Chemical Evolution of Galaxies

237 Pages·2012·8.749 MB·English

by  Francesca Matteucci (auth.)

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•
Francesca Matteucci
Chemical Evolution
of Galaxies
123
FrancescaMatteucci
Universita`diTrieste
Dipto.Fisica
ViaG.B.Tiepolo11
34131Trieste
Italy
[email protected]
ISSN0941-7834
ISBN978-3-642-22490-4 e-ISBN978-3-642-22491-1
DOI10.1007/978-3-642-22491-1
SpringerHeidelbergDordrechtLondonNewYork
LibraryofCongressControlNumber:2011942161
(cid:2)c Springer-VerlagBerlinHeidelberg2012
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Coverfigure:Galaxiesofdifferentmorphologicaltype
Credit:AustralianAstronomicalObservatory/DavidMalinImages
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To themostimportantwomen of mylife:
MariaFrancescaandRaffaella
•
Preface
This book deals with the chemical evolution of galaxies. The term chemical
evolutionisimmediatelyassociatedwithBeatriceTinsley,whogreatlycontributed
to this important astrophysicalfield. Simultaneously with R.J.Jr. Talbot and D.W.
Arnett,shedevelopedaformalismthatallowedustoexploregalaxyevolutionfrom
thepointofviewofelementproductionanddistributioninsideandoutsidegalaxies.
DuringtheBigBang,onlylightelements(H,D,He,andLi)wereproducedandall
theheavierones(fromcarbontotheheaviest)originatedinstars.Thehistoryofstar
formationisoneofthemaindriversofchemicalevolutionsincethenumberofstars
formed(the star formationrate) and the distribution of stars as a functionof their
mass (the initial mass function) are regulating the rate of chemical enrichment at
anycosmic time. Then,the stellar evolutionandnucleosynthesisprovidesus with
fundamental information about element production and their restoration into the
interstellar medium. As time passes, more and more stellar generations succeed
one another and the new ones form out of gas enriched in heavy elements by the
previousgenerations.Thisistheprocesswecallchemicalevolution.BernardPagel
alsogreatlycontributedtothestudyofgalacticchemicalevolution,firstofallasa
carefulandexperiencedobserverandalsobymodelingchemicalevolution,witha
preferenceforanalyticalmodels,whoseresultsareeasiertounderstandthanthose
of numericalmodels. However,numericalmodelsbecamenecessaryto followthe
evolutionofsinglechemicalelementsproducedbystarsindifferentmassrangesand
bydifferentnucleosyntheticprocesses.The hypothesisof instantaneousrecycling,
which allows us to solve analytically the chemical evolution equations, assumes
thatthestellarlifetimesarenegligibleandthatallstarsrestoretheirprocessedand
unprocessedmaterialinstantaneously.Whilethishypothesiscanstillbeacceptable
forthosechemicalelementswhichareproducedonveryshorttimescalescompared
to the age of the Universe (massive stars), it is a very poor assumption for those
elements restored into the interstellar medium on timescales ranging from several
hundred million to billion years. One such element is iron, which is used as the
maintracerofthe“metallicity”(i.e.,thesumoftheabundancesofalltheelements
fromcarbonandheavier)instars.Webelievenowthatmostoftheironisproduced
in longer living stars such as those which end their lives as carbon–oxygenwhite
dwarfs.Adoptingtheinstantaneousrecyclingapproximationimpliesthattheratioof
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viii Preface
everytwochemicalelements,suchasoxygenandiron,wouldbeconstantacrossthe
lifeoftheUniverse.Aswewillsee,thisisnotthecaseformanypairsofelements,
inparticularforoxygenandiron.
The topic of galactic chemical evolution is a highly challenging one since it
involvesmanyphysicalprocessesthatwehavenotyetfullyunderstood.Oneofthe
mostcommoncriticismsofthechemicalevolutionmodelsisthattheycontaintoo
manyfreeparameters.Thisistrue,butmyanswertothiscriticismhasalwaysbeen
thatwhatmattersisthenumberofobservationalconstraintsthatwecanreproduce:
in fact, this number should be much larger than the number of free parameters.
Besides the star formation history and stellar nucleosynthesis, the important pro-
cesses regulating the chemical evolution of galaxies are the feedback between
supernovaeandtheinterstellarmedium,namely,thetransferofenergyfromstarsto
theinterstellargas,andthegasflowswhichcanbeenteringorleavinggalaxies.The
balancebetweenalloftheseprocesseswilleventuallydeterminethemetalcontent
and the abundance patterns we observe in stars and gas. The interpretationof the
behavior of abundance ratios in galaxies is one of the main topics of this book
and it is based on the different roles played in the chemical enrichment by stars
with differentlifetimeswhich producedifferentelements. For example,oxygenis
producedmainly in core-collapse supernovaeemanating from short-livedmassive
stars, whereas iron is mainly produced in thermonuclear supernovae (Type Ia),
which are believed to originate from white dwarfs in binary systems which can
evolveoverlongtimescales.Thedifferenceinthetimescalesforelementproduction
produces the observed abundance patterns; this interpretation is known as “time-
delaymodel.”Onthebasisofthetime-delaymodelwecantrytoestimatethenature
andtheagesofhighredshiftobjectsofunknownmorphology,justbyanalysingtheir
observedabundancesandabundanceratios.Ingeneral,modelsofgalacticchemical
evolution can be helpful in constraining both stellar nucleosynthesis and galaxy
formation mechanisms. For example, the study of the abundance gradients along
the thin disk of the Milky Way leads us to suggest the disk formed inside-out, a
mechanismwhichshouldbesharedalsobyotherspiraldisks.
Many researchershave contributedin the last 30 years to the understandingof
galacticchemicalevolution.Manyof them are quotedin this book.I apologizeto
thoseresearcherswhohavenotbeenmentioned,butthenumberofreferencesina
bookshouldbesmallerthaninresearcharticlesand,therefore,Ihavemadeaneffort
ofconstrainthenumberofcitations.
Theorganizationofthisbookisasfollows.
In Introduction, I discuss the concept and definition of chemical abundances
startingfromtheabundancesderivedfortheSun.Then,Idescribethemainstellar
populationsin our Galaxy, characterized by their chemical and kinematical prop-
erties. The Hubble sequence of galaxies is presented and a possible interpretation
ofthissequenceintermsofdifferentstarformationhistoriesisattempted.Someof
themainobservablesingalaxiesarereviewedand,finally,differentmethodological
approachestomodelinggalaxyevolution,aredescribed.
Chapter 2 dealswith the basic ingredientsfor constructingmodelsof chemical
evolution.Theyare:(1)theinitialconditions,(2)thestarformationhistory,(3)the
Preface ix
stellar evolution and nucleosynthesis, the so-called stellar yields, and (4) the gas
flows.
In Chap.3, I present several analytical solutions to models assuming instanta-
neousrecyclingapproximationand refertothese modelsas “simple models.”The
varioussolutionsarecriticallydiscussed.
Chapter 4 containsthe completeequationsof chemicalevolutionwhich can be
solvedonlynumerically.Recipesforthevariousingredientsaregiven.Ishowhow
to compute the supernova and nova rates in galaxies. Finally, I describe possible
methodsofsolutionforthebasicequations.
WithinChap.5,thechemicalevolutionoftheMilkyWayisstudiedindetail.The
comparisonbetween modelresults and observationsis used to impose constraints
onthemechanismsofformationofthedifferentcomponents:halo,thickdisk,thin
disk, and bulge. At the end of this chapter, some considerations on the chemical
evolutionofotherspiralsoftheLocalGrouparepresented.
IdescribeinChap.6thechemicalevolutionofspheroids,inparticular,ellipticals,
bulges, and dwarf spheroidals. The connection between ellipticals and quasars is
alsodiscussed.
In Chap.7, I presentthe chemicalevolutionof irregulargalaxies,includingthe
MagellanicCloudsandthebluecompactgalaxies,andtheirconnectiontodamped
Lyman-˛systems.
Finally,inChap.8,Iintroducetheso-calledcosmicchemicalevolution,namely,
theevolutionofacomovingunitaryvolumeoftheUniverse,wheregalaxiesofall
types are present. The methods and the results obtained so far are discussed. The
subjectofthecosmicstarformationrateaswellasthecosmicgamma-rayburstrate
istouchedupon.
This book is aimed at graduate students and young researchers who want to
enterinthefieldofmodelinggalacticchemicalevolutionandtrytounderstandthe
complexprocessesgoverninggalaxiesthroughcomparingobservationsandmodel
results.Othersimilarbooksalreadyexists,theyare“NucleosynthesisandChemical
Evolutionof Galaxies” by BernardPagel (1997)and “The ChemicalEvolutionof
theGalaxy”bymyself(2001).
Iwanttowarmlythankmanypeoplewhohavehelpedmeinproducingthisbook.
Inparticular,Iwouldliketomentionmycollaboratorswhohavegreatlycontributed
to the material presented in this book: Francesco Calura, Gabriele Cescutti,
CristinaChiappini,AnnibaleD’Ercole,BradGibson,GustavoLanfranchi,Monica
Marcon-Uchida, Antonio Pipino, Simone Recchi, Donatella Romano, Emanuele
Spitoni, Monica Tosi, and Jun Yin. I am also indebted to some of my students
who read extremely carefully the manuscript finding typos and repetitions that
I would never have found, they are: Valentina Grieco, Shaji Vattakunnel, and
LucaVincoletto.A specialthanksgoto my husbandJohnDanziger,who patiently
improvedtheEnglishofsomeChapters.
Trieste,Italy FrancescaMatteucci