We present ALMA Cycle-0 observations of the CO (6-5) line emission and of the 435um dust continuum emission in the central kpc of NGC 1614, a local luminous infrared galaxy (LIRG) at a distance of ...67.8 Mpc (1 arcsec = 329 pc). The CO emission is well resolved by the ALMA beam (0".26 x 0".20) into a circum-nuclear ring, with an integrated flux of f_{CO(6-5)} = 898 (+-153) Jy km/s, which is 63(+-12)% of the total CO(6-5) flux measured by Herschel. The molecular ring, located between 100pc < r < 350pc from the nucleus, looks clumpy and includes seven unresolved (or marginally resolved) knots with median velocity dispersion of 40 km/s. These knots are associated with strong star formation regions with \Sigma_{SFR} 100 M_\sun/yr/kpc^{2} and \Sigma_{Gas} 1.0E4 M_\sun/pc^{2}. The non-detections of the nucleus in both the CO (6-5) line emission and the 435um continuum rule out, with relatively high confidence, a Compton-thick AGN in NGC 1614. Comparisons with radio continuum emission show a strong deviation from an expected local correlation between \Sigma_{Gas} and \Sigma_{SFR}, indicating a breakdown of the Kennicutt-Schmidt law on the linear scale of 100 pc.
Interactions between pairs of isolated dwarf galaxies provide a critical window into low-mass hierarchical, gas-dominated galaxy assembly and the build-up of stellar mass in low-metallicity systems. ...We present the first Very Large Telescope/Multi Unit Spectroscopic Explorer (VLT/MUSE) optical integral field unit (IFU) observations of the interacting dwarf pair dm1647+21 selected from the TiNy Titans survey. The Ha emission is widespread and corresponds to a total unobscured star formation rate (SFR) of 0.44 M-circle dot yr(-1), which is 2.7 times higher than the SFR inferred from Sloan Digital Sky Survey (SDSS) data. The implied specific SFR (sSFR) for the system is elevated by more than an order of magnitude above non-interacting dwarfs in the same mass range. This increase is dominated by the lower-mass galaxy, which has a sSFR enhancement of > 50. Examining the spatially resolved maps of classic optical line diagnostics, we find that the interstellar medium (ISM) excitation can be fully explained by star formation. The velocity field of the ionized gas is not consistent with simple rotation. Dynamical simulations indicate that the irregular velocity field and the stellar structure is consistent with the identification of this system as an ongoing interaction between two dwarf galaxies. The widespread, clumpy enhancements in the star formation in this system point to important differences in the effect of mergers on dwarf galaxies, compared to massive galaxies; rather than the funneling of gas to the nucleus and giving rise to a nuclear starburst, starbursts in low-mass galaxy mergers may be triggered by large-scale ISM compression, and thus may be more distributed.
Abstract
We describe a
Herschel Space Observatory
194–671
μ
m spectroscopic survey of a sample of 121 local luminous infrared galaxies and report the fluxes of the CO
J
to
J
–1 rotational transitions ...for
, the N
ii
205
μ
m line, the C
i
lines at 609 and 370
μ
m, as well as additional and usually fainter lines. The CO spectral line energy distributions (SLEDs) presented here are consistent with our earlier work, which was based on a smaller sample, that calls for two distinct molecular gas components in general: (i) a cold component, which emits CO lines primarily at
J
≲ 4 and likely represents the same gas phase traced by CO (1−0), and (ii) a warm component, which dominates over the mid-
J
regime (4 <
J
≲ 10) and is intimately related to current star formation. We present evidence that the CO line emission associated with an active galactic nucleus is significant only at
J
> 10. The flux ratios of the two C
i
lines imply modest excitation temperatures of 15–30 K; the C
i
370
μ
m line scales more linearly in flux with CO (4−3) than with CO (7−6). These findings suggest that the C
i
emission is predominantly associated with the gas component defined in (i) above. Our analysis of the stacked spectra in different far-infrared (FIR) color bins reveals an evolution of the SLED of the rotational transitions of
vapor as a function of the FIR color in a direction consistent with infrared photon pumping.
Abstract
We study the ionization and excitation structure of the interstellar medium in the late-stage gas-rich galaxy merger NGC 6240 using a suite of emission-line maps at ∼25 pc resolution from ...the Hubble Space Telescope, Keck/NIRC2 with Adaptive Optics, and the Atacama Large Millimeter/submillimeter Array (ALMA). NGC 6240 hosts a superwind driven by intense star formation and/or one or both of two active nuclei; the outflows produce bubbles and filaments seen in shock tracers from warm molecular gas (H
2
2.12
μ
m) to optical ionized gas (O
iii
, N
ii
, S
ii
, and O
i
) and hot plasma (Fe
XXV
). In the most distinct bubble, we see a clear shock front traced by high O
iii
/H
β
and O
iii
/O
i
. Cool molecular gas (CO(2−1)) is only present near the base of the bubble, toward the nuclei launching the outflow. We interpret the lack of molecular gas outside the bubble to mean that the shock front is not responsible for dissociating molecular gas, and conclude that the molecular clouds are partly shielded and either entrained briefly in the outflow, or left undisturbed while the hot wind flows around them. Elsewhere in the galaxy, shock-excited H
2
extends at least ∼4 kpc from the nuclei, tracing molecular gas even warmer than that between the nuclei, where the two galaxies’ interstellar media are colliding. A ridgeline of high O
iii
/H
β
emission along the eastern arm aligns with the southern nucleus’ stellar disk minor axis; optical integral field spectroscopy from WiFeS suggests this highly ionized gas is centered at systemic velocity and likely photoionized by direct line of sight to the southern active galactic nucleus.
ABSTRACT Compact groups of galaxies provide a unique environment to study the evolution of galaxies amid frequent gravitational encounters. These nearby groups have conditions similar to those in the ...earlier universe when galaxies were assembled and give us the opportunity to witness hierarchical formation in progress. To understand how the compact group environment affects galaxy evolution, we examine the gas and dust in these groups. We present new single-dish GBT neutral hydrogen (H i) observations of 30 compact groups and define a new way to quantify the group H i content as the H i-to-stellar mass ratio of the group as a whole. We compare the H i content with mid-IR indicators of star formation and optical g − r color to search for correlations between group gas content and star formation activity of individual group members. Quiescent galaxies tend to live in H i-poor groups, and galaxies with active star formation are more commonly found in H i-rich groups. Intriguingly, we also find "rogue" galaxies whose star formation does not correlate with group H i content. In particular, we identify three galaxies (NGC 2968 in RSCG 34, KUG 1131+202A in RSCG 42, and NGC 4613 in RSCG 64) whose mid-IR activity is discrepant with the H i. We speculate that this mismatch between mid-IR activity and H i content is a consequence of strong interactions in this environment that can strip H i from galaxies and abruptly affect star formation. Ultimately, characterizing how and on what timescales the gas is processed in compact groups will help us understand the interstellar medium in complex, dense environments similar to the earlier universe.
We describe a Herschel Space Observatory 194-671 m spectroscopic survey of a sample of 121 local luminous infrared galaxies and report the fluxes of the CO J to J-1 rotational transitions for , the N ...ii 205 m line, the C i lines at 609 and 370 m, as well as additional and usually fainter lines. The CO spectral line energy distributions (SLEDs) presented here are consistent with our earlier work, which was based on a smaller sample, that calls for two distinct molecular gas components in general: (i) a cold component, which emits CO lines primarily at J 4 and likely represents the same gas phase traced by CO (1−0), and (ii) a warm component, which dominates over the mid-J regime (4 < J 10) and is intimately related to current star formation. We present evidence that the CO line emission associated with an active galactic nucleus is significant only at J > 10. The flux ratios of the two C i lines imply modest excitation temperatures of 15-30 K; the C i 370 m line scales more linearly in flux with CO (4−3) than with CO (7−6). These findings suggest that the C i emission is predominantly associated with the gas component defined in (i) above. Our analysis of the stacked spectra in different far-infrared (FIR) color bins reveals an evolution of the SLED of the rotational transitions of vapor as a function of the FIR color in a direction consistent with infrared photon pumping.
The enormous increase in mid-IR sensitivity and spatial and spectral resolution provided by the JWST spectrographs enables, for the first time, detailed extragalactic studies of molecular vibrational ...bands. This opens an entirely new window for the study of the molecular interstellar medium in luminous infrared galaxies (LIRGs). We present a detailed analysis of rovibrational bands of gas-phase CO, H\(_2\)O, C\(_2\)H\(_2\) and HCN towards the heavily-obscured eastern nucleus of the LIRG VV 114, as observed by NIRSpec and MIRI MRS. Spectra extracted from apertures of 130 pc in radius show a clear dichotomy between the obscured AGN and two intense starburst regions. We detect the 2.3 \(\mu\)m CO bandheads, characteristic of cool stellar atmospheres, in the star-forming regions, but not towards the AGN. Surprisingly, at 4.7 \(\mathrm{\mu}\)m we find highly-excited CO (\(T_\mathrm{ex} \approx 700-800\) K out to at least rotational level \(J = 27\)) towards the star-forming regions, but only cooler gas (\(T_\mathrm{ex} \approx 200\) K) towards the AGN. We conclude that only mid-infrared pumping through the rovibrational lines can account for the equilibrium conditions found for CO and H\(_2\)O in the deeply-embedded starbursts. Here the CO bands probe regions with an intense local radiation field inside dusty young massive star clusters or near the most massive young stars. The lack of high-excitation molecular gas towards the AGN is attributed to geometric dilution of the intense radiation from the bright point source. An overview of the relevant excitation and radiative transfer physics is provided in an appendix.
We study the ionization and excitation structure of the interstellar medium in the late-stage gas-rich galaxy merger NGC 6240 using a suite of emission line maps at \(\sim\)25 pc resolution from the ...Hubble Space Telescope, Keck NIRC2 with Adaptive Optics, and ALMA. NGC 6240 hosts a superwind driven by intense star formation and/or one or both of two active nuclei; the outflows produce bubbles and filaments seen in shock tracers from warm molecular gas (H\(_2\) 2.12\(\mu\)m) to optical ionized gas (O III, N II, S II, O I) and hot plasma (Fe XXV). In the most distinct bubble, we see a clear shock front traced by high O III/H\(\beta\) and O III/O I. Cool molecular gas (CO(2-1)) is only present near the base of the bubble, towards the nuclei launching the outflow. We interpret the lack of molecular gas outside the bubble to mean that the shock front is not responsible for dissociating molecular gas, and conclude that the molecular clouds are partly shielded and either entrained briefly in the outflow, or left undisturbed while the hot wind flows around them. Elsewhere in the galaxy, shock-excited H\(_2\) extends at least \(\sim\)4 kpc from the nuclei, tracing molecular gas even warmer than that between the nuclei, where the two galaxies' interstellar media are colliding. A ridgeline of high O III/H\(\beta\) emission along the eastern arm aligns with the south nucleus' stellar disk minor axis; optical integral field spectroscopy from WiFeS suggests this highly ionized gas is centered at systemic velocity and likely photoionized by direct line-of-sight to the south AGN.
Compact groups of galaxies provide a unique environment to study the evolution of galaxies amid frequent gravitational encounters. These nearby groups have conditions similar to those in the earlier ...universe when galaxies were assembled and give us the opportunity to witness hierarchical formation in progress. To understand how the compact group environment affects galaxy evolution, we examine the gas and dust in these groups. We present new single-dish GBT neutral hydrogen (HI) observations of 30 compact groups and define a new way to quantify the group HI content as the HI-to-stellar mass ratio of the group as a whole. We compare the HI content with mid-IR indicators of star formation and optical g-r color to search for correlations between group gas content and star formation activity of individual group members. Quiescent galaxies tend to live in HI-poor groups, and galaxies with active star formation are more commonly found in HI-rich groups. Intriguingly, we also find "rogue" galaxies whose star formation does not correlate with group HI content. In particular, we identify three galaxies (NGC 2968 in RSCG 34, KUG 1131+202A in RSCG 42, and NGC 4613 in RSCG 64) whose mid-IR activity is discrepant with the HI. We speculate that this mismatch between mid-IR activity and HI content is a consequence of strong interactions in this environment that can strip HI from galaxies and abruptly affect star-formation. Ultimately, characterizing how and on what timescales the gas is processed in compact groups will help us understand the interstellar medium in complex, dense environments similar to the earlier Universe.
We describe a Herschel Space Observatory 194-671 micron spectroscopic survey of a sample of 121 local luminous infrared galaxies and report the fluxes of the CO \(J\) to \(J\)-1 rotational ...transitions for \(4 \leqslant J \leqslant 13\), the NII 205 um line, the CI lines at 609 and 370 um, as well as additional and usually fainter lines. The CO spectral line energy distributions (SLEDs) presented here are consistent with our earlier work, which was based on a smaller sample, that calls for two distinct molecular gas components in general: (i) a cold component, which emits CO lines primarily at \(J \lesssim 4\) and likely represents the same gas phase traced by CO (1-0), and (ii) a warm component, which dominates over the mid-\(J\) regime (\(4 < J < 10\)) and is intimately related to current star formation. We present evidence that the CO line emission associated with an active galactic nucleus is significant only at \(J > 10\). The flux ratios of the two CI lines imply modest excitation temperatures of 15 to 30 K; the CI 370 um line scales more linearly in flux with CO (4-3) than with CO (7-6). These findings suggest that the CI emission is predominately associated with the gas component defined in (i) above. Our analysis of the stacked spectra in different far-infrared (FIR) color bins reveals an evolution of the SLED of the rotational transitions of water vapor as a function of the FIR color in a direction consistent with infrared photon pumping.
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