55 research outputs found
The colour-magnitude relation of Globular Clusters in Centaurus and Hydra - Constraints on star cluster self-enrichment with a link to massive Milky Way GCs
We investigate the colour-magnitude relation of metal-poor globular clusters,
the 'blue tilt', in the Hydra and Centaurus galaxy clusters and constrain the
primordial conditions for star cluster self-enrichment. We analyse U,I
photometry for about 2500 globular clusters in the central regions of Hydra and
Centaurus, based on FORS1@VLT data. We convert the measured colour-magnitude
relations into mass-metallicity space and obtain a scaling of Z \propto M^{0.27
\pm 0.05} for Centaurus GCs and Z \propto M^{0.40 \pm 0.06} for Hydra GCs,
consistent with results in other environments. We find that the GC
mass-metallicity relation already sets in at present-day masses of a few 10^5
solar masses and is well established in the luminosity range of massive MW
clusters like omega Centauri. We compare the mass-metallicity relation with
predictions from the star cluster self-enrichment model by Bailin & Harris
(2009). For this we include effects of dynamical and stellar evolution and a
physically well motivated primordial mass-radius scaling. The self-enrichment
model reproduces the observed relations well for average primordial half-light
radii r_h ~ 1-1.5 pc, star formation efficiencies f_* ~ 0.3-0.4, and
pre-enrichment levels of [Fe/H] ~ -1.7 dex. Within the self-enrichment
scenario, the observed blue tilt implies a correlation between GC mass and
width of the stellar metallicity distribution. We find that this implied
correlation matches the trend of width with GC mass measured in Galactic GCs,
including extreme cases like omega Cen and M54. We conclude that 1. A
primordial star cluster mass-radius relation provides a significant improvement
to the self-enrichment model fits. 2. Broadenend metallicity distributions as
found in some massive MW globular clusters may have arisen naturally from
self-enrichment processes, without the need of a dwarf galaxy progenitor.Comment: 15 pages, 13 figures. Language edited version of paper accepted for
publication in Astronomy & Astrophysics. Colour-composite in Figure 1 reduced
in resolutio
Ionization processes in a local analogue of distant clumpy galaxies: VLT MUSE IFU spectroscopy and FORS deep images of the TDG NGC 5291N
We present IFU observations with MUSE@VLT and deep imaging with FORS@VLT of a
dwarf galaxy recently formed within the giant collisional HI ring surrounding
NGC 5291. This TDG-like object has the characteristics of typical z=1-2
gas-rich spiral galaxies: a high gas fraction, a rather turbulent clumpy ISM,
the absence of an old stellar population, a moderate metallicity and star
formation efficiency. The MUSE spectra allow us to determine the physical
conditions within the various complex substructures revealed by the deep
optical images, and to scrutinize at unprecedented spatial resolution the
ionization processes at play in this specific medium. Starburst age, extinction
and metallicity maps of the TDG and surrounding regions were determined using
the strong emission lines Hbeta, [OIII], [OI], [NII], Halpha and [SII] combined
with empirical diagnostics. Discrimination between different ionization
mechanisms was made using BPT--like diagrams and shock plus photoionization
models. Globally, the physical conditions within the star--forming regions are
homogeneous, with in particular an uniform half-solar oxygen abundance. At
small scales, the derived extinction map shows narrow dust lanes. Regions with
atypically strong [OI] emission line immediately surround the TDG. The [OI] /
Halpha ratio cannot be easily accounted for by photoionization by young stars
or shock models. At larger distances from the main star--forming clumps, a
faint diffuse blue continuum emission is observed, both with the deep FORS
images and MUSE data. It does not have a clear counterpart in the UV regime
probed by GALEX. A stacked spectrum towards this region does not exhibit any
emission line, excluding faint levels of star formation, nor stellar absorption
lines that might have revealed the presence of old stars. Several hypotheses
are discussed for the origin of these intriguing features.Comment: 13 pages, 15 figures, accepted for publication in A&
A contribution of star-forming clumps and accreting satellites to the mass assembly of z ⌠2 galaxies
We investigate the contribution of clumps and satellites to the galaxy mass assembly. We analysed spatially resolved HubbleSpace Telescope observations (imaging and slitless spectroscopy) of 53 star-forming galaxies at z ⌠1â3. We created continuum and emission line maps and pinpointed residual âblobsâ detected after subtracting the galaxy disc. Those were separated into compact (unresolved) and extended (resolved) components. Extended components have sizes âŒ2 kpc and comparable stellar mass and age as the galaxy discs, whereas the compact components are 1.5 dex less massive and 0.4 dex younger than the discs. Furthermore, the extended blobs are typically found at larger distances from the galaxy barycentre than the compact ones. Prompted by these observations and by the comparison with simulations, we suggest that compact blobs are in situ formed clumps, whereas the extended ones are accreting satellites. Clumps and satellites enclose, respectively, âŒ20 perâcent and âČ80 perâcent of the galaxy stellar mass, âŒ30 perâcent and âŒ20 perâcent of its star formation rate. Considering the compact blobs, we statistically estimated that massive clumps (Mâ âł 109 Mâ) have lifetimes of âŒ650 Myr, and the less massive ones (108 < Mâ < 109 Mâ) of âŒ145 Myr. This supports simulations predicting long-lived clumps (lifetime âł 100 Myr). Finally, âČ30 perâcent (13 perâcent) of our sample galaxies are undergoing single (multiple) merger(s), they have a projected separation âČ10 kpc, and the typical mass ratio of our satellites is 1:5 (but ranges between 1:10 and 1:1), in agreement with literature results for close pair galaxies
A titanic interstellar medium ejection from a massive starburst galaxy at redshift 1.4
Feedback-driven winds from star formation or active galactic nuclei might be a relevant channel for the abrupt quenching of star formation in massive galaxies. However, both observations and simulations support the idea that these processes are non-conflictingly co-evolving and self-regulating. Furthermore, evidence of disruptive events that are capable of fast quenching is rare, and constraints on their statistical prevalence are lacking. Here we present a massive starburst galaxy at redshift z = 1.4, which is ejecting 46 ± 13% of its molecular gas mass at a startling rate of âł10,000 Mâ yrâ1. A broad component that is red-shifted from the galaxy emission is detected in four (low and high J) CO and [C i] transitions and in the ionized phase, which ensures a robust estimate of the expelled gas mass. The implied statistics suggest that similar events are potentially a major star-formation quenching channel. However, our observations provide compelling evidence that this is not a feedback-driven wind, but rather material from a merger that has been probably tidally ejected. This finding challenges some literature studies in which the role of feedback-driven winds might be overstated
Molecular and Ionized Gas in Tidal Dwarf Galaxies: The Spatially Resolved Star-Formation Relation
Tidal dwarf galaxies (TDGs) are low-mass objects that form within tidal
and/or collisional debris ejected from more massive interacting galaxies. We
use CO() observations from ALMA and integral-field spectroscopy from MUSE
to study molecular and ionized gas in three TDGs: two around the collisional
galaxy NGC 5291 and one in the late-stage merger NGC 7252. The CO and H
emission is more compact than the HI emission and displaced from the HI
dynamical center, so these gas phases cannot be used to study the internal
dynamics of TDGs. We use CO, HI, and H data to measure the surface
densities of molecular gas (), atomic gas () and star-formation rate (), respectively. We confirm
that TDGs follow the same spatially integrated relation of regular galaxies, where , even though they are HI dominated. We find a more complex
behaviour in terms of the spatially resolved relation on sub-kpc scales. The majority (60) of SF regions in
TDGs lie on the same relation of normal
spiral galaxies but show a higher dispersion around the mean. The remaining
fraction of SF regions (40) lie in the starburst region and are
associated with the formation of massive super star clusters, as shown by
Hubble Space Telescope images. We conclude that the local SF activity in TDGs
proceeds in a hybrid fashion, with some regions comparable to normal spiral
galaxies and others to extreme starbursts.Comment: 11 pages, 6 figures, 3 tables, Accepted for publication in MNRA
Stellar feedback in a clumpy galaxy at z ⌠3.4
Giant star-forming regions (clumps) are widespread features of galaxies at z â 1â4. Theory predicts that they can play a crucial role in galaxy evolution, if they survive to stellar feedback for >50 Myr. Numerical simulations show that clumpsâ survival depends on the stellar feedback recipes that are adopted. Up to date, observational constraints on both clumpsâ outflows strength and gas removal time-scale are still uncertain. In this context, we study a line-emitting galaxy at redshift z â 3.4 lensed by the foreground galaxy cluster Abell 2895. Four compact clumps with sizes âČ280 pc and representative of the low-mass end of clumpsâ mass distribution (stellar masses âČ2 Ă 108âMâ) dominate the galaxy morphology. The clumps are likely forming stars in a starbursting mode and have a young stellar population (âŒ10 Myr). The properties of the Lyman-α (Lyα) emission and nebular far-ultraviolet absorption lines indicate the presence of ejected material with global outflowing velocities of âŒ200â300 kmâsâ1. Assuming that the detected outflows are the consequence of star formation feedback, we infer an average mass loading factor (η) for the clumps of âŒ1.8â2.4 consistent with results obtained from hydrodynamical simulations of clumpy galaxies that assume relatively strong stellar feedback. Assuming no gas inflows (semiclosed box model), the estimates of η suggest that the time-scale over which the outflows expel the molecular gas reservoir (â7 Ă 108âMâ) of the four detected low-mass clumps is âČ50 Myr
Spectroscopic study of MATLAS-2019 with MUSE:An ultra-diffuse galaxy with an excess of old globular clusters
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 a low-surface brightness. Among these so-called ultra-diffuse
galaxies, a few seem to host an excess of globular clusters. With the
integral-field unit spectrograph MUSE we have observed one of these galaxies -
MATLAS J15052031+0148447 (MATLAS-2019) - located towards the nearby group NGC
5846 and measured its systemic velocity,age, and metallicity, and that of its
globular clusters candidates. For the stellar body of MATLAS-2019 we derive a
metallicity of -1.33+0.19-0.01 dex and an age of 11.2+1.8-0.8 Gyr. For some of
the individual GCs and the stacked GC population, we derive consistent ages and
metallicities. From the 11 confirmed globular clusters and using a Markov Chain
Monte Carlo approach we derived a dynamical mass-to-light ratio of
4.2+8.6-3.4M/L. This is at the lower end of the luminosity-mass scaling
relation defined by the Local Group dwarf galaxies. Furthermore, we couldn't
confirm nor reject the possibility of a rotational component of the GC system.
If present, this would further modify the inferred mass. Follow-up observations
of the globular cluster population and of the stellar body of the galaxy are
needed to assess whether this galaxy is lacking dark matter like it was
suggested for the pair of dwarf galaxies in the field of NGC 1052, or if this
is a miss-interpretation arising from systematic uncertainties of the method
commonly used for these systems and the large uncertainties of the individual
globular cluster velocities.Comment: 10 pages, 9 figures, 2 tables, accepted for publication in A&
The limited role of galaxy mergers in driving stellar mass growth over cosmic time
© The Author(s) 2017. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. Published by Oxford University Press on behalf of the Royal Astronomical Society.A key unresolved question is the role that galaxy mergers play in driving stellar mass growth over cosmic time. Recent observational work hints at the possibility that the overall contribution of `major' mergers (mass ratios 1:4) to cosmic stellar mass growth may be small, because they enhance star formation rates by relatively small amounts at high redshift, when much of today's stellar mass was assembled. However, the heterogeneity and relatively small size of today's datasets, coupled with the difficulty in identifying genuine mergers, makes it challenging to quantify the merger contribution to stellar mass growth. Here, we use Horizon-AGN, a cosmological hydrodynamical simulation, to comprehensively quantify the contribution of mergers to the star formation budget over the lifetime of the Universe. We show that: (1) both major and minor mergers enhance star formation to similar amounts, (2) the fraction of star formation directly attributable to merging is small at all redshifts (e.g. 35 and 20 per cent at z3 and z1 respectively) and (3) only 25 per cent of today's stellar mass is directly attributable to galaxy mergers over cosmic time. Our results suggest that smooth accretion, not merging, is the dominant driver of stellar mass growth over the lifetime of the Universe.Peer reviewedFinal Published versio
Normal black holes in bulge-less galaxies: the largely quiescent, merger-free growth of black holes over cosmic time
© 2018 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (http://creativecommons.org/licenses/by/4.0/).Understanding the processes that drive the formation of black holes (BHs) is a key topic in observational cosmology. While the observed M BH-M Bulge correlation in bulge-dominated galaxies is thought to be produced by major mergers, the existence of an M BH-M* relation, across all galaxy morphological types, suggests that BHs may be largely built by secular processes. Recent evidence that bulge-less galaxies, which are unlikely to have had significant mergers, are offset from the M BH-M Bulge relation, but lie on the M BH-M* relation, has strengthened this hypothesis. Nevertheless, the small size and heterogeneity of current data sets, coupled with the difficulty in measuring precise BH masses, make it challenging to address this issue using empirical studies alone. Here, we use Horizon-AGN, a cosmological hydrodynamical simulation to probe the role of mergers in BH growth over cosmic time. We show that (1) as suggested by observations, simulated bulge-less galaxies lie offset from the main M BH-M Bulge relation, but on the M BH-M* relation, (2) the positions of galaxies on the M BH-M* relation are not affected by their merger histories, and (3) only ~35 per cent of the BH mass in today's massive galaxies is directly attributable to merging - the majority (~65 per cent) of BH growth, therefore, takes place gradually, via secular processes, over cosmic time.Peer reviewedFinal Published versio
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