Abstract
Ever since a thick disk was proposed to explain the vertical distribution of the Milky Way disk stars, its origin has been a recurrent question. We aim to answer this question by inspecting ...19 disk galaxies with stellar mass greater than 10
10
M
⊙
in recent cosmological high-resolution zoom-in simulations:
galactica
and
NewHorizon
. The thin and thick disks are reasonably reproduced by the simulations with scale heights and luminosity ratios as observed. We then spatially classify the thin and thick disks and find that the thick disk stars are older, metal-poorer, kinematically hotter, and higher in accreted star fraction, while both disks are dominated by the stars formed in situ. Half of the in situ stars in the thick disks are formed before the galaxies develop their disks, and the rest are formed in spatially and kinematically thinner disks and then thickened with time by heating. However, the 19 galaxies have various properties and evolutionary routes, highlighting the need for statistically large samples to draw general conclusions. We conclude from our simulations that the thin and thick disk components are not entirely distinct in terms of formation processes but rather markers of the evolution of galactic disks. Moreover, as the combined result of the thickening of the existing disk stars and the continued formation of young thin disk stars, the vertical distribution of stars does not change much after the disks settle, pointing to the modulation of both orbital diffusion and star formation by the same confounding factor: the proximity of galaxies to marginal stability.
The origin of the disk and spheroid of galaxies has been a key open question in understanding their morphology. Using the high-resolution cosmological simulation New Horizon, we explore kinematically ...decomposed disk and spheroidal components of 144 field galaxies with masses greater than at z = 0.7. The origins of stellar particles are classified according to their birthplace (in situ or ex situ) and their orbits at birth. Before disk settling, stars form mainly through chaotic mergers between protogalaxies and become part of the spheroidal component. When disk settling starts, we find that more massive galaxies begin to form disk stars from earlier epochs; massive galaxies commence to develop their disks at z ∼ 1-2, while low-mass galaxies do after z ∼ 1. The formation of disks is affected by accretion as well, as mergers can trigger gas turbulence or induce misaligned gas infall that hinders galaxies from forming corotating disk stars. The importance of accreted stars is greater in more massive galaxies, especially in developing massive spheroids. A significant fraction of the spheroids come from the disk stars that are perturbed, and this becomes more important at lower redshifts. Some (∼12.5%) of our massive galaxies develop counter-rotating disks from the gas infall misaligned with the existing disk plane, which can last for more than a gigayear until they become the dominant component and flip the angular momentum of the galaxy in the opposite direction. The final disk-to-total ratio of a galaxy needs to be understood in relation to its stellar mass and accretion history. We quantify the significance of the stars with different origins and provide them as guiding values.
Building galaxy merger trees from a state-of-the-art cosmological hydrodynamical simulation, Horizon-AGN, we perform a statistical study of how mergers and diffuse stellar mass acquisition processes ...drive galaxy morphologic properties above z > 1. By diffuse mass acquisition here, we mean both accretion of stars by unresolved mergers (relative stellar mass growth smaller than 4.5 per cent) as well as in situ star formation when no resolved mergers are detected along the main progenitor branch of a galaxy. We investigate how stellar densities, galaxy sizes and galaxy morphologies (defined via shape parameters derived from the inertia tensor of the stellar density) depend on mergers of different mass ratios. We investigate how stellar densities, effective radii and shape parameters derived from the inertia tensor depend on mergers of different mass ratios. We find strong evidence that diffuse stellar accretion and in situ formation tend to flatten small galaxies over cosmic time, leading to the formation of discs. On the other hand, mergers, and not only the major ones, exhibit a propensity to puff up and destroy stellar discs, confirming the origin of elliptical galaxies. We confirm that mergers grow galaxy sizes more efficiently than diffuse processes (r sub( 0.5)...M... and r sub( 0.5) ... M... on average, respectively) and we also find that elliptical galaxies are more susceptible to grow in size through mergers than disc galaxies with a size-mass evolution r sub( 0.5) ... M... instead of r sub( 0.5) ... M...-M super( 0.5) for discs depending on the merger mass ratio. The gas content drives the size-mass evolution due to merger with a faster size growth for gas-poor galaxies r sub( 0.5) ... M... than for gas-rich galaxies r sub( 0.5) ... M sub( s). (ProQuest: ... denotes formulae/symbols omitted.)
Radio AGN in spiral galaxies Kaviraj, Sugata; Shabala, Stanislav S; Deller, Adam T ...
Monthly notices of the Royal Astronomical Society,
12/2015, Letnik:
454, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Radio AGN in the nearby Universe are more likely to be found in galaxies with early-type morphology, the detection rate in spiral or late-type galaxies (LTGs) being around an order of magnitude ...lower. We combine the mJy Imaging VLBA Exploration at 20 cm (mJIVE-20) survey with the Sloan Digital Sky Survey, to study the relatively rare population of AGN in LTGs that have nuclear radio luminosities similar to that in their early-type counterparts. The LTG AGN population is preferentially hosted by galaxies that have high stellar masses (M* > 1010.8 M⊙), red colours and low star formation rates, with little dependence on the detailed morphology or local environment of the host LTG. The merger fraction in the LTG AGN is ∼4 times higher than that in the general LTG population, indicating that merging is an important trigger for radio AGN in these systems. The red colours of our systems extend recent work which indicates that merger-triggered AGN in the nearby Universe appear after the peak of the associated starburst, implying that they do not strongly regulate star formation. Finally, we find that in systems where parsec-scale jets are clearly observed in our very long baseline interferometry images, the jets are perpendicular to the major axis of the galaxy, indicating strong alignment between the accretion disc and the host galaxy stellar disc.
We exploit multiwavelength photometry of early-type galaxies (ETGs) in the Cosmological Evolution Survey (COSMOS) to demonstrate that the low-level star formation activity in the ETG population at ...intermediate redshift is likely to be driven by minor mergers. Splitting the ETGs into galaxies that show disturbed morphologies indicative of recent merging and those that appear relaxed, we find that similar to 32 per cent of the ETG population appears to be morphologically disturbed. While the relaxed objects are almost entirely contained within the UV red sequence, their morphologically disturbed counterparts dominate the scatter to blue UV colours, regardless of luminosity. Empirically and theoretically determined major-merger rates in the redshift range z < 1 are several times too low to account for the fraction of disturbed ETGs in our sample, suggesting that minor mergers represent the principal mechanism driving the observed star formation activity in our sample. The young stellar components forming in these events have ages between 0.03 and 0.3 Myr and typically contribute less than or equal to 10 per cent of the stellar mass of the remnant. Together with recent work which demonstrates that the structural evolution of nearby ETGs is consistent with one or more minor mergers, our results indicate that the overall evolution of massive ETGs may be heavily influenced by minor merging at late epochs and highlights the need to systematically study this process in future observational surveys.
A major amendment in recent models of hierarchical galaxy formation is the inclusion of so-called active galactic nucleus (AGN) feedback. The energy input from an active central massive black hole is ...invoked to suppress star formation in early-type galaxies at later epochs. A major problem is that this process is poorly understood, and compelling observational evidence for its mere existence is still missing. In search for signatures of AGN feedback, we have compiled a sample of 16 000 early-type galaxies in the redshift range 0.05 < z < 0.1 from the Sloan Digital Sky Survey (SDSS) data base (MOSES: Morphologically Selected Ellipticals in SDSS). Key in our approach is the use of a purely morphological selection criterion through visual inspection which produces a sample that is not biased against recent star formation and nuclear activity. Based on the nebular emission-line characteristics we separate between star formation activity, black hole activity, the composite of the two and quiescence. We find that emission is mostly LINER (low ionization nuclear emission line region) like in high-mass galaxies (σ > 200 km s−1) and roughly evenly distributed between star formation and AGN at intermediate and low (σ < 100 km s−1) masses. The objects with emission (∼20 per cent) are offset from the red sequence and form a well-defined pattern in the colour–mass diagram. Star-forming early-types inhabit the blue cloud, while early-types with AGN are located considerably closer to and almost on the red sequence. Star formation–AGN composites are found right between these two extremes. We further derive galaxy star formation histories using a novel method that combines multiwavelength photometry from near-ultraviolet (UV) to near-infrared (IR) and stellar absorption indices. We find that in those objects deviating from the red sequence star formation occurred several 100 Myr in the past involving 1–10 per cent of the total stellar mass. We identify an evolutionary sequence from star formation via nuclear activity to quiescence. This transition process lasts about 1 Gyr, and the peak AGN phase occurs roughly half a Gyr after the starburst. The most likely interpretation is that star formation is suppressed by nuclear activity in these objects before they settle on the red sequence. This is empirical evidence for the occurrence of AGN feedback in early-type galaxies at recent epochs.
We exploit multiwavelength photometry of early-type galaxies (ETGs) in the Cosmological Evolution Survey (COSMOS) to demonstrate that the low-level star formation activity in the ETG population at ...intermediate redshift is likely to be driven by minor mergers. Splitting the ETGs into galaxies that show disturbed morphologies indicative of recent merging and those that appear relaxed, we find that ∼32 per cent of the ETG population appears to be morphologically disturbed. While the relaxed objects are almost entirely contained within the UV red sequence, their morphologically disturbed counterparts dominate the scatter to blue UV colours, regardless of luminosity. Empirically and theoretically determined major-merger rates in the redshift range z < 1 are several times too low to account for the fraction of disturbed ETGs in our sample, suggesting that minor mergers represent the principal mechanism driving the observed star formation activity in our sample. The young stellar components forming in these events have ages between 0.03 and 0.3 Myr and typically contribute ≤ 10 per cent of the stellar mass of the remnant. Together with recent work which demonstrates that the structural evolution of nearby ETGs is consistent with one or more minor mergers, our results indicate that the overall evolution of massive ETGs may be heavily influenced by minor merging at late epochs and highlights the need to systematically study this process in future observational surveys.
Recent work (Schawinski et al.) indicates that star-forming early-type galaxies residing in the blue cloud migrate rapidly to the red sequence within around a Gyr, passing through several phases of ...increasingly strong active galactic nucleus (AGN) activity in the process. We show that natural depletion of the cold gas reservoir through star formation (i.e. in the absence of any feedback from the AGN) induces a blue-to-red reddening rate that is several factors lower than that observed by Schawinski et al. This is because the gas depletion rate due to star formation alone is too slow, implying that another process needs to be invoked to remove cold gas from the system and accelerate the reddening rate. We develop a simple phenomenological model, in which a fraction of the AGN's luminosity couples to the gas reservoir over a certain 'feedback time-scale' and removes part of the cold gas mass from the galaxy, while the remaining gas continues to contribute to star formation. We use the model to investigate scenarios which yield migration times consistent with the results of Schawinski et al. We find that acceptable models have feedback time-scales ≲0.2 Gyr. The mass fraction in young stars in the remnants is ≲5 per cent and the residual cold gas fractions are less than 0.6 per cent, in good agreement with the recent literature. At least half of the initial cold gas reservoir is removed as the galaxies evolve from the blue cloud to the red sequence. If we restrict ourselves to feedback time-scales similar to the typical duty cycles of local AGN (a few hundred Myr) then a few tenths of a per cent of the luminosity of an early-type Seyfert (∼1011 L⊙) must couple to the cold gas reservoir in order to produce migration times that are consistent with the observations.
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