While the formation of stellar clumps in distant galaxies is usually attributed to gravitational violent disk instabilities, we show here that major mergers also represent a competitive mechanism to ...form bright clumps. Using ∼0.1″ resolution ACS F814W images in the entire COSMOS field, we measured the fraction of clumpy emission in 109 main sequence (MS) and 79 Herschel-detected starbursts (off-MS) galaxies at 0.5 < z < 0.9, representative of normal versus merger induced star-forming activity, respectively. We additionally identify merger samples from visual inspection and from Gini-M20 morphological parameters. Regardless of the merger criteria adopted, the clumpiness distribution of merging systems is different from that of normal isolated disks at a > 99.5% confidence level. The former reaches higher clumpiness values up to 20% of the total galaxy emission. We confirm the merger induced clumpiness enhancement with novel hydrodynamical simulations of colliding galaxies with gas fractions typical of z ∼ 0.7. Multi-wavelength images of three starbursts in the CANDELS field support the young nature of clumps, which are likely merger products rather than older preexisting structures. Finally, for a subset of 19 starbursts with existing near-infrared rest frame spectroscopy, we find that the clumpiness is mildly anti-correlated with the merger phase, which decreases toward final coalescence. Our result can explain recent ALMA detections of clumps in hyperluminous high-z starbursts, while normal objects are smooth. This work raises a question as to the role of mergers on the origin of clumps in high redshift galaxies in general.
ABSTRACT
High-redshift star-forming galaxies often have irregular morphologies with giant clumps containing up to 108−109 solar masses of gas and stars. The origin and evolution of giant clumps are ...debated both theoretically and observationally. In most cosmological simulations, high-redshift galaxies have regular spiral structures or short-lived clumps, in contradiction with many idealized high-redshift disc models. Here, we test whether this discrepancy can be explained by the low gas fractions of galaxies in cosmological simulations. We present a series of simulations with varying gas fractions, from 25 per cent, typical of galaxies in most cosmological simulations, to 50 per cent, typical of observed galaxies at 1.5 < z < 3. We find that gas-poor models have short-lived clumps, that are unbound and mostly destroyed by galactic shear, even with weak stellar feedback. In contrast, gas-rich models form long-lived clumps even with boosted stellar feedback. This shows that the gas mass fraction is the primary physical parameter driving violent disc instabilities and the evolution of giant clumps on ∼108 yr time-scales, with lower impact from the calibration of the stellar feedback. Many cosmological simulations of galaxy formation have relatively gas-poor galactic discs, which could explain why giant clumps are absent or short-lived in such models. Similar baryonic and dark matter mass distribution could produce clumpy galaxies with long-lived clumps at z ∼ 2 if the gas fraction was in better agreement with observations.
ABSTRACT
High-redshift star-forming galaxies have very different morphologies compared to nearby ones. Indeed, they are often dominated by bright star-forming structures of masses up to 108–9 M⊙ ...dubbed ‘giant clumps’. However, recent observations questioned this result by showing only low-mass structures or no structure at all. We use Adaptative Mesh Refinement hydrodynamical simulations of galaxies with parsec-scale resolution to study the formation of structures inside clumpy high-redshift galaxies. We show that in very gas-rich galaxies star formation occurs in small gas clusters with masses below 107–8 M⊙ that are themselves located inside giant complexes with masses up to 108 and sometimes 109 M⊙. Those massive structures are similar in mass and size to the giant clumps observed in imaging surveys, in particular with the Hubble Space Telescope. Using mock observations of simulated galaxies, we show that at very high resolution with instruments like the Atacama Large Millimeter Array or through gravitational lensing, only low-mass structures are likely to be detected, and their gathering into giant complexes might be missed. This leads to the non-detection of the giant clumps and therefore introduces a bias in the detection of these structures. We show that the simulated giant clumps can be gravitationally bound even when undetected in mocks representative for ALMA observations and HST observations of lensed galaxies. We then compare the top-down fragmentation of an initially warm disc and the bottom-up fragmentation of an initially cold disc to show that the process of formation of the clumps does not impact their physical properties.
The so-called ultra-diffuse galaxy NGC 1052-DF2 was announced to be a galaxy lacking dark matter based on a spectroscopic study of its constituent globular clusters. Here we present the first ...spectroscopic analysis of the stellar body of this galaxy using the MUSE integral-field spectrograph at the (ESO) Very Large Telescope. The MUSE datacube simultaneously provides DF2’s stellar velocity field and systemic velocities for seven globular clusters (GCs). We further discovered three planetary nebulae (PNe) that are likely part of this galaxy. While five of the clusters had velocities measured in the literature, we were able to confirm the membership of two more candidates through precise radial velocity measurements, which increases the measured specific frequency of GCs in DF2. The mean velocity of the diffuse stellar body, 1792.9
+1.4
−1.8
km s
−1
, is consistent with the mean globular cluster velocity. We detect a weak but significant velocity gradient within the stellar body, with a kinematic axis close to the photometric major axis, making it a prolate-like rotator. We estimate a velocity dispersion from the clusters and PNe of
σ
int
= 10.6
−2.3
+3.9
km s
−1
. The velocity dispersion
σ
DF2⋆
(
R
e
) for the stellar body within one effective radius is 10.8
+3.2
−4.0
km s
−1
. Considering various sources of systemic uncertainties, this central value varies between 5 and 13 km s
−1
, and we conservatively report a 95% confidence upper limit to the dispersion within one
R
e
of 21 km s
−1
. We provide updated mass estimates based on these dispersions corresponding to the different distances to NGC 1052-DF2 that have been reported in the recent literature.
While interstellar gas is known to be supersonically turbulent, the injection processes of this turbulence are still unclear. Many studies suggest a dominant role of gravitational instabilities. ...However, their effect on galaxy morphology and large-scale dynamics varies across cosmic times, in particular, due to the evolution of the gas fraction of galaxies. In this paper, we propose numerical simulations to follow the isothermal turbulent cascade of purely gravitationally driven turbulence from its injection scale down to 0.095 pc for a gas-poor spiral disk and a gas-rich clumpy disk. For this purpose, and to lift the memory-footprint technical lock of sufficiently resolving the interstellar medium of a galaxy, we developed an encapsulated zoom method that allows us to self-consistently probe the self-generated turbulence cascade over three orders of magnitude on spatial scales. We followed this cascade for 10 Myr. We find that the turbulent cascade follows the same scaling laws in both setups. Namely, in both cases, the turbulence is close to equipartition between its compressive and solenoidal modes, the velocity power spectrum follows the Burgers scaling, and the density power spectrum is rather shallow, with a power-law slope of −0.7. Last, gravitationally bound substructures follow a mass distribution with a −1.8 slope, similar to that of CO clumps. These simulations thus suggest that gravity-driven isothermal turbulent cascades are universal in disk galaxies across cosmic time.
Recent advances in deep dedicated imaging surveys over the past decade have uncovered a surprisingly large number of extremely faint low surface brightness galaxies with large physical sizes called ...ultra diffuse galaxies (UDGs) in clusters and, more recently, in lower density environments. As part of the Mass Assembly of early-Type GaLAxies with their fine Structures (MATLAS) survey, a deep imaging large program at the Canada-France-Hawaii Telescope (CFHT), our team has identified 2210 dwarf galaxies, 59 (∼3%) of which qualify as UDGs. Averaging over the survey area, we find ∼0.4 UDG per square degree. They are found in a range of low to moderate density environments, although 61% of the sample fall within the virial radii of groups. Based on a detailed analysis of their photometric and structural properties, we find that the MATLAS UDGs do not show significant differences from the traditional dwarfs, except from the predefined size and surface brightness cut. Their median color is as red as the one measured in galaxy clusters, albeit with a narrower color range. The majority of the UDGs are visually classified as dwarf ellipticals with log stellar masses of ∼6.5 − 8.7. The fraction of nucleated UDGs (∼34%) is roughly the same as the nucleated fraction of the traditional dwarfs. Only five (∼8%) UDGs show signs of tidal disruption and only two are tidal dwarf galaxy candidates. A study of globular cluster (GC) candidates selected in the CFHT images finds no evidence of a higher GC specific frequency
S
N
for UDGs than for classical dwarfs, contrary to what is found in most clusters. The UDG halo-to-stellar mass ratio distribution, as estimated from the GC counts, peaks at roughly the same value as for the traditional dwarfs, but spans the smaller range of ∼10 − 2000. We interpret these results to mean that the large majority of the field-to-group UDGs do not have a different formation scenario than traditional dwarfs.
We present the photometric properties of 2210 newly identified dwarf galaxy candidates in the MATLAS fields. The Mass Assembly of early Type gaLAxies with their fine Structures (MATLAS) deep imaging ...survey mapped ˜142 deg2 of the sky around nearby isolated early type galaxies using MegaCam on the Canada-France-Hawaii Telescope, reaching surface brightnesses of ˜ 28.5 - 29 in the g-band. The dwarf candidates were identified through a direct visual inspection of the images and by visually cleaning a sample selected using a partially automated approach, and were morphologically classified at the time of identification. Approximately 75% of our candidates are dEs, indicating that a large number of early type dwarfs also populate low density environments, and 23.2% are nucleated. Distances were determined for 13.5% of our sample using pre-existing zspec measurements and HI detections. We confirm the dwarf nature for 99% of this sub-sample based on a magnitude cut Mg = -18. Additionally, most of these (˜90%) have relative velocities suggesting that they form a satellite population around nearby massive galaxies rather than an isolated field sample. Assuming that the candidates over the whole survey are satellites of the nearby galaxies, we demonstrate that the MATLAS dwarfs follow the same scaling relations as dwarfs in the Local Group as well as the Virgo and Fornax clusters. We also find that the nucleated fraction increases with Mg, and find evidence of a morphology-density relation for dwarfs around isolated massive galaxies.
ABSTRACT
We present a photometric study of the dwarf galaxy population in the low to moderate density environments of the MATLAS (Mass Assembly of early-Type gaLAxies with their fine Structures) deep ...imaging survey. The sample consists of 2210 dwarfs, including 508 nucleated. We define a nucleus as a compact source that is close to the galaxy photocentre (within 0.5 $R_\mathrm{ e}$) which is also the brightest such source within the galaxy’s effective radius. The morphological analysis is performed using a 2D surface brightness profile modelling on the g-band images of both the galaxies and nuclei. Our study reveals that, for similar luminosities, the MATLAS dwarfs show ranges in the distribution of structural properties comparable to cluster (Virgo and Fornax) dwarfs and a range of sizes comparable to the Local Group and Local Volume dwarfs. Colour measurements using the r- and i-band images indicate that the dwarfs in low and moderate density environments are as red as cluster dwarfs on average. The observed similarities between dwarf ellipticals in vastly different environments imply that dEs are not uniquely the product of morphological transformation due to ram-pressure stripping and galaxy harassment in high density environments. We measure that the dwarf nuclei are located predominantly in massive, bright and round dwarfs and observe fewer nuclei in dwarfs with a faint centre and a small size. The colour of the galaxy nucleus shows no clear relation to the colour of the dwarf, in agreement with the migration and wet migration nucleus formation scenarios. The catalogues of the MATLAS dwarfs photometric and structural properties are provided.
The MATLAS deep imaging survey has uncovered a plethora of dwarf galaxies in the low density environment it has mapped. A fraction of them are unusually extended and have low surface brightness. ...Among these so-called ultra-diffuse galaxies, a few seem to host an excess of globular clusters (GCs). With the integral field unit spectrograph MUSE we have observed one of these galaxies – MATLAS J15052031+0148447 (MATLAS-2019) – located toward the nearby group NGC 5846 and measured its systemic velocity, age, and metallicity, and that of its GC candidates. For the stellar body of MATLAS-2019 we derive a metallicity of −1.33
−0.01
+0.19
dex and an age of 11.2
−0.8
+1.8
Gyr. For some of the individual GCs and the stacked GC population, we derive consistent ages and metallicities. From the 11 confirmed GCs and using a Markov Chain Monte Carlo approach we derived a dynamical mass-to-light ratio of 4.2
−3.4
+8.6
M
⊙
/
L
⊙
. This is at the lower end of the luminosity-mass scaling relation defined by the Local Group dwarf galaxies. Furthermore, we could not confirm or reject the possibility of a rotational component in the GC system. If present, this would further modify the inferred mass. Follow-up observations of the GC population and of the stellar body of the galaxy are needed to assess whether this galaxy lacks dark matter, as was suggested for the pair of dwarf galaxies in the field of NGC 1052, or if this is a misinterpretation arising from systematic uncertainties of the method commonly used for these systems and the large uncertainties of the individual GC velocities.
Stellar shells around galaxies could provide precious insights into their assembly history. However, their formation mechanism remains poorly empirically constrained, regarding in particular the type ...of galaxy collisions at their origin. We present MUSE at VLT data of the most prominent outer shell of NGC 474, to constrain its formation history. The stellar shell spectrum is clearly detected, with a signal-to-noise ratio of ∼65 pix
−1
. We used a full spectral fitting method to determine the line-of-sight velocity and the age and metallicity of the shell and associated point-like sources within the MUSE field of view. We detect six globular cluster (GC) candidates and eight planetary nebula (PN) candidates that are all kinematically associated with the stellar shell. We show that the shell has an intermediate metallicity, M/H = −0.83
−0.12
+0.12
, and a possible
α
-enrichment,
α
/Fe ∼ 0.3. Assuming the material of the shell comes from a lower mass companion, and that the latter had no initial metallicity gradient, such a stellar metallicity would constrain the mass of the progenitor at around 7.4 × 10
8
M
⊙
, implying a merger mass ratio of about 1:100. However, our census of PNe and earlier photometry of the shell would suggest a much higher ratio, around 1:20. Given the uncertainties, this difference is only significant at the ≃1
σ
level. We discuss the characteristics of the progenitor, and in particular whether the progenitor could also be composed of stars from the low-metallicity outskirts of a more massive galaxy. Ultimately, the presented data do not allow us to put a firm constraint on the progenitor mass. We show that at least two GC candidates possibly associated with the shell are quite young, with ages below 1.5 Gyr. We also note the presence of a young (∼1 Gyr) stellar population in the center of NGC 474. The two may have resulted from the same event.
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