Gas-rich and gas-poor dwarf galaxies:on the origin of the different dwarf galaxy types

Carme Gallart & LCID Instituto de Astrofísica de Canarias

LEO I

Dwarf Irregular (dIrr): Gas, HII regions, recent star formation, exponential profiles.

Dwarf spheroidal (dSph): No gas, no recent star formation, exponential or King profiles

IC 1613

Dwarf elliptical (dE): No gas, no recent star formation, R1/4 profiles

Transition (dT): Little or no gas, little recent star formation, exponential profiles.

M32 Phoenix

µ0 21

M 109 M○

Z 0.1 Z○

MB > -15

The different dwarf galaxy types (dSph, dIrr, dT) share some similarities, and show some differences. Some ‘textbook knowledge”, not fully accurate, is:

-similar relationships btw σ, rc, μC, MV

(Kormendy85); -all can be fitted by exponential profiles (Faber&Lin83)

- (most) dIrr rotate while (most) dSph do not-morphology-density relation (with exceptions)

-differences in gas content

All dwarfs have old stars, consistent with a common initial epoch of star formation.

They all must have looked like dIrr galaxies.And then, at some point, some lost their gas

Morphological transformation “dIrr”-> dSph ?(this is a quite common implicit assumption; there is even a ´transition class’ of dwarf galaxies: dT)

Were the precursors of dSph like today’s dIrr? (“different by nurture”)OR

Were dSph and dIrr born different? (“different by nature”)

DIFFERENT BY “NURTURE”?

Most (mostly theoretical) research on the transformation “dIrr” -> dSph

has focused on answering the question:

How did dSph galaxies lose their gas? Internal processes: -Gas expulsion through star formation (Larson’74; Dekel&Silk’86, McLow & Ferrara1999, Ferrara & Tolstoy 2000, Salvadori et al. 2008, Sawala et al. 2010)

-Gas exhaustion by star formation (though difficult to remove 100% of gas)

External processes (supported by the morphology-density relation): -Tidal stirring (Mayer et al. 01; 07; Klimentowski et al. 09): can transform infalling disky dwarfs into dSph-like objects, in ~several Gyr -Tidal stirring + gas stripping (ram-pressure + tidal), to produce really low gas fraction (Mayer et al. 01, Passeto et al. 2003)

-What about isolated dSph? (Cetus & Tucana: D’Onghia‘09, Sales et al.‘07, Kazantzidis et al.

‘11)

-Internal/External + UV background: enhancing gas loss/stripping(Sawala et al. 10, Mayer 10)

OR, DIFFERENT BY “NATURE”?

In order to answer the question:

Were dSph and dIrr born different?

Key information: Early Star Formation Histories

If accretion onto a big halo, or interaction with another dwarf galaxy was responsible for stopping star formation in a gas-rich dwarf that otherwise would have evolved to a “normal” dIrr galaxy,

then one would expect to see, in the star formation

histories:

- similar early SFHs for dSph and dIrr

- a variety of durations/epochs for the end of star

formation in dSph

Main goal:

To obtain, for the first time, color-magnitude diagrams (CMDs) reaching the oldest main-sequence turnoffs for a sample of isolated dwarf galaxies, i.e. beyond the Milky Way satellite system

FIRST EVER oMSTO CMDs FOR A dIRR

LOCAL COSMOLOGY FROM ISOLATED DWARFS (LCID) PROJECT:

TUCANA PHOENIX LEO-A

CETUS LGS-3 IC1613

dSph dIrr

MORE GAS CONTENT

tran

LCID CMDs sample

Compl.> 90%

TUCANA PHOENIX LEO-A

CETUS LGS-3 IC1613

MORE GAS CONTENT

dSph tran dIrr

Monelli et al (2010)

Monelli et al (2010)

Hidalgo et al (2011)

Hidalgo et al (2009)

Skillman et al (2013)

Hidalgo et al (2013)

LCID SFH results

TUCANA PHOENIX LEO-A

CETUS LGS-3 IC1613

MORE GAS CONTENT

dSph tran dIrr

Monelli et al (2010)

Monelli et al (2010)

Hidalgo et al (2010)

Hidalgo et al (2009)

Skillman et al (2010)

Hidalgo et al (2010)

LCID SFH results

Most likely Blue Straggler stars:Monelli et al. 2012

TUCCET

LGS3PHO

I1613LEOA

[M/H

]

Look-back time

Ψ(t

)VERY DIFFERENT EARLY SFHs FOR dIRR & dSph/dT

The dIrrs in the sample don’t show a dominant early burst of star formation. Also, slower metallicity enrichment.

dIrr SFHs “similar” to MCs:

Meschin et al. 2013 LCID 2013

Noël et al. 2009

If accretion onto a big halo, or interaction with another dwarf galaxy was responsible for stopping star formation in a gas-rich dwarf that otherwise would have evolved to a “normal” dIrr galaxy,

then one would expect to see, in the star formation

histories:

- similar early SFHs for dSph and dIrr

- a variety of durations/epochs for the end of star

formation in dSph

Look-back time

TUCCET

LGS3

Look-back time

Ψ(t

)

Tests with ‘mock’ stellar populations:Observational errors tend to broaden the measured features of the SFH.

How reliable are the recovered ages at early times?

VERY SIMILAR AGE OF END OF SF IN LCID dSph/dT

Tucana

Look-back time

Cetus LGS3

Re-ionization epoch TUCCET

LGS3

Look-back time

Ψ(t

)

Reionization was proposedto stop star formation in small galaxies (one way out of the “missing satellite problem”).

Reionization seems to have not stopped star formationin any of these galaxies.

VERY SIMILAR AGE OF END OF SF IN LCID dSph/dT

However, all three galaxies formed the bulk of their stars before z≈3, or 11 Gyr

What about the Milky Way dSph satellites?

BaSTI isochrones (Pietrinferni et al. 2004) of 9 &13 Gyr, Z=0.0003, superimposed

VERY SIMILAR AGE OF END OF SF IN LCID dSph/dT

What about the Milky Way dSph satellites?

BaSTI isochrones (Pietrinferni et al. 2004) of 9 &13 Gyr, Z=0.0003, superimposed

“Blue plumes” are most likely blue straggler stars sequences(Monelli et al. 2012, Mapelli et al. 2007, 2009, Momany et al. 2008)

VERY SIMILAR AGE OF END OF SF IN LCID dSph/dT

The odd guys

If accretion onto a big halo, or interaction with another dwarf galaxy was responsible for stopping star formation in a gas-rich dwarf that otherwise would have evolved to a “normal” dIrr galaxy,

then one would expect to see, in the star formation

histories:

- similar early SFHs for dSph and dIrr

- a variety of durations/epochs for the end of star

formation in dSph

Need a different scenario

Re-ionization epoch TUCCET

LGS3

Mac Low & Ferrara (1999)

Look-back time

If re-ionization didn’t stop star formation, then what did it? SNe Feedback? Models by Mac Low & Ferrara (1999) for SNe feedback indicate that this mechanism may not remove completely the gas in these galaxies.

SNe Feedback + UV background? Models by Sawala et al. (2010) indicate that UV background + feedback should have halted star formation at z ∼ 6, for M<8x108M. Above, self-shielding becomes effective, allowing SF to continue beyond z=6 Strigari et al (2008): M300≈107M ; MT≈109M

Walker et al. (2009): MT compatible with ≈109M

Ψ(t

)

Sawala et al. (2010)

Note that the LCID galaxies are isolated!!

Re-ionization epoch TUCCET

LGS3

Mac Low & Ferrara (1999)

Look-back time

Ψ(t

)

Sawala et al. (2010)

‘External effects’ not totally ruled out:

Radial velocities of Tucana and Cetus (Fraternali et

al. 2009; Lewis et al. 2007) not incompatible with passage through the inner parts of the Local Group (possible gas stripping: Mayer et al 2001, 2006)

Resonant stripping (D’Onghia 2009 )

Cosmic ménage à trois (Sales et al. 2007): Tucana & Cetus could be the lighter member of a satellite pair ejected to a highly energetic orbit

Merger of disky dwarfs: (Kazantzidis et al. 2011)

But seems a bit contrived to me: Occam’s razor?

Note that the LCID galaxies are isolated!!

Stellar populations in MW dSph possibly in place before tidal stirring and stripping could start acting! (Sawala et

al. 2010)

EVIDENCE FROM LCID PROJECT (+ literature):1) Different early SFH of dIrr and dSph

2) Sincronicity in end of star formation time in dSph & dT galaxies

dIrr and dSph different by NATURE

PROPOSED SCENARIO(Compatible with morphology-density relation)

-’Milder’ early star formation mode in dIrr galaxies as compared with dSph. Possibly because they are born in lower density environment, with no interactions triggering SF? They have an initially lower density? (Carraro

et al. 2001) Other?

-‘Milder’ star formation activity milder feedback less gas loss?(however, models have difficulty in producing mild SFR: Sawala et al. 2012)

-The high level early star formation activity in dSph (possibly triggered by environmental effects), aided with some extra energy from the UV reionization background, responsible for gas loss in dSph? (Sawala et al. 2010)

-Tidal effects (stirring, stripping) playing a role in the final removal of gas?

THE END

External processes (supported by the morphology-density relation):

----Tidal interaction with a large host galaxy----

Don’t explain the ‘isolated dSph’ such as Cetus or Tucana

Ways out:

Radial velocities of Tucana and Cetus (Fraternali et al. 2009; Lewis et al. 2007) not incompatible with passage through the inner parts of the Local Group (possible gas stripping)

Resonant stripping (D’Onghia )

Cosmic ménage à trois (Sales et al. 2007): Tucana & Cetus , lighter member of a satellite pair ejected to a highly energetic orbit

Merger of disky dwarfs: (Kazantzidis et al. 2011)

DIFFERENT BY “NURTURE”?

External processes

-TAKE TIME: a best case scenario to produce an ‘old’ dSph:

“[with enhanced gas stripping due to the UV ionizing background] we can argue that if the progenitors of Draco & Umi fell into the MW at z>1.5-2 [10 Gyr], then ram-pressure combined with tides was able to remove their entire gas content in a couple of orbits [2-3 Gyr]. Mayer (2010)”

-expected variations of time of end of star formation depending on infall time and orbit (in principle supported by the “variety of star formation histories of MW dSph satellites”: but see later)

Ways out:

?

DIFFERENT BY “NURTURE”?

SFH of LCID dIrr, (including MCs?)FRACTION of Old pops (RR Lyrae diagram)Compare early SFHs of one dSph & one dIrr of similar mass: Tucana & Leo A?=>=> NOT THE SAME!!!!!

HYPOTHESIS: dIrr started SF in a ‘mild’ mode while dSph started violently.Influence of environment?? This would still explain the morphology-density relation.

Probably this is an ingredient that is still lacking in galaxy evolution models: SFHs of dIrr by Sawala: high initial SFR.

Conclusion

II

observed CMD

model CMD

Look-back time (Gyr)

The distribution of stars in the observed CMD is compared with that of a number of simple populations in a model CMD.

By using a merit function, we obtain the combination of simple stellar populations that best reproduces the observed CMD, i.e., the star formation rate and the chemical evolution law, as a function of time.

Aparicio & Gallart (2004)Aparicio & Hidalgo (2009)

Stellar populations of dSph possibly in place before tidal stirring and stripping could start

acting! (Sawala et al. 2010)