Extremely Metal-Poor Galaxies: The Environment

0.73 MB·English

by  M. E. Filho

DraftversionJanuary28,2015
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
EXTREMELYMETAL-POORGALAXIES:THEENVIRONMENT
M.E.Filho1,2,3,4,5,J.Sa´nchezAlmeida2,3,C.Mun˜oz-Tun˜o´n2,3,S.E.Nuza6,F.Kitaura6andS.Heß6
DraftversionJanuary28,2015
ABSTRACT
Wehaveanalyzedbibliographicalobservationaldataandtheoreticalpredictions,inordertoprobetheenvi-
5 ronmentinwhichextremelymetal-poordwarfgalaxies(XMPs)reside. WehaveassessedtheH Icomponent
1 anditsrelationtotheopticalgalaxy,thecosmicwebtype(voids,sheets,filamentsandknots),theoverdensity
0 parameterandanalyzedthe nearestgalaxyneighbours. The aim isto understandthe role ofinteractionsand
2 cosmologicalaccretionflows in the XMP observationalproperties, particularlythe triggeringand feedingof
n the star formation. We find that XMPs behave similarly to Blue Compact Dwarfs; they preferably populate
a low-densityenvironmentsinthelocalUniverse: 60%occupyunderdenseregions,and 75%resideinvoids
∼ ∼
J andsheets. Thisismoreextremethanthedistributionofirregulargalaxies,andincontrasttothoseregionspre-
7 ferredbyellipticalgalaxies(knotsandfilaments).Wefurtherfindresultsconsistentwithpreviousobservations;
2 while theenvironmentdoesdeterminethe fractionof a certaingalaxytype, it doesnotdeterminethe overall
observational properties. With the exception of five documented cases (four sources with companions and

] onerecentmerger),XMPs donotgenerallyshowsignaturesofmajormergersandinteractions;we findonly
A oneXMPwithacompaniongalaxywithinadistanceof100kpc,andtheH I gasinXMPsistypicallywell-
G behaved,demonstratingasymmetriesmostlyintheoutskirts.Weconcludethatmetal-pooraccretionflowsmay
be drivingthe XMP evolution. Such cosmologicalaccretion could explainall the major XMP observational
.
h properties: isolation,lackofinteraction/mergersignatures,asymmetricopticalmorphology,largeamountsof
p unsettled,metal-poorHIgas,metallicityinhomogeneities,andlargespecificstarformation.
-
Subjectheadings:galaxies:dwarf–galaxies:formation–galaxies:evolution–galaxies:interaction–galaxies:
o
r starburst
t
s
a
1. INTRODUCTION Thetheorypredictsthatthesecosmologicalfilamentsarethe
[
mainprocessbywhichstar formationisfedandtriggeredat
Thestandardmodelof galaxyformationandevolutionin-
1 all cosmological times (Brooks et al. 2009; Verbeke et al.
cludes hierarchical merging/clustering, where smaller struc-
v 2014). Cosmological gas accretion should have been more
tures are used as building blocks of more massive galaxies,
9 commonat high redshift, but in the local Universe, it is hy-
and evolution through accretion from cosmological gas fila-
0 pothesized to intervene in the evolution of individual low-
ments in the cosmic web (e.g., Combes 2004; Dekel et al.
7 mass galaxies in low-density environments, via small-scale
2009; Dekel, Sari & Ceverino 2009; Nuza et al. 2014a;
6 gasstreams,facilitatingthearrivalofgasfromlargedistances
Sa´nchez Almeida et al. 2014b). Because merger events are
0
(Keresˇ et al. 2005; Dekel & Birnboim 2006; Dekel et al.
. generallyresponsibleforthedisruptionofgasandstellarag-
1 gregates,theyhavebeentraditionallyassociatedwiththetrig- 2009;Dekel,Sari&Ceverino2009;Brooksetal. 2009;Cev-
0 erino,Dekel&Bournaud2010;Ceverinoetal.2014;Nuzaet
gering of supermassive black hole accretion at the center of
5 al. 2014a).
galaxies(ActiveGalacticNucleiactivity;Springeletal.2005;
1 There has been some observationalevidence for these ac-
Hopkinsetal. 2008;DiMatteoetal. 2008),andwiththegen-
: cretion flows at high redshift, from the detection of inverse
v erationofstar formationepisodes(e.g.,Sanderset al. 1988;
metallicity gradients (lower metallicity in the central star-
i Robainaetal. 2009).However,thecontributionofcosmolog-
X formingregionsthan in the peripheryof the sources; Cresci
ical cold-accretionflows to galaxy evolution in general, and
r star formation in particular, is recently gaining ground. In- et al. 2010) and from high-column density H I absorbers
a (e.g., van de Voort et al. 2012). However, in the local Uni-
deed,cosmologicalaccretionontogalaxiesofexternalmetal-
verse,therehasonlybeenindirectevidencefortheseevents,
poorgasispredictedbynumericalsimulationstobethemain
mainly from the study of star formation in dwarf galaxies.
modeofgalaxyanddiskassembly,athigh,andpossiblyalso
For example, for galaxies of the same mass, the metallicity
atlow,redshift(Birnboim&Dekel2003;Brooksetal. 2009).
wasshown to decreasewhenstar formationincreases(Man-
nucci et al. 2010; Lara-Lo´pez et al. 2010; Pe´rez-Montero
email:mfi[email protected] et al. 2013). It was also found that metallicities in qui-
1Universidad deLasPalmas deGranCanaria-Universidad deLaLa-
escent blue compact dwarf (BCD) galaxies are higher than
guna, CIE Canarias: Tri-Continental Atlantic Campus, Canary Islands,
Spain those of star-formingBCDs (Sa´nchezAlmeida 2008, 2009).
2InstitutoAstrof´ısicadeCanarias,38200LaLaguna,Tenerife,Spain More recently, observations of tadpole galaxies in the Kiso
3DepartamentodeAstrof´ısica, UniversidadLaLaguna,38206LaLa- survey of UV-bright galaxies (Elmegreen et al. 2013) and
guna,Tenerife,Spain extremely metal-poor dwarf star-forming galaxies (XMPs),
4CentrodeAstrof´ısicadaUniversidadedoPorto,RuadasEstrelass/n, haveshownthattheregionswherestarformationoccurspos-
4150-762Porto,Portugal
sessalowermetallicitythantheunderlyinggalaxy(Sa´nchez
5InstitutodeAstrof´ısicaeCieˆnciasdoEspac¸o,UniversidadedeLisboa,
OAL,TapadadaAjuda,1349-018Lisboa,Portugal Almeida2013,2014a). FromtheH Idata,ithasbeenshown
6Leibniz-Institutfu¨rAstrophysikPotsdam(AIP),AnderSternwarte16, thatlocalBCDsandXMPsaregenerallysurroundedbypools
D-14482Potsdam,Germany and streams of H I gas, over three times larger than the op-
2 M.E.Filhoetal.
tical extension, which frequently display, in the peripheries, DigitalSkySurvey(SDSS) DataRelease (DR)7 (Abazajian
distorted and lopsided H I distributions and velocities (Be- etal.2009)andliterature.For19XMPs,archiveHIimaging,
gum&Chengalur2003;Youngetal. 2003;Thuan,Hibbard at resolutions higher than 1, are available via interferomet-
′
& Le´vrier 2004; Ekta, Chengalur & Pustilnik 2006; Begum ric Very Large Array (VLA) and/or Giant Metrewave Radio
etal. 2006;Chengaluretal. 2006;Walter etal. 2008;Ekta, Telescope(GMRT)observations;theseareusedtoprobethe
Chengalur& Pustilnik2008,2009; Ekta&Chengalur2010; nearbyenvironment.Thecosmicwebenvironmentisinferred
Ottetal.2012;Lellietal. 2012a,b;Lelli;Verheijan&Frater- fromthe constrainednumericalsimulationsby Heß, Kitaura
nali2014a,b; Beccarietal. 2014). Furthermore,significant &Gottlo¨ber(2013)andNuzaetal. (2014b),andthecatalog
quantitiesofmetal-poor,large-scaleHIgaswerefoundinthe ofSDSSgalaxyfilamentsassembledbyTempeletal. (2014).
subset of XMPs (Filho et al 2013; hereinafter Paper I). Be- WehavealsosearchedforclosecompanionstotheXMPsin
cause the gas in galaxy disks is diluted in a relatively small theSDSSdatabase.
timescale(oftheorderofonegalaxyrotation;e.g.,deAvillez Thepaperisorganizedasfollows. InSection2wepresent
& Mac Low 2002; Yang & Krumholz 2012), this cumula- a summary of the optical and H I properties of XMPs with
tiveevidencesuggeststhatthegashadtohavebeenaccreted archivalhigh resolutionH I observations,whichincludesan
recently,mostlikelyasvestigialstreamsofcosmologicalac- analysis of the small-scale environment. Section 3 contains
cretion (Sa´nchez Almeida 2014b), like the streams modeled our investigationof the large-scale environmentof XMPs in
inhigh-massgalaxies(Dekeletal2009;Dekel,Sari&Cev- lightofthreediagnostictools: cosmicwebtype,overdensity
erino 2009; Ceverino, Dekel& Bournaud2010; Ceverinoet and nearest neighbours. In Section 4 we discuss the results
al. 2014). Therationaleisthatthemetallicityoftheaccreted andpresenttheconclusions. TheAppendixcontainsaquali-
HIgasislow. Asthisgasisfedtothestarformationregions tativedescriptionofthearchivalhighresolutionHImorphol-
ofthesource,theoverallmetallicityinthisregionislowered, ogyandvelocityfieldofindividualXMPs.
dueto dilutionofthegas,while elsewherein thegalaxy,the
HIgashasbeenenrichedwithmetalsfromstellarwindsand 2. SMALL-SCALEENVIRONMENT
supernovaefrompreviousstarformationepisodes.
Of the 140 local XMPs in the parent sample, 29 sources
Thus, XMPs may constitute the ideal local laboratoriesto
withnoH Iinformationhavebeenobservedwiththesingle-
probetheroleofcosmologicalaccretionflowsingalaxyevo-
dishEffelsbergradiotelescope,resultingin11newdetections
lution and star formation. XMPs comprise over 0.1% of
(PaperI).Overall,53XMPspossessHIdatainliterature;but
the galaxies in the local volume (Morales-Luiset al. 2011),
of these, only 19 have been observed at high spatial resolu-
andrepresentthelow-metallicityendofthemostmetal-poor
tion(.1),witheithertheGMRTortheVLA.We usethese
galaxies,belowanarbitrarilydefinedmetallicityofone-tenth ′
archivalHIobservationstoinvestigatethespatialanddynam-
solar, or 12 + log(O/H) 7.65. Although most XMPs are
≤ icaldistributionofthe H I gas, andits relationto theoptical
a subsample of star-forming BCDs (Sargent& Searle 1970;
component, which can provide clues as to the origin of the
Thuan & Martin 1981; Papaderos et al. 1996a; Papaderos
observedXMPproperties.
etal. 1996b;Telles&Terlevich1997;Kunth&O¨stlin2000;
Cairo´setal. 2001;Bergvall&O¨stlin2002;Cairo´setal.2003;
2.1. XMPsWithArchivalHighResolutionHIObservations
Noeskeetal. 2003;GildePaz&Madore2005;Amor´ınetal.
2007,2009;Michevaetal. 2013),someXMPsaremoredif- Lelli, Verheijan & Fraternali (2014a, b) have presented a
fuseandcorrespondtothecategoryofdwarfirregulargalax- detailed quantitative assessment of the H I properties (from
ies(e.g.,Skillmanetal.2013).Theycontaincopiousamounts newandarchivaldata)of18starburstdwarfgalaxies,fourof
ofmetal-poor,unsettled,H Igas(Begum&Chengalur2003; whichare also in ourXMP sample(UGC 4483,UGC 6456,
Young et al. 2003; Thuan, Hibbard & Le´vrier 2004; Ekta, IZw18,andSBS1415+437). Inthispaper,weattemptonly
Chengalur& Pustilnik 2006; Begumet al. 2006; Chengalur a qualitative description of the high resolution H I data in
etal. 2006;Walter etal. 2008;Ekta, Chengalur& Pustilnik literature, in order to obtain global statistical properties for
2008, 2009; Ekta & Chengalur 2010; Ott et al. 2012; Lelli theXMPsample.
etal. 2012a,b; PaperI;Lelli, Verheijan&Fraternali2014a, InTable 1 we presenta summaryof the opticalproperties
b; Beccari et al. 2014). Moreover, in 80% of the cases, ofthesampleof19localXMPswitharchivalhighresolution
XMPsshowevidenceforopticalasymme∼try(Papaderosetal. interferometric H I observations. The data is adapted from
2008;Morales-Luisetal.2011;PaperI).Thelopsidedoptical Table3ofPaperI,withsomerevisionanddescriptionofthe
morphology has been commonly interpreted as asymmetric opticalmorphology. Table 2 providesa summaryof the rel-
starformation,occurringinagalaxy/diskthatisinatransient evantH I properties,atseveralscales, ofthe 19localXMPs
phase(Papaderosetal. 2008;Elmegreen2009;Elmegreen& witharchivalhighresolutioninterferometricHIobservations.
Elmegreen2010),arareoccurrenceatlowredshift. The propertiesare a compilationof interferometricH I data
Theobjectiveofthispaper(PaperII)is toanalyzethe en- fromliterature,namelyVLAandGMRTdata.Lowresolution
vironment of the XMPs, with the aim of understanding the mapsdisplayresolutions&30′′,whileintermediateresolution
role of cosmological accretion flows in the galaxy/disk as- display 30 – 15′′ and high resolution display . 15′′. In the
semblyandtriggering/feedingofthestarformationinXMPs. Appendix, we present a detailed description of the H I high
Archival high resolution interferometric H I data are used resolutiondataforeachXMP.
to investigate the nearby environment, whereas cosmologi- IfwecomparetheXMPHImorphologyandvelocityfield
calsimulationsconstrainedbyobservations,andtheobserved with the classification given in Lelli, Verheijan & Fraternali
distribution of galaxies, provideinformationon the environ- (2014b),weconcludethatalmostalloftheXMPsfallwithin
mentonlargescales. ThesourcesarethesameasinPaperI, theirclassificationofTypeA/Bsources–galaxieswithregu-
and haven been taken from the parent sample of 140 local larlyrotatingH Idisks/galaxieswithkinematicallydisturbed
XMPs,selectedbyMorales-Luisetal.(2011),fromtheSloan H I disks. That is, the XMPs generally show an H I disk-
type morphology, with a velocity field typical of a rotating
TheEnvironmentofExtremelyMetal-PoorGalaxies 3
TABLE1
Summaryoftheopticaldataforthe19extremelymetal-poorgalaxiesinthelocalUniversewitharchivalhigh
resolutioninterferometricHIobservations.
Source RA(J2000) DEC(J2000) mg dopt PA O/H Optical Note
Name hms mag Morphology
◦ ′ ′′ ′′ ◦
(1) (2) (3) (4) (5) (6) (7) (8) (9)
J0119-0935 011914 -093546 19.5 21.8 115 7.31 cometary ...
UM133 014442 +045342 15.4 54.7 105 7.63 cometary 1
SBS0335-052W 033738 -050237 19.0 5.0 ... 7.11 symmetric ...
SBS0335-052E 033744 -050240 16.3 8.3 45 7.31 cometary ...
UGC4305 081905 +704312 ... 26.5 115 7.65 multi-knotdisk 2
HS0822+03542 082555 +353231 17.8 13.9 45 7.35 cometary ...
DD053 083407 +661054 20.3 71.2 55 7.62 multi-knotdisk 3
UGC4483 083703 +694631 15.1 61.8 80 7.58 cometary 4
IZw18 093402 +551425 16.4 18.5 45 7.17 two-knotcometary 5
LeoA 095926 +304447 19.0 272.0 10 7.30 multi-knotdisk ...
SextansB 100000 +051956 20.5 269.0 25 7.53 multi-knotdisk 6
SextansA 101100 -044134 ... 264.0 45 7.54 multi-knotdisk/symmetric 7
UGC6456 112800 +785939 ... 93.0 115 7.35 multi-knotdisk 8
SBS1129+576 113202 +572246 16.7 45.6 75 7.36 cometary ...
UGCA292 123840 +324601 18.9 57.0 165 7.28 multi-knotdisk 9
GR8 125840 +141303 17.9 78.0 130 7.65 multi-knotdisk 10
SBS1415+437 141701 +433005 17.8 55.5 120 7.43 two-knotcometary 11
SagDig 192959 -174041 ... 116.0 170 7.44 multi-knotdisk/symmetric 12
J2104-0035 210455 -003522 17.9 13.1 110 7.05 cometary ...
Columns:
Col.1:Sourcename.
Col.2:RightAscension(J2000).
Col.3:Declination(J2000).
Col.4:SDSSDR7g-band(Petrosian)magnitude,fromMorales-Luisetal.(2011).
Col.5:Opticaldiameteralongthemajoraxis,obtainedfromtheSDSSDR7r-bandorDigitizedSkySurvey(DSS)-II
R-banddata,measuredat25magarcsec 2.
−
Col. 6: Approximateopticalpositionangle,alongthelargestextension, measuredcounterclockwise fromtheWest,
fromtheSDSSDR10orDSSimages.
Col.7:Metallicity,12 + log(O/H),fromMorales-Luisetal.(2011).
Col.8:Opticalmorphology,obtainedfromtheSDSSDR10orDSSimages.Symmetric(overallsphericallysymmetric
structure),disk(overalldisk-likestructure),two-knot(structurewithtwoprominentstarformationknots),multi-knot
(structurewithmultiplestarformationknots)andcometary(overallhead-tail structure, withatleastoneprominent
starformationknot),areused.
Col.9:NoteontheopticalstructurefromtheSDSSDR10orDSSimages.
Noteincolumn9:
1–TheextendedcometarytailiscurvedtotheWest,ina”boomerang-type”structure.
2 – There is a faint nodule of emission to the Northwest, a bright ”loop-shape” structure to the East, and a faint
filament,beginningneartheloop,andcurvingtotheSouthwest. Thebrighteststarformationknotsarefoundonthe
Easternedgeofdisk,neartheloopandfilament.
3–Thereisseconddisk-likeemissiontotheWest,atapositionangleofapproximately55 ,similartothemaindisk.
◦
ThebrighteststarformationknotsarefoundontheNorthwesterntipofthemaindisk.
4–Althoughtheoverallshapeiscometary, thereappearstobeafainterX-shapepattern, withthebrighterandthe
fainterarmatapositionangleofapproximately95and60 ,respectively.
◦
5–Thetwobrighteststarformationknotsareeachattheendofthecometarystructure.
6–ThereisafainttailofemissiontotheWest,atapositionangleofapproximately160 .
◦
7–ThebrighteststarformationknotsarefoundontheEasterntipofthedisk/symmetricstructure.
8–Thereisfaintextendedemissionalongapositionangleofapproximately75 . Thebrighteststarformationknots
◦
arefoundontheEasternedgeofthedisk.
9–Thediskhasa”boomerang”shape,withthebrighteststarformationknotsinthecenterandontheEasterntipof
thedisk.
10–Thediskshowscurvatureatbothends,ina”boomerang-type”morphology.
11–Thetwobrighteststarformationknotsareattheheadofthecometarystructure. Theextendedcometarytailis
curvedtotheWest.
12–ThebrighteststarformationknotsarefoundontheEasterntipofthedisk/symmetricstructure.
disk,whichmaypresentsomesmallasymmetriesandirregu- or”cradling”brightstar-formingregions,orshowingsignsof
laritiesintheoutskirts,andwhichmaybe(onlyslightly)of- ”clearing” around the star-formationknots (Appendix). Sag
fcenterrelativetotheopticalpeakand/orstar-formingknots; Dig has been suggested to be the result of a merger process
statistically,wedonotobservesignificantperturbationsinthe (Beccari et al. 2014 and Appendix). Its H I morphology
large-scale H I gas componentof the XMP population(Ap- is ring-like, with an offcenter depression, and the velocity
pendix). High spatial resolutionobservationsof the H I gas field displays no sign of rotation. These features are gener-
suggest feeding of the star formation regions; the gas is re- allynotfoundinXMPs, whichweinterpretasevidencethat
solved into complexsubstructure, with the gas”pointingat” the XMPs, as a class, are currently not undergoing a major
4 M.E.Filhoetal.
TABLE2
SummaryoftheHIdataofthe19extremelymetal-poorgalaxiesinthelocalUniversewitharchivalhigh
resolutioninterferometricHIobservations.
Source PAopt HI/optRatio HI/optOffset PAHI PAVG Note Ref
Name
◦ ′′ ◦ ◦
(1) (2) (3) (4) (5) (6) (7) (8)
J0119-935 115 4 10/10 120–110 120 ... 1
UM133 105 2 0/10 105 105 ... 1
SBS0335-052Wa ... 6 10/10 150 NE-SW–N-S P 2
SBS0335-052Ea 45 4 10/10 145–20 NE-SW P 2
UGC4305 115 5 0/0 ... 100 ... 3
HS0822+03542 45 10 10/10 70-80 70 P 4
DD053 55 3 10/10 150 45 ... 3,5
UGC4483 80 2 10/0 80–60–80 90–120 ... 6,7,8,9
IZw18 45 7 10/0 45 50–55 P 8,9,10
LeoA 10 3 ... 10 ... ... 5
SextansB 25 3 10/10 25 130 ... 6
SextansA 45 2 10/0 135 150 ... 6
UGC6456 115 2 10/0 75 90 ... 8,9,11
SBS1129+576 75 2 0/0 75–70 70 P 12
UGCA292 165 3 0/0 175 145 ... 6,13
GR8 130 2 10/10 165 80 ... 5,6,13,14
SBS1415+437 120 2 10/0 115 115 ... 8,9
SagDig 170 6 0/0 ... ... M 5,15
J2104-0035 110 5 10/10 90 90 ... 16
aSBS 0335-052E and SBS 0335-052W are an interacting pair. Due to lack of resolution, the PAVG is
merelyindicative.
Columns:
Col.1:Sourcename.
Col. 2:Approximatepositionangleofthelargestopticalextension,measuredcounterclockwisefromtheWest,
fromtheSDSSDR10orDSSimages.
Col. 3:ApproximateHI-to-opticalratio,wheretheHIextensionismeasuredinthelowestresolutioninterfero-
metricmap.
Col.4:ApproximateoffsetbetweentheHIpeakandtheopticalcenter/brighteststar-formingknot.
Col.5:ApproximatepositionangleofthelargestHIextension,measuredcounterclockwisefromtheWest(from
thelowesttohighestresolutionmap).
Col.6:ApproximatepositionangleoftheHIvelocitygradient,measuredcounterclockwisefromtheWest(from
thelowesttohighestresolutionmap).
Col.7:Noteonthepossibleinteraction/mergeroftheXMP.”P”designatesagalaxythatbelongstoaconfirmed
interactingpair.”M”designatesagalaxythatmayhaveundergonearecentmerger.
Col.8:Reference,withinterferometerdataand/orsurvey.
Instrument,interferometerandsurveyacronym:
GMRT–GiantMetrewaveRadioTelescope
VLA–VeryLargeArray
THINGS–TheHINearbyGalaxySurvey
ACS–AdvancedCameraforSurveys
ANGST–ACSNearbyGalaxySurveyTreasury
Referenceincolumn8:
1–Ekta&Chengalur(2010);GMRT
2–Ekta,Pustilnik&Chengalur(2009);GMRT
3–Walteretal.(2008);VLATHINGS
4–Chengaluretal.(2006);GMRT
5–Begumetal.(2006);GMRT
6–Ottetal.(2012);VLAANGST
7–Lellietal.(2012b);VLA
8–Lelli,Verheijan&Fraternali(2014a);VLA
9–Lelli;Verheijan&Fraternali(2014b);VLA
10–Lellietal.(2012a);VLA
11–Thuan,Hibbard&Le´vrier(2004);VLA
12–Ekta,Chengalur&Pustilnik(2006);GMRT
13–Youngetal.(2003);VLA
14–Begum&Chengalur(2003);VLA
15–Beccarietal.(2014);VLA
16–Ekta,Chengalur&Pustilnik(2008);GMRT
TheEnvironmentofExtremelyMetal-PoorGalaxies 5
interactionormerger. Rather,theasymmetriesandirregular- environment(comicweb type and overdensity)in whichthe
itiesoftheouterenvelopeareunderstoodassignsofweaker XMPsreside. Spectroscopicredshiftsfromthe SDSS DR10
interactions caused by minor mergers, gas accretion or gas (Ahnetal. 2012)areavailablefor 70%ofthe140XMPs.
∼
outflowsdrivenbygalacticwinds. For the remaining XMPs, photometric redshifts from the
SDSSDR10orNASA/IPACExtragalacticDatabase7 (NED)
3. LARGE-SCALEENVIRONMENT
wereused.
InordertoinvestigatetheenvironmentinwhichXMPsoc- In Figure 1, we have compared the cosmic web type en-
cur,andtheinfluencethatcosmologicalaccretionflowsmay vironment distribution of XMPs with other galaxy morpho-
haveontheirevolutionandobservedproperties,wehavecom- logicaltypes,namely,ellipticals(E),lenticulares(S0),spirals
puted the likelihood of an XMP to inhabit a certain type of (Sp) and irregular galaxies (Irrs), as assigned by the 2MRS
environmentusingthreediagnostics: cosmicwebtype,over- team (Huchra et al. 2012). Although the Poisson errorbars
densityandnearestneighbours. are largerfor the XMPsample, it is apparentthat the XMPs
Theenvironmentcan beclassified accordingtolocalindi- follow similar trends as Irr galaxies, but with a more pro-
cators, such as the local density, or according to non-local nounced behaviour (Fig. 1). The XMP prevalence in voids
indicators,based,forexample,onthetidal-fieldtensor. Inthe andsheets,andtheavoidanceofknots,suggeststhat,statisti-
firstcase,onecanestimatethelocaloverdensitybasedsolely callyandonalargescale( 1Mpc),theXMPsarerelatively
∼
on observations, defined as the number density of galaxies isolated(Fig.1).
relativetothebackgroundnumberdensity,orbycharacteriz- Asaconsistencycheck,wehavealsoestimatedthecosmic
ingthenearestneighbours(type,mass,numberanddistance). webtypedistributionforalargesampleofcontrolgalaxies(
∼
However, a classification of the environmentinto its cosmic 700BCDs,withZ>0.1Z )fromtheSDSSDR8(Aiharaet
webcomponentsrequiresknowledgeofthelarge-scaleenvi- al. 2011). Wefindasimila⊙rbehaviouramongtheBCDsand
ronment. Thesignoftheeigenvaluesofthetidal-fieldtensor XMPsample, which is markedlydifferentfrom the environ-
indicate pure contracting regions, such as clusters or knots, mentsfoundforE,S0andSpgalaxies,andmoreextremethan
or expanding regions in three, two, or one dimensions, rep- the behaviour of the Irr galaxies in the zero redshift 2MRS
resentingvoids,sheets,orfilaments,respectively(Hahnetal. sample(tobeincludedinafuturepaper).
2007).Whilenumericalsimulationsprovideallthenecessary Figure2showstheprojectedspatialdistributionofthe140
information to perform a cosmic web analysis, observations local XMPs, color-coded for cosmic web type. The sizes
requireapriorreconstructionofthelarge-scalematterdensity of the circles trace the overdensity (δ), such that, the larger
field. Theparticularreconstructiontechniquewilldetermine the circle, the largerthe overdensity. The valuesrangefrom
theaccuracyofthecosmicwebclassification. the most underdense (δ = 0.9, for PHL 293B) to the most
overdense(δ=376,forVC−C0428,aBCDintheVirgoclus-
3.1. CosmicWebTypeandOverdensity
ter; Meyeretal. 2014)sourceenvironment. Figure 3shows
WehererelyonpreciseconstrainedN-bodysimulationsof the overdensity distribution (log(1+δ)), color-coded by cos-
the Local Universe, using the Two-Micron All-Sky Galaxy micwebtype,fortheXMPsample. Arrows,color-codedby
RedshiftSurvey(2MRS;Huchraetal. 2012),withinvolumes cosmicwebtype,showthemeanoverdensity(log(1+<δ>))
of 180h 3 Mpc3 (Heß, Kitaura & Gottlo¨ber 2013). Cosmo- for each distribution. Accordingto the overdensitydistribu-
−
logicalparametersonlyprovideastatisticaldescriptionofthe tion, 60% of the XMPs are found in underdense regions
Universe, so that an infinite number of realizations are con- (δ .∼0) of the reconstruction (Fig. 2 and 3). On the other
sistent with the same set of parameters. The procedurepro- hand, most of the XMP galaxies ( 75%) reside in the less
∼
videsacosmologicalsimulation,whichreproduces,indetail, dense cosmic web type environments ( 40% are in sheets
∼
thespatialdistributionofgalaxiesinthelocaluniverseasde- and 34%invoids),while 25%canbefoundinfilaments,
scribedby2MRS.Themethodtoreconstructtheinitialfluctu- and.∼1%inknots(Fig.1,2∼and3).
ationsusedin thesesimulationsisbasedonaself-consistent Intheanalysisbelow,becauseweareconstrainedbysmall
Bayesian machine-learning algorithm, reaching an accuracy numberstatisticsandsystematicerrorsthatmayaffectthede-
of 2Mpch 1 (Kitauraetal. 2012;Kitaura2013;Heß,Ki- terminationof the physicalparameters, the trends should be
−
∼
taura&Gottlo¨ber2013). Thecosmicwebclassificationused takenwithcaution.
inthisstudyhasbeenperformedfollowingForero-Romeroet WehaveinvestigatedwhethertheH Imass(M ),dynam-
HI
al. (2009), based on the best correlated constrained N-body ical mass (M ) or stellar mass (M ) of the XMPs depends
Dyn
simulation with the 2MRS galaxyfield (Nuza et al. 2014b). on the environment in which they r∗eside (Fig. 4). The H I
The overdensity (δ) used in this study has been computed andstellarmassesweretakenfromTable5ofPaperI.Inthis
from the same constrained simulation. The preference of a Paper II, we have estimated the dynamical masses with re-
type of galaxy to be located in a particular environment is vised optical radius measurements. There is a large overlap
quantifiedbyη(τ, ǫ), anexcessprobabilityratio, whereτ is in dynamical, H I and stellar mass for a range of overdensi-
the galaxy type and ǫ is the cosmic web environment type ties (Fig. 4). However, XMPs in filaments and sheets show
(voids, sheets, filamentsandknots; Nuza etal. 2014b). The thelargestrangeinH Imass,particularlyextendingtolower
environment(overdensityorcosmicwebtype)ofasourcecan H I masses, while XMPs in voidsshow tendentiouslylarger
beassignedaccordingtothecellinthereconstructionwhere H I masses, abovelog(MHI/M ) 8(Fig.4a). Fivesources
thesourceislocated. Eachcellis characterizedbythreeco- in voids and sheets show the l⊙arg≃est dynamical masses (log
ordinates: rightascension,declination,andredshift,withthe (M /M )&9;Fig.4b).Intermsofstellarmass,mostXMPs
Dyn
redshiftparameterizingthedistance,includingthepropermo- in filamen⊙ts foundin overdenseregions(δ & 0) show stellar
tions caused by local tidal fields, taken consistently into ac- masses below log (M /M ) 7, with a tail of XMPs in less
count. dense filamentary env∗iron⊙me≃nts towards high stellar masses
BecausewepossesscoordinatesforourXMPgalaxies,we
haveusedtheconstrainedsimulationtodeterminethetypeof 7http://ned.ipac.caltech.edu/
6 M.E.Filhoetal.
Fig.1.—TheexcessprobabilitydistributionofXMPs,andE,S0,SpandIrrgalaxiesofthezeroredshift2MRSsample(Huchraetal.2012;Nuzaetal.2014b).
Thefourpanelscorrespondtosourcesfoundinvoids,sheets,filamentsandknots.
Fig.2.—Theprojectedspatialdistributionofthe140localXMPs,color- Fig.3.—Theoverdensitydistributionforthe140localXMPsample,color-
codedbycosmicwebtype(blueforvoids,redforsheets,greenforfilaments codedbycosmicwebtype(blueforvoids,redforsheets,greenforfilaments
andcyanforknots). Thesizesofthecirclestracetheoverdensity,suchthat, andcyanforknots).Arrows,color-codedbycosmicwebtype,showthemean
thelargerthecircle, thelargertheoverdensity. Thevalues rangefromthe overdensity(log(1+<δ>))foreachdistribution.
mostunderdense(δ= 0.9,forPHL293B)tothemostoverdense(δ=376,
forVCC0428,aBCD−intheVirgocluster)sourceenvironment.
(Fig.5a), SFR (Fig. 5b)and gasconsumptionrate (Fig.5d).
However, XMPs in voids show a slight tendency for larger
(log (M /M ) & 8; Fig. 4c). The largest H I-to-stellar mass valuesofsSFR(logsSFR& 8yr 1;Fig.5c). Cometaryand
−
ratiobel∗ong⊙stoanXMPresidinginavoid(Fig.4d).Overall, multi-knotXMPsareroughly−equally(fractionally)prevalent
we see no clear relation between the environment in which invoids,sheetsandfilaments(Fig.5eand5f),whiletwo-knot
XMPsresideandtheirdynamical,stellarorHImass. sources are roughly equally (fractionally) prevalent in voids
We have further investigated if there is an environment andsheets(Fig.5e and5f). Symmetricsourcesarefoundto
trend with metallicity (12 + log(O/H)), star formation rate bemorecommoninvoids(Fig.5eand5f). Thetwosources
(SFR),specificstarformationrate(sSFR SFR/M ),HIgas with morphologicalinformation,andcontainedin knots, are
consumptionrate (1/t SFR/M ), an≡d optical∗morphol- bothcometary(Fig.5dand5e).
MHI ≡ HI
ogy(symmetric,cometary,two-knotandmulti-knot;Fig.5). In summary, we find that XMPs tend to reside in low-
These values were taken from Table 3 and 4 of Paper I. We densityenvironments,andincosmicwebtypescharacterized
findnoclearcorrelationbetweenenvironmentandmetallicity asvoidsandsheets. WefurtherfindthatXMPs,BCDsandIrr
TheEnvironmentofExtremelyMetal-PoorGalaxies 7
(a) (b)
(c) (d)
Fig.4.—Theoverdensityasafunctionof(a)HImass,(b)dynamicalmass,(c)stellarmass,and(d)HI-to-stellarmassratio,color-codedbycosmicwebtype
(blueforvoids,redforsheets,greenforfilamentsandcyanforknots). TheHIandstellarmassesweretakenfromTable5ofPaperI.InthisPaperII,wehave
estimatedthedynamicalmasseswithrevisedopticalradiusmeasurements.
galaxiestendtoinhabitthesametypeofenvironment,which Hence,thedistanceofthenearestgalaxyandmorphological
isinstarkcontrastwiththebehaviourofE,S0,Spgalaxiesin filamentcanbeusedasaproxyforthedistanceoftheXMPs
thezeroredshift2MRSsample. to morphologicalfilaments. In order to identify our sample
XMPsintheA3,wesearchedtheA3forgalaxieswithinD
∼
3.2. ProximitytoSDSSMorphologicalFilaments 10kpcoftheXMPpositions;nonewerefound.Weare,there-
fore,confidentthatthenearestgalaxyidentifiedforeachXMP
Wehaveinvestigatedtheproximity(orinclusion)ofXMPs
is truly a separate galaxy. We stress that A3 contains 500
tomorphologicalfilaments,i.e.,”chains”ofgalaxies,asem- ∼
000 SDSS DR8 sources in a contiguous field, which could
piricallydefinedusinggalaxiesinthespectroscopicsampleof
miss the XMPs and some of their possible neighbours (see
the SDSS DR8 dataset. Tempelet al. (2014)use a marked-
below).
pointprocesswithinteractions,calledtheBisousmodel(Sto-
Figure 6 contains the XMP distance to the nearest galaxy
ica,Gregori&Mateu2005),todetectgalaxyfilamentsinthe
(D ) in A3, as a function of the distance between the
spectroscopicsampleoftheSDSSDR8dataset(Tempel,Tago galaxy
nearestgalaxyin A3andthe nearestmorphologicalfilament
&Liivama¨gi2012). Aredshiftintervalof0.009 z 0.155
(D ),color-codedbycosmicwebtypeoftheXMP.The
was used so that the lower redshift limit exclude≤s th≤e Local filament
figure also contains solid black lines, indicating the radii
Supercluster. The authors look for morphological filaments
of the morphological filaments, so that XMPs in galaxy fil-
witha radiusof0.5h−1 Mpc. Theyfindthattheshortestfil- aments should display D . 0.5 Mpc and D .
amentsintheirfilamentcatalogare3–5h 1 Mpcinlength, galaxy filament
− 0.5 Mpc. Only three of the XMPs are found within the
whiletypicalfilamentlengthsareof60h−1Mpc. D . 0.5 Mpc and D . 0.5 Mpc region, and only
We have mined the catalog of galaxies (hereinafter A3), galaxy filament
marginally(Fig.6). Therefore,wefindthatXMPsaregener-
used to generate the morphological filaments, to search for
allynotcontainedwithinthemorphologicalfilamentsdefined
XMP identifications, or, alternatively, to find the nearest
byTempeletal. (2014).
galaxy to each XMP. Distances between the XMPs and A3
galaxies were estimated neglecting galaxy proper motions,
3.3. NearestGalaxiesandPotentialPerturbers
sincethelocalflowforpairsofnearbygalaxiesshouldbeof
thesameorder. A3alsoincludesthedistanceofeachgalaxy AccordingtothegalaxiesintheA3catalog,onlytwoXMPs
to the nearest morphologicalfilament (Tempel et al. 2014). have a galaxy within D . 100 kpc, with one of these
galaxy
8 M.E.Filhoetal.
(a) (b)
(c) (d)
(e) (f)
Fig.5.—Theoverdensity asafunction of(a)metallicity, (b)SFR,(c)sSFR,(d)H Igasconsumption rate(SFR/MHI), and(eandf)optical morphology
(symmetric,cometary,two-knotandmulti-knot),color-codedbycosmicwebtype(blueforvoids,redforsheets,greenforfilamentsandcyanforknots).These
parametersweretakenfromTable3and4ofPaperI.
XMPs residing in a filament and another in a void (Fig. 6). (D &10Mpc)distances(Fig.6).
galaxy
Indeed,therearefourdocumentedXMPsthatareknowntobe In order to complementthe results based on A3, we have
interactingwithnearbycompanions(IZw18,SBS0335-052, also looked for nearest galaxy neighbours, as potential per-
HS0822+03542andSBS1129+576;Sect.2andAppendix). turbers,usingthe fullSDSSDR9 (Ahnetal. 2012)catalog.
Overall, the nearest galaxies in A3 show a large range in The search was performed within a 12 radius around each
′
distances to the XMPs (D 0.1 – 100 Mpc), which XMP,correspondingtoadistanceofD 200kpcatz=0.01.
galaxy
∼ ∼
does not seem to depend strongly on the XMP environment Wefurtherconstrainedthegalaxiestohavespectroscopicred-
(Fig.6). However,thereisatendencyforXMPsinfilaments shiftsof z < 0.1, resultingin 1 000galaxies, potentialper-
∼
to show a somewhat bimodalbehaviour: there is a group of turbersto104XMPs. Wehavealsoestimatedthestellarmass
(nearest) galaxies at smaller (D . 1 Mpc) and larger ofthepotentialperturbersfromtheirSDSSDR9g rcolours,
galaxy
−
TheEnvironmentofExtremelyMetal-PoorGalaxies 9
Fig.6.—ThedistancebetweentheXMPandthenearestgalaxyinA3,as Fig.7.—ThedistancetothenearestpotentialperturberintheSDSSDR9
afunctionofthedistancebetweenthenearestgalaxyinA3andthenearest (withinD.300MpcoftheXMPs),asafunctionofstellarmass,color-coded
morphological filament, color-codedbycosmicwebtypeoftheXMP.The bythecosmicwebtypeoftheXMP(blueforvoids, redforsheets, green
solid black lines indicate the radii of the morphological filaments, sothat forfilamentsandcyanforknots). Thestellarmassofthenearestpotential
0X.M5MPspicn.gTalhaexyblfiaclakmdeansthsesdholiunledidnidspiclaatyesDagarlaexpyre.se0n.t5atMivepcdaisntdanDcefilafmoerntth.e pmearstus-rtboe-rlighhastrbaetieonsefrsotimmaBteedllf&rodmeJthoengS(D2S0S01D).R9g−r colours, usingthe
nearestgalaxy,inordertoexertsomegravitationalinfluenceovertheXMP
(seeSect.3.3and3.4).
usingthemass-to-lightratiosfromBell&deJong(2001).
gerand/orfeedthe star formation. As a first approximation,
In Figure 7 we plot the stellar mass (M ) and distance
wehaveanalyzedtheconditionsforperturbationinasystem
(D ) of the nearest potential perturbe∗r within D .
perturber of a satellite galaxy(XMP) on a circular orbitarounda per-
300 Mpc of the XMPs, color-coded by cosmic web type of
turber.Letusconsiderthatthesatellitegalaxyhasmassmand
the XMP.Only oneXMPshowsa potentialperturberwithin
radiusr,orbitingaperturbergalaxyofmass M,atadistance
D . 100 kpc, with a stellar mass of M 107 M
perturber ofD,andwherem<< M.
(Fig.7). Wecannot,however,excludethepresen∗ce∼ofpoten⊙-
Minormergers/interactions(whenm<< M)occurthrough
tialperturberswithnomeasuredspectroscopicredshiftinthe
dynamical friction, when the perturber and satellite share a
SDSS, norless luminous(andless massive)than the XMPs,
common dark matter halo. This dynamical friction causes
dueto the magnitudecuttoffofthe SDSS (magnitudeinr &
orbital decay of the satellite galaxy within the so-called dy-
17.7mag).ComparingFig.6andFig.7,weconcludethatthe
namicalfrictiontimescale(e.g.,Pattonetal. 2000;Binney&
potentialbiases in the A3 sample do not modifythe conclu-
Tremaine2008):
sionthatXMPsgenerallydonothaveclosecompaniongalax-
ies. AboveaD 1MpcdistancetotheXMP,thestel-
larmassesofthpeertnurebaerre∼stpotentialperturbersshowachange; T 2.64 105Gyr D2v , (1)
the stellar masses not onlyspan a larger range, but potential fric ≃ · × mln Λ
perturberswithstellarmassesofuptoM 1011 M canbe where D is the distance between the satellite galaxyand the
found(Fig.7). Thenearest,leastmassive∗,∼andmost⊙massive perturber (in kpc), v is the circular speed of the satellite (in
potential perturbers belong to XMPs in all three major cos- kms 1),misthemassofthesatellite(inM ),andlnΛisthe
−
micwebtypes(voids,sheetsandfilaments). Wefurthernote Coulomblogarithm. Thisexpressiondoesn⊙ottakeintocon-
abreakinperturbermassdistributionat Dperturber 10Mpc; siderationanymassloss(fromtidalstripping)thatthesatellite
∼
above this distance, stellar masses are skewed towards the maysufferduringtheorbitaldecay.
largerrange(M &109 M ),consistentwiththedetection,at If we assume a typical distance to the perturber of D
largerdistances,∗ofmorel⊙uminous,andhence,moremassive 100 kpc (Fig. 6 and 7), a satellite mass of m 108 M≃
galaxies(Fig.7). (Fig.4c),aCoulomblogarithmof2(e.g.,Binney&≃Tremain⊙e
Overall, with the exception of the already documented 2008;Filhoetal. 2014),anda circularvelocityofthesatel-
casesofXMPswithcompanions(Sect.2andAppendix),sta- lite of v √GM/D, where G is the gravitational constant,
tistically we do not find evidence for the presence of poten- ≃
this then yields a dynamical friction timescale of T
fric
tial perturbers (with measured spectroscopic redshifts in the ≃
50 Gyr, for M m, or 150 Gyr, for M 10m. This esti-
SDSS) within D . 100kpcoftheXMPs. We caution,how- ≃ ≃
mateprovidesatimescaleforthesatellitetospiralintowards
ever,thatduetoalackofspectroscopicredshiftandthespec-
the perturber. The resulting timescales are too long for the
troscopicfluxlimitoftheSDSS,wemaybemissingpotential merger/interactionprocesstobesignificantinXMPs.
perturbersofverylowmassandluminosity.
Asthesatelliteapproachestheperturberonthisorbitalde-
cay,thetidalforcesatworkonthesatellitebecomemoreim-
3.4. PerturbationsandInteractions
portant. If we now consider a test particle at a distance ∆r
Becausethenearestgalaxiesto XMPsaregenerallyfound from the surface of the satellite (where ∆r << r), then the
to be at distances D & 100 kpc (Fig. 6 and 7), we have at- perturberwillexertaperturbativetidalforceonthetestparti-
tempted to evaluate the gravitational influence these could cle(relativetotheforceexertedatthecenterofthesatellite),
haveontheXMPs,particularlyiftheycouldpotentiallytrig- thatisgivenby(e.g.,Binney&Tremaine2008):
10 M.E.Filhoetal.
webof E, S0 andSp galaxies, butinhabitingregionssimilar
tothoseofIrrsandBCDs(Sect.3.1).
We find results similar to those found for nearby (rela-
tively) luminous galaxies (Blanton & Moustakas 2009) and
Hα-emittinggalaxies(Darvishetal. 2014),wherebytheen-
vironmentdoesnotdeterminetheoverallobservedproperties
(mass,luminosity,SFR,etc.) ofacertaingalaxytype. How-
ever,thefractionofacertaingalaxytype(inthiscase,XMPs),
doesappeartobedeterminedbytheenvironmentinwhichit
resides(Sect.3.1).
The evidence for occupation of low-density environments
is furtherstrengthenedwhen we investigatethe proximityto
morphologicalfilaments(Sect.3.2),andgalaxiesinthevicin-
ityofthe XMPs(Sect.3.3). We findnosignificantevidence
forthepresenceofgalaxyfilamentswithinD 0.5Mpcofan
∼
XMP,andonlyonepotentialperturber(ofmassandluminos-
Fig.8.—Thedisplacement producedbytidalforces, asafunctionofthe ityatleastequaltothatofanXMP,andwithameasuredspec-
ratiobetweentheXMPradiusandthedistancetotheperturber,color-coded troscopicredshiftintheSDSS)isfoundwithin D 100kpc
bytherelativeXMP/perturbermass: M/m=0.1(cyan),1(blue),10(red), ofanXMP.However,evenatthesedistances,wehav∼edemon-
100(green). Theblacklinecorrespondsto∆r/r=1,thelowerlimitcriteria stratedthatapotentialperturberisveryunlikelytocausegrav-
fortidalstripping.
itationaldisturbancecapableof drawingfreshgasor driving
thestarformation(Sect.3.4).
We have also analyzedthe small-scaleenvironment,prob-
ing for signatures of galaxy merger/interactions in the pub-
2GM(r+∆r) 2GMr lished H I data of XMPs (Sect. 2 and Appendix). IZw 18
F . (2)
M ≃ D3 ≃ D3 (Lelli et al. 2012a; Lelli, Verheijan & Fraternali 2014a,
b), SBS 0335-052WandSBS 0335-052E(Ekta, Pustilnik&
Thesatellitewillalsoexertarestoringforceonthetestparti-
Chengalur 2009), HS 0822+03542 (Chengalur et al. 2006)
cle, if tidalforcesdisplaceit fromits equilibriumconfigura-
andSBS1129+576(Ekta,Chengalur&Pustilnik2006)have
tionatr,namely,(e.g.,Binney&Tremaine2008):
beenconfirmedtobeinteractingwithcompaniongalaxies,all
within D .25kpc(Sect.2andAppendix). Moreover,ithas
Gm∆r
Fm = r3 , (3) been suggested that the H I properties observed in Sag Dig
couldbetheresultofarecentmergerbetweendwarfgalaxies
wherewehaveneglectedthepressuresupportinthesatellite (Beccariet al. 2014). However,if Sag Dig is the resultof a
(Adamsetal. 2011). InequilibriumF F ;therefore,the merger(Beccarietal. 2014;Sect.2andAppendix),thenthe
M m
displacementproducedbytidalforcesisg≃ivenby: mergersignaturespresentin Sag Dig (ring-likeH I distribu-
tion versus disk-like optical morphology,clumpy H I distri-
∆r M r 3 bution with no optical correspondence, large offset between
2 . (4)
r ≃ m (cid:18)D(cid:19) the optical center and the depression in the H I distribution,
no clear rotationalmotion) are generally absent in the XMP
The condition for tidal disruption of the test particle by the
population.Indeed,wefindnostatisticalevidencefor(major)
perturberisproducingalargeperturbation,i.e.,∆r/r&1.
interactionswith (gas-rich)galaxy companionsin the recent
In Figure 8 we show the displacement produced by tidal
past (Sect. 2 and Appendix; see also Brosch, Almoznino &
forces (Eq. 4), as a function of the ratio between the XMP
Heller2004,Holwerdaetal. 2013andKreckeletal. 2015),a
radius and the distance to the perturber, color-coded by the
mechanismwhichistraditionallyinvokedasadriverforstar
relativeXMP/perturbermass. AscanbeseenfromFigure8,
formationandoverallgalaxyevolution(e.g.,Keeletal. 1985;
inorderfortidalstrippingtooccur,theperturberisrequiredto
Robainaetal. 2009).Althoughsuchaninteractioncouldpro-
haveahighmassand/orbelocatedatasmalldistancetothe
videlargequantitiesoffreshHIgas,andcreateconditionsfor
satellite. For a perturberand satellite of similar mass (M
triggering/feedingtheobservedstarformation,itcannotex-
≃
m),thedistancemustbeoftheorderofthesizeofthegalaxy
plainwhythegasweobserveismetal-poor(PaperI),norcan
(D r). Because XMPs have radii of a few kpc, D must
it explain the metallicity inhomogeneitiesobserved in some
≃
also be a few kpc in order to exert appreciable gravitational
XMPs(Sa´nchezAlmeidaetal. 2013,2014a),anditdoesnot
influenceontheXMP.
explain why we do not systematically observe signatures of
Therefore, although a significant number of XMPs show
major disturbances/interactions in the H I gas, and particu-
potential perturberswithin D 300 Mpc, it is unlikely that
∼ larlyintheH I-to-opticalmorphological/kinematicalrelation
thesewillcausemajorgravitationaldisturbance(ortidalstrip-
(Sect. 2 and Appendix). In fact, the central regions of the
ping)oftheXMP,whichcouldinstigategastransferandsus-
HIstructuresarerelativelywell-behaved;inmanycasesthey
tainstarformation.
areorganizedinadiskwithclearrotationalmotion,inwhich
thekinematicalHIaxisisvirtuallyaligned(within15 )with
4. DISCUSSIONANDCONCLUSIONS ◦
theH I andopticalaxis, wherethe peakofthe H I emission
XMPsarefoundtotendentiouslyresideinlow-densityen- ispositionallycoincident(within10 ) with thecenterofthe
′′
vironments ( 60%) and in cosmic web types characterized opticaldisk or the brighteststar formationknot(Appendix),
asvoidsand∼sheets ( 75%; Sect. 3.1). Inthisrespect, their and where the velocity offsets between the stellar and H I
behaviourisextreme,∼incontrasttothelocationinthecosmic components are absent or small (< 40 km s 1; of the order
−