We investigate how ram pressure of intragroup and intracluster medium can influence the spatial and temporal variations of star formation (SF) of disc galaxies with halo masses (M
h) ranging from ...1010 to 1012 M (i.e. from dwarf irregular to Milky Way-type) in groups and clusters with 1013 ≤ M
h/M ≤ 1015 by using numerical simulations with a new model for time-varying ram pressure. The long-term evolution of SF rates and Hα morphologies corresponding to the distributions of star-forming regions are particularly investigated for different model parameters. The principal results are as follows. Whether ram pressure can enhance or reduce SF depends on M
h of disc galaxies and inclination angles of gas discs with respect to their orbital directions for a given orbit and a given environment. For example, SF can be moderately enhanced in disc galaxies with M
h = 1012 M at the pericentre passages in a cluster with M
h = 1014 M whereas it can be completely shut down ('quenching') for low-mass discs with M
h = 1010 M. Ram pressure can reduce the Hα-to-optical-disc-size ratios of discs and the level of the reduction depends on M
h and orbits of disc galaxies for a given environment. Disc galaxies under strong ram pressure show characteristic Hα morphologies such as ring-like, one-sided and crescent-like distributions.
We investigate the time evolution of dust properties, molecular hydrogen (H2) contents and star formation histories in galaxies by using our original chemodynamical simulations. The simulations ...include the formation of dust in the stellar winds of supernovae and asymptotic giant branch (AGB) stars, the growth and destruction processes of dust in the interstellar medium (ISM), the formation of polycyclic aromatic hydrocarbon (PAH) dust in carbon-rich AGB stars, the H2 formation on dust grains and the H2 photodissociation due to far-ultraviolet light in a self-consistent manner. We focus mainly on disc galaxies with the total masses ranging from 1010 to 1012 M in this preliminary study. The principal results are as follows. The star formation histories of disc galaxies can be regulated by the time evolution of interstellar dust, mainly because the formation rates of H2 can be controlled by dust properties. The observed correlation between dust-to-gas-ratios (D) and gas-phase oxygen abundances A
O ≡ 12 + log (O/H) can be reproduced reasonably well in the present models. The discs show negative radial gradients (i.e. larger in inner regions) of H2 fraction (
), PAH-to-dust mass ratio (f
PAH), D and A
O, and these gradients evolve with time. The surface mass densities of dust (Σdust) are correlated more strongly with the total surface gas densities (Σgas) than with those of H2 (
). Local gaseous regions with higher D are more likely to have higher
in individual discs and total H2 masses (
) correlate well with total dust masses (M
dust). More massive disc galaxies are more likely to have higher D, f
PAH and
, and smaller dust-to-stellar mass ratios (R
dust = M
dust/M
star). Early-type E/S0 galaxies formed by major galaxy merging can have lower R
dust than isolated late-type disc galaxies. We also compare between galactic star formation histories in the metallicity-dependent and dust-dependent star formation models and find no major differences. Based on these results, we discuss the roles of dust in chemical and dynamical evolution of galaxies.
We numerically investigate whether and how gaseous ejecta from AGB stars can be converted into new stars within originally massive star clusters (MSCs) in order to understand the origin of multiple ...stellar populations in globular clusters (GCs). We adopt a scenario in which (i) MSCs with masses of M
s can be formed from high-mass, high-density giant molecular clouds (GMCs) in their host galactic building blocks embedded in dark matter haloes at high redshifts, and (ii) their evolution therefore can be significantly influenced by M
s, their initial locations and physical properties of their hosts. Our 3D hydrodynamical simulations show that gaseous ejecta from AGB stars can be retained within MSCs and consequently converted into new stars very efficiently in the central regions of MSCs, only if M
s exceeds a threshold mass (M
th) of ≈106 M⊙. The new stars can correspond to the 'second generation (SG)' of stars with higher Na and lower O abundances observed in GCs. Star formation efficiencies during the formation of SG stars within MSCs with M
s≥M
th can be rather high (0.3-0.9) so that very compact new clusters within original MSCs can be formed. M
s should be as large as 106-107 M⊙ to explain the observed large fraction of SG stars in the present ordinary Galactic GCs, because new stars can consist of only 1-4 per cent among all stars for the standard initial mass function. Nuclear MSCs are found to retain much more effectively the AGB ejecta and convert more efficiently the gas into new stars, owing to the much deeper gravitational potential of their hosts. Capture and accretion of cold molecular gas (or small GMCs) by forming MSCs themselves can be mechanisms for mixing (i.e., dilution) of AGB ejecta with cold pristine gas. We suggest that both M
s and their locations within their hosts can determine whether abundance spread can be seen only in light elements or even in heavy ones. We discuss how and in what time-scale MSCs preferentially lose old stars owing to tidal stripping by their host galactic building blocks. We also suggest that the origin of the intermediate-age GCs with possible age spread of ∼100 Myr yet apparently no/little abundance spread in light elements in the LMC is closely associated with their incapability to retain the AGB ejecta owing to their low masses.
Abstract
Recent observations have shown that P/Fe in the Galactic stars decreases with increasing Fe/H for Fe/H ≳ − 1 whereas it is almost subsolar for Fe/H ≲ −2. These P/Fe trends with Fe/H have not ...been well reproduced by previous theoretical models incorporating phosphorus (P) enrichment only by core collapse supernoave. We here show, for the first time, that the trends can be naturally explained by our new models incorporating P enrichment by oxygen–neon (ONe) novae, which occur at the surface of massive white dwarfs whose masses are larger than 1.25
M
⊙
with a metallicity-dependence rate. We also show that the observations can be better reproduced by the models by assuming that (i) the total mass of gaseous ejecta per ONe nova (
M
ej
) is as high as 4 × 10
−5
M
⊙
and (ii) the number of such novae per unit mass (
N
ONe
) is as large as 0.01 at Fe/H ≈ −3. The assumed
M
ej
is consistent with observations, and the high
N
ONe
is expected from recent theoretical models for ONe nova fractions. We predict that (i) P/Fe increases with increasing Fe/H for −2 ≲ Fe/H ≲ −1 and (ii) P/Fe and Cl/Fe trends with Fe/H are very similar to each other due to very large yields of P and Cl from ONe novae. It is thus worthwhile for future observations to assess the validity of the proposed P enrichment by ONe novae by confirming or ruling out these two predictions.
Bound orbits have traditionally been assigned to the Large and Small Magellanic Clouds (LMC and SMC, respectively) in order to provide a formation scenario for the Magellanic Stream (MS) and its ...Leading Arm (LA), two prominent neutral hydrogen (HI) features connected to the LMC and SMC. However, Hubble Space Telescope (HST) measurements of the proper motions of the LMC and SMC have challenged the plausibility of bound orbits, causing the origin of the MS to re-emerge as a contested issue. We present a new tidal model in which structures resembling the bifurcated MS and elongated LA are able to form in a bound orbit consistent with the HST proper motions. The LMC and SMC have remained bound to each other only recently in our model despite being separately bound to the Milky Way for more than 5 Gyr. We find that the MS and LA are able to form as a consequence of LMC-dominated tidal stripping during the recent dynamical coupling of the LMC and SMC. Our orbital model depends on our assumption that the Milky Way has a constant circular velocity of V
cir= 250 km s−1 up to 160 kpc, which implies a massive isothermal halo that is not completely rejected by observations.
We propose that the observed stellar halo around the globular cluster (GC) NGC 1851 is evidence of its formation in the central region of its defunct host dwarf galaxy. We numerically investigate the ...long-term dynamical evolution of a nucleated dwarf galaxy embedded in a massive dark matter halo under the strong tidal field of the Galaxy. The dwarf galaxy is assumed to have a stellar nucleus (or a nuclear star cluster) that could be the progenitor for NGC 1851. We find that although the dark matter halo and the stellar envelope of the host dwarf of NGC 1851 can be almost completely stripped during its orbital evolution around the Galaxy, a minor fraction of stars in the dwarf can remain trapped by the gravitational field of the nucleus. The stripped nucleus can be observed as NGC 1851 with no/little dark matter, whereas stars around the nucleus can be observed as a diffuse stellar halo around NGC 1851. The simulated stellar halo has a symmetric distribution with a power-law density slope of ∼−2 and shows no tidal tails within ∼200 pc from NGC 1851. We show that two GCs can merge with each other to form a new nuclear GC embedded in field stars owing to the low stellar velocity dispersion of the host dwarf. This result makes no assumption on the ages and/or chemical abundances of the two merging GCs. Thus, the observed stellar halo and characteristic multiple stellar populations in NGC 1851 suggest that NGC 1851 could have formed initially in the central region of an ancient dwarf galaxy. We predict that the stellar halo of NGC 1851 may have at least three different stellar populations. We also suggest some Galactic GCs with diffuse haloes, such as NGC 1904 and 5694, could be formed in a similar way to NGC 1851. We discuss the importance of GC merging within dwarfs in the formation of multiple stellar populations with abundance spreads in heavy elements in some Galactic GCs, such as M22 and NGC 2419. We also discuss other possible scenarios for the formation of the stellar halo around NGC 1851.
Abstract
Understanding massive cluster formation is one of the important issues of astronomy. By analyzing the H i data, we have identified that the two H i velocity components (L- and D-components) ...are colliding toward the H i Ridge, in the southeastern end of the Large Magellanic Cloud (LMC), which hosts the young massive cluster R136 and ∼400 O/Wolf–Rayet stars (Doran et al. 2013, A&A, 558, A134) including the progenitor of SN 1987A. The collision is possibly evidenced by bridge features connecting the two H i components and by complementary distributions between them. We frame a hypothesis that the collision triggered the formation of R136 and the surrounding high-mass stars as well as the H i Ridge and the Molecular Ridge. Fujimoto and Noguchi (1990, PASJ, 42, 505) advocated that the last tidal interaction between the LMC and the Small Magellanic Cloud (SMC) induced collision of the L- and D-components about 0.2 Gyr ago. This model is consistent with numerical simulations (Bekki & Chiba 2007a, MNRAS, 381, L16). We suggest that a dense H i, cloud of 106 M
⊙ partly including CO, a precursor of R136, was formed at the shock-compressed interface between the colliding L- and D-components. We suggest that part of the low-metallicity gas from the SMC was mixed in the tidal interaction based on the Planck/IRAS data of dust optical depth (Planck Collaboration 2014, A&A, 571, A11).
ABSTRACT
We use Milky Way-like chemodynamical simulations with a new treatment for dust destruction and growth to investigate how these two processes affect the properties of the interstellar medium ...in galaxies. We focus on the role of two specific parameters, namely fdes (a new parameter that determines the fraction of dust destroyed in a single gas particle vicinity of a supernova) and Cs (the probability that a metal atom or ion sticks to the dust grain after colliding, i.e. the sticking coefficient), in regulating the amount and distribution of dust, cold gas and metals in galaxies. We find that simulated galaxies with low fdes and/or high Cs values not only produce more dust, but they also have a shallower correlation between the dust surface density and the total gas surface density, and a steeper correlation between the dust-to-gas ratio and the metallicity. Only for values of fdes between 0.01 and 0.02, and of Cs between 0.5 and 1 do our simulations produce an average slope of the dust-to-gas ratio versus metallicity relationship that is consistent with observations. fdes values correspond to a total fraction of dust destroyed by a single supernova ranging between 0.42 and 0.44. Finally, we compare predictions of several simulations (with different star formation recipes, gas fractions, central metallicities, and metallicity gradients) with the spatially resolved M101 galaxy, and conclude that metallicity is the primary driver of the spatial distribution of dust, while the dust-to-gas ratio controls the cold gas distribution, as it regulates the atomc-to-molecular hydrogen conversion rate.
ABSTRACT We show for the first time that H2 formation on dust grains can be enhanced in disk galaxies under strong ram pressure (RP). We numerically investigate how the time evolution of H i and H2 ...components in disk galaxies orbiting a group/cluster of galaxies can be influenced by the hydrodynamical interaction between the gaseous components of the galaxies and the hot intracluster medium. We find that compression of H i caused by RP increases H2 formation in disk galaxies before RP rapidly strips H i, cutting off the fuel supply and causing a drop in H2 density. We also find that the level of this H2 formation enhancement in a disk galaxy under RP depends on the mass of its host cluster dark matter halo, the initial positions and velocities of the disk galaxy, and the disk inclination angle with respect to the orbital plane. We demonstrate that dust growth is a key factor in the evolution of the H i and H2 mass in disk galaxies under strong RP. We discuss how the correlation between H2 fractions and surface gas densities of disk galaxies evolves with time in the galaxies under RP. We also discuss whether galaxy-wide star formation rates (SFRs) in cluster disk galaxies can be enhanced by RP if the SFRs depend on H2 densities.
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