301 research outputs found
Unravelling the physics of multiphase AGN winds through emission line tracers
Observations of emission lines in active galactic nuclei (AGNs) often find fast (ā¼1000 kmāsā1) outflows extending to kiloparsec scales, seen in ionized, neutral atomic and molecular gas. In this work we present radiative transfer calculations of emission lines in hydrodynamic simulations of AGN outflows driven by a hot wind bubble, including non-equilibrium chemistry, to explore how these lines trace the physical properties of the multiphase outflow. We find that the hot bubble compresses the line-emitting gas, resulting in higher pressures than in the ambient interstellar medium or that would be produced by the AGN radiation pressure. This implies that observed emission line ratios such as [OāIV]25Ī¼m / [NeāII]12Ī¼mā , [NeāV]14Ī¼m / [NeāII]12Ī¼mā , and [NāIII]57Ī¼m / [NāII]122Ī¼m constrain the presence of the bubble and hence the outflow driving mechanism. However, the line-emitting gas is under-pressurized compared to the hot bubble itself, and much of the line emission arises from gas that is out of pressure, thermal and/or chemical equilibrium. Our results thus suggest that assuming equilibrium conditions, as commonly done in AGN line emission models, is not justified if a hot wind bubble is present. We also find that ā³50 per cent of the mass outflow rate, momentum flux, and kinetic energy flux of the outflow are traced by lines such as [NāII]122Ī¼m and [NeāIII]15Ī¼m (produced in the 104K phase) and [CāII]158Ī¼m (produced in the transition from 104K to 100 K)
Stellar feedback sets the universal acceleration scale in galaxies
It has been established for decades that rotation curves deviate from the Newtonian gravity expectation given baryons alone below a characteristic acceleration scale gā ā¼10ā»āø cm sā»Ā²ā , a scale promoted to a new fundamental constant in MOND. In recent years, theoretical and observational studies have shown that the star formation efficiency (SFE) of dense gas scales with surface density, SFE ā¼ Ī£/Ī£_(crit) with Ī£_(crit)ā¼āØpĖ/māā©/(ĻG)ā¼1000 M_ā pcā»Ā² (where āØpĖ/māā© is the momentum flux output by stellar feedback per unit stellar mass in a young stellar population). We argue that the SFE, more generally, should scale with the local gravitational acceleration, i.e. that SFE ā¼g_(tot)/g_(crit) ā” (GM_(tot)/RĀ²)/āØpĖ/māā©ā , where M_(tot) is the total gravitating mass and g_(crit) = āØpĖ/māā© = ĻGĪ£_(crit) ā 10ā»āø cm sā»Ā² ā gā . Hence, the observed gā may correspond to the characteristic acceleration scale above which stellar feedback cannot prevent efficient star formation, and baryons will eventually come to dominate. We further show how this may give rise to the observed acceleration scaling g_(obs) ā¼ (g_(baryon)gā )^(1/2) (where g_(baryon) is the acceleration due to baryons alone) and flat rotation curves. The derived characteristic acceleration gā can be expressed in terms of fundamental constants (gravitational constant, proton mass, and Thomson cross-section): gā ā¼0.1Gmp_/Ļ_Tā
The HI covering fraction of Lyman Limit Systems in FIRE haloes
Atomic hydrogen (HI) serves a crucial role in connecting galactic-scale
properties such as star formation with the large-scale structure of the
Universe. While recent numerical simulations have successfully matched the
observed covering fraction of HI near Lyman Break Galaxies (LBGs) and in the
foreground of luminous quasars at redshifts , the low-mass end
remains as-of-yet unexplored in observational and computational surveys. We
employ a cosmological, hydrodynamical simulation (FIREbox) supplemented with
zoom-in simulations (MassiveFIRE) from the Feedback In Realistic Environments
(FIRE) project to investigate the HI covering fraction of Lyman Limit Systems
( cm) across a wide range of
redshifts () and halo masses ( at ,
at ) in the absence of feedback from active
galactic nuclei (AGN). We find that the covering fraction inside haloes
exhibits a strong increase with redshift, with only a weak dependence on halo
mass for higher-mass haloes. For massive haloes (), the radial profiles showcase scale-invariance and
remain independent of mass. The radial dependence is well-captured by a fitting
function. The covering fractions in our simulations are in good agreement with
measurements of the covering fraction in LBGs. Our comprehensive analysis
unveils a complex dependence with redshift and halo mass for haloes with
that future observations aim to
constrain, providing key insights into the physics of structure formation and
gas assembly
Probing bursty star formation by cross-correlating extragalactic background light and galaxy surveys
Understanding the star formation rate (SFR) variability and how it depends on
physical properties of galaxies is important for developing and testing the
theory of galaxy formation. We investigate how statistical measurements of the
extragalactic background light (EBL) can shed light on this topic and
complement traditional methods based on observations of individual galaxies.
Using semi-empirical models of galaxy evolution and SFR indicators sensitive to
different star formation timescales (e.g., H and UV continuum
luminosities), we show that the SFR variability, quantified by the joint
probability distribution of the SFR indicators (i.e., the bivariate conditional
luminosity function), can be characterized as a function of galaxy mass and
redshift through the cross-correlation between deep, near-infrared maps of the
EBL and galaxy distributions. As an example, we consider combining upcoming
SPHEREx maps of the EBL with galaxy samples from Rubin/LSST. We demonstrate
that their cross-correlation over a sky fraction of can
constrain the joint SFR indicator distribution at high significance up to
for mass-complete samples of galaxies down to
. These constraints not only allow models of
different SFR variability to be distinguished, but also provide unique
opportunities to investigate physical mechanisms that require large number
statistics such as environmental effects. The cross-correlations investigated
illustrate the power of combining cosmological surveys to extract information
inaccessible from each data set alone, while the large galaxy populations
probed capture ensemble-averaged properties beyond the reach of targeted
observations towards individual galaxies.Comment: 12 pages, 7 figures, MNRAS accepte
Seen and unseen: bursty star formation and its implications for observations of high-redshift galaxies with JWST
Both observations and simulations have shown strong evidence for highly
time-variable star formation in low-mass and/or high-redshift galaxies, which
has important observational implications because high-redshift galaxy samples
are rest-UV selected and therefore particularly sensitive to the recent star
formation. Using a suite of cosmological "zoom-in" simulations at from
the Feedback in Realistic Environments (FIRE) project, we examine the
implications of bursty star formation histories for observations of
high-redshift galaxies with JWST. We characterize how the galaxy observability
depends on the star formation history. We also investigate selection effects
due to bursty star formation on the physical properties measured, such as the
gas fraction, specific star formation rate, and metallicity. We find the
observability to be highly time-dependent for galaxies near the survey's
limiting flux due to the SFR variability: as the star formation rate
fluctuates, the same galaxy oscillates in and out of the observable sample. The
observable fraction at and to for a JWST/NIRCam survey reaching a limiting
magnitude of -, representative of
surveys such as JADES and CEERS. JWST-detectable galaxies near the survey limit
tend to have properties characteristic of galaxies in the bursty phase: on
average, they show approximately 2.5 times higher cold, dense gas fractions and
20 times higher specific star formation rates at a given stellar mass than
galaxies below the rest-UV detection threshold. Our study represents a first
step in quantifying selection effects and the associated biases due to bursty
star formation in studying high-redshift galaxy properties.Comment: 8 pages, 4 figures, resubmitted after incorporating referee's
comments; analysis expanded to include more galaxies and some quantitative
results correcte
A Flat Photoionization Rate at 2<z<4.2: Evidence for a Stellar-Dominated UV Background and Against a Decline of Cosmic Star Formation Beyond z~3
We investigate the implications of our measurement of the Lyman-alpha forest
opacity at redshifts 2<z<4.2 from a sample of 86 high-resolution quasar spectra
for the evolution of the cosmic ultraviolet luminosity density and its sources.
The derived hydrogen photoionization rate is remarkably flat over this redshift
range, implying an increasing comoving ionizing emissivity with redshift.
Because the quasar luminosity function is strongly peaked near z~2,
star-forming galaxies likely dominate the ionizing emissivity at z>~3. Our
measurement argues against a star formation rate density declining beyond z~3,
in contrast with existing state-of-the-art determinations of the cosmic star
formation history from direct galaxy counts. Stellar emission from galaxies
therefore likely reionized the Universe.Comment: 5 pages, including 1 figure, published by Ap
When Feedback Fails: The Scaling and Saturation of Star Formation Efficiency
We present a suite of 3D multi-physics MHD simulations following star
formation in isolated turbulent molecular gas disks ranging from 5 to 500
parsecs in radius. These simulations are designed to survey the range of
surface densities between those typical of Milky Way GMCs (\sim 10^2
M_\odot\,pc^{-2}}) and extreme ULIRG environments (\sim 10^2
M_\odot\,pc^{-2}}) so as to map out the scaling of the cloud-scale star
formation efficiency (SFE) between these two regimes. The simulations include
prescriptions for supernova, stellar wind, and radiative feedback, which we
find to be essential in determining both the instantaneous per-freefall
() and integrated () star formation
efficiencies. In all simulations, the gas disks form stars until a critical
stellar surface density has been reached and the remaining gas is blown out by
stellar feedback. We find that surface density is a good predictor of
, as suggested by analytic force balance arguments from
previous works. SFE eventually saturates to at high surface density.
We also find a proportional relationship between and
, implying that star formation is feedback-moderated even over
very short time-scales in isolated clouds. These results have implications for
star formation in galactic disks, the nature and fate of nuclear starbursts,
and the formation of bound star clusters. The scaling of with
surface density is not consistent with the notion that is
always on the scale of GMCs, but our predictions recover the value for GMC parameters similar to those found in sprial galaxies,
including our own.Comment: 21 pages, 7 figures. Accepted to MNRA
Can magnetized turbulence set the mass scale of stars?
Understanding the evolution of self-gravitating, isothermal, magnetized gas is crucial for star formation, as these physical processes have been postulated to set the initial mass function (IMF). We present a suite of isothermal magnetohydrodynamic (MHD) simulations using the GIZMO code that follow the formation of individual stars in giant molecular clouds (GMCs), spanning a range of Mach numbers found in observed GMCs (ā Mā¼10ā50ā ). As in past works, the mean and median stellar masses are sensitive to numerical resolution, because they are sensitive to low-mass stars that contribute a vanishing fraction of the overall stellar mass. The mass-weighted median stellar mass Mā
ā becomes insensitive to resolution once turbulent fragmentation is well resolved. Without imposing Larson-like scaling laws, our simulations find Mā
āāā¼MāMā»Ā³Ī±_(turb)SFE^(1/3) for GMC mass Mā, sonic Mach number Mā , virial parameter Ī±_(turb), and star formation efficiency SFE = Mā/Mā. This fit agrees well with previous IMF results from the RAMSES, ORION2, and SPHNG codes. Although Mā
ā has no significant dependence on the magnetic field strength at the cloud scale, MHD is necessary to prevent a fragmentation cascade that results in non-convergent stellar masses. For initial conditions and SFE similar to star-forming GMCs in our Galaxy, we predict Mā
ā to be >20Māā , an order of magnitude larger than observed (ā ā¼2Māā ), together with an excess of brown dwarfs. Moreover, Mā
ā is sensitive to initial cloud properties and evolves strongly in time within a given cloud, predicting much larger IMF variations than are observationally allowed. We conclude that physics beyond MHD turbulence and gravity are necessary ingredients for the IMF
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