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Title: COLD MOLECULAR GAS IN MERGER REMNANTS. I. FORMATION OF MOLECULAR GAS DISKS

Abstract

We present the ≲1 kpc resolution {sup 12}CO imaging study of 37 optically selected local merger remnants using new and archival interferometric maps obtained with ALMA, CARMA, the Submillimeter Array, and the Plateau de Bure Interferometer. We supplement a sub-sample with single-dish measurements obtained at the Nobeyama Radio Observatory 45 m telescope for estimating the molecular gas mass (10{sup 7} {sup –} {sup 11} M {sub ☉}) and evaluating the missing flux of the interferometric measurements. Among the sources with robust CO detections, we find that 80% (24/30) of the sample show kinematical signatures of rotating molecular gas disks (including nuclear rings) in their velocity fields, and the sizes of these disks vary significantly from 1.1 kpc to 9.3 kpc. The size of the molecular gas disks in 54% of the sources is more compact than the K-band effective radius. These small gas disks may have formed from a past gas inflow that was triggered by a dynamical instability during a potential merging event. On the other hand, the rest (46%) of the sources have gas disks that are extended relative to the stellar component, possibly forming a late-type galaxy with a central stellar bulge. Our new compilation of observationalmore » data suggests that nuclear and extended molecular gas disks are common in the final stages of mergers. This finding is consistent with recent major-merger simulations of gas-rich progenitor disks. Finally, we suggest that some of the rotation-supported turbulent disks observed at high redshifts may result from galaxies that have experienced a recent major merger.« less

Authors:
; ; ; ; ; ;  [1];  [2];  [3];  [4];  [5];  [6];  [7];  [8]
  1. National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka,Tokyo 181-8588 (Japan)
  2. Department of Astronomy, University of Massachusetts, Amherst, MA 01003 (United States)
  3. Ritter Astrophysical Research Center, University of Toledo, Toledo, OH 43606 (United States)
  4. Department of Astronomy, Haverford College, 370 Lancaster Avenue, Haverford, PA 19041 (United States)
  5. Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577 (Japan)
  6. Institute of Astronomy, The University of Tokyo, 2-21-1 Osawa, Mitaka,Tokyo 181-0015 (Japan)
  7. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
  8. Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588 (Japan)
Publication Date:
OSTI Identifier:
22340175
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal, Supplement Series; Journal Volume: 214; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; GALAXIES; INTERFEROMETERS; MAPS; MASS; RED SHIFT; RESOLUTION; ROTATION; SIMULATION; TELESCOPES

Citation Formats

Ueda, Junko, Iono, Daisuke, Komugi, Shinya, Espada, Daniel, Hatsukade, Bunyo, Matsuda, Yuichi, Kawabe, Ryohei, Yun, Min S., Crocker, Alison F., Narayanan, Desika, Kaneko, Hiroyuki, Tamura, Yoichi, Wilner, David J., and Pan, Hsi-An, E-mail: junko.ueda@nao.ac.jp. COLD MOLECULAR GAS IN MERGER REMNANTS. I. FORMATION OF MOLECULAR GAS DISKS. United States: N. p., 2014. Web. doi:10.1088/0067-0049/214/1/1.
Ueda, Junko, Iono, Daisuke, Komugi, Shinya, Espada, Daniel, Hatsukade, Bunyo, Matsuda, Yuichi, Kawabe, Ryohei, Yun, Min S., Crocker, Alison F., Narayanan, Desika, Kaneko, Hiroyuki, Tamura, Yoichi, Wilner, David J., & Pan, Hsi-An, E-mail: junko.ueda@nao.ac.jp. COLD MOLECULAR GAS IN MERGER REMNANTS. I. FORMATION OF MOLECULAR GAS DISKS. United States. doi:10.1088/0067-0049/214/1/1.
Ueda, Junko, Iono, Daisuke, Komugi, Shinya, Espada, Daniel, Hatsukade, Bunyo, Matsuda, Yuichi, Kawabe, Ryohei, Yun, Min S., Crocker, Alison F., Narayanan, Desika, Kaneko, Hiroyuki, Tamura, Yoichi, Wilner, David J., and Pan, Hsi-An, E-mail: junko.ueda@nao.ac.jp. Mon . "COLD MOLECULAR GAS IN MERGER REMNANTS. I. FORMATION OF MOLECULAR GAS DISKS". United States. doi:10.1088/0067-0049/214/1/1.
@article{osti_22340175,
title = {COLD MOLECULAR GAS IN MERGER REMNANTS. I. FORMATION OF MOLECULAR GAS DISKS},
author = {Ueda, Junko and Iono, Daisuke and Komugi, Shinya and Espada, Daniel and Hatsukade, Bunyo and Matsuda, Yuichi and Kawabe, Ryohei and Yun, Min S. and Crocker, Alison F. and Narayanan, Desika and Kaneko, Hiroyuki and Tamura, Yoichi and Wilner, David J. and Pan, Hsi-An, E-mail: junko.ueda@nao.ac.jp},
abstractNote = {We present the ≲1 kpc resolution {sup 12}CO imaging study of 37 optically selected local merger remnants using new and archival interferometric maps obtained with ALMA, CARMA, the Submillimeter Array, and the Plateau de Bure Interferometer. We supplement a sub-sample with single-dish measurements obtained at the Nobeyama Radio Observatory 45 m telescope for estimating the molecular gas mass (10{sup 7} {sup –} {sup 11} M {sub ☉}) and evaluating the missing flux of the interferometric measurements. Among the sources with robust CO detections, we find that 80% (24/30) of the sample show kinematical signatures of rotating molecular gas disks (including nuclear rings) in their velocity fields, and the sizes of these disks vary significantly from 1.1 kpc to 9.3 kpc. The size of the molecular gas disks in 54% of the sources is more compact than the K-band effective radius. These small gas disks may have formed from a past gas inflow that was triggered by a dynamical instability during a potential merging event. On the other hand, the rest (46%) of the sources have gas disks that are extended relative to the stellar component, possibly forming a late-type galaxy with a central stellar bulge. Our new compilation of observational data suggests that nuclear and extended molecular gas disks are common in the final stages of mergers. This finding is consistent with recent major-merger simulations of gas-rich progenitor disks. Finally, we suggest that some of the rotation-supported turbulent disks observed at high redshifts may result from galaxies that have experienced a recent major merger.},
doi = {10.1088/0067-0049/214/1/1},
journal = {Astrophysical Journal, Supplement Series},
number = 1,
volume = 214,
place = {United States},
year = {Mon Sep 01 00:00:00 EDT 2014},
month = {Mon Sep 01 00:00:00 EDT 2014}
}
  • Since the violent relaxation in hierarchical merging is incomplete, elliptical galaxies retain a wealth of information about their formation pathways in their present-day orbital structure. Recent advances in integral field spectroscopy, multi-slit infrared spectroscopy, and triaxial dynamical modeling techniques have greatly improved our ability to harvest this information. A variety of observational and theoretical evidence indicates that gas-rich major mergers play an important role in the formation of elliptical galaxies. We simulate 1:1 disk mergers at seven different initial gas fractions (f{sub gas}) ranging from 0% to 40%, using a version of the TreeSPH code Gadget-2 that includes radiative heatingmore » and cooling, star formation, and feedback from supernovae and active galactic nuclei. We classify the stellar orbits in each remnant and construct radial profiles of the orbital content, intrinsic shape, and orientation. The dissipationless remnants are typically prolate-triaxial, dominated by box orbits within r{sub c} {approx} 1.5 R{sub e} , and by tube orbits in their outer parts. As f{sub gas} increases, the box orbits within r{sub c} are increasingly replaced by a population of short-axis tubes (z-tubes) with near zero net rotation, and the remnants become progressively more oblate and round. The long-axis tube (x-tube) orbits are highly streaming and relatively insensitive to f{sub gas}, implying that their angular momentum is retained from the dynamically cold initial conditions. Outside r{sub c} , the orbital structure is essentially unchanged by the gas. For f{sub gas} {approx}> 15%, gas that retains its angular momentum during the merger re-forms a disk that appears in the remnants as a highly streaming z-tube population superimposed on the hot z-tube distribution formed by the old stars. In the 15%-20% gas remnants, this population appears as a kinematically distinct core (KDC) within a system that is slowly rotating or dominated by minor-axis rotation. These remnants show an interesting resemblance, in both their velocity maps and intrinsic orbital structure, to the KDC galaxy NGC 4365. At 30%-40% gas, the remnants are rapidly rotating, with sharp embedded disks on {approx}1 R{sub e} scales. We predict a characteristic, physically intuitive orbital structure for 1:1 disk merger remnants, with a distinct transition between 1 and 3 R{sub e} that will be readily observable with combined data from the two-dimensional kinematics surveys SAURON and SMEAGOL. Our results illustrate the power of direct comparisons between N-body simulations and dynamical models of observed systems to constrain theories of galaxy formation.« less
  • We present a high-resolution (down to 0.''18), multi-transition imaging study of the molecular gas in the z = 4.05 submillimeter galaxy GN20. GN20 is one of the most luminous starburst galaxy known at z>4, and is a member of a rich proto-cluster of galaxies at z = 4.05 in GOODS-North. We have observed the CO 1-0 and 2-1 emission with the Very Large Array (VLA), the CO 6-5 emission with the Plateau de Bure Interferometer, and the 5-4 emission with Combined Array for Research in Millimeter Astronomy. The H{sub 2} mass derived from the CO 1-0 emission is 1.3 xmore » 10{sup 11}({alpha}/0.8) M{sub sun}. High-resolution imaging of CO 2-1 shows emission distributed over a large area, appearing as partial ring, or disk, of {approx}10 kpc diameter. The integrated CO excitation is higher than found in the inner disk of the Milky Way, but lower than that seen in high-redshift quasar host galaxies and low-redshift starburst nuclei. The CO 4-3 integrated line strength is more than a factor of 2 lower than expected for thermal excitation. The excitation can be modeled with two gas components: a diffuse, lower excitation component with a radius {approx}4.5 kpc and a filling factor {approx}0.5, and a more compact, higher excitation component (radius {approx}2.5 kpc, filling factor {approx}0.13). The lower excitation component contains at least half the molecular gas mass of the system, depending on the relative conversion factor. The VLA CO 2-1 image at 0.''2 resolution shows resolved, clumpy structure, with a few brighter clumps with intrinsic sizes {approx}2 kpc. The velocity field determined from the CO 6-5 emission is consistent with a rotating disk with a rotation velocity of {approx}570 km s{sup -1} (using an inclination angle of 45{sup 0}), from which we derive a dynamical mass of 3 x 10{sup 11} M{sub sun} within about 4 kpc radius. The star formation distribution, as derived from imaging of the radio synchrotron and dust continuum, is on a similar scale as the molecular gas distribution. The molecular gas and star formation are offset by {approx}1'' from the Hubble Space Telescope I-band emission, implying that the regions of most intense star formation are highly dust obscured on a scale of {approx}10 kpc. The large spatial extent and ordered rotation of this object suggests that this is not a major merger, but rather a clumpy disk accreting gas rapidly in minor mergers or smoothly from the proto-intracluster medium. Qualitatively, the kinematic and structural properties of GN20 compare well to the most rapid star formers fed primarily by cold accretion in cosmological hydrodynamic simulations. Conversely, if GN20 is a major, gas-rich merger, then some process has managed to ensure that the star formation and molecular gas distribution has not been focused into one or two compact regions.« less
  • We used the SPIRE/FTS instrument aboard the Herschel Space Observatory to obtain the Spectral Line Energy Distributions (SLEDs) of CO from J = 4-3 to J = 13-12 of Arp 193 and NGC 6240, two classical merger/starbursts selected from our molecular line survey of local Luminous Infrared Galaxies (L {sub IR} ≥ 10{sup 11} L {sub ☉}). The high-J CO SLEDs are then combined with ground-based low-J CO, {sup 13}CO, HCN, HCO{sup +}, CS line data and used to probe the thermal and dynamical states of their large molecular gas reservoirs. We find the two CO SLEDs strongly diverging frommore » J = 4-3 onward, with NGC 6240 having a much higher CO line excitation than Arp 193, despite their similar low-J CO SLEDs and L {sub FIR}/L {sub CO,} {sub 1} {sub –0}, L {sub HCN}/L {sub CO} (J = 1-0) ratios (proxies of star formation efficiency and dense gas mass fraction). In Arp 193, one of the three most extreme starbursts in the local universe, the molecular SLEDs indicate a small amount (∼5%-15%) of dense gas (n ≥ 10{sup 4} cm{sup –3}) unlike NGC 6240 where most of the molecular gas (∼60%-70%) is dense (n ∼ (10{sup 4}-10{sup 5}) cm{sup –3}). Strong star-formation feedback can drive this disparity in their dense gas mass fractions, and also induce extreme thermal and dynamical states for the molecular gas. In NGC 6240, and to a lesser degree in Arp 193, we find large molecular gas masses whose thermal states cannot be maintained by FUV photons from Photon-Dominated Regions. We argue that this may happen often in metal-rich merger/starbursts, strongly altering the initial conditions of star formation. ALMA can now directly probe these conditions across cosmic epoch, and even probe their deeply dust-enshrouded outcome, the stellar initial mass function averaged over galactic evolution.« less
  • We have carried out observations of Zeeman splitting of the H I 21 cm emission line from shocked atomic gas in the supernova remnants (SNRs) IC 443 and W51C using the Arecibo telescope. The observed shocked atomic gas is expanding at {approx}100 km s{sup -1} and this is the first Zeeman experiment of such fast-moving, shocked atomic gas. The emission lines, however, are very broad and the systematic error due to baseline curvature hampers an accurate measurement of field strengths. We derive an upper limit of 100-150 {mu}G on the strength of the line-of-sight field component. These two SNRs aremore » interacting with molecular clouds, but the derived upper limits are considerably smaller than the field strengths expected from a strongly shocked dense cloud. We discuss the implications and conclude that either the magnetic field within the telescope beam is mostly randomly oriented or the high-velocity H I emission is from a shocked interclump medium of relatively low density.« less
  • We present Herschel Spectral and Photometric Imaging Receiver (SPIRE) Fourier Transform Spectrometer (FTS) observations of the Antennae (NGC 4038/39), a well-studied, nearby (22 Mpc), ongoing merger between two gas-rich spiral galaxies. The SPIRE-FTS is a low spatial ( FWHM ∼ 19''-43'') and spectral (∼1.2 GHz) resolution mapping spectrometer covering a large spectral range (194-671 μm, 450-1545 GHz). We detect five CO transitions (J = 4-3 to J = 8-7), both [C I] transitions, and the [N II] 205 μm transition across the entire system, which we supplement with ground-based observations of the CO J = 1-0, J = 2-1, andmore » J = 3-2 transitions and Herschel Photodetecting Array Camera and Spectrometer (PACS) observations of [C II] and [O I] 63 μm. Using the CO and [C I] transitions, we perform both a local thermodynamic equilibrium (LTE) analysis of [C I] and a non-LTE radiative transfer analysis of CO and [C I] using the radiative transfer code RADEX along with a Bayesian likelihood analysis. We find that there are two components to the molecular gas: a cold (T {sub kin} ∼ 10-30 K) and a warm (T {sub kin} ≳ 100 K) component. By comparing the warm gas mass to previously observed values, we determine a CO abundance in the warm gas of x {sub CO} ∼ 5 × 10{sup –5}. If the CO abundance is the same in the warm and cold gas phases, this abundance corresponds to a CO J = 1-0 luminosity-to-mass conversion factor of α{sub CO} ∼ 7 M {sub ☉} pc{sup –2} (K km s{sup –1}){sup –1} in the cold component, similar to the value for normal spiral galaxies. We estimate the cooling from H{sub 2}, [C II], CO, and [O I] 63 μm to be ∼0.01 L {sub ☉}/M {sub ☉}. We compare photon-dominated region models to the ratio of the flux of various CO transitions, along with the ratio of the CO flux to the far-infrared flux in NGC 4038, NGC 4039, and the overlap region. We find that the densities recovered from our non-LTE analysis are consistent with a background far-ultraviolet field of strength G {sub 0} ∼ 1000. Finally, we find that a combination of turbulent heating, due to the ongoing merger, and supernova and stellar winds are sufficient to heat the molecular gas.« less