19 research outputs found
On probing the properties of QSOs through their proximity effects on the intergalactic medium
The proximity effect (PE) of QSOs is believed to be useful in constraining
the QSO lifetime. Observations on the PE so far, however, give apparently
contradictory results -- some are consistent with a long QSO lifetime (>~ a few
10^7 yr), but others appear to be only consistent with a short QSO lifetime <~
10^6 yr. In this paper, we show that this apparent contradiction may be solved
by simultaneously taking into account both the effect due to the density
enhancement in the QSO near zones and that due to the obscuration of the tori
associated with the QSOs, using a large number of Monte-Carlo generated
synthetic Lyman alpha forest spectra. We demonstrate that the QSO properties
and environment can be constrained simultaneously by the transverse PE and the
line of sight PE of bright type 1 QSOs together. The current available
measurements on the PEs of type 1 QSOs suggest that (1) the density is
significantly enhanced in the vicinity of the QSOs; (2) the QSO lifetime is
consistent with being as large as a few 10^7 yr and a substantially shorter
lifetime (e.g., <~10^6 yr) is not required; and (3) the half opening angle of
the tori associated with QSOs is ~60 deg, consistent with some other
independent estimates. Our simulations also show that the TPE of type 2 QSOs
can be significantly different from that of type 1 QSOs, which may be useful to
put further constraints on the QSO properties and the QSO environment.Comment: 14 pages, 11 figures, to appear in Ap
Implications of the measured parameters of PSR J1903+0327 for its progenitor neutron star
Using the intrinsic PSR J1903+0327 parameters evaluated from radio
observations (mass, rotation period and dipole magnetic field deduced from the
timing properties) we calculate the mass of its neutron star progenitor, M_i,
at the onset of accretion. Simultaneously, we derive constraints on average
accretion rate Mdot and the pre-accretion magnetic field B_i. Spin-up is
modelled by accretion from a thin disk, using the magnetic-torque disk-pulsar
coupling model proposed by Kluzniak and Rappaport (2007), improved for the
existence of relativistic marginally-stable circular orbit. Orbital parameters
in the disk are obtained using the space-time generated by a rotating neutron
star in the framework of General Relativity. We employ an observationally
motivated model of the surface magnetic field decay. We also seek for the
imprint of the poorly known equation of state of dense matter on the spin-up
tracks - three equations of state of dense matter, consistent with the
existence of 2 Msun neutron star, are considered. We find that the minimum
average accretion rate should be larger than 2-8 10^(-10) Msun/yr, the highest
lower bound corresponding to the stiffest equation of state. We conclude that
the influence of magnetic field in the "recycling" process is crucial - it
leads to a significant decrease of spin-up rate and larger accreted masses, in
comparison to the B=0 model. Allowed B_i-dependent values of M_i are within
1.0-1.4 Msun, i.e., much lower than an oversimplified but widely used B=0
result, where one gets M_i>1.55 Msun. Estimated initial neutron-star mass
depends on the assumed dense-matter equation of state.Comment: 11 pages, 10 figures; A&A accepte
Pulsar spin-velocity alignment from single and binary neutron star progenitors
The role of binary progenitors of neutron stars in the apparent distribution
of space velocities and spin-velocity alignment observed in young pulsars is
studied. A Monte-Carlo synthesis of pulsar population from single and binary
stars with different assumptions about the NS natal kick model (direction
distribution, amplitude, and kick reduction in binary progenitors which
experienced mass exchange due to Roche lobe overflow with initial masses on the
main sequence from the range 8-11 ) is performed. The calculated
spin-velocity alignment distributions are compared with observational data
obtained from radio polarization measurements. The observed space velocity of
pulsars is found to be mostly shaped by the natal kick velocity form and its
amplitude; the fraction of binaries is not important here for reasonably large
kicks. The distribution of kick direction relative to the spin axis during the
formation of a NS is found to affect strongly the spin-velocity correlation of
pulsars. Comparison with observed pulsar spin-velocity angles favours a
sizeable fraction of binary progenitors and the kick-spin angle . The form of the initial binary mass ratio distribution does not
affect our results.Comment: 20 pages, 8 figures; Submitted to MNRA
Recommended from our members
The origin of metal-poor stars on prograde disk orbits in FIRE simulations of Milky Way-mass galaxies
Abstract
In hierarchical structure formation, metal-poor stars in and around the Milky Way (MW) originate primarily from mergers of lower-mass galaxies. A common expectation is therefore that metal-poor stars should have isotropic, dispersion-dominated orbits that do not correlate strongly with the MW disk. However, recent observations of stars in the MW show that metal-poor ([Fe/H]âČ â2) stars are preferentially on prograde orbits with respect to the disk. Using the FIRE-2 suite of cosmological zoom-in simulations of MW/M31-mass galaxies, we investigate the prevalence and origin of prograde metal-poor stars. Almost all (11 of 12) of our simulations have metal-poor stars on preferentially prograde orbits today and throughout most of their history: we thus predict that this is a generic feature of MW/M31-mass galaxies. The typical prograde-to-retrograde ratio is âŒ2â:â1, which depends weakly on stellar metallicity at [Fe/H]âČ â1. These trends predicted by our simulations agree well with MW observations. Prograde metal-poor stars originate largely from a single LMC/SMC-mass gas-rich merger 7 â 12.5Gyr ago, which deposited existing metal-poor stars and significant gas on an orbital vector that sparked the formation of and/or shaped the orientation of a long-lived stellar disk, giving rise to a prograde bias for all low-metallicity stars. We find sub-dominant contributions from in-situ stars formed in the host galaxy before this merger, and in some cases, additional massive mergers. We find few clear correlations between any properties of our MW/M31-mass galaxies at z = 0 and the degree of this prograde bias as a result of diverse merger scenarios
Cosmic-Ray Driven Outflows to Mpc Scales from L* Galaxies
We study the effects of cosmic rays (CRs) on outflows from star-forming galaxies in the circum and intergalactic medium (CGM/IGM), in high-resolution, fully cosmological FIRE-2 simulations (accounting for mechanical and radiative stellar feedback, magnetic fields, anisotropic conduction/viscosity/CR diffusion and streaming, and CR losses). We showed previously that massive (â Mhaloâł1011Mââ ), low-redshift (z âČ 1â2) haloes can have CR pressure dominate over thermal CGM pressure and balance gravity, giving rise to a cooler CGM with an equilibrium density profile. This dramatically alters outflows. Absent CRs, high gas thermal pressure in massive haloes âtrapsâ galactic outflows near the disc, so they recycle. With CRs injected in supernovae as modelled here, the low-pressure halo allows âescapeâ and CR pressure gradients continuously accelerate this material well into the IGM in âfastâ outflows, while lower-density gas at large radii is accelerated in situ into âslowâ outflows that extend to >Mpc scales. CGM/IGM outflow morphologies are radically altered: they become mostly volume-filling (with inflow in a thin mid-plane layer) and coherently biconical from the disc to >Mpc. The CR-driven outflows are primarily cool (â TâŒ105 K) and low velocity. All of these effects weaken and eventually vanish at lower halo masses (â âČ1011Mââ ) or higher redshifts (z âł 1â2), reflecting the ratio of CR to thermal + gravitational pressure in the outer halo. We present a simple analytical model that explains all of the above phenomena. We caution that these predictions may depend on uncertain CR transport physics
Realistic mock observations of the sizes and stellar mass surface densities of massive galaxies in FIRE-2 zoom-in simulations
The galaxy sizeâstellar mass and central surface densityâstellar mass relationships are fundamental observational constraints on galaxy formation models. However, inferring the physical size of a galaxy from observed stellar emission is non-trivial due to various observational effects, such as the mass-to-light ratio variations that can be caused by non-uniform stellar ages, metallicities, and dust attenuation. Consequently, forward-modelling light-based sizes from simulations is desirable. In this work, we use the skirtâ dust radiative transfer code to generate synthetic observations of massive galaxies (â MââŒ1011Mâ at z = 2, hosted by haloes of mass MhaloâŒ1012.5Mââ ) from high-resolution cosmological zoom-in simulations that form part of the Feedback In Realistic Environments project. The simulations used in this paper include explicit stellar feedback but no active galactic nucleus (AGN) feedback. From each mock observation, we infer the effective radius (Re), as well as the stellar mass surface density within this radius and within 1kpc (ÎŁe and ÎŁ1, respectively). We first investigate how well the intrinsic half-mass radius and stellar mass surface density can be inferred from observables. The majority of predicted sizes and surface densities are within a factor of 2 of the intrinsic values. We then compare our predictions to the observed sizeâmass relationship and the ÎŁ1âMâ and ÎŁeâMâ relationships. At z âł 2, the simulated massive galaxies are in general agreement with observational scaling relations. At z âČ 2, they evolve to become too compact but still star forming, in the stellar mass and redshift regime where many of them should be quenched. Our results suggest that some additional source of feedback, such as AGN-driven outflows, is necessary in order to decrease the central densities of the simulated massive galaxies to bring them into agreement with observations at z âČ 2
Recommended from our members
Gas kinematics in FIRE simulated galaxies compared to spatially unresolved HI observations.
The shape of a galaxy's spatially unresolved, globally integrated 21-cm emission line depends on its internal gas kinematics: galaxies with rotationally supported gas discs produce double-horned profiles with steep wings, while galaxies with dispersion-supported gas produce Gaussian-like profiles with sloped wings. Using mock observations of simulated galaxies from the FIRE project, we show that one can therefore constrain a galaxy's gas kinematics from its unresolved 21-cm line profile. In particular, we find that the kurtosis of the 21-cm line increases with decreasing V/Ï and that this trend is robust across a wide range of masses, signal-to-noise ratios, and inclinations. We then quantify the shapes of 21-cm line profiles from a morphologically unbiased sample of ~2000 low-redshift, HI-detected galaxies with M star = 107-11 Mâ and compare to the simulated galaxies. At M star âł 1010 Mâ, both the observed and simulated galaxies produce double-horned profiles with low kurtosis and steep wings, consistent with rotationally supported discs. Both the observed and simulated line profiles become more Gaussian like (higher kurtosis and less-steep wings) at lower masses, indicating increased dispersion support. However, the simulated galaxies transition from rotational to dispersion support more strongly: at M star 108-10 M, most of the simulations produce more Gaussian-like profiles than typical observed galaxies with similar mass, indicating that gas in the low-mass simulated galaxies is, on average, overly dispersion supported. Most of the lower-mass-simulated galaxies also have somewhat lower gas fractions than the median of the observed population. The simulations nevertheless reproduce the observed line-width baryonic Tully-Fisher relation, which is insensitive to rotational versus dispersion support
The bursty origin of the Milky Way thick disc
Abstract
We investigate thin and thick stellar disc formation in Milky-Way-mass galaxies using twelve FIRE-2 cosmological zoom-in simulations. All simulated galaxies experience an early period of bursty star formation that transitions to a late-time steady phase of near-constant star formation. Stars formed during the late-time steady phase have more circular orbits and thin-disc-like morphology at z = 0, whilst stars born during the bursty phase have more radial orbits and thick-disc structure. The median age of thick-disc stars at z = 0 correlates strongly with this transition time. We also find that galaxies with an earlier transition from bursty to steady star formation have a higher thin-disc fractions at z = 0. Three of our systems have minor mergers with LMC-size satellites during the thin-disc phase. These mergers trigger short starbursts but do not destroy the thin disc nor alter broad trends between the star formation transition time and thin/thick disc properties. If our simulations are representative of the Universe, then stellar archaeological studies of the Milky Way (or M31) provide a window into past star-formation modes in the Galaxy. Current age estimates of the Galactic thick disc would suggest that the Milky Way transitioned from bursty to steady phase âŒ6.5 Gyr ago; prior to that time the Milky Way likely lacked a recognisable thin disc