Kilo-parsec Molecular Gas Disks! In Merger Remnants

Transcription

1 Kilo-parsec Molecular Gas Disks! In Merger Remnants!! Collaborators: D. Iono, M. S. Yun, A. F. Crocker, D. Narayanan,! S. Komugi, D. Espada, B. Hatsukade, H. Kaneko, Y. Matsuda,! Y. Tamura, D. J. Wilner, R. Kawabe, H. -A. Pan!

2 Galaxy Evolution via a Merging event The ul'mate goal is to reveal how galaxies have formed and evolved. (Bridge+10) Why mergers? Galaxy merger is one of the primary mechanisms of the forma'on and evolu'on of galaxies, as illustrated by the increasing galaxy merger rate at higher redshi<s (e.g., Bundy+09). Inves6ga6ng the effect of galaxy merger leads to understanding the galaxy evolu6on.

3 Galaxy Evolution via a Merging event

4 Scenarios of Galaxy Evolu3on Major mergers result in a forma'on of early- type galaxies (ETGs) (e.g., Toomre 77; Barnes & Hernquist 92). Classical Scenario Merger Forma'on of gas disks ETG (E+S0) Recent Simula'ons Not all of mergers will become an ETG, but some will reemerge as a late- type galaxy (LTG), forming an extended gas disk (e.g., Springel & Hernquist 05; Robertson & Bullock 08; Xu et al. 10). LTG (Sa+Sb)

5 Formation of an extended gas disk Gas falls to the galac'c center, and then it contributes to a forma'on of the spheroidal component. Gas that does not lose significant angular momentum through merging will reform an extended gas disk. Distribu6on of stars vs. gas in a merger remnant (Springel & Hernquist+05) Stars Gas Distribu6on of gas components during the merging event (Cox+08)

6 Depending on Gas Mass Fractions A large gas mass frac6on (M H2 /M * ) leads to a more efficient forma6on of an extended gas disk. (Hopkins+09)

7 Scenarios of Galaxy Evolu3on Major mergers result in a forma'on of early- type galaxies (ETGs) (e.g., Toomre 77; Barnes & Hernquist 92). Classical Scenario Merger Forma'on of gas disks ETG (E+S0) Recent Simula'ons Not all of mergers will become an ETG, but some will reemerge as a late- type galaxy (LTG), forming an extended gas disk (e.g., Springel & Hernquist 05; Robertson & Bullock 08; Xu et al. 10). LTG (Sa+Sb)

8 Sample of Merger Remnants K- band images of our 37 merger remnant sample (k- band Images: Rothberg & Joseph 2004) Our sample is taken from the op#cally- selected merger remnant sample (Rothberg & Joseph 2004) according to the following criteria: 1. Op'cal morphology ('dal tails, loops, and shells) 2. Single nucleus & no nearby companion

9 Interferometric CO Observations & Data Telescope No. of sources CO line Resolu'on [arcsec] Noise level [mjy/beam] Observa'ons ALMA 20 CO (1-0) SMA 5 CO (2-1) CARMA 2 CO (1-0) Archival Data SMA 7 CO (2-1) CO (3-2) PdBI 2 CO (1-0) ALMA 1 CO (1-0)

10 The CO velocity fields of the merger remnants The CO detec'on: 30/37 sources Among sources with robust CO detec6ons, 4 kpc 4 kpc 4 kpc 0.5 kpc 0.5 kpc 0.5 kpc 80 % (24/30) show kinema6cal signatures of molecular gas disks (including nuclear rings). The sizes of the disks: 1.1 kpc to 9.3 kpc.

11 The relative size of the Molecular Gas Disk Inves'ga'ng the rela've size of the molecular gas disk to the stellar spheroidal component. R ra'o = R 80 R eff R 80 : the radius which contains 80 % of the total CO flux R eff : the K- band effec've radius = the radius of the isophote containing half of the total K- band luminosity (Rothberg & Joseph 2004) star gas star gas R eff R 80 R eff R 80

12 The relative size of the Molecular Gas Disk 8 54 % (R ra'o < 1) : 46 % (R ra'o > 1) 6 Number R ratio (= R 80 /R eff )

13 Comparison the properties of molecular gas We expect merger remnants to show the proper'es similar to ETGs and LTGs, depending on which galaxy type they evolve into. Number Number Number Molecular gas disk size 15 (a) Merger remnants (b) Early-type galaxies (d) Late-type galaxies log R 80 /R bulge Merger remnant 38 ETGs (ATLAS 3D ) Alatal+13, Davis LTGs (BIMA- SONG) Regan+01, Helfer (a) Merger remnants Gas mass frac'on (b) Early-type galaxies (c) Late-type galaxies Log (M H2 /M * )

14 Early- type galaxies (ETGs) Late- type galaxies (LTGs) Merger Remnants

15 Implication for Galaxies at z = 1 to 3 Understanding the process of disk forma'on is par'cularly important at high- redshi< when morphological segrega'on into different Hubble types begins. Recent observa'ons reveal the presence of molecular and ionized gas disks in star- forming galaxies at high redshi<. (Förster Schreiber+09) While the surrounding environment may be different at low and high redshi<s, we suggest the possibility that a good frac'on of what are iden'fied as disk galaxies at z = 1-3 through rota'onal kinema'cs may contain a substan'al subset of mergers.

16 Summary of this study We have conducted a CO imaging study of merger remnants in the local Universe using sub/millimeter interferometers. Our new compila'on of observa'onal data suggests that molecular gas disks are common in the final stages of mergers. The sizes of these disks vary significantly from 1. to 9.3 kpc. 54 % of the sample have compact gas disks, but a few sources have gas disks which are extended rela've to the stellar component, possibly forming a late- type galaxy. We suggest that some of the rota'on- supported disks observed at z=1-3 may result from galaxies that have experienced a recent major merger. A part of this study was accepted to ApJS and posted to astrp- ph (arxiv: ).

17 URL: hop://alma- intweb.mtk.nao.ac.jp/~jueda/data/mr/index.html