We study resolution effects in numerical simulations of gas-rich and gas-poor major mergers, and show that the formation of slowly rotating elliptical galaxies often requires a resolution that is ...beyond the present-day standards to be properly modelled. Our sample of equal-mass merger models encompasses various masses and spatial resolutions, ranging from about 200 pc and 105 particles per component (stars, gas and dark matter), i.e. a gas mass resolution of ∼105 M⊙, typical of some recently published major merger simulations, to up to 32 pc and ∼103 M⊙ in simulations using 2.4 × 107 collisionless particles and 1.2 × 107 gas particles, among the highest resolutions reached so far for gas-rich major merger of massive disc galaxies. We find that the formation of fast-rotating early-type galaxies, that are flattened by a significant residual rotation, is overall correctly reproduced at all such resolutions. However, the formation of slow-rotating early-type galaxies, which have a low-residual angular momentum and are supported mostly by anisotropic velocity dispersions, is strongly resolution-dependent. The evacuation of angular momentum from the main stellar body is largely missed at standard resolution, and systems that should be slow rotators are then found to be fast rotators. The effect is most important for gas-rich mergers, but is also witnessed in mergers with an absent or modest gas component (0–10 per cent in mass). The effect is robust with respect to our initial conditions and interaction orbits, and originates in the physical treatment of the relaxation process during the coalescence of the galaxies. Our findings show that a high-enough resolution is required to accurately model the global properties of merger remnants and the evolution of their angular momentum. The role of gas-rich mergers of spiral galaxies in the formation of slow-rotating ellipticals may therefore have been underestimated. Moreover, the effect of gas in a galaxy merger is not limited to helping the survival/rebuilding of rotating disc components: at high resolution, gas actively participates in the relaxation process and the formation of slowly rotating stellar systems.
Approximately 20 per cent of early-type galaxies host small nuclear stellar discs that are tens to a few hundred parsecs in size. Such discs are expected to be easily disrupted during major galactic ...encounters, hence their age serve to constrain their assembly history. We use VIsible MultiObject Spectrograph integral-field spectroscopic observations for the intermediate-mass E0 galaxy NGC 4458 and age-date its nuclear disc via high-resolution fitting of various model spectra. We find that the nuclear disc is at least 6 Gyr old. A clue to gain narrow limits to the stellar age is our knowledge of the nuclear disc contribution to the central surface brightness. The presence of an old nuclear disc, or the absence of disruptive encounters since z ∼ 0.6, for a small galaxy such as NGC 4458 which belongs to the Virgo cluster, may be consistent with a hierarchical picture for galaxy formation where the smallest galaxies assembles earlier and the crowded galactic environments reduce the incidence of galaxy mergers. On the other hand, NGC 4458 displays little or no bulk rotation except for a central kpc-scale kinematically decoupled core. Slow rotation and decoupled core are usually explained in terms of mergers. The presence and age of the nuclear disc constraint these mergers to have happened at high redshift.
The hypervelocity OB stars in the Milky Way Galaxy were ejected from the central regions some 10–100 million years ago. We argue that these stars, as well as many more abundant bound OB stars in the ...innermost few parsecs, were generated by the interactions of an AGN jet from the central black hole with a dense molecular cloud. Considerations of the associated energy and momentum injection have broader implications for the possible origin of the Fermi bubbles and for the enrichment of the intergalactic medium.
Nuclear stellar discs (NSDs), of a few tens to hundreds of parsec across, are a common and yet poorly studied feature of early-type galaxies. Still, such small discs represent a powerful tool to ...constrain the assembling history of galaxies, since they can be used to trace to the epoch when galaxies experienced their last major merger event. By studying the fraction and stellar age of NSDs, it is thus possible to test the predictions for the assembly history of early-type galaxies according to the current hierarchical paradigm for galaxy formation. In this paper we have produced the most comprehensive census of NSDs in nearby early-type galaxies by searching for such discs in objects within 100 Mpc and by using archival images from the Hubble Space Telescope. We found that NSDs are present in approximately 20 per cent of early-type galaxies, and that the fraction of galaxies with NSDs depends neither on their Hubble type nor on their galactic environment, whereas the incidence of NSDs appears to decline in the most massive systems. Furthermore, we have separated the light contribution of 12 such discs from that of their surrounding stellar bulge in order to extract their physical properties. This doubles the number of decomposed NSDs and although the derived values for their central surface brightness and scalelength are consistent with previous studies, they also give a hint of possible different characteristics due to different formation scenario between nuclear discs and other kinds of large galactic discs.
We combine deep optical and NIR (UBVRIzJK) photometry from the Multiwavelength Survey by Yale–Chile (MUSYC) with redshifts from the COMBO-17 survey to perform a large-scale study of the rest-frame ...ultraviolet (UV) properties of 674 high-redshift (0.5 < z < 1) early-type galaxies, drawn from the Extended Chandra Deep Field-South (E-CDFS). Galaxy morphologies are determined through visual inspection of Hubble Space Telescope (HST) images taken from the GEMS survey. We harness the sensitivity of the UV to young (<1-Gyr old) stars to quantify the recent star formation history of early-type galaxies across a range of luminosities −23.5 < M(V) < −18. Comparisons to simple stellar populations forming at high redshift indicate that ∼1.1 per cent of early-types in this sample are consistent with purely passive ageing since z= 2 – this value drops to ∼0.24 per cent and ∼0.15 per cent for z= 3 and 5, respectively. Parametrizing the recent star formation (RSF) in terms of the mass fraction of stars less than a Gyr old, we find that the early-type population as a whole shows a typical RSF between 5 and 13 per cent in the redshift range 0.5 < z < 1. Early-types on the broad UV ‘red sequence’ show RSF values less than 5 per cent, while the reddest early-types (which are also the most luminous) are virtually quiescent with RSF values of ∼1 per cent. In contrast to their low-redshift (z < 0.1) counterparts, the high-redshift early-types in this sample show a pronounced bimodality in the rest-frame UV–optical colour, with a minor but significant peak centred on the blue cloud. Furthermore, star formation in the most active early-types is a factor of 2 greater at z∼ 0.7 than in the local universe. Given that evolved sources of UV flux (e.g. horizontal branch stars) should be absent at z > 0.5, implying that the UV is dominated by young stars, we find compelling evidence that early-types of all luminosities form stars over the lifetime of the Universe, although the bulk of their star formation is already complete at high redshift. This ‘tail-end’ of star formation is measurable and not negligible, with luminous −23 < M(V) < −20.5 early-types potentially forming 10–15 per cent of their mass since z= 1, with less luminous early-types M(V) > −20.5 potentially forming 30–60 per cent of their mass after z= 1. This, in turn, implies that intermediate-age stellar populations should be abundant in local early-type galaxies, as expected in hierarchical cosmology.
We investigate the evolution of the faint-end slope of the luminosity function, alpha , using semianalytical modeling of galaxy formation. In agreement with observations, we find that the slope can ...be fitted well by alpha (z) = alpha + bz, with alpha = -1.13 and b = -0.1. The main driver for the evolution in a is the evolution in the underlying dark matter mass function. Sub-L unk galaxies reside in dark matter halos that occupy a different part of the mass function. This part of the mass function is steeper at high redshifts than at low redshifts, and hence a is steeper. Supernova feedback in general causes the same relative flattening with respect to the dark matter mass function. The faint-end slope at low redshifts is dominated by field galaxies, and at high redshifts by cluster galaxies. The evolution of alpha (z) in each of these environments is different, with field galaxies having a slope b = -0.14 and cluster galaxies having a slope b = -0.05. The transition from a cluster-dominated to a field-dominated faint-end slope occurs roughly at a redshift z. unk 2 and suggests that a single linear fit to the overall evolution of alpha (z) might not be appropriate. Furthermore, this result indicates that tidal disruption of dwarf galaxies in clusters cannot play a significant role in explaining the evolution of alpha (z) at z < z. In addition, we find that different star formation efficiencies alpha . in the Schmidt-Kennicutt law and supernova-feedback efficiencies e generally do not strongly influence the evolution of alpha (z).
We use semianalytical modeling of galaxy formation to predict the redshift-size evolution of elliptical galaxies. Using a simple model in which relative sizes of elliptical galaxies of a given mass ...correlate with the fraction of stars formed in a starburst during a major merger event, we are able to reproduce the observed redshift-size evolution. The size evolution is a result of the amount of cold gas available during the major merger. Mergers at high redshifts are gas-rich and produce ellipticals with smaller sizes. In particular, we find a power-law relation between the sizes at different redshifts, with the power-law index giving a measure of the relative amount of dissipation during the mergers that lead to the formation of an elliptical. The size evolution is found to be stronger for more massive galaxies as they involve more gas at high redshifts when they form, compared to less massive ellipticals. Local ellipticals more massive than 5 x 10 super(11) M sub( )will be approximately 4 times larger than their counterparts at z = 2. Our results indicate that the scatter in the size of similar massive present-day elliptical galaxies is a result of their formation epoch, with smaller ellipticals being formed earlier.
We present a detailed two-dimensional stellar dynamical analysis of a sample of 44 cosmological hydrodynamical simulations of individual central galaxies with stellar masses of 2 × 1010 M⊙ ≲ M
...* ≲ 6 × 1011 M⊙. Kinematic maps of the stellar line-of-sight velocity, velocity dispersion and higher order Gauss–Hermite moments h
3 and h
4 are constructed for each central galaxy and for the most massive satellites. The amount of rotation is quantified using the λR-parameter. The velocity, velocity dispersion, h
3 and h
4 fields of the simulated galaxies show a diversity similar to observed kinematic maps of early-type galaxies in the ATLAS3D survey. This includes fast (regular), slow and misaligned rotation, hot spheroids with embedded cold disc components as well as galaxies with counter-rotating cores or central depressions in the velocity dispersion. We link the present-day kinematic properties to the individual cosmological formation histories of the galaxies. In general, major galaxy mergers have a significant influence on the rotation properties resulting in both a spin-down as well as a spin-up of the merger remnant. Lower mass galaxies with significant (≳18 per cent) in situ formation of stars since z ≈ 2, or with additional gas-rich major mergers – resulting in a spin-up – in their formation history, form elongated (ϵ ∼ 0.45) fast rotators (λR ∼ 0.46) with a clear anticorrelation of h
3 and v/σ. An additional formation path for fast rotators includes gas-poor major mergers leading to a spin-up of the remnants (λR ∼ 0.43). This formation path does not result in anticorrelated h
3 and v/σ. The formation histories of slow rotators can include late major mergers. If the merger is gas rich, the remnant typically is a less flattened slow rotator with a central dip in the velocity dispersion. If the merger is gas poor, the remnant is very elongated (ϵ ∼ 0.43) and slowly rotating (λR ∼ 0.11). The galaxies most consistent with the rare class of non-rotating round early-type galaxies grow by gas-poor minor mergers alone. In general, more massive galaxies have less in situ star formation since z ∼ 2, rotate slower and have older stellar populations. We discuss general implications for the formation of fast and slowly rotating galaxies as well as the weaknesses and strengths of the underlying models.
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