1. Comparison of E+A and “Normal Green galaxies” in the local Universe
Jinrong Li
USTC

2. OUTLINE Summary Introduction Sample: E+As & green galaxies
Comparison between E+As and Green galaxies
Morphology
Structure
Star Formation History
Discussion
Origin of violent starburst in E+As
SF Quenching mechanisms
Connection between E+As and ETGs
Summary

3. Introduction: E+A galaxies
Prominent Balmer absorption lines
Weak or no emission lines associating with current star formation
Simply, their spectra can be well fitted by a combination of just two SSPs with the age of K-type (typical population of Elliptical galaxies) and A-type stars.
This fig comes from Yan et al. MNRAS 2009

4. Introduction: Green Valley
Baldry et al. 2004
Yan et al. 2009
Pan et al. 2013
The morphological and spectral properties of green galaxies are consistent with the transitioning population between the blue cloud and the red sequence. (Pan et al. apj,2013)
What is the difference between E+A and “Normal Green” galaxies?

5. Sample: E+A galaxies SDSS DR8 Data:
0.01 < z <0.25, z_warning=0
r_petro < 17.77, err_r,u,m<0.2
r_fiber-r_petro<2.50
sn_median > 10
b/a>0.6
There is an evident spur in the equivalent diagram of Recent sSFR vs. current sSFR
The median u-r color of the spur at the bottom-right corner is green.
Final E+A sample after visually checking the spectra : 1452

6. Sample: Visually removed “E+A” from large catalog

7. Sample: E+A in the CMD
Most of E+As are in the green valley (dash lines encompass 68% dots)
The color of E+As are correlated with their stellar masses
E+As account for ~1-6% green valley members

8. Sample: normal green galaxies matched to E+As
In the color-mass-redshift space, we search the nearest galaxy for each E+A.
Normal Green Galaxies (NGGs)

9. Morphology: the observability of low-brightness features
apj-1982 Schweizer-f2
apj-2004 YangYJ-f11
Arxiv Ji,Peirani-F8
-- The detective of low-brightness features require deep and high-resolution images.
-- Even so, the nearest galaxies in the SDSS images have relative deep exposure and high resolution.

10. Morphology (high-mass galaxies)

11. Morphology (mid-mass galaxies)

12. Morphology (low-mass galaxies)

13. Comparison: morphologies
- Massive E+As are mostly post-mergers
- E+As are more concentrated than NGGs, almost all of them are ETGs.
- more than half of massive NGGs have disk component, especially for low-mass objects
- E+As are central bright, most of low-mass NGGs have no bright center

14. Comparison: Structural properties
E+As are structurally more concentrated and inner bluer than NGGs.
However, E+As are not more compact than old ETGs (Dn4000>1.6 & CI > 2.6)

15. Comparison: SFH recovered from StarLight analysis
YSP mass Fractioin ~ 0.1% (sSFR~10^-11) for both E+A and NGGs  quenched or quenching with negligible residual star formation
E+As have bursty SFH and the recent starburst contribute a median mass fraction of ~40%
E+As almost have no tail of residual star formation after the main starburst  require the quenching process rapidly enough
The flux at 4020 Ang (or g-band) of E+As is completely dominated by ISP (0.1<t<2.0 Gyr)  pure green

16. Comparison: SFH in color-color and spectral index diagrams
E+As locate outside of the main members
E+As experienced a violent starburst Gyr ago
NGGs experienced continuous SFH in the past life
E+As近期经历了剧烈暴发式的恒星形成活动; NGG具有长期累积的恒星形成历史。

17. Discussion: origin of the violent starburst in E+As
Necessary conditions for the bulk of new stars:
A large amount of cold gas;
strong trigger.
Main mechanism
Gas-rich major mergers or strong interactions evidenced by morphology and environment (Zabludof 1996, Goto 2005, Yang 2008)
Other mechanism
In rich cluster , gas is squeezed by strong interaction between the galaxy and dense ICM (Poggianti 2004,2009; Ma 2008; Mahajan 2013)
Violent Disk Instability (VDI) ?
Ma 2008

18. Discussion: quenching of massive E+As
Quasar feedback, RPS, compact starburst, Halo quench
Diamond-Stanic 2012
Tremonti
-- Local E+As are not so compact as high-z ones, they are even larger than local old ETGs at the same stellar mass.
-- Just a small fraction of massive E+As are in rich cluster in the local universe.
-- Halo quench is not rapid enough
-- Candidate mechanism: Quasar feedback; else, residual SF (simulation)
Canalizo et al. 2001
apj-2012 Matsuoka

19. Discussion: quenching of low-mass E+As
SN feedback, RPS, strangulation, starvation
Poggianti 2004
mn-2007 Kaviraj: if SN feedback dominate, then SN 注入的能量与新生恒星的质量成正比,斜率=2
Mahajan 2013
-- SN feedback for central galaxies
-- ICM process for satellites of rich cluster

20. Discussion: quenching of NGGs
Quenching of NGG (Schawinski):
LTG: driven by secular and/or environmental processes (halo); their SFR decline very slowly.
ETG: consequence of a major merger; rapid quenching accompanied by a morphological transformation. merger fraction?
Pan: The inflow of cool gas can fuel an AGN or trigger star formation activity in the center of ETGs without the requirement of a strong starburst triggered by a merger. VDI ?
Pan et al ApjL
Schawinski et al MNRAS

21. Summary
E+As and NGGs are quenched or quenching galaxies, sSFR <= yr -1
Massive E+As are originated from gas-rich major mergers.
E+As: similar structures as ETGs, but morphological disturbances and younger ages at inner region  Early-ETGs
NGGs: structurally extended than E+As and local ETGs, the latest SF took place at outer region
Roads into the dead red sequence in the local universe:
E+As represent a sub-branch in which galaxies rapidly transformed their color and morphologies.
NGGs are the main-branch in which galaxies teminated their continuous gentle SFH (dominated by secular evolution).

22. Thanks for your attention.