KASI Optical Astronomy Division Journal Club

1. Presentation by the speaker

2. Discussion on the paper

3. Basic questions (off-time?)

Main JC

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, 2011Joon Hyeop Lee

- Color-magnitude relation (red sequence) of early-type galaxies

: “Brighter early-type galaxies tend to be redder.”Kodama et al. (1998)- 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.

M87

- 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

Bernardi et al. (2011b)

→ Another factors may affect CMRs in addition to the basic metal enrichment.

→ Merger- Major?

Minor?- Wet?

Dry?

- 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 ① sizes, 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

- 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.

- 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.

- The relation between Mdyn ( ∝ R σ2) 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.

- 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).

+ =

+ =

- 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.

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)

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⊙]

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⊙]

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⊙]

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.

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.

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.

- 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.

- 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.

- 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

- 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)