wolf Rayet stars are good candi-dates to explain the He+2emission in the Lynx arc(they could also be responsible for the out?owing wind discovered by(FOSB03)).The non detection of the typical Wolf Rayet bumps found in Wolf Rayet galaxies(e.g.(Vacca&Conti 1992))is simply due to the fact that the stellar population is not detected at all(see§3.3).

We have used two of the SEDs that best reproduce the Lynx spectrum(Z stars=20%Z⊙and3.4Myr;Z stars=40% Z⊙and3.0Myr,see§2.2)to predict the number of WR stars in the Lynx arc.CMK04models predict2.9×10?4 and2.5×10?4Wolf Rayet stars per solar mass for these two https://www.sodocs.net/doc/0d16225337.html,ing the total stellar burst masses estimated from the Hβluminosity(see§3.3),we obtain2.7×104or2.1×104 WR stars for the20%Z⊙and40%Z⊙SEDs respectively. This is in the range of values found for low redshift WR galaxies,where the presence of100to100000WR stars has been inferred((Vacca&Conti1992)).

WR stars have been proposed to explain the He+2emis-sion in stellar ionized nebulae.However,this is an unsolved problem in nearby star forming objects.For this reason,we

c 2001RAS,MNRAS000,1–??

A metal poor HII galaxy at z=3.367 NVλ1240≤0.090.010.010.010.030.010.130.27

SIVλλ1394≤0.090.130.120.120.100.120.130.09

SIVλλ1402≤0.090.060.060.060.060.060.100.07

NIV]λ14860.42±0.050.520.180.520.470.520.280.06

CIVλ1549 3.65 3.63 3.90 3.69 3.33 3.63 1.970.60

HeIIλ16400.11±0.030.0080.0080.0080.0040.0050.0120.023

OIII]λ16650.56±0.040.520.540.520.460.520.370.17

NIII]λ17500.18±0.020.170.050.170.130.170.170.13

SiIII]λ18830.09±0.020.120.110.110.110.120.030.11

SiIII]λ18920.06±0.020.070.050.070.070.070.020.08

CIII]λ19090.59±0.060.940.880.910.740.990.840.84

[OII]λ3727≤0.250.080.080.080.060.080.030.43

[NeIII]λ38690.69±0.060.620.630.620.590.620.580.37

[NeIII]λ3968≤0.220.190.190.190.180.190.170.11

HeIIλ46860.015-0.0250.00110.00110.00110.00060.00070.00160.0024

[OIII]λ50077.50±0.38.258.368.287.978.257.8 5.43

Hβ 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

100001.73+0.50

?0.70

1.64 1.64 1.65 1.59 1.64 1.48 1.31

Z stars/Z⊙Lynx20%20%20%20%20%20% Age stars1Myr2Myr3Myr 3.4Myr4Myr5Myr Z neb/Z⊙10%10%10%10%10%10% N/N⊙10%30%30%30%30%30% log(ff)-1.7-1.5-0.5-1.50.00.0

t 1.73+0.50

?0.70

1.67 1.62 1.56 1.62 1.50 1.28

WR

8Villar-Mart′?n,Cervi?n o&Gonz′a lez Delgado

NVλ1240≤0.090.010.0220.120.160.200.24

SIVλλ1394≤0.090.120.110.110.090.060.08

SIVλλ1402≤0.090.060.060.060.050.040.07

NIV]λ14860.420.450.480.530.520.510.03

CIVλ1549 3.65 3.13 3.34 3.69 3.69 3.770.50

HeIIλ16400.110.0070.0060.100.140.180.02

OIII]λ16650.560.480.480.480.440.400.07

NIII]λ17500.180.180.150.140.090.040.06

SiIII]λ18830.090.130.100.090.060.0180.19

SiIII]λ18920.060.080.060.060.040.0130.12

CIII]λ19090.590.980.840.700.500.210.77

[OII]λ3727≤0.250.090.080.080.060.03 2.17

[NeIII]λ38690.690.600.600.590.570.570.20

[NeIII]λ3968≤0.220.180.180.180.170.170.06

HeIIλ46860.015-0.0250.0010.00080.0120.0170.0220.002

Hβ 1.00 1.00 1.00 1.00 1.00 1.00

[OIII]λ50077.507.957.977.857.597.8 3.01

Hβ 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Table5.Z stars=0.008=40%Z⊙.All models assume N/O～3×(N/O⊙).In spite of the He+2problem(see text)the agreement between the models and the observations is remarkably good for an age of3.3Myr or less.The best models are those with age～3Myr, when WR stars appear and ionize He+.Models of4Myr or more produce strong discrepancies with the observations.

NVλ1240≤0.090.020.040.150.320.160.58

SIVλλ1394≤0.090.120.090.100.100.100.16

SIVλλ1402≤0.090.060.050.060.060.060.14

NIV]λ14860.420.470.490.510.520.520.046

CIVλ1549 3.65 3.26 3.48 3.57 3.73 3.90.79

HeIIλ16400.110.0120.0090.130.260.0380.071

OIII]λ16650.560.480.450.450.420.440.06

NIII]λ17500.180.160.110.120.080.080.04

SiIII]λ18830.090.100.090.090.080.050.17

SiIII]λ18920.060.060.050.060.050.030.14

CIII]λ19090.590.900.630.620.470.440.49

[OII]λ3727≤0.250.080.040.090.090.05 2.81

[NeIII]λ38690.690.600.610.560.530.570.12

[NeIII]λ3968≤0.220.180.180.170.160.170.03

HeIIλ46860.015-0.0250.00160.00120.0170.0330.00460.0063

[OIII]λ50077.508.008.27.497.07.5 1.53

Hβ 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Table6.Z stars=0.020=Z⊙.All models assume N/O～3×(N/O⊙).In spite of the He+2problem(see text)the agreement between the models and the observations is remarkably good for ages of5Myr or less.The best model corresponds to2.8Myr of age,when Wolf Rayet are responsible for the ionization of He+.Models of6Myr or more produce strong discrepancies with the observations.

c 2001RAS,MNRAS000,1–??

A metal poor HII galaxy at z=3.369

do not reject the possibility that another unknown ioniza-tion source is present.

As an example,evidence for the existence of Wolf Rayet stars in the metal poor galaxy I Zw18(Z=1/50Z⊙)has been reported by several authors((Izotov et al.1997),(Legrand et al.1997)).(Stasi′n ska&Schaerer(1999))showed that the He+2nebular intensity can be reproduced using a radiation ?eld consistent with the observed Wolf-Rayet spectral fea-tures in this object(see also(de Mello et al.1998)).However, it is not clear whether WR stars can explain the spatial dis-tribution of the He+2emission((V′ilchez&Iglesias-P′a ramo 1998)).

Izotov et al.(2004)have recently discovered the high ionization emission line[NeV]λ3426in the spectrum of the blue compact low metallicity galaxy TOL1214-277.Fricke et al.(2001)detected the[FeV]λ4227line.Izotov et al.(2004) conclude that the stellar radiation is too soft to explain the existence of these high ionization species and propose that fast shocks or high mass X-ray binary systems could be re-sponsible.The possibility that ultra-luminous X-ray sources (ULX)are responsible for the He+2emission in the dwarf ir-regular galaxy Holmberg II has been proposed by(Pakull& Mirioni(2002))and(Kaaret,Ward&Zezas(2004)).ULXs may be be accreting normal mass(<20M⊙)black holes or neutron stars.An alternative possibility is that ULXs are high-mass X-ray binaires with super-Eddington mass trans-fer rates((King et al.2001)).

Peimbert,Sarmiento&Fierro(1991)showed that the emission line spectra of giant HII regions can be altered by the presence of shock waves produced by stellar winds or su-pernova events.This is also the case in star forming galaxies depending on the evolutionary phase of the stellar cluster ((Viegas,Contini&Contini1999)).The out?owing wind in the Lynx arc discovered by(FOSB03)provides evidence for the possible presence of shocks in the Lynx arc.Such kind of e?ects are not included in our synthesis models(where the supernova X-ray emission is the time-average contribution of the events).

A?nal possibility is the presence of super-soft X-ray sources((Rappaport,di Stefano&Smith1994)).One of the physical models proposed to explain the nature of these sources involves mass transfer from a main-sequence or sub-giant donor star to the surface of a white dwarf in a binary system((van den Heuvel et al.1992)).If this is the case in the Lynx arc,an additional stellar population with an age older than50Myrs must be present.

Therefore,we do not discard the possibility that alter-native mechanisms to WR stars are responsible for the ion-ization of He+in the Lynx arc.

3.2The nitrogen overabundance

We have found that the strength of the NIII]and NIV] lines cannot be explained assuming solar abundances.As ex-plained above,a possible explanation for this discrepancy is the assumption of constant density,rather than taking into account the presence of a gradient(see§2.1.3)(Luridiana et al.2004,in preparation).

An alternative explanation is that N is overabundant by a factor～2-3relative to the solar abundance(see§2.1.3 and§2.2).Signi?cant nitrogen excesses,although rare,have been found in some star forming objects.Particularly in-teresting are the?ve extremely metal poor BCGs with large nitrogen excesses(see(Pustilnik et al.(2004))for a more de-tailed discussion on this issue).The authors propose that the nitrogen excesses could be a consequence of merger events and a short powerful starburst phase when many WR stars contribute to a fast enrichment of the ISM.

The morphology((FOSB03))of the Lynx arc is remi-niscent of a merging system(several clumps A,B,C and D joined by a very faint arc),but this is misleading since it is strongly distorted by the e?ects of the lensing cluster.In their strong lensing model,(FOSB03)assume that the Lynx arc consists of two di?erent clumps(A and C)of the same source,while B and D are mirror images of A and C re-spectively.A and C(or B and D)might be two components of a merging system.The evidence,however,is not strong enough to con?rm this.

3.3The continuum problem

We investigate here whether the starburst luminosity re-quired to excite the strong emission lines in the Lynx arc is consistent with the non detection of the stellar component.

(FOSB03)found a discrepancy(a factor of～20)be-tween the mass of the starburst needed to power the Hβlu-mininosity of the Lynx arc and the mass of a starburst whose continuum would be just marginally detected,in agreement with the fact that the observed continuum is mostly nebular. We have done a similar exercise using the SED models that best reproduce the spectroscopic properties of the Lynx arc.

Using the SEDs by(CMK04)we have calculated the stellar mass of a burst that can generate the observed Lynx Hβluminosity L(Hβ)=4.3×1042erg s?1(corrected for magni?cation,(FOSB03)).The results are shown in Table 7for two of the SEDs that best reproduce the emission line spectroscopic properties of the Lynx arc(see§2.2).

The calculations have been done assuming covering fac-tor CC=1((FOSB03)).We obtain M stars～9×107M⊙. This is a lower limit,since we have assumed that all ionizing photons are absorbed by the gas and there is no dust red-dening.It is important to note that the uncertainties on the assumed magni?cation factorμare large.(FOSB03)found a rather wide range of models that successfully describe the Lynx arc system.This introduces serious uncertainties in the estimated cluster mass.

We have then calculated the stellar continuum at～1600?A(rest frame)emitted by these instantaneous bursts and compared it with the continuum level measured for the Lynx arc which is～0.15μJy(observed frame value cor-rected for magni?cation,(FOSB03)).This corresponds to Fν=7.89×10?32erg s?1cm?2Hz?1(rest frame).The results are shown in Table7.The theoretical values are a factor of ～10higher than the value measured for the Lynx arc.The discrepancy is even higher if we take the nebular continuum into account,which according to(FOSB03)is the dominant continuum component.

As(FOSB03),we therefore conclude that the stellar burst that reproduces the measured Hβluminosity would produce an observable continuum much(～10times)brighter than that detected from the Lynx arc.Adding the nebular contribution,the continuum should be even brighter.The conclusion is not a?ected by uncertainties on the magni?-cation factorμif the continuum and the Hβ?ux scale in

c 2001RAS,MNRAS000,1–??

10Villar-Mart′?n,Cervi?n o&Gonz′a lez Delgado

20% 3.4 3.2×10349.4×107 6.2×10?31

40% 3.0 3.5×10348.6×107 6.4×10?31

Lynx7.9×10?32

A metal poor HII galaxy at z=3.3611 than the expectation for a normal cluster of hot stars with

the same total mass and a Salpeter IMF.Bromm,Kudritzki

&Loeb(2001)estimated that a cluster of106M⊙of popIII

stars with a heavy IMF would produce～2.3×1042erg s?1

in the HeIIλ1640line and a spectral luminosity per unit

frequency of1.8×1027erg s?1Hz?1.For the observed lu-

minosity of the HeIIλ1640in the Lynx arc(～4.4×1042erg

s?1)a cluster of～2×106M⊙of primordial stars would be re-

quired.Such a cluster would produce a spectral?ux per unit

frequency of3.1×10?32erg s?1cm?2Hz?1at the distance

of the Lynx arc.This is a factor of～2.3below the contin-

uum measured for the Lynx arc.The discrepancy,therefore,

disappears and this result is consistent with the conclusion

by(FOSB03)that the continuum is mostly nebular.

The possibility that the Lynx arc is a popIII object was

proposed and discussed in detail by(FOSB03).It is di?cult

to explain,however,nebular abundances as high as～10%if

the stellar population is primordial.

4SUMMARY AND CONCLUSIONS

We have characterized the physical properties(electron tem-

perature,density,chemical abundances)of the ionized gas

and the ionizing stellar population in the Lynx arc,a gravi-

tationally ampli?ed HII galaxy at z=3.36.The temperature

sensitive ratio[OIII]λλ1661,1666/λ5007implies an electron

temperature T e=17300+500

?700K,in good agreement within the

errors with photoionization model predictions.The UV dou-blets imply the existence of a density gradient in this ob-ject,with a highly ionized high density region(0.1-1.0×105 cm?3)and a low density region(<3200cm?3)with lower ionization state.

Both the photoionization modeling and standard tech-niques of chemical abundance determination imply that the gas metallicity is～10±3%Z⊙.Both methods suggest that nitrogen is overabundant with N/O～2.0-3×[N/O]⊙,unless a density gradient produces this apparent e?ect.We do not ?nd evidence for Si overabundance as Fosbury et al.(2003). The reason is the di?erent shape of the ionizing continuum assumed by these authors(80000K black body,much hotter than the dominant ionizing stars in our models).

Photoionization models imply that the ionizing stars have very young ages<～5Myr.Since the emission lines trace the properties of the present burst only,nothing can be said about the possible presence of an underlying old stellar pop-ulation.Instantaneous burst models with Z star>5%Z⊙and ages～2.8-3.4Myr(depending on Z star),are in excellent agreement with the Lynx spectrum,including the strong He+2emission.At this age Wolf Rayet stars make an impor-tant contribution to the hard ionizing luminosity and they are responsible for the excitation of the He+2emission.In such case,we infer the existence of～2.5×104WR stars in the Lynx arc.Alternative excitation mechanisms for He+2, however,cannot be discarded.

Therefore,the Lynx arc is a low metallicity HII galaxy that is undergoing a burst of star formation of<～5Myr age. One possible scenario that explains the emission line spec-trum of the Lynx arc,the strength of the nitrogen lines and the strong He+2emission is that the object has experienced a merger event that has triggered a powerful starburst phase.Wolf Rayet stars have been formed and contribute to a fast chemical enrichment of the interstellar medium.

As Fosbury et al.(2003),we?nd a factor of>10discrep-ancy between the mass of the instantaneous burst implied by the luminosity of the Hβline and the mass implied by the continuum level measured for the Lynx arc.We have dis-cussed several possible solutions to this problem.The most satisfactory explanation is that gas and stars have di?erent spatial distribution so that the emission lines and the stellar continuum su?er di?erent gravitational ampli?cation by the intervening cluster.Detailed gravitional lensing models are needed to test the vailidity of this scenario. ACKNOWLEDGMENTS

We thank an anonymous referee for providing very useful comments that helped to improve this paper substantially. We thank Valentina Luridiana for useful scienti?c discus-sions and Andrew Humphrey for providing Figure1.M. Villar-Mart′?n and M.Cervi?n o are supported by the Spanish National program Ram′o n y Cajal.We acknowledge support by the Spanish Ministry of Science and Technology(MCyT) through grant AYA-2001-3939-C02-01.

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c 2001RAS,MNRAS000,1–??

从实践的角度探讨在日语教学中多媒体课件的应用

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新视野大学英语全部课文原文

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/usr/sbin:超级用户的一些管理程序。 /usr/doc:linux文档。 /usr/include:linux下开发和编译应用程序所需要的头文件。 /usr/lib:常用的动态链接库和软件包的配置文件。 /usr/man:帮助文档。 /usr/src:源代码,linux内核的源代码就放在/usr/src/linux 里。 /usr/local/bin:本地增加的命令。 /usr/local/lib:本地增加的库根文件系统。 通常情况下,根文件系统所占空间一般应该比较小,因为其中的绝大部分文件都不需要经常改动,而且包括严格的文件和一个小的不经常改变的文件系统不容易损坏。除了可能的一个叫/vmlinuz标准的系统引导映像之外,根目录一般不含任何文件。所有其他文件在根文件系统的子目录中。 1. /bin目录 /bin目录包含了引导启动所需的命令或普通用户可能用的命令(可能在引导启动后。这些命令都是二进制文件的可执行程序(bin是binary的简称,多是系统中重要的系统文件。 2. /sbin目录 /sbin目录类似/bin,也用于存储二进制文件。因为其中的大部分文件多是系统管理员使用的基本的系统程序,所以虽然普通用户必要且允许时可以使用,但一般不给普通用户使用。 3. /etc目录

新视野大学英语2课文翻译

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让你知道C盘的每个文件夹代表什么,其作用是什么

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