KASI Optical Astronomy Division Journal Club

1. KASI Optical Astronomy Division Journal Club Main JC 1. Presentation by the speaker 2. Discussion on the paper 3. Basic questions (off-time?)

2. KASI Optical Astronomy Division Journal Club Evidence of Major Dry Mergers at M* > 2x1011 M⊙from Curvature in Early-Type Galaxy Scaling Relations? - M. Bernardi et al. 2011, MNRAS, 412, L6 - June 23, 2011 Joon Hyeop Lee

3. Color-magnitude relation (red sequence) of early-type galaxies • : “Brighter early-type galaxies tend to be redder.” • Thought to be mainly because the metal abundance depends on galaxy mass. • More massive → Easier to maintain metal rich gas against galactic winds → Redder color. Kodama et al. (1998) M87

4. Bernardi et al. (2011b): Color-magnitude relation (CMR) of early-type galaxies differs significantly from a pure power law. • Curving downwards at low L (Mr > –20.5): previously reported (Graham 2008; Skelton et al. 2009) • Curving upwards at high L (Mr < –22.5): first reported by Bernardi et al. (2011). Not due to stellar population effects → Another factors may affect CMRs in addition to the basic metal enrichment. → Merger - Major? Minor? - Wet? Dry? Bernardi et al. (2011b)

5. Early-type galaxy selection • 1. Hyde & Bernardi (2009) selection: b/a > 0.6, fracDev = 1 (almost purely ellipticals) • 2. Cr > 2.86 (containing many edge-on discs) • For the 2 samples • Investigated the ①sizes, ② velocity dispersions (σ), ③ surface brightnesses (μ), ④ colors, ⑤ axis ratios (b/a) and ⑥ color gradients as functions of stellar mass (M*). • Focusing on the curvature at high L

7. None of the correlations is pure power laws. • At faint, low-mass end (log M*/M⊙<10.5): • Curve towards bluer colors, larger sizes, fainter μ, smaller axis ratios and color gradients. • At bright, high-mass end (log M*/M⊙>11.3): • Curve towards redder colors, larger sizes, smaller than expected σ, fainter μ, smaller axis ratios and color gradients.

9. M* is replaced with σ. • Little curvature at log σ/km s-1 > 2.2. • Correlations vs. μ, color gradient and axis ratio are almost flat.

10. The relation between Mdyn (∝ Rσ2) and M*is well described by a single power law over the entire range: the curvature in the sizes and σ cancel. → Because the objects are virialized, whatever their merger histories.

11. Major dissipationless (= dry) mergers • Change the sizes in proportion to the masses. • Leave σ and colors unchanged. • -W = 2K, K+W = K – 2K = -K = W/2 • Equial mass merging: -mv2/2 - mv2/2 = -2m (v2/2) • Minor dissipationless mergers • Produce larger fractional changes in size than in mass. • Decrease σ and colors. + = + = < 1 • The curvature in the correlations between M*, size, σ and color have all been discussed in this context (e.g. Davies et al. 1983; Bernardi et al. 2007, 2010, 2011).

12. What’s new in this letter • All of the curvature occur at the same mass scale. • This mass scale is also important for axis raitos and color gradients. • The curvature is absent when M* is replaced by σ.

13. Axis ratio • Van der Wel et al. (2009): the width of the b/a distribution changes at log(M*/M⊙) ~ 10.5 → “Above this mass, assembly histories are dominated by major mergers.” → This is not the full story. Van der Wel et al. (2009)

14. Axis ratio • Comparison of the 2 early-type samples: HB09 (b/a>0.6) and Cr cut (no b/a limit) • → The Cr cut sample – considerably smaller b/a at small M* • → Primarily due to an increased incidence of disks and contamination by Sa galaxies: morphological mix! Log M* [M⊙]

15. Axis ratio • The real feature of interest is the drop in b/a at at log(M*/M⊙) > 11.3, where morphological mix is no longer an issue. (Van der Wel also found it, but dismissed it.) • The narrowing of the distribution at log(M*/M⊙) ~ 10.5 marks the transition from dissipational (wet) to dissipationless (dry) histories, or a change in relative importance of SNe and AGN feedback. Log M* [M⊙]

16. Axis ratio • The decrease in b/a at log(M*/M⊙) > 11.3 marks the transition to major dry mergers. • Has been expected for some time (e.g. Gonzalez-Garcia & van Albada 2005; Ragone-figueroa et al. 2010) • First found by Bernardi et al. (2008) • Indicates an increasing incidence of major radial mergers, resulting in more prolate objects. Log M* [M⊙]

17. Color Gradient • Defined as model color – Petrosian color (approximately half-light radius color – total color). • Maximal at log(M*/M⊙) > 11.3 : consistent with previous estimates (e.g. Roche et al. 2010; Bernardi et al. 2011). • → Onset of major dry merger? • Major mergers: decrease color gradients (di Matteo et al. 2009) • Minor mergers: should not change the gradients significnatly (Kobayashi 2004) or may enhance them slightly • → The smaller bluer object involved in the minor merger is expected to deposit most of its stars at larger distances from the center.

18. Color Gradient • Major mergers double M*, but do not change σ. • → Color gradient vs. σ plot should show less of a feature than when gradients are ploted vs. M*. • The great scatter at σ > 150 km s-1: a range of merger histories, hence gradients, can all have the same σ. • Even more dramatic increase of the scatter in the gradient–M*relation? – Not easily explained.

19. Color Gradient • Major merger picture provides a natural explanation for why none of the scaling relations in Fig. 2 shows any feature at log σ / km s-1 > 2.2, and those that are most clearly sensitive to merger histories are almost completely flat. • Feedback picture • Merger is not the only way to produce or alter color gradients • Gradients are related to feedback and winds (pipino et al. 2010). • Interesting challenge: producing the changes in the parameters – M / σ relations.

20. Because the major dry merger model provides a simple framework for understanding all these relations, the results suggest that M* > 2x1011 M⊙is the scale above which major dry mergers dominate the assembly history. • This particular mass scale also appears in analyses of a local sample (higher quality data, but significantly smaller sample). • Kormendy et al. (2009); Guo & White (2008); Hopkins et al. (2008); Eliche-Moral et al. (2010) • This analysis suggests that above this mass scale, the mergers were not just dry – they were major.

21. Tension: recent work argues that although mass in the central kpc or so of early-type galaxies has not grown since z~2, the half-light radii have increased by more than a factor of two. • → Inside-out scenario: since z~2, mass has been added to the outer regions only • → Minor mergers? • - Not necessarily! • Tiret et al. (2010): the observation of constant mass in the central regions does not, by itself, exclude major mergers. • Gao et al. (2004): in major dry mergers, the mix of particles in the central regions can change dramatically, even though the total mass in the central regions remains constant.

22. BCGs (Brightest Cluster Galaxies) • The most massive objects in the local Universe – how they fit into this picture? • Slightly redder than non-BCGs of similar M or L, smaller color gradients (e.g. Roche et al. 2010) → in agreement with the major merger picture • slightly larger sizes (Bernardi 2009) → more size growth than is usually associated with major mergers

23. BCGs (Brightest Cluster Galaxies) • Two step process? • 1. Major mergers making a BCG erase its color gradient and decrease b/a. • 2. Minor mergers puff up its size. • → in good agreement with a detailed analysis of the age, metallicity and aundance gradients of BCG NGC 4889 (Coccato et al. 2010)