Dwarfs Gobbling Dwarfs: A Stellar Tidal Stream Around NGC 4449 and Hierarchical Galaxy Formation on Small Scales

0.56 MB·English

by  David Martinez-Delgado

AstrophysicalJournalLetters, in press: received 9 December 2011,accepted31 January2012
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
DWARFS GOBBLING DWARFS: A STELLAR TIDAL STREAM AROUND NGC 4449
AND HIERARCHICAL GALAXY FORMATION ON SMALL SCALES
David Mart´ınez-Delgado1,13, Aaron J. Romanowsky2, R. Jay Gabany3, Francesca Annibali4,
Jacob A. Arnold2, Ju¨rgen Fliri5,6, Stefano Zibetti7, Roeland P. van der Marel8,
Hans-Walter Rix1, Taylor S. Chonis9, Julio A. Carballo-Bello10, Alessandra Aloisi8,
Andrea V. Maccio`1, J. Gallego-Laborda11, Jean P. Brodie2, Michael R. Merrifield12
Astrophysical Journal Letters,in press: received 9 December 2011, accepted 31 January 2012
2
1 ABSTRACT
0 Acandidatediffusestellarsubstructurewaspreviouslyreportedinthehaloofthenearbydwarfstar-
2
burstgalaxyNGC 4449by Karachentsevetal. We mapandanalyzethis feature usinga unique com-

n bination of deep integrated-lightimages from the Black Bird 0.5-meter telescope, and high-resolution
a wide-fieldimagesfromthe8-meterSubarutelescope,whichresolvethenebulosityintoastreamofred
J giantbranchstars,andconfirmits physicalassociationwithNGC 4449. The propertiesofthe stream

1 imply a massive dwarf spheroidalprogenitor,which after complete disruption will deposit an amount
3 of stellar mass that is comparable to the existing stellar halo of the main galaxy. The ratio between
luminosity or stellar-mass between the two galaxies is ∼ 1:50, while the indirectly measured dynam-

] ical mass-ratio, when including dark matter, may be ∼ 1:10–1:5. This system may thus represent a
O
“stealth”merger,whereaninfallingsatellitegalaxyisnearlyundetectablebyconventionalmeans,yet
C has a substantial dynamical influence on its host galaxy. This singular discovery also suggests that
. satellite accretioncanplay a significantrole in building up the stellar halos of low-massgalaxies,and
h possibly in triggering their starbursts.
p
-
o
1. INTRODUCTION vations (Lovell et al. 2012; Boylan-Kolchinet al. 2012;
r
st A fundamental characteristic of the modern cold dark Ferrero et al. 2012).
There has been relatively little work on substruc-
a matter (ΛCDM) cosmology (Mo et al. 2010) is that
[ galaxies assemble hierarchically under the influence of ture and merging in the halos around low-mass,
“dwarf” galaxies. Many cases of extended stellar ha-
gravity, continually accreting smaller DM halos up un-
2 los around dwarfs have been identified observationally
til the present day. If those halos contain stars, then
v (Stinson et al.2009),butitisnotclearifthesestarswere
they will be visible as satellites around their host galax-
4
accreted, or formed in-situ. Star formation in dwarfs is
5 ies, eventually disrupting through tidal forces into dis-
thought to occur in stochastic episodes (Tolstoy et al.
1 tinct streams and shells before phase-mixing into obscu-
2009;Weisz et al.2011),whichcouldbe triggeredby ac-
2 rity (Cooper et al. 2010). This picture seems to explain
cretion events.
. the existence of satellites and substructures observed
2 An iconic galaxyin this context is NGC 4449,a dwarf
around massive galaxies (Arp 1966; Schweizer & Seitzer
1 irregular in the field that has been studied intensively
1988; McConnachie et al. 2009; Mart´ınez-Delgado et al.
1 as one of the strongest galaxy-wide starbursts in the
2010). However, quantitative confirmation of this as-
1 nearbyuniverse. Its absolute magnitude of M =−18.6
pect of ΛCDM has been more elusive, with linger- V
: makesitanLMC-analogue(andnotformallyadwarfby
v ing doubts provoked by small-scale substructure obser-
i somedefinitions),butwithamuchhigherstarformation
X rate. It is strongly suspected to have recently interacted
ar 12MUnaixv-ePrslaitnycko-fInCstaitliuftorfnuiraAOstbrsoenrovmatyo,riHese,id1e1lb5e6rgH,iGghermStarneyet, winigthpaecnuoltihaerrkginaleamxaytbicasseind oitnsvcaorldiougsassigannadtuHrIeIsrinegcilounds-
SantaCruz,CA95064,USA
3BlackBirdObservatory,Mayhill,NewMexico,USA (Hartmann et al. 1986; Hunter et al. 1998), but the na-
4OsservatorioAstronomicodiBologna,INAF,ViaRanzani1, ture of this interaction is unknown.
I-40127Bologna,Italy An elongated dwarf galaxy or stream candidate near
5LERMA, CNRS UMR 8112, Observatoire de Paris, 61 Av-
NGC 4449 was first noticed by Karachentsev et al.
enuedel’Observatoire,75014Paris,France
6GEPI, CNRS UMR 8111, Observatoire de Paris, 5 Place (2007) from Digitized Sky Survey (POSS-II) plates (ob-
JulesJanssen,92195Meudon,France jectd1228+4358),andis visibleinthe SloanDigitalSky
7Dark Cosmology Centre, Niels Bohr Institute - University Survey (SDSS)14. Here we present deep, wide-field opti-
of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen,
Denmark cal imaging that supplies the definitive detection of this
8Space Telescope Science Institute, 3700 San Martin Drive, ongoingaccretionevent involvinga smaller galaxy,lead-
Baltimore,MD21218 ingtointerestingimplicationsabouttheevolutionofthis
9Department of Astronomy, University of Texas at Austin,
system and of dwarf galaxies in general.
Texas,USA
10Instituto deAstrofisicadeCanarias,Tenerife,Spain
11Fosca Nit Observatory, Montsec Astronomical Park, Ager,
Spain
12School of Physics and Astronomy, University of Notting-
ham,UniversityPark,NottinghamNG72RD,England
13Alexander vonHumboldtFellowforAdvanced Research 14 http://www.sdss.org/
2
2. OBSERVATIONSANDDATAREDUCTION 3. STREAMMORPHOLOGY
Our observations of NGC 4449 and its surroundings Figure1(left) showsaBBROimagesubsection,where
consist of two main components. The first is imaging with an adopted distance of 3.82 Mpc (A+08), 1′ corre-
from a small robotic telescope, to confirm the presence sponds to 1.1 kpc. Clearly visible near the minor axis
of a low-surface-brightness substructure and provide its of NGC 4449, ∼ 10 kpc to the southeast, is a very elon-
basic characteristics (similar techniques were used with gated, S-shaped feature of dimensions ∼ 1.5 × 7 kpc,
larger galaxies in Mart´ınez-Delgado et al. 2008, 2010). which we designate “the stream”. The stream’s position
The second is follow-up imaging with the Subaru tele- does not overlap with any of the complex HI-gas fea-
scope to map out the resolvedstellar populations. tures surroundingNGC 4449(Hunter et al. 1998). Also,
We obtained very deep images with the f/8.3 Ritchey- it is on the opposite side of the galaxy with respect to
Chretien 0.5-meter telescope of the Black Bird Remote aninteresting starcluster that may be linkedto another
Observatory(BBRO)15 duringdifferentdark-skyobserv- past accretion event (Annibali et al. 2012). The main
ingrunsovertheperiods2010-04-13through2010-06-10, galaxy’s existing stellar halo is also apparent, including
and 2011-01-13through 2011-01-28 (UT). We used a 16 shell-likefeatures inthe southwestpreviouslynoticedby
mega-pixel Apogee Imaging Systems U16M CCD cam- Hunter et al. (1999).
era,with31.3′×31.3′ field-of-viewand0.46arcsecpix−1 The right-hand panel shows a Suprime-Cam zoom-in
plate-scale. We acquired 18 hours of imaging data in ofthestream. Itisclearlyresolvedintostarsandhasthe
half-hoursub-exposures,usinganon-infraredclearlumi- general appearance of a dwarf spheroidal (dSph) galaxy
nance (λ = 3500–8500˚A) Astrodon E-series filter. Each that is elongated by tides. This classification (defined
sub-exposurewasreducedfollowingstandardprocedures by a spheroidal morphology, stellar mass M⋆ < 108M⊙,
for dark-subtraction, bias-correction, and flat-fielding and the absence of star formation and cold gas) is also
(Mart´ınez-Delgado et al. 2009). supported by the HI non-detection of Huchtmeier et al.
The resulting image was calibrated photometrically (2009).
to SDSS using the brighter regions of NGC 4449 (see To complement these integrated surface-brightness
Mart´ınez-Delgado et al. 2010). The final image has 5-σ maps, we construct stellar-density maps around
g-band surface-brightness detection limits from 26.4 to NGC4449usingindividualRGBstarsdowntog′ =25.8
27.5 mag arcsec−2 for seeing-limited and large-scale dif- from Suprime-Cam. We discuss the RGB selection and
fuse features, respectively. modelinglater,butingeneralthesestarstraceapopula-
Wesubsequentlyobtainedimagesfromthe 8.2-mSub- tionolderthan1Gyr. We define agridacrossthe image
aru Telescope and the Suprime-Cam wide-field imager with 130×130 pc2 bins, and count the number of stars
(34′×27′ field-of-view,0.202′′ pixel-scale;Miyazaki et al. in each bin, subsequently applying a Gaussian smooth-
2002)on2011-01-05(UT).Conditionswerephotometric, ing kernel with σ = 130 pc. There are typically
smooth
and we took dithered exposures in r′ and i′ bands, with 7–9 stars per bin in the stream.
total exposure times of 225 s per filter. We reduced the Figure 2(a) shows the resulting Subaru-based stellar-
data using a modified SDFRED pipeline (Ouchi et al. density map, where the overallstream morphologyis in-
2004), including bias subtraction, flat-fielding, and dis- distinguishable from the BBRO integrated-light results.
tortion correction. Each frame was re-projected to a Since there is no evidence for resolved young stars (see
commonastrometriccoordinatesystemfollowedbyback- nextsection),weinferthatthevisiblelightisdominated
ground rectification and image co-addition using Mon- by RGB stars and hence by a population older than 1
tage16. Gyr.
The exquisite image quality (∼ 0.5′′ FWHM) al- Panel (b) shows a higher-contrast version of the same
lows us to resolve individual stars in the outer regions map, demonstrating that the main galaxy’s pre-existing
of NGC 4449. We carried out point-spread-function stellar halo extends out to at least ∼ 10 kpc. We can
photometry using DAOPHOT II/ALLSTARS (Stetson furthermore discern a very faint feature that seems to
1987), and identified stars as objects with sharpness extend the stream’s angular path in a loop-like struc-
parameter |S| < 1.0. We calibrated the photometry ture. This loop is also apparent in the BBRO image,
based on two central images from the Hubble Space and we infer that the stream is stretched out over at
Telescope Advanced Camera for Surveys (HST/ACS) leasthalf its orbit, with the projectedturning point ata
(Annibali et al. 2008, hereafter A+08). galactocentric radius of 13 kpc.
The ACS photometry was originally in F555W Wenextwishtolocatethedisruptingsatellitegalaxy’s
and F814W, and recalibrated to Johnson-Cousins VI original center or nucleus. In both the BBRO and Sub-
(A+08). We used fairly bright, red stars in common be- aruimagesthereis adensity enhancementnearthe mid-
tween the datasets to derive linear transformationequa- point of the “S”,as wouldbe expected if the “arms”are
tionsfromr′i′ instrumentalmagnitudestoVI,including leading and trailing tidal tails around a still marginally-
foreground extinction corrections of E(B −V) = 0.019 bound, or just disrupting, main body. We construct
(Schlegel et al.1998). Ourfinalstar cataloghasstatisti- a stellar-density contour map for the stream region,
calinternalerrorsinV −I colorof∼0.11magand∼0.14 using RGB stars and σ = 270 pc. The cen-
smooth
mag at V=25 and V=26, respectively. tral stellar clump is visible in Figure 2(d), with a po-
sition slightly offset from the stream’s main ridgeline
(α = 12 28 43.32,δ = +43 58 30.0; positional
J2000 J2000
15 BBRO was originallysituated in the Sacramento Mountains uncertainty ∼ 6′′). An off-center nucleus is also seen
(NewMexico,USA),andlatermovedtotheSierraNevadaMoun- in a tidally-disrupting Milky Way dSph, Ursa Major
tains(California,USA).
16 http://montage.ipac.caltech.edu/ (Mart´ınez-Delgado et al. 2001).
3
Fig.1.— NGC 4449 and its halo stream. Left: image from BBRO, showing a 19.0′ ×24.5′ (21×27 kpc) field. Right: 5.5′×8.6′
(6×9.5 kpc) subsection of the Subaru/Suprime-Cam data, showing the stream resolved into stars. In both panels, shallower BBRO
exposuresinred/green/blue filtersprovideindicativecolors.
4. STELLARPOPULATIONS trace the data reasonably well. The Z = 0.001 models
appear significantly bluer than the mean RGB color (by
Figure 3 shows the color-magnitude diagram (CMD)
&0.13mag: oursystematiccoloruncertaintiesare.0.1
for point sources in the stream region. The RGB
mag), although a Z =0.001, 10 Gyr model is consistent
stars are the dominant feature, along with a few
with the blue edge of the observed RGB. Presumably a
brighter,redderstarsthatmaybeoxygen-orcarbon-rich
10 Gyr model with Z ∼ 0.002 would also be consistent
thermally-pulsating asymptotic giant branch stars from
withthedata,whileperhapsprovidingabettermatchto
an intermediate-age or old population (Marigo et al.
the observed CMD slope. We conclude that, if the bulk
2003). We find no blue stars that would trace recent
of the RGB stars are old (> 10 Gyr), the metallicity
star formation.
rangeisroughlyZ =0.001–0.004,whileforyoungerages
The detection of the tip of the RGB (TRGB) per-
the metallicity range shifts to higher values.
mits a distance estimate. Using techniques from A+08
These results are comparable to the RGB analysis
and Cioni et al. (2000), we find a TRGB magnitude of
of the main body of NGC 4449 by A+08 (their fig-
I = 24.06 ± 0.04 (random) ±0.08 (systematic).
TRGB ure17;seealsoRy´s et al.2011). Thereforeboththemain
The random error was estimated using bootstrapping
galaxyandthe streamcontainsimilar old, intermediate-
techniques; the systematic error is dominated by the
metallicity populations, although the main galaxy also
magnitude-transformationuncertainties. Themainbody
contains very young stars, as well as more metal-poor
of NGC 4449 was found by A+08 to have I =
TRGB starsasinferredfromitsglobularclusters(Strader et al.
24.00 ± 0.01 (random) ±0.04 (systematic). Thus the
2012).
stream is at the same distance as the main body, to
We provide a preliminary overview of spatial stellar
within ∼180kpc, and we conclude that there is a phys-
population variations by splitting the RGBs into color-
ical association rather than a chance superposition.
basedsubpopulations,usingthe4Gyr,Z =0.004model
Although the RGB is affected by the well-known age-
as a boundary. We then create stellar-density maps as
metallicity degeneracy, this feature can still be used to
before,forthetwosubpopulationsseparately. Usingblue
constrain the properties of stars older than ∼ 1 Gyr. In
and green color-coding to represent the subpopulations
Figure3weoverplotthePaduaisochrones(Girardi et al.
leftandrightofthemodelboundary,weshowtheresults
2002) for ages of 2, 4, and 10 Gyr, for both Z = 0.004
inFigure 2(c). The stream’sbrightpartshaveno visible
and Z = 0.001. For Z = 0.004 the 2–4 Gyr isochrones
4
Fig.2.—Stellar-densitymapsofNGC4449anditsstream,basedonRGBcounts. (a)30′×30′ (33×33kpc)imagewithlinearscaling.
Acompactdensityenhancementwithinthestreammaybetheprogenitorgalaxy’sremnantnucleus. (b)Thesamedatawithlessdynamic
range. The stream shows a loop-like structure curving back toward the main galaxy, which in turn also shows a shell-likeoverdensity in
itshalotowardthesouthwest,reachingsimilarprojectedgalactocentricradiiasthestream. (c)Compositemapofthestream,color-coded
blueforbluerRGBcolors,andgreenforreddercolors(seemaintext). TheRGBsinthestream’smainparthavesimilaraveragecolorsto
themaingalaxy’sinnerhalostars,withredderpopulationsapparentontheoutskirtsofboththehaloandstream. (d)Star-countcontour
mapofthestream,withcontour levelscorrespondingto5–10starsper50arcsec2 bin,inintervalsof1starperbin.
RGB color gradient, and have an overallcolor similar to globalcolorcorrespondingto the centralSDSS measure-
the main galaxy’s halo at radii of ∼ 3–5 kpc. Both the ment, g − r = 0.45± 0.1. We find a stream mass of
stream’s faint-loop continuation, and the halo at ∼ 5– M⋆ =1.5+−00..86×107M⊙,implyingastellarmass-ratiobe-
10 kpc, are redder, implying older or more metal-rich tweenstreamandhostof ∼1:50,forany uniformIMF.17
stars.
The second mass-estimation approach uses the CMD,
5. STELLARANDDYNAMICALMASS comparing observed star-counts to predicted numbers
fromastellarpopulationsmodel. Forthestream,weuse
We now proceed to estimate the luminosities and the I-band stellar luminosity function near the TRGB,
masses of NGC 4449 and its stream. For NGC 4449, and normalize it to Monte Carlo simulations drawn as-
it is straightforward to add up the SDSS pixel fluxes suming Z =0.001–0.004,and ages 2–10 Gyr. We obtain
inside the “optical radius” (µr = 25 mag arcsec−2). M⋆ ∼ (2–5.5)×107M⊙ for the stream (a lower-limit be-
We find an extinction-corrected Mr = −17.8. For the cause of incompleteness), which agrees with the color-
stream, we use the SDSS-calibrated BBRO image, inte- based results. For the main galaxy, a similar approach
grating the flux within the faintest isophote that closes wasfollowedbyMcQuinn et al.(2010),whoseresultsim-
wµith=ou2t6.i7n5clmudaigngartchseecm−a2inangdalaaxsyt,rweahmichsecmori-rmesapjoonrdasxtios ply M⋆ = (1.2±0.2)×109M⊙. This mass is somewhat
g higher than by using colors,but the CMD-based stellar-
distance of 3 kpc. We find a stream magnitude of M =
r mass-ratio comes out to be similar, ∼ 1:40.
−13.5,whichiscomparabletothebrightestLocalGroup
Remarkably, the mass of NGC 4449’s stellar halo is
dSphs, Fornax and And VII. Such galaxies have typical
similar to the stream’s mass: based on the RGB counts
projected half-light radii of ∼ 0.4–1.0 kpc (Brodie et al.
of Ry´s et al. (2011) and their normalization to K-band
2011), which is consistent with the stream’s ∼ 0.8 kpc
surface-brightness photometry, we estimate M ∼ 2 ×
⋆
half-width. 107M⊙ for the halo betweenprojectedradiiof5–10kpc.
Forbothgalaxies,theseluminosityestimatesarelower-
This halo could have therefore been built up directly by
limits since they do not include potential extended low-
oneora fewpastaccretioneventssimilartothe present-
surface-brightnessfeatures. Theimpliedluminosity-ratio
day stream.
is ∼ 1:50.
We next consider the dynamical masses of the host
To calculate stellar masses M , we use two differ-
⋆ galaxy and its stream, including DM. The quantity that
entapproaches,adoptingaChabrier(2003)initial-mass-
is arguably the most relevant to the current stream-
function (IMF; final mass, including stellar remnants).
galaxy interaction is the dynamical mass-ratio within
Thefirstisbasedontheintegratedopticalcolors,follow-
the interaction region: the ∼ 14 kpc galactocentric ra-
ing the relations betweencolorand stellarmass-to-light-
dius. Based on the HI gas kinematics, we estimate an
ratio (Υ⋆) from Zibetti et al. (2009, Table B1). This inclination-corrected circular velocity of v ≃62 km s−1
c
paper also introduced a technique for mapping out lo-
cal Υ⋆ and M⋆ variations pixel-by-pixel, which we ap- 17 Thesimilarityoftheluminosity-andmass-ratiosimpliesthat
ply to NGC 4449, and after integrating, find a total the luminosity-weighted estimate of Υ⋆ for NGC 4449 happened
M⋆ = 7.46 × 108M⊙. For the stream, we assume a toturnoutthesameasinferredforthestream.
5
Fig.3.— Color-magnitude diagram of the stream region, centered at (αJ2000 = 12 28 43,δJ2000 = +43 58 30.0), with a 250′′×540′′
(4.6×10.0 kpc) field-of-view, using r′i′ photometry transformed to VI. RGB isochrones are overlaid for metallicities Z = 0.001 (blue
curves)andZ=0.004(redcurves). ForeachZ,theisochronesshowagesof2,4,and10Gyr,fromlefttoright,respectively.
at this radius (Bajaja et al. 1994; Hunter et al. 2002), including DM. Such an extreme circumstance could be
which means a dynamical mass for the main galaxy of comparedwith models of satellite disruption and poten-
Mdyn(r < 15 kpc) ≃ 1.1×1010M⊙. The HI gas mass tiallydiscriminate betweenΛCDMandalternativetheo-
is ∼ 109M⊙ (Hunter et al. 1998), so this region is DM- ries (McGaugh & Wolf 2010).
dominated. Note that the v and M values together
suggest that NGC 4449 is intcermediat⋆e in mass to the 6. DISCUSSION
LMC and SMC (cf. Besla et al. 2010). We have detected and analyzed a stellar tidal stream
For the stream mass, we have no direct measure- in the halo of NGC 4449 which we interpret as the on-
ments, and instead turn to a plausibility argument going disruption of a dSph galaxy by a larger dwarf (an
based on Local Group dSphs, where the brightest LMC/SMC analogue18). This appears to be the lowest-
cases have estimated circular velocities of v ∼ 15– mass primary galaxy with a verified stellar stream.
c
20 km s−1 on ∼ 1–3 kpc scales (Walker & Pen˜arrubia We suggest some implications for galaxy evolution. It
2011; Boylan-Kolchinet al. 2012). Then if we assume has been proposedthat dSph’s orbiting massive galaxies
the v values for both stream and main galaxyare fairly such as the Milky Way were “pre-processed” from gas-
c
constant with radius, the ratio of v2 yields the dynam- rich dwarfs by tidal effects within dwarf-galaxy groups
c
ical mass-ratio. This ratio is ∼ 1:20–1:10, and thus the (D’Onghia et al. 2009). We may be witnessing such a
stream may be significantly perturbing the main galaxy. transformationin-action, with the HI streams surround-
A final metric is the ratio of total (virial) halo masses ing NGC 4449 representing additional tidal debris.
M , which are not directly measurable but may be in- We also suspect it is not just a coincidence that such
vir
ferred on a statistical basis, assuming a ΛCDM frame- a novel stream was found first around one of the most
work. In this context, it is well-established that the intensely star-forming nearby galaxies. The accretion
total mass-to-light-ratios of dwarf galaxies increase dra- eventmaywellbethestarbursttrigger. Theperiodofel-
matically at lower luminosities. Current estimates of evatedstarformationappearsto havestarted∼ 0.5Gyr
M⋆–Mvir and luminosity–Mvir relations (Moster et al. ago (McQuinn et al. 2010), which is suggestively similar
2010; Tollerud et al. 2011) would imply Mvir ∼ (1– to the stream’s ∼ 1–2 Gyr orbital period19 (and to any
5)×1011M⊙forNGC4449,andapre-infallmassof∼(1– processthatislinkedtothedynamicaltime on∼30kpc
10)×1010M⊙ forthestreamprogenitor—whichalthough
very uncertain, plausibly implies an initial virial mass- 18 The LMC and SMC may also have a history of interaction,
ratio of ∼ 1:10–1:5. withastellarmass-ratioof∼1:15(Beslaetal.2010).
19 Given a projected apocentric radius of Ra = 13 kpc,
Thus what appears to be a very minor merger in visi-
a circular orbit provides a lower-limit for the period of T =
ble light may actually be closer to a major merger when 2πRa/vc≃1.3Gyr.
6
scales). Regardlessof the implications for starbursts,dSph ac-
Are such accretionevents frequent among other dwarf cretion appears to be an increasingly viable avenue for
galaxies in recent epochs? We suspect that exact ana- direct assembly of dwarf galaxies’ stellar halos—as wit-
loguestothisstreamarenotverycommon,ortheywould nessedbyNGC4449,andbyFornax,whichshowstraces
have been noticed already in DSS/SDSS images. How- of swallowing an even smaller dSph (Coleman et al.
ever, if the stream had been only a bit fainter, more 2005). Future observational determinations of dwarf
diffuse, or at a larger radius, it could have been missed, streamfrequencyincombinationwiththeoreticalmodels
and thus there may be many more dwarf-hosted stellar may provide clues to the general substructure problem.
streams awaiting detection.
In theory, the history of DM halo assembly should be
fairly scale-free, and ∼ 1:10 mergers are expected to be We thank Jay Strader for a preview of his paper, and
themostgenerallydominantcontributorstomassgrowth James Bullock, Pavel Kroupa, Jorge Pen˜arrubia, Mon-
(Stewart et al. 2008). It is also increasingly recognized ica Tosi, and the referee for comments. Based on data
that such relatively minor mergers can have important collectedat SubaruTelescope (operated by the National
effects onthe largergalaxies,suchasinciting globaldisk AstronomicalObservatoryof Japan), via Gemini Obser-
instabilities (Purcell et al. 2011). vatory time exchange (GN-2010B-C-204). FA received
IfstreamsasinNGC4449arecommonindwarfs,they partial financial support from ASI, through contracts
re-ignite classic ideas about galaxy interactions trigger- COFIS ASI-INAF I/016/07/0 and I/009/10/0. The
ing starbursts. Given the high rates of star formation in Dark Cosmology Centre is funded by the Danish Na-
dwarfgalaxies,itisnaturaltoaskifsatellitesarerespon- tionalResearchFoundation. WorksupportedbytheNa-
sible. Surveys along these lines have produced mixed tional Science Foundation (Grants AST-0808099, AST-
results (Noeske et al. 2001; Brosch et al. 2004; Li et al. 0909237,AST-1109878, Graduate Research Fellowship),
2008),butuntilnow,low-surface-brightnessobjectssuch byNASA/SpitzergrantJPL-1310512,andbytheUCSC-
as dSphs would have been missed. UARC Aligned Research Program.
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