arXiv:0911.2328v1 [astro-ph.CO] 12 Nov 2009 - inSPIRE

C hem ical A bundances in the U niverse: C onnecting First Stars to P lanets c 2009 InternationalA stronom icalU nion P roceedings IA U Sym posium N o. 265, 2009 K . C unha, M . Spite & B . B arbuy, eds. D O I:00.0000/X 000000000000000X

T he C osm ic C hem icalE volution as seen by the B rightest E vents in the U niverse Sandra Savaglio1

arXiv:0911.2328v1 [astro-ph.CO] 12 Nov 2009

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M ax Planck Institute for ExtraterrestrialPhysics, 85748 G arching beiM unchen,G erm any em ail:savaglio@ m pe.m pg.de

A bstract.G am m a-ray bursts (G R B s) are the brightest eventsin the universe.T hey have been used in the last ve years to study the cosm ic chem ical evolution, from the local universe to the rst stars.T he sam ple size is stillrelatively sm allw hen com pared to eld galaxy surveys. H ow ever,G R B s show a universe that is surprising.A t z > 2,the cold interstellar m edium in galaxies is chem ically evolved,w ith a m ean m etallicity of about 1/10 solar. A t low er redshift (z < 1),m etallicities ofthe ionized gas are relatively low ,on average 1/6 solar.N otonly is there no evidence ofredshift evolution in the interval0 < z < 6:3,but also the dispersion in the 30 objects is large.T his suggests that the m etallicity ofhost galaxies is not the physicalquantity triggering G R B events.From the investigation ofothergalaxy param eters,item ergesthatactive star-form ation m ightbea strongerrequirem entto producea G R B .Severalrecentstriking results strongly support the idea that G R B studies open a new view on our understanding of galaxy form ation and evolution,back to the very prim ordialuniverse at z 8. K eyw ords.G am m a rays:bursts,observations,ISM :abundances,cosm ology:observations.

1. Introduction D uring the last decade,the chem icalevolution ofthe universe has been investigated using a new classofobjects:gam m a-ray bursts (G R B s).G R B s are the brightestsources in the universe,butwere rstdetected only in 1967 by a U S m ilitary satellite(K lebesadel etal.1973),because theirem ission doesnotlastlong.Forthisreason,theircosm ological origin wasdem onstrated only in 1997,w hen the rst redshift was m easured (M etzger et al.1998).Today,after m ore than twelve years,the num ber ofevents w ith spectroscopic redshift is still relatively low , about 200. N evertheless, on A pril 23 2009 the highest spectroscopic redshifteverwasm easured,and thishappened to be a G R B ,G R B 090423, at z = 8:2 (Salvaterra et al.2009;Tanvir et al.2009).T his is not only a very exciting success for G R B science,but it also dem onstrates that the eld is stillpotentially and e ectively crucialfor the understanding ofour universe. G R B s are very lum inous,but do not shine for very long (it cannot be any di erent, otherw isewe would notbe here to tell).T heir -ray em ission lastsatm osta few m inutes, during w hich they radiatethe sam eenergy em itted by the Sun overitsentirelife,10 G yr. Long-duration G R B s(m ore than a few seconds,the m ajority ofthose detected)originate from the nalcorecollapseofa m assivestar,a supernova (W oosley,1993).Short-duration G R B s(shorterthan a few seconds)havelikely a di erentprogenitor(K atz& C anel1996): the coalescence oftwo com pact objects (neutron stars or black holes).In both classes, rotation isthe key ingredientproducing the collim ated em ission.T he G R B rate isofthe order of one event every 105 years in a galaxy.T his m eans that, integrating over the entire universe and considering the collim ated em ission,few events are detectable from -ray satellites. D uring the lasttwo years,a num ber ofparticularly interesting discoverieshave show n 119

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thatthe universe probed by G R B s is surprisingly exciting.A partfrom the already m entioned G R B 090423,in M arch 2008 the brightest source ever was recorded.T his was G R B 080319 at z = 1:9 (7.5 G yrs after the B ig B ang), nicknam ed the \naked eye" G R B because it had an optical m agnitude m = 5:6 at its m axim um (B loom et al. 2009).In Septem ber2008,the at-the-tim e second m ostdistantobjecteverwasdetected, G R B 080913B at z = 6:7 (G reiner et al.2009). T hanks to this rich phenom enology and the large redshift range spanned,there is no doubtthatG R B sarevery e ectively probing,am ong otherthings,thechem icalevolution ofthe universe,allthe way from the localuniverse to the epoch of rststars,m ore than 13 G yr ago.In this paper,we w illsum m arize the results obtained in the last ve years.

2. T he cosm ic chem ical enrichm ent w ith G R B s T here are basically two distinct m ethods providing inform ation on the chem icalenrichm ent in galaxies and its redshift evolution using G R B s.In one case,rest-fram e U V absorption lines detected in the opticalafterglow spectra give m easurem entsin the neutralgas(T < 1000 K ) for z > 2 hostgalaxies(e.g.,Savaglio et al.2003;Prochaska et al. 2007;Fynbo et al.2009).In the other,rest-fram e opticalem ission lines from integrated spectra ofz < 1 hosts probe the ionized gas (T > 5000 K ;e.g.,Soderberg et al.2004; G orosabelet al.2005;T hone 2008).T hese two com plem entary m ethods did not give so far results sim ultaneously for the sam e G R B event for lack ofsuitable instrum entation. H owever,the new ly com m issioned optical-N IR spectrograph X -Shooteratthe ESO Very Large Telescope and the C osm ic O rigin Spectrograph recently installed on H ubble Space Telescope should llthe redshift desert soon.T he form er already delivered interesting ndings for the z = 3:372 event G R B 090313 (de U garte Postigo et al.2009). T heabsorption system sseen in high-z G R B sarecalled G R B -D LA s,asthey aresim ilar to dam ped Lym an- system s (D LA s) detected in Q SO spectra.O ne di erence is that in the form er case,the D LA is in the host galaxy,w hile in the latter case the D LA is generally not associated w ith the Q SO and distributed along its sight line. M oreover, colum n densities in G R B -D LA s are generally higher than in Q SO -D LA s (Fig.1),indi-

QSO-DLAs GRB-DLAs

F igure 1.Fraction ofG R B -D LA s ( lled histogram s) and Q SO -D LA s (em pty histogram s) per H Iand ZnIIcolum n-density bin (left-and right-hand side panels,respectively).T he Q SO -D LA histogram s are com plete for log N H I > 20:2 and log N Z nII > 12:4. T he com pleteness level for G R B -D LA s is not w ell determ ined. It is apparent that colum n densities in G R B -D LA s are generally higher than in Q SO -D LA s. T his can either indicate that G R B -D LA s originate in bigger galaxies, or that the volum e density of the gas is higher (e.g., the G R B sightline is crossing a region closer to the galaxy center) than Q SO -D LA s,or both.

JD 11. C osm ic C hem icalEvolution with Brightest Events

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cating that the neutral-gasregionscrossed by G R B s are larger,ordenser,or both,than those crossed by Q SO s.In Fig.2 we show the m etalabundances m easured in the D LA s detected in G R B hosts and in Q SO sight lines. It was claim ed, from a sm aller sam ple,that G R B -D LA s have generally higher m etallicity than Q SO -D LA s (B erger et al. 2006;Savaglio 2006;Prochaska etal.2007).T he m ostup-to-date sam ple ofG R B -D LA s, show n in Fig.2,contains 17 m easurem ents and two lower lim its in the redshift interval 2 < z < 6:3.T he average value (and statistical dispersion) for the 15 G R B -D LA s in 2:0 < z < 4:5 is< [Z=H ]> = 1:0 0:7,w hereasforthe 156 Q SO -D LA sin the sam e redshift intervalthis is < [Z=H ]> = 1:4 0:6.T he new large sam ple ofG R B -D LA s tends to show still a higher m etalcontent than Q SO -D LA s, but the gap is getting sm aller. T his indicates that the observationalbias that prevents us from m easuring abundances w hen m etallines are too weak m ight a ect our results.T he di erence w ith Q SO -D LA s is that G R B afterglow s,w hen spectroscopically observed,are on average severalm agnitudes fainter than the typicalQ SO ,and they cannot be observed for too long because they disappear quickly. For lower redshift, z < 1, m etallicities are m easured w ith em ission lines from H II regionsin the hostgalaxy.Em ission linem etallicitiesrely on di erentcalibrators(K ew ley & Ellison 2008)used depending on thesetoflinesavailable,according to theG R B redshift

F igure 2. R edshift evolution of the m etallicity relative to solar values,for 17 G R B -D LA s at z > 2, 16 G R B hosts at z < 1 and 250 Q SO -D LA s in the interval 0 < z < 4:4. Error bars are not available for allG R B -D LA s.Errors for G R B hosts are not estim ated.Errors for Q SO -D LA s are generally sm aller than 0.2 dex.T he dashed line is the best- t linear correlation for Q SO -D LA s. T he solid line is the m ean m etallicity predicted by sem i-analytic m odels for galaxy form ation (Som erville et al. 2001). T he G R B -D LA s m etallicity in 2 < z < 4:5 is on average 2.5 tim es higher than the average value in Q SO -D LA s in the sam e redshift interval.

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and instrum entsetting.R esultson lessthan 20 hostsindicate m etallicitiesbetween solar and 1/14 tim es solar values (Savaglio,G lazebrook & Le B orgne 2009;Levesque et al. 2009).T heaveragevalueand dispersion in 16 hosts(m edian redshiftz = 0:44)is< [Z=H ]> = 0:75 0:29 (Fig.2;Savaglio etal.2009).T hisissom ehow surprising,aswe do notsee evidence ofredshiftevolution from G R B -D LA m etallicitiesatz > 2.O n the otherhand, evolution isobserved in Q SO -D LA s,w here m etallicity atz < 1 is< [Z=H ]> = 0:3 0:5, a factor ofat least 10 tim es higher than at z > 2. Itwasrecently proposed thatthe di erentm etallicitiesin G R B -D LA sand Q SO -D LA s could be due to the di erentregionsprobed by the two populations.G R B stend to occur in regions w ith high star-form ation, therefore in regions closer to the galaxy center, w here m etallicity is on average larger than in a random galaxy sightline. Q SO s are background sources not associated w ith the galaxy hosting the D LA , therefore their sightline is crossing the galaxy in a random location,not necessarily close to a region ofstar form ation (Fynbo et al.2008).T his is con rm ed by the larger dust content and extinction m easured in G R B -D LA sw ith respectto Q SO -D LA s(K ann etal.2006;K ruhler et al.2008;Prochaska et al.2009).H owever,such a sensible conclusion collides w ith the relatively low m etallicities found in low -z G R B hosts.T he large dispersion ofthe m etal contentin G R B hosts in a large redshiftintervalis indicative that perhapsm etallicity is notdriving the G R B phenom enon.Forthis reason,we considerin the follow ing sections the othertwo fundam entalphysicalquantitiescharacterizing galaxies:the star-form ation rate (SFR ) and the stellar m ass M .

3. Star-form ation rate and stellar m ass of G R B hosts Fruchter et al. (2006) found that m ost G R B s occur preferentially in the brightest regionsofgalaxies.T hisissim ilarto supernovaeoftype Ic (K elly etal.2007).G R B hosts have very often som e sign of star form ation.SFR s are m easured from opticalnebular em ission lines (generally H and [O II]) up to redshift z = 1:4. For higher redshift, w hen nebular em ission lines are redshifted to the m ore di cult N IR ,the rest-fram e U V em ission (observed optical)can beused astheeasiest(butm oreuncertain)star-form ation indicator.Values m easured in the interval0 < z < 3:4 span a large range,from 0:01 M yr 1 to 40 M yr 1 (Savaglio et al.2009).T he average value of 2.5 M yr 1 is relatively high, ve tim es higher than in the Large M agellanic C loud.H owever,real SFR s m ight be a ected by dust obscuration in large portions of the host.SFR s from subm illim eter uxes (not a ected by dust) in four z 1 G R B hosts are m uch higher, 150 M yr 1,m ore than an order of m agnitude larger than the optical/U V values (M ichalow skiet al. 2008).T he e ect of undetected dust extinction is still not totally understood. T he stellar m ass ofG R B hosts is derived by tting the observed spectralenergy distribution (SED ) over a large wavelength range,w hich should include detections in the rest-fram e N IR ,beyond the 4000A B alm er break (M ichalow skiet al.2008;Savaglio et al.2009).T hisisbecause the bulk ofthe stellarm assisin sm alland cold starsw hich are m ostly em itting in the N IR .T he sam ple for w hich the stellar m ass is determ ined is still relatively low ,45 objects in the redshift interval0 < z < 3:4 (average redshift z = 0:96); the m ajority ofthem are at z < 2 (Savaglio et al.2009).O n average the stellar m ass is low ,ofthe order ofthe stellar m ass ofthe Large M agellanic C loud:M = 109:3 M . From the observed stellar m ass and m etallicities, one can ask w hether G R B hosts behave like norm al eld galaxies.In particular,one can consider the m ass-m etallicity (M Z) or lum inosity-m etallicity relations for galaxies,as a function of redshift. So far, allattem pts trying to identify the two relations in G R B hosts and sim ilarities w ith eld


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galaxies have failed (e.g. B erger et al. 2007; C hen et al. 2009; Levesque et al. 2009; Savaglio et al.2009).T he m ain problem seem s to be the sm allnum ber statistics. W e can com pare m edian values ofstellar m ass and m etallicity for z < 1 G R B hosts w ith the M Z relationsfor eld galaxiesby Trem ontietal.(2004)atz 0:07 and Savaglio etal.(2005)atz 0:7.A tz 6 0:45 (9 G R B hosts,m edian redshiftz = 0:17)the m edian m etallicity and stellar m ass are logZ=Z = 0:56 and M = 109:21 M ,respectively. In the interval0:55 6 z 6 0:97 (7 G R B hosts,m edian redshift z = 0:69) the m edian m etallicity and stellar m ass are logZ=Z = 1:06 and M = 109:73 M ,respectively. To com pare these valuesw ith eld-galaxy relations,we convertthe M Z relationsusing the new ly published converters ofm etallicity calibrators (K ew ley & Ellison 2008).T he expected m etallicities in the two redshiftand stellarm assbins (z = 0:17;0:69 and M = 109:21;109:73 M ) are higher than in G R B hosts,in both cases logZ=Z = 0:16.T he di erence is signi cant, especially for the high-redshift bin. T his issue needs further investigations w ith m ore objects,especially at higher redshifts,w here the M Z relation show sa strong redshiftevolution (Erb etal.206;M aiolino etal.2007).X -Shooteris the best instrum ent available at this tim e to m easure m etallicity in high-z G R B hosts. From the m ass and the SFR , it is possible to derive a m eaningful galaxy physical param eter:the speci c star-form ation rate SSFR = SFR =M ,that is the SFR per unit stellar m ass.Its inverse,the grow th tim e-scale = M =SFR ,gives the tim e interval required to a galaxy to reach the observed stellar m ass, assum ing that the m easured

GRB hosts

F igure 3.G row th tim escale = M =SFR (lefty-axis)oritsinverse,the speci c star-form ation rate SSFR = SFR =M (righty-axis)asa function ofredshift(Savaglio etal.2009).Filled circles and triangles are G R B hosts w ith SFR s m easured from em ission lines and U V lum inosities, respectively. Sm all, m edium , and large sym bols are hosts w ith M 6 109:0 M , 109:0 M < M 6 109:7 M ,and M > 109:7 M ,respectively.T he curve show s the age ofthe universe as a function ofredshift,and indicates the transition from bursty to quiescent m ode for galaxies. D ots are eld galaxies at 0:5 < z < 1:7 (Juneau et al.2005).C rosses are Lym an break galaxies at1:3 < z < 3 (R eddy etal.2006).T he big and sm allstarsatzero redshiftrepresentthe grow th tim escale for the M ilky W ay and the Large M agellanic C loud,respectively.

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SFR isconstantoverits pasthistory.Fig.3 show s and SSFR asa function ofredshift for G R B hosts,and the com parison w ith eld galaxies.G R B hosts are alm ost allstarform ing galaxies,halfofthem are in the bursty regim e.

4. Star form ation history of the universe w ith G R B s A s m ost G R B s are associated w ith m assive stars,therefore regions ofstar form ation, they are interesting candidates to study the SFR density (SFR D ) ofthe universe.T his exercise,recently attem pted by C hary,B erger,& C ow ie (2007),isbased on the idea that the G R B rate in galaxies at di erent epochs is proportionalto the SFR and that the ratio doesnotchangew ith redshift.T he norm alization isdone by taking the SFR density value at low redshift for w hich the density ofthe G R B rate is estim ated. K istleretal.(2009)havecom pared SFR D fordi erent eld galaxy sam plesw ith SFR D derived from G R B s (Fig.4).T he recent G R B 080913 at z = 6:7 and G R B 090423 at z = 8:2 have further extended the redshift intervalw here this can be done,in a regim e never explored before. A t z = 8, G R B SFR D is consistent w ith Lym an-break galaxy (LB G )m easurem entsafteraccounting forunseen galaxiesatthe faint-end U V lum inosity function.T his im plies that not allstar-form ing galaxies at these redshifts are currently being accounted for in deep surveys.G R B s provide the contribution to the SFR D from sm allgalaxies.A n interesting im plication is that the typicalG R B host at high redshift m ight be a sm allstar form ing galaxy.T his is not totally obvious,because it has been

F igure 4. T he cosm ic star form ation density of the universe (from K istler et al.2009).Light circles are the data from H opkins & B eacom (2006).C rosses are contributions from Lym anem itters (LA Es;O ta et al.2008).D ow n and up triangles are Lym an-break galaxies (LB G s) for tw o U V lum inosity functions: integration dow n to 0.2L at z = 3 (B ouw ens et al. 2008) and com plete (up triangles),respectively.T he latter show s a better m atch w ith values inferred from G R B s(red diam onds;K istleretal.2009),indicating the strong contribution from sm allgalaxies generally not accounted for in the observed LB G lum inosity function.


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established that the SFR D in m assive galaxies was m uch higher in the past than it is today (at z 2 a factor of6 higher than at z 0),w hereas the redshift evolution has been m ilder for low -m ass galaxies (Juneau et al.2005).A t z > 5,we m ight expect SFR to be m ainly in m assive galaxies.Finally,the SFR D from G R B s does not show a clear decline for z > 5,and it is m uch higher than in Lym anem itters (LA Es).

5. C onclusions Itiswellknow n thatG R B sshine through a universe thatishard to see in otherways. T hey are incredibly bright and last only a short tim e.T hese two properties m ake them very di erent from Q SO s,w hich are not as bright,and do not fade away,m aking the investigation ofnearby galaxiesm uch m orecom plicated.From lum inousG R B afterglow s, it is possible to m easure the redshift and localize faint galaxies. M ost G R B s are associated w ith the death ofa m assive star,thus w ith a star-form ing region.Itiswellknow n thatthe SFR ofthe universe wasm uch higherin the pastthan it is today (H opkins & B eacom 2006),therefore G R B s m ight be the m ost e cient way of identifying the evolution ofthe SFR density.W e also know thatthe SFR D is dom inated by sm all star-form ing galaxies, w hich are probably the m ost com m on galaxies in the distant universe (Pozzettiet al.2009). Identi cation ofdistant galaxies w ith G R B s is a ected by a di erent bias than traditional galaxy surveys, because G R B s are not detected through optical instrum ents, but w ith -ray and X -ray satellites.Som e ofthese galaxies can be faint,because dust extinguished,orbecause too far,or because intrinsically faint.W ith G R B s it is possible to explore extrem e regim esofgalaxy param eters,thusthey are im portantto understand galaxy form ation and evolution.G R B hosts identi ed in the opticaland N IR at z < 2 are generally sm all (on average the stellar m ass of the Large M agellanic C loud) and star-form ing galaxies,although SFR s span a large interval. G R B s are probes of the state of the chem icalenrichm ent of the universe,from the localuniverse,back to the tim e ofthe form ation of rst stars.M etallicities ofthe cold ISM in host galaxies at z > 2 is not low .T he m easured average value is 1/10 solar,and the dispersion is large,about a factor of ve. R elatively high m etallicity is con rm ed also for the highest redshift detections (Savaglio 2006;Totaniet al.2006;Price et al. 2007),w hich m eansthat there is no indication ofredshiftevolution.Low m etallicities of the G R B progenitor are theoretically predicted (no m ass loss) in order to keep a high angular m om entum ,and have a highly collim ated jet. R elatively high m etallicities, in this case from ionized gas of the host galaxies, are con rm ed also at z < 1.T he average value is 1/6 solar,w ith a dispersion of a factor of two,indicating that the m etalcontent in host galaxies is not evolving so fast.T he sam ple is not very large and system atic uncertainties are stillnot totally under control, therefore m ore observationsand detections are very im portant.N evertheless,the lack of evidence ofredshift evolution and the observed large dispersion suggest that G R B s do nothappen necessarily in m etalpoorgalaxies.Starform ation,on the otherhand,m ight be a m ore im portant physicaltrigger. G R B s are extrem ely im portant for our understanding ofthe prim ordialuniverse and theform ation and evolution ofheavy elem ents.T heenlightening discoveriesofthelastfew yearsare a clearindication thatthe investigation isa ected by ourtechnicalcapabilities w hich have dram atically im proved recently. D edicated instrum ents and observational program shave opened a new w indow in the hidden universe and show that this is m ore surprising and fascinating than expected.

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