Understanding the size growth of massive (spheroidal) galaxies through stellar populations

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1 Department of Space and Climate Physics Mullard Space Science Laboratory Understanding the size growth of massive (spheroidal) galaxies through stellar populations Ignacio Ferreras Mullard Space Science Laboratory University College London IAU S319 Galaxies at high redshift and their evolution over cosmic time Honolulu, HI, USA August 14th, 2015

2 The SFHs of massive ETGs The stellar populations of the central parts of ETGs refect a fast, efcient process of star formation at early times (e.g. Trager+2000; Thomas+2005,2010; Graves+2009; Rogers+2010) z~0 de la Rosa et al. (2011, MNRAS, 418, L74) Ferreras et al. (2009, ApJ, 706,158)

3 Size evolution of massive galaxies However, evidence obtained over the past decade has shown that early-type massive galaxies at moderate/high redshift are more compact than at z~0 (Daddi et al. 2005; Trujillo et al. 2007; van Dokkum et al. 2008; etc). Cassata et al. (2013)

4 Possible growth channels: Gas-driven processes: AGN activity (Fan et al. 2008; 2010; Ragone- Figueroa & Granato 2011) Supernova winds (Damjanov et al. 2009) Merger-driven processes: Major mergers (Ciotti & van Albada 2001; Boylan-Kolchin et al. 2006; Naab et al. 2007; Nipoti et al. 2010) Minor mergers (Khochfar & Burkert 2006; Maller et al. 2006; Hopkins et al. 2009; Naab et al. 2009; Sommer-Larsen & Toft 2010; Oser et al. 2010)

5 Stellar age on the mass-size plane A comparison of size evolution between nearby (SDSS; triangles) and distant galaxies (ACS; dots) shows no segregation with respect to stellar age (red/blue) Only the downsizing trend is apparent: lower mass galaxies are younger. Therefore, no signifcant star formation should be involved in the process. Trujillo, IF, et al. (2011, MNRAS, 415, 3903)

6 Massive galaxy evolution: SHARDS Tracing the properties of merger progenitors through the analysis of the stellar populations of massive galaxies and close companions SHARDS survey (Pérez-González et al. 2013), 10.4m GTC, deep (26.5AB), 130 arcmin2 Medium band (170Å) flters, low-res spectroscopy (R~50) Highly accurate photo-zs (photo-spectra) Ferreras et al. (2014, MNRAS, 444, 906)

7 Massive galaxy evolution: SHARDS Select merger progenitors of massive galaxies (e.g. Patton et al. 2000; Lin et al. 2010; López-SanJuan et al. 2011) All galaxies within 100kpc of a massive (>1011M ) galaxy 100kpc Ferreras et al. (2014, MNRAS, 444, 906)

8 Quantifying the merger growth rate % of mass in satellites The fraction of close-satellites in massive galaxies is fairly constant from z~1.5 Most of the mass growth originates from massive satellites Role of very-minor mergers not so important Ferreras et al. (2014, MNRAS, 444, 906) (SDSS z=0; Ruiz et al. 2014, MNRAS, 442, 347)

9 The nature of the accreted stellar material Two additional constraints can be considered for the contribution from major/minor mergers: Age of merger progenitor eventual age gradient Dynamical friction less efcient merging with minor mergers Merger mass growth rate: ~8% per Gyr. Ferreras et al. (2014, MNRAS, 444, 906)

10 Age and Metallicity gradients Mʘ M ʘ The fat age profles in massive ETGs at low redshift confrm this issue. (see also simulations by Hirschmann et al. 2015) Mʘ La Barbera, IF, et al. (2012, MNRAS, 426, 2300)

11 An additional channel Note that massive ETGs represent a relatively small proportion of the local galaxy census: There are similar numbers of very massive & compact **bulges** residing in late-type galaxies (Graham 2013; Graham et al. 2015) A possible evolution channel can therefore be: Massive Compact Galaxy Massive Compact Bulge in a LTG z~2 Sample: Mendel et al. (2014, ApJS, 210, 3) z~0 de la Rosa et al. (submitted)

12 Another possible channel of compact massive galaxy evolution: Compact massive ulges de la Rosa et al (submitted)

13 Conclusions The size growth of massive ETGs cannot be driven by gas flows (no significant star formation involved). An analysis of close pairs involving a massive galaxy (as merger progenitors) reveal a constant fraction of merger systems out to z~1.5, with a dominant contribution from massive mergers. Very-minor mergers (below 1:10) do not contribute much to the mass growth budget. The metallicity AND age gradient of massive ETGs also rule out a large contribution from very minor mergers. Additional channel for massive compact galaxies Compact/massive bulges at low redshift.