The Progenitors of the Compact Early- Type Galaxies at High Redshift or, the evolution from z=∞ to z≈2 Mauro Giavalisco University of Massachusetts Amherst With Christina Williams, BoMee Lee, Paolo Cassata, Elena Tundo, Yicheng Guo.

Galaxy properties largely bi-modal Schawinski+ “Population” features: Early types: passive, evolved, spheroidal, v/σ<1 Late types: star forming, young, disk, v/σ>1 When did this differentiation begin? Lee, MG+, 2012, press release

Rodighiero et al Clear Trends in the ways galaxies form stars: Main Sequence Galaxies: continnuous mode of SF Starburst Galaxies: large SFR from major merging (they have not quenched yet, however) Passive galaxies Quenching Noeske Daddi Rodighiero+2011 Elbaz Many others… Lee, MG+ 2014

Environment Quenching Peng et al. 2010, 2013; Renzini 2009 Mass Quenching Gas strangulation? Tidal stripping? Shock heating? AGN feedback? Star formation Feedback?

Ellipticals: key testing grounds Include the oldest, most massive galaxies Formed the bulk of their stellar mass at high redshift, on short time scale: ≈90% at z>2 (Renzini 2006) Probes of the physics of early star formation Evolved passively since Dispersion of properties (color/age gradients, light profiles) probes how environment drives galaxy evolution

Early Type Galaxies at High Redshift Observations showed that early-type galaxies, i.e. non star-forming galaxies with low sSFR and “spheroidal looking” morphology (sSFR 2.5), can indeed be massive, but they are also much more compact than their local counterpart of the same mass: x5 smaller r e, x10 2 higher ρ star (Daddi, Cimatti, van Dokkum, Trujillo, Nipoti, Saracco, Valentinuzzi, Poggianti, Huerdas-Company, Cassata, Guo, Williams….) Compact: Σ 50 ≥3x10 9 M  kpc -1 Ultra compact: Σ 50 ≥1.2x10 10 M  kpc -1

The Evolution of compact ETG Compact massive ETG first galaxies to passivize They dominate passive galaxies at high z cETG appear to peak at z≈1 Less compact galaxies evolve monotonically The formation of ETG continues at all times The formation of cETG stopped at z≈1 cETG only destroyed afterward? Today, ultra-compact galaxies are very rare (but see Kormendy+ 2009) Cassata, MG Compact: Σ 50 ≥3x10 9 M  kpc -1 Ultra compact: Σ 50 ≥1.2x10 10 M  kpc -1

The nature of the compact ETG at z≈2 –Are they really passive? Are they spheroids? –Little disagreement they are passively evolving (sSFR< yr -1 ) –Light profile fit to Sersic generally yields n≥2 (very often n>3); axial ratio≈1 –Compact ones are barely resolved by HST –Today, fraction of disk dominated SDSS galaxies with sSFR≤ yr -1 and M * ≥10 10 M  with n>2 is <5% –Dynamical properties being explored; certainly consistent with being massive, compact spheroids (Onodera et al. 2012; van Dokkum et al. 2011) –Some suggest that they include a significant (25-50%) fraction of compact disks (Bruce et al. 2012) or even that they are mostly compact disks (van der Wel et al. 2011) –The details of the selection vary. One must make sure they are equivalent –Especially at ground-based resolution, it remains very difficult to recognize the kinematical signature of compact disks and spheroids at high redshift

Panchromatric SED and spectra consistent with passively evolving (or quenching) stellar populations Today’s ellipticals occasionally show some emission lines (e.g. [OII]) indicative of SF activity (either residual or due to episodes of rejuvination)

The nature of the compact ETG? Onodera et al. 2012

Early Type Galaxies at High Redshift Key questions: what are the cETG and how do they relate to the evolution of ETG in general? Do all ETG go through the compact phase and then become “normally-sized”? Or are these galaxies a separate class of ETG? Why are they so compact? Compactness seems a good predictor of passivity (e.g. Bell et al. 2012). Why? Are they telling us about a different formation mechanism for massive galaxies (e.g. Dekel et al. 2009; Wuyts et al. 2011; Sales et al. 2012)? Compactness suggests that a highly dissipative mechanism was at work during the assembly of their stellar mass. How? Lots of efforts trying to understand the evolution from z≈2 to the present.

A possibility is that these objects formed through a highly dissipative process that involved essentially only gas e.g. accretion of cold gas directly from the cosmic Web Are compact galaxies (both passive and SF) direct evidence of galaxy formation by cold accretion? Dekel et al. 2009a,b Expected to have strong dependence on environment : Cold accretion ends sooner in denser environment

Do compact ETG evolve by growing inside-out (e.g., by accretion, star formation)? Inner core seems to evolve at constant size Total size and mass increase by accretion of, SF in, outer envelope Van Dokkum+ 2011

ETG also grow in size because of the addition of newer, larger members of the population of quenched galaxies Carollo+ 2013

The morphology of cETG The z≈2 compact ETGs with M star >3x10 10 M  do not appear to have extended structure or halos around them Subtracting the residuals of the best-fit Sersic model only reveals intervening galaxies. Cassata, MG+ 2013, Williams, MG There are NO low SB structures or extended halos around the compact core of cETG at z≈2 (e.g. tidal tails, companions) Stack of all ETGs in the CANDELS sample in GOODS Residuals consistent with NORMAL surface density of intervening sources All but passive sources masked out Possibly, there is an excess of ETG companions (≈2.7σ)

How did the cETG form? So far most efforts focused on how cETG evolved from z≈2 to z≈0 But perhaps more importantly is how they evolved from z≈∞ to z≈2 These galaxies might be the best evidence so far that cold accretion (in ways we do not fully understand) was actually a key mode of galaxy formation

(cfr. Mancini et al. 2011, who do find evidence of “normal sizes” and “halos” around some ultra massive, M star >2x10 11 M , ETG at z≈1.6) Williams, MG et al. 2013, ApJ, in press Light profile of stack show no evidence for diffuse light

Stacks show no evidence of diffuse light (halo) or structures No diffuse light light around compact ETG, both around individual galaxies and from stacks No evidence that extended halos and/or tidal debris are common If these objects formed via wet mergers, a diffuse light profile is expected from the violent relaxation of the dissipationless component (e.g. Hopkins et al. 2008) The physical message here is that high-z ETG have generally NOT experienced any major merging of galaxies with a sizeable stellar component. Whatever process has put the baryons in such small volume, it was characterized by a very high amount of dissipation, i.e. the baryons were mostly in the form of gas when that happened. The stars formed after the gas was in place (Dekel et al. 2009, Wuyts et al. 2010)

How did such objects form? Is wet merging (f gas ≥50%) viable? Mergers needs to be wet (f gas >40%) and compact. Still, sims cannot reproduce the observations, remnants are too large, have too much light at large radii. Wuyts et al. et al simulations by T.J. Cox

Radial profiles: Results from the Sersic fits to data and sims Sims of merging produce remnants that are too large Williams, MG et al. et al. 2012

Hopkins+, in prep. New sims show that neither merging nor other dissipative processes seem to work… very puzzling!

How do SB galaxies look like? MG+ in prep. z≈2 SB stack z≈2 cETG stack z≈2 SBs are our best candidates for merger remnants Provide empirical information on morphology of major merger remnants We staked all 5x SB above MS We also stacked those that are “compact” Stack of SBs is “disky” (n=1.7) and large (re= 2 kpc). = 6.3x10 10 M  Stack of cETG is “spheroidal” (n=3.6) and small (re= 1 kpc). = 6.4x10 10 M  Core-normalized difference

cETG and their candidate progenitors reasonably well described by steep, “spheroidal” (n>3) Sersic profile Sersic profile of core of SB galaxies is “disky” (n=1.7) Removal leaves residual (halos) Mergers scatter stars and form halos (stars are not dissipative)

The progenitors of compact ETG If cETG formed in situ via some highly dissipative process (cold accretion?), then their progenitors must be among compact star-forming galaxies at z≥3. Are there any reasonable candidates? We used pure UV selection (LBG), UV/Opt selection (VJL) as well as SED selection. Here results for LBG at z≈3 in GOODS-S. Search criteria are: 1. Redshift such that SF ended ≈1+ Gyr prior observation epoch of cETG (consistent with estimated age of stars, see Onodera et al. 2012) 2. Compact morphology; (from the WFC3 CANDELS images) We estimate the projected stellar mass density from the WFC3 H-band images (rest frame optical): Σ 50 = M * /2πr e 2 Cassata et al.’s stellar density criteria for normal, compact, ultra-compact 3. SFR and stellar mass such that, after including the extra mass formed during quench at z<z obs, they reproduce the stellar mass distribution of ETG observed at 1.5<z< Must be passive, i.e. have SSFR ≈1.6

Modeling the cessation of SF For each galaxy, we used a simple exponentially declining SFH, e -t/τ Quenching starts right after z obs We measured SFR from UV continuum + Calzetti dust How do we constrain τ q, the quenching time scale? τ q set by the difference between the redshift of the cETG and that of the progenitor candidates To end the SF activity across the whole structure, the minimum τ q must be of the order of the sound crossing time: The maximum τ q set by the requirement SSFR< yr -1 at z=1.5

The progenitors of the massive compact ETG Candidates must have the right SFR, stellar mass, and SFH, to be observed as cETG at z≈2 Must have right morphology, too: a SF disk at z≈3 is still visible at z≈2 even after quenching We used conservative choice of τ q. Larger τ q result in larger mass and more candidates Because of the LBG simplicity and high efficiency, we looked for candidates first among z~3 LBG (U-band dropouts). We will extend the search to other redshift epochs and selection criteria Williams, MG et al. 2013

The progenitors of the compact ETG: star-forming galaxies at z≈3 Galaxy Type Co-moving volume density Compact ETG 3.5 x [Mpc -3 ] Compact z≈3 LBG 2.3 x [Mpc -3 ] The number depends on the assumptions on the quenching history, τ q More candidates expected when other types of SF galaxies will be included

WFC3 H-band images of the candidate progenitors (compact z≈3 LBG)

UV/Opt SED of ETG progenitor candidates vs. non candidates: SED differs only in the UV part Candidate progenitors have slightly larger D4000 and redder UV Optical part is virtually identical Consistent with higher metallicity or an older, more evolved burst (not supported by emission lines) Repeating the stacks after eliminating all galaxies with [OII] and [OIII] in any of the used bands yields the same result The UV SED of candidates is redder

IR SED of ETG progenitor candidates vs. non candidates (MIPS 24  m & Herschel 160  m) Both candidates and non candidates seem to have similar MIR luminosity. i.e. similar dust-emission properties (24  m at z≈3 is about 6  m, i.e. thermal IR emission by warm/hot dust) Non-candidates seem to have higher luminosity

Effects of AGE and DUST on the UV/Opt SED of Star-Forming Galaxies: models

Effects of AGE and DUST on the UV/Opt SED of Star-Forming Galaxies: observations Williams, MG Difference between the average points of candidates and non-candidates is along the the age line

Stacked spectra show that: Candidates have stronger UV features, consistent with higher metallicity (see Rix et al. 2004). They also seem to have larger galactic winds Williams+ in prep.

Candidates have both wider and more blueshifted interstellar lines: More powerful outflows and more turbulent ISM MG+ in prep.

AGN quenching? Barro et al. report that the AGN fraction (X-ray detection) in the compact SF galaxies is 30% vs. 1% in non compact ones at 2<z<3. At z>3, we do not observe the same Very few detections of AGN in our sample (<6%) Similar rates for candidates and non-candidates No individual galaxy is detected in 4M Chandra image We used stacks

Stacks of the CXO 4-Msec X-ray images (Tundo+ in prep.) Candidates Hard band Candidates Soft band Non candidates Hard band Non candidates Soft band

Stacks using the CXO 4-Msec X-ray images revealed no obvious AGN activity (Tundo et al. in prep.) Galaxy Type Average X-ray flux Compact z≈3 LBG, soft 1.9 ± 2.5 cnt/source Compact z≈3 LBG, hard 0.5 ± 2.5 cnt/source Non compact z≈3 LBG, soft 2.0 ± 0.9 cnt/source Non compact z≈3 LBG, hard -0.2 ± 1.1 cnt/source Detection rate of individual sources is about 6% for both compact, 4% for non compact Consistent with the small incidence of AGN among LBG Absence of evidence is NOT evidence of absence, but presence of AGN does not seem obvious

We do not know how galaxies quench, i.e. their s.f.h. to passivity. Remember that compact galaxies quench sooner than non compact ones All our SF candidates progenitors sit high on the MS. These galaxies should be the one with the fastest rise in the SFR, followed by a quick quenching time-scale τ q (Renzini 2009) Quick τ q  internal process Slow τ q  environmental process Maybe the best predictor of a galaxy future S.F.H. is its position on the MS (if measured well) Better than fitting a S.F.H. and then extrapolating it…

Compactness (n,  *) a good predictor of passivity Bell Cheung McGrath U-B

The Quenching rate of compact galaxies MG et al. in prep. At any given epoch, the number of cETG depends on 1)the rate at which compact star forming galaxies appear and 2)the rate at which they become passive (quenched) Only galaxies with M star >10 10 M  and Σ 50 ≥1.2x10 10 M  kpc -1 shown (ultra-compact and massive as per Cassata et al.’s definition)

Conclusions Compact Early-Type Galaxies unlikely to form by merging of pre-existing galaxies Their compactness require highly dissipative gas process; they may be the telltale of galaxy formation by “cold accretion”. However, current models seem too crude. They are the first passive galaxies to appear in the universe. The place to go to understand underlying physics of quenching. Compactness seems to play a role Dependence of compact systems (ETG & SF) on environment as a function of redshift is key (Lani+ 2013; Poggianti+ 2013) Will help constrain the cold accretion idea Will help testing if merging and interaction are responsible for the disappearance of compact galaxies Progenitor candidates: SF galaxies at z≈3 whose stellar density is as high as cETG at z≈2 and have the same (projected) mass Progenitors appear to have higher metallicity. Consistent with properties of ETG They sit high on the MS: more likely to quench sooner, more rapidly. AGN? no obvious difference of X-ray properties b/w compact and non-compact SFG (candidate and non- candidate progenitors) ISM kinematics seems more extreme. Stellar feedback, i.e. transfer of energy, momentum to ISM efficient quenching agent in high density environment? (e.g. see Krumolz, Quataer, other)