Abstract
We investigate the fraction of close pairs and morphologically identified mergers on and above the star-forming main sequence (MS) at 0.2 ≤ z ≤2.0. The novelty of our work lies in the use of ...a non-parametric morphological classification performed on resolved stellar mass maps, reducing the contamination by non-interacting, high-redshift clumpy galaxies. We find that the merger fraction rapidly rises to ≥70 per cent above the MS, implying that – already at z ≳ 1 – starburst (SB) events (ΔMS ≥ 0.6) are almost always associated with a major merger (1:1 to 1:6 mass ratio). The majority of interacting galaxies in the SB region are morphologically disturbed, late-stage mergers. Pair fractions show little dependence on MS offset and pairs are more prevalent than late-stage mergers only in the lower half of the MS. In our sample, major mergers on the MS occur with a roughly equal frequency of ∼5–10 per cent at all masses ≳ 1010 M⊙. The MS major merger fraction roughly doubles between z = 0.2 and 2, with morphological mergers driving the overall increase at z ≳ 1. The differential redshift evolution of interacting pairs and morphologically classified mergers on the MS can be reconciled by evolving observability time-scales for both pairs and morphological disturbances. The observed variation of the late-stage merger fraction with ΔMS follows the perturbative 2-Star Formation Mode model, where any MS galaxy can experience a continuum of different star formation rate enhancements. This points to an SB–merger connection not only for extreme events, but also more moderate bursts which merely scatter galaxies upward within the MS, rather than fully elevating them above it.
The Stephan's Quintet (hereafter SQ) is a template source to study the impact of galaxies interaction on the physical state and energetics of their gas. We report on IRAM single-dish CO observations ...of the SQ compact group of galaxies. These observations follow up the Spitzer discovery of bright mid-IR H sub(2) rotational line emission (L(H sub(2)) approx = 10 super(35) W) from warm (10 super(2-3) K) molecular gas, associated with a 30 kpc long shock between a galaxy, NGC 7318b, and NGC 7319's tidal arm. We detect CO(1-0), (2-1) and (3-2) line emission in the inter-galactic medium (IGM) with complex profiles, spanning a velocity range of approx =1000 km s super(-1). The spectra exhibit the pre-shock recession velocities of the two colliding gas systems (5700 and 6700 km s super(-1)), but also intermediate velocities. This shows that much of the molecular gas has formed out of diffuse gas accelerated by the galaxy-tidal arm collision. CO emission is also detected in a bridge feature that connects the shock to the Seyfert member of the group, NGC 7319, and in the northern star forming region, SQ-A, where a new velocity component is identified at 6900 km s super(-1), in addition to the two velocity components already known. Assuming a Galactic CO(1-0) emission to H sub(2) mass conversion factor, a total H sub(2) mass of approx =5 x 10 super(9) M sub(middot in circle) is detected in the shock. The ratio between the warm H sub(2) mass derived from Spitzer spectroscopy, and the H sub(2) mass derived from CO fluxes is approx =0.3 in the IGM of SQ, which is 10--100 times higher than in star-forming galaxies. The molecular gas carries a large fraction of the gas kinetic energy involved in the collision, meaning that this energy has not been thermalized yet. The kinetic energy of the H sub(2) gas derived from CO observations is comparable to that of the warm H sub(2) gas from Spitzer spectroscopy, and a factor approx =5 greater than the thermal energy of the hot plasma heated by the collision. In the shock and bridge regions, the ratio of the PAH-to-CO surface luminosities, commonly used to measure the star formation efficiency of the H sub(2) gas, is lower (up to a factor 75) than the observed values in star-forming galaxies. We suggest that turbulence fed by the galaxy-tidal arm collision maintains a high heating rate within the H sub(2) gas. This interpretation implies that the velocity dispersion on the scale of giant molecular clouds in SQ is one order of magnitude larger than the Galactic value. The high amplitude of turbulence may explain why this gas is not forming stars efficiently.
Abstract
We present deep ALMA CO(5−4) observations of a main-sequence, clumpy galaxy at z = 1.5 in the HUDF. Thanks to the ∼0
${^{\prime\prime}_{.}}$
5 resolution of the ALMA data, we can link ...stellar population properties to the CO(5−4) emission on scales of a few kiloparsec. We detect strong CO(5−4) emission from the nuclear region of the galaxy, consistent with the observed L
IR–
$L^{\prime }_{\rm CO(5-4)}$
correlation and indicating ongoing nuclear star formation. The CO(5−4) gas component appears more concentrated than other star formation tracers or the dust distribution in this galaxy. We discuss possible implications of this difference in terms of star formation efficiency and mass build-up at the galaxy centre. Conversely, we do not detect any CO(5−4) emission from the UV-bright clumps. This might imply that clumps have a high star formation efficiency (although they do not display unusually high specific star formation rates) and are not entirely gas dominated, with gas fractions no larger than that of their host galaxy (∼50 per cent). Stellar feedback and disc instability torques funnelling gas towards the galaxy centre could contribute to the relatively low gas content. Alternatively, clumps could fall in a more standard star formation efficiency regime if their actual star formation rates are lower than generally assumed. We find that clump star formation rates derived with several different, plausible methods can vary by up to an order of magnitude. The lowest estimates would be compatible with a CO(5−4) non-detection even for main-sequence like values of star formation efficiency and gas content.
AbstractWe use hydrodynamical simulations of a Cartwheel-like ring galaxy, modelled as a nearly head-on collision of a small companion with a larger disc galaxy, to probe the evolution of the gaseous ...structures and flows, and to explore the physical conditions setting the star formation activity. Star formation is first quenched by tides as the companion approaches, before being enhanced shortly after the collision. The ring ploughs the disc material as it radially extends, and almost simultaneously depletes its stellar and gaseous reservoir into the central region, through the spokes, and finally dissolve 200 Myr after the collision. Most of star formation first occurs in the ring before this activity is transferred to the spokes and then the nucleus. We thus propose that the location of star formation traces the dynamical stage of ring galaxies, and could help constrain their star formation histories. The ring hosts tidal compression associated with strong turbulence. This compression yields an azimuthal asymmetry, with maxima reached in the side furthest away from the nucleus, which matches the star formation activity distribution in our models and in observed ring systems. The interaction triggers the formation of star clusters significantly more massive than before the collision, but less numerous than in more classical galaxy interactions. The peculiar geometry of Cartwheel-like objects thus yields a star (cluster) formation activity comparable to other interacting objects, but with notable second order differences in the nature of turbulence, the enhancement of the star formation rate, and the number of massive clusters formed.
Abstract
We combine James Webb Space Telescope (JWST) and Hubble Space Telescope imaging with Atacama Large Millimeter Array CO(2–1) spectroscopy to study the highly turbulent multiphase ...intergalactic medium (IGM) in Stephan’s Quintet on 25–150 pc scales. Previous Spitzer observations revealed luminous H
2
line cooling across a 45 kpc-long filament, created by a giant shock wave, following the collision with an intruder galaxy, NGC 7318b. We demonstrate that the Mid-Infrared Instrument/F1000W/F770W filters are dominated by 0–0 S(3) H
2
and a combination of polycyclic aromatic hydrocarbon and 0–0 S(5) H
2
emission. These observations reveal the dissipation of kinetic energy as massive clouds experience collisions, interactions, and likely destruction/recycling within different phases of the IGM. In 1 kpc-scaled structure, warm H
2
was seen to form a triangular-shaped head and tail of compressed and stripped gas behind a narrow shell of cold H
2
. In another region, two cold molecular clumps with very different velocities are connected by an arrow-shaped stream of warm, probably shocked, H
2
suggesting a cloud–cloud collision is occurring. In both regions, a high warm-to-cold molecular gas fraction indicates that the cold clouds are being disrupted and converted into warm gas. We also map gas associated with an apparently forming dwarf galaxy. We suggest that the primary mechanism for exciting strong mid-IR H
2
lines throughout Stephan’s Quintet is through a fog of warm gas created by the shattering of denser cold molecular clouds and mixing/recycling in the post-shocked gas. A full picture of the diverse kinematics and excitation of the warm H
2
will require future JWST mid-IR spectroscopy. The current observations reveal the rich variety of ways that different gas phases can interact with one another.
Diffuse Galactic light has been observed in the optical since the 1930s. We propose that, when observed in the optical with deep imaging surveys, it can be used as a tracer of the turbulent cascade ...in the diffuse interstellar medium (ISM), down to scales of about 1 arcsec. Here we present a power spectrum analysis of the dust column density of a diffuse cirrus at high Galactic latitude (l ≈ 198°, b ≈ 32°) as derived from the combination of a MegaCam g-band image, obtained as part of the MATLAS large programme at the CFHT, with Planck radiance and WISE 12 μm data. The combination of these three datasets have allowed us to compute the density power spectrum of the H i over scales of more than three orders of magnitude. We found that the density field is well described by a single power law over scales ranging from 0.01 to 50 pc. The exponent of the power spectrum, γ = −2.9 ± 0.1, is compatible with what is expected for thermally bi-stable and turbulent H i. We did not find any steepening of the power spectrum at small scales indicating that the typical scale at which turbulent energy is dissipated in this medium is smaller than 0.01 pc. The ambipolar diffusion scenario that is usually proposed as the main dissipative agent, is consistent with our data only if the density of the cloud observed is higher than the typical values assumed for the cold neutral medium gas. We discuss the new avenue offered by deep optical imaging surveys for the study of the low density ISM structure and turbulence.
Massive star clusters observed in galaxy mergers are often suggested to be progenitors of globular clusters. To study this hypothesis, we performed the highest resolution simulation of a gas-rich ...galaxy merger so far. The formation of massive star clusters of 105 to 107 M⊙, triggered by the galaxy interaction, is directly resolved in this model. We show that these clusters are tightly bound structures with little net rotation, due to evolve into compact long-lived stellar systems. Massive clusters formed in galaxy mergers are thus robust candidates for progenitors of long-lived globular clusters. The simulated cluster mass spectrum is consistent with theory and observations. Tidal dwarf galaxies of 108–9 M⊙ can form at the same time, and appear to be part of a different class of objects, being more extended and rotating.
Shells are fine stellar structures that are identified by their arc-like shapes around a galaxy. They are currently thought to be vestiges of galaxy interactions and/or mergers. The study of their ...number, geometry, stellar populations, and gas content can help us to derive the interaction or merger history of a galaxy. Numerical simulations have proposed a mechanism of shell formation through phase wrapping during a radial minor merger. Alternatively, there could be merely a space wrapping, when particles have not yet radially oscillated, but are bound by their radial expansion, or produce an edge-brightened feature. These can be distinguished because they are expected to keep a high radial velocity. While shells are first a stellar phenomenon, HI and CO observations have revealed neutral gas associated with shells. Some of the gas, the most diffuse and dissipative, is expected to be quickly driven to the center if it is traveling on nearly radial orbits. Molecular gas, which is distributed in dense clumps, is less dissipative, and may be associated with shells. It can then determine the shell velocity, which is too difficult to obtain from stars. We present here a search for molecular gas in nine shell galaxies with the IRAM-30 m telescope. Six of them are detected in their galaxy center, and in three galaxies, we clearly detect molecular gas in shells. The derived amount of molecular gas varies from 1.5 × 108 to 3.4 × 109 M⊙ in the shells. For two of them (Arp 10 and NGC 3656), the shells are characteristic of an oblate system. Their velocity is nearly systemic, and we conclude that these shells are phase wrapped. In the third galaxy (NGC 3934), the shells appear to participate in the rotation. Follow-up with higher spatial resolution is required to conclude.
Context.
Early-type galaxies (ETGs) are divided into slow and fast rotators (FRs and SRs) according to the degree of ordered rotation of their stellar populations. Cosmological hydrodynamical ...simulations indicate that galaxies form as FRs before their rotational support decreases, usually because of mergers.
Aims.
We aimed to investigate this process observationally for galaxies outside of clusters.
Methods.
We made use of the fact that different merger types leave different traces that have different lifetimes. We statistically analyzed multiple characteristics of galaxies that are expected to be influenced by mergers, such as tidal features, kinematically distinct cores, and stellar ages. They were taken from the MATLAS and ATLAS
3D
databases. Through multilinear regression we identified the quantities that, at a fixed mass and environmental density of the galaxy, significantly correlate with a measure of the ordered rotation of the galaxy,
λ
R
e
N
.
Results.
We found a negative correlation of the rotational support with the occurrence of tidal disturbances and kinematic substructures, and a positive correlation with metallicity and metallicity gradients. For massive galaxies, the rotational support correlates negatively with the abundance of
α
-elements, and for the galaxies in low-density environments, it correlates negatively with the central photometric cuspiness. These and additional literature observational constraints are explained the easiest if the mergers that decreased the rotational support of ETGs were typically minor, wet, and happening at
z
≈ 2. They did not form the currently observed tidal features. The observed frequency of tidal features implies a merging rate of 0.07–0.2 per Gyr. This is insufficient to explain the observed growth of the radii of ETGs with redshift by mergers.
Context.
Observations of neutral hydrogen (H I) and molecular gas show that 50% of all nearby early-type galaxies contain some cold gas. Molecular gas is always found in small gas discs in the ...central region of the galaxy, while neutral hydrogen is often distributed in a low-column density disc or ring typically extending well beyond the stellar body. Dust is frequently found in early-type galaxies as well.
Aims.
The goal of our study is to understand the link between dust and cold gas in nearby early-type galaxies as a function of H I content.
Methods.
We analyse deep optical
g
−
r
images obtained with the MegaCam camera at the Canada-France-Hawaii Telescope for a sample of 21 H I-rich and 41 H I-poor early-type galaxies.
Results.
We find that all H I-rich galaxies contain dust seen as absorption. Moreover, in 57% of these H I-rich galaxies, the dust is distributed in a large-scale spiral pattern. Although the dust detection rate is relatively high in the H I-poor galaxies (∼59%), most of these systems exhibit simpler dust morphologies without any evidence of spiral structures. We find that the H I-rich galaxies possess more complex dust morphology extending to almost two times larger radii than H I-poor objects. We measured the dust content of the galaxies from the optical colour excess and find that H I-rich galaxies contain six times more dust (in mass) than H I-poor ones. In order to maintain the dust structures in the galaxies, continuous gas accretion is needed, and the substantial H I gas reservoirs in the outer regions of early-type galaxies can satisfy this need for a long time. We find that there is a good correspondence between the observed masses of the gas and dust, and it is also clear that dust is present in regions further than 3
R
eff
.
Conclusions.
Our findings indicate an essential relation between the presence of cold gas and dust in early-type galaxies and offer a way to study the interstellar medium in more detail than what is possible with H I observations.
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