12 research outputs found
An uncertainty principle for star formation -- III. The characteristic emission time-scales of star formation rate tracers
We recently presented a new statistical method to constrain the physics of
star formation and feedback on the cloud scale by reconstructing the underlying
evolutionary timeline. However, by itself this new method only recovers the
relative durations of different evolutionary phases. To enable observational
applications, it therefore requires knowledge of an absolute 'reference
time-scale' to convert relative time-scales into absolute values. The logical
choice for this reference time-scale is the duration over which the star
formation rate (SFR) tracer is visible because it can be characterised using
stellar population synthesis (SPS) models. In this paper, we calibrate this
reference time-scale using synthetic emission maps of several SFR tracers,
generated by combining the output from a hydrodynamical disc galaxy simulation
with the SPS model SLUG2. We apply our statistical method to obtain
self-consistent measurements of each tracer's reference time-scale. These
include H and 12 ultraviolet (UV) filters (from GALEX, Swift, and
HST), which cover a wavelength range 150-350 nm. At solar metallicity, the
measured reference time-scales of H are Myr
with continuum subtraction, and 6-16 Myr without, where the time-scale
increases with filter width. For the UV filters we find 17-33 Myr, nearly
monotonically increasing with wavelength. The characteristic time-scale
decreases towards higher metallicities, as well as to lower star formation rate
surface densities, owing to stellar initial mass function sampling effects. We
provide fitting functions for the reference time-scale as a function of
metallicity, filter width, or wavelength, to enable observational applications
of our statistical method across a wide variety of galaxies.Comment: 24 pages, 18 figures, 7 tables (including Appendices); published in
MNRA
Metal and dust evolution in ALMA REBELS galaxies: insights for future JWST observations
ALMA observations revealed the presence of significant amounts of dust in the
first Gyr of Cosmic time. However, the metal and dust buildup picture remains
very uncertain due to the lack of constraints on metallicity. JWST has started
to reveal the metal content of high-redshift targets, which may lead to firmer
constraints on high-redshift dusty galaxies evolution. In this work, we use
detailed chemical and dust evolution models to explore the evolution of
galaxies within the ALMA REBELS survey, testing different metallicity scenarios
that could be inferred from JWST observations. In the models, we track the
buildup of stellar mass by using non-parametric SFHs for REBELS galaxies.
Different scenarios for metal and dust evolution are simulated by allowing
different prescriptions for gas flows and dust processes. The model outputs are
compared with measured dust scaling relations, by employing
metallicity-dependent calibrations for the gas mass based on the [CII]158micron
line. Independently of the galaxies metal content, we found no need for extreme
dust prescriptions to explain the dust masses revealed by ALMA. However,
different levels of metal enrichment will lead to different dominant dust
production mechanisms, with stardust production dominant over other ISM dust
processes only in the metal-poor case. This points out how metallicity
measurements from JWST will significantly improve our understanding of the dust
buildup in high-redshift galaxies. We also show that models struggle to
reproduce observables such as dust-to-gas and dust-to-stellar ratios
simultaneously, possibly indicating an overestimation of the gas mass through
current calibrations, especially at high metallicities.Comment: 16 pages + appendices, 9 Figures, 1 Table. Resubmitted to MNRAS after
moderate revisio
The lifecycle of molecular clouds in nearby star-forming disc galaxies
It remains a major challenge to derive a theory of cloud-scale (â âČ100âpc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially resolved (âŒ100âpc) CO-to-Hâα flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically 10â30Myrâ , and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities ÎŁH2â„8Mâpcâ2â , the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at ÎŁH2â€8Mâpcâ2 GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by Hâα (75-90 perâcent of the cloud lifetime), GMCs disperse within just 1â5Myr once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4-10 perâcent. These results show that galactic star formation is governed by cloud-scale, environmentally dependent, dynamical processes driving rapid evolutionary cycling. GMCs and HâII regions are the fundamental units undergoing these lifecycles, with mean separations of 100â300pc in star-forming discs. Future work should characterize the multiscale physics and mass flows driving these lifecycles.MC and JMDK gratefully acknowledge funding
from the Deutsche Forschungsgemeinschaft (DFG, German
Research Foundation) through an Emmy Noether Research Group
(grant number KR4801/1-1) and the DFG Sachbeihilfe (grant
number KR4801/2-1). JMDK, APSH, SMRJ, and DTH gratefully
acknowledge funding from the European Research Council
(ERC) under the European Unionâs Horizon 2020 research and
innovation programme via the ERC Starting Grant MUSTANG
(grant agreement number 714907). MC, JMDK, SMRJ, and DTH
acknowledge support from the Australia-Germany Joint Research
Cooperation Scheme (UA-DAAD, grant number 57387355).
APSH, SMRJ, and DTH are fellows of the International Max
Planck Research School for Astronomy and Cosmic Physics
at the University of Heidelberg (IMPRS-HD). BG gratefully
acknowledges the support of the Australian Research Council
as the recipient of a Future Fellowship (FT140101202). CNC,
AH, and JP acknowledge funding from the Programme National
âPhysique et Chimie du Milieu Interstellaireâ (PCMI) of the Centre
national de la recherche scientifique/Institut national des sciences
de lâUnivers (CNRS/INSU) with the Institut de Chimie/Institut de
Physique (INC/INP), co-funded by the Commissariat a lâ ` energie ÂŽ
atomique et aux energies alternatives (CEA) and the Centre ÂŽ
national dâetudes spatiales (CNES). AH acknowledges support ÂŽ
by the Programme National Cosmology et Galaxies (PNCG) of
CNRS/INSU with the INP and the Institut national de physique
nucleaire et de physique des particules (IN2P3), co-funded by ÂŽ
CEA and CNES. PL, ES, CF, DL, and TS acknowledge funding
from the ERC under the European Unionâs Horizon 2020 research
and innovation programme (grant agreement No. 694343).
The work of AKL, JS, and DU is partially supported by the
National Science Foundation (NSF) under Grants No. 1615105,
1615109, and 1653300. AKL also acknowledges partial support
from the National Aeronautics and Space Administration
(NASA) Astrophysics Data Analysis Program (ADAP) grants
NNX16AF48G and NNX17AF39G. ER acknowledges the support
of the Natural Sciences and Engineering Research Council of
Canada (NSERC), funding reference number RGPIN-2017-03987.
FB acknowledges funding from the ERC under the European
Unionâs Horizon 2020 research and innovation programme (grant
agreement No. 726384). GB is supported by the Fondo de Fomento
al Desarrollo CientŽıfico y Tecnologico of the Comisi Ž on Nacional de Ž
Investigacion Cient Ž Žıfica y Tecnologica (CONICYT/FONDECYT), Ž
Programa de Iniciacion, Folio 11150220. SCOG acknowledges ÂŽ
support from the DFG via SFB 881 âThe Milky Way Systemâ
(subprojects B1, B2, and B8) and also via Germanyâs
Excellence Strategy EXC-2181/1â390900948 (the Heidelberg
STRUCTURES Excellence Cluster). KK gratefully acknowledges
funding from the DFG in the form of an Emmy Noether
Research Group (grant number KR4598/2-1, PI Kreckel). AU
acknowledges support from the Spanish funding grants AYA2016-79006-P (MINECO/FEDER) and PGC2018-094671-B-I00
(MCIU/AEI/FEDER)
Cloud-Scale Molecular Gas Properties in 15 Nearby Galaxies
We measure the velocity dispersion, , and surface density, ,
of the molecular gas in nearby galaxies from CO spectral line cubes with
spatial resolution - pc, matched to the size of individual giant
molecular clouds. Combining galaxies from the PHANGS-ALMA survey with
targets from the literature, we characterize independent
sightlines where CO is detected at good significance. and
show a strong positive correlation, with the best-fit power law slope close to
the expected value for resolved, self-gravitating clouds. This indicates only
weak variation in the virial parameter
, which is - for
most galaxies. We do, however, observe enormous variation in the internal
turbulent pressure , which spans
across our sample. We find , , and
to be systematically larger in more massive galaxies. The
same quantities appear enhanced in the central kpc of strongly barred galaxies
relative to their disks. Based on sensitive maps of M31 and M33, the slope of
the - relation flattens at
, leading to high for a given
and high apparent . This echoes results found in
the Milky Way, and likely originates from a combination of lower beam filling
factors and a stronger influence of local environment on the dynamical state of
molecular gas in the low density regime.Comment: Accepted for publication in ApJ. 45 pages, 11 figures, 8 tables, 4
Appendices; key results summarized in Figure 10. Machine-readable table can
be downloaded at http://www.astronomy.ohio-state.edu/~sun.1608/datafile3.txt
prior to publication. For a brief video describing the main results of this
paper, please see https://www.youtube.com/watch?v=-_eL7t1PVq8&
The ALMA REBELS Survey: Dust Continuum Detections at z > 6.5
We report 18 dust continuum detections () at and out of 49 ultraviolet(UV)-bright galaxies (, observed by the Cycle-7 ALMA Large Program, REBELS
and its pilot programs. This has more than tripled the number of dust continuum
detections known at . Out of these 18 detections, 12 are reported for
the first time as part of REBELS. In addition, 15 of the dust continuum
detected galaxies also show a [CII] emission line,
providing us with accurate redshifts. We anticipate more line emission
detections from six targets (including three continuum detected targets) where
observations are still ongoing. The dust continuum detected sources in our
sample tend to have a redder UV spectral slope than the ones without a dust
continuum detection. We estimate that all of the sources have an infrared (IR)
luminosity () in a range of , except
for one with . Their
fraction of obscured star formation is significant at . Some of
the dust continuum detected galaxies show spatial offsets ()
between the rest-UV and far-IR emission peaks. These separations appear to have
an increasing trend against an indicator that suggests spatially decoupled
phases of obscured and unobscured star formation. REBELS offers the best
available statistical constraints on obscured star formation in UV-bright,
massive galaxies at .Comment: 17 pages, 9 figures, submitted to MNRA
The lifecycle of molecular clouds in nearby star-forming disc galaxies
It remains a major challenge to derive a theory of cloud-scale (â âČ100âpc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially resolved (âŒ100âpc) CO-to-Hâα flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically 10â30 Myrâ , and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities ÎŁ_(Hâ) â„ 8 M_â pcâ»ÂČâ , the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at ÎŁ_(Hâ) †8 M_â pcâ»ÂČ GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by Hâα (75â90 perâcent of the cloud lifetime), GMCs disperse within just 1â5 Myr once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4â10 perâcent. These results show that galactic star formation is governed by cloud-scale, environmentally dependent, dynamical processes driving rapid evolutionary cycling. GMCs and HâII regions are the fundamental units undergoing these lifecycles, with mean separations of 100â300 pc in star-forming discs. Future work should characterize the multiscale physics and mass flows driving these lifecycles
An uncertainty principle for star formation - II. A new method for characterising the cloud-scale physics of star formation and feedback across cosmic history
The cloud-scale physics of star formation and feedback represent the main uncertainty in galaxy formation studies. Progress is hampered by the limited empirical constraints outside the restricted environment of the Local Group. In particular, the poorly-quantified time evolution of the molecular cloud lifecycle, star formation, and feedback obstructs robust predictions on the scales smaller than the disc scale height that are resolved in modern galaxy formation simulations. We present a new statistical method to derive the evolutionary timeline of molecular clouds and star-forming regions. By quantifying the excess or deficit of the gas-to-stellar flux ratio around peaks of gas or star formation tracer emission, we directly measure the relative rarity of these peaks, which allows us to derive their lifetimes. We present a step-by-step, quantitative description of the method and demonstrate its practical application. The method's accuracy is tested in nearly 300 experiments using simulated galaxy maps, showing that it is capable of constraining the molecular cloud lifetime and feedback time-scale to dex precision. Access to the evolutionary timeline provides a variety of additional physical quantities, such as the cloud-scale star formation efficiency, the feedback outflow velocity, the mass loading factor, and the feedback energy or momentum coupling efficiencies to the ambient medium. We show that the results are robust for a wide variety of gas and star formation tracers, spatial resolutions, galaxy inclinations, and galaxy sizes. Finally, we demonstrate that our method can be applied out to high redshift () with a feasible time investment on current large-scale observatories. This is a major shift from previous studies that constrained the physics of star formation and feedback in the immediate vicinity of the Sun
The headlight cloud in NGC 628: An extreme giant molecular cloud in a typical galaxy disk
International audienceContext. Cloud-scale surveys of molecular gas reveal the link between giant molecular cloud properties and star formation across a range of galactic environments. Cloud populations in galaxy disks are considered to be representative of the normal star formation process, while galaxy centers tend to harbor denser gas that exhibits more extreme star formation. At high resolution, however, molecular clouds with exceptional gas properties and star formation activity may also be observed in normal disk environments. In this paper we study the brightest cloud traced in CO(2â1) emission in the disk of nearby spiral galaxy NGC 628.Aims. We characterize the properties of the molecular and ionized gas that is spatially coincident with an extremely bright HâŻII region in the context of the NGC 628 galactic environment. We investigate how feedback and large-scale processes influence the properties of the molecular gas in this region.Methods. High-resolution ALMA observations of CO(2â1) and CO(1â0) emission were used to characterize the mass and dynamical state of the âheadlightâ molecular cloud. The characteristics of this cloud are compared to the typical properties of molecular clouds in NGC 628. A simple large velocity gradient (LVG) analysis incorporating additional ALMA observations of 13CO(1â0), HCO+(1â0), and HCN(1â0) emission was used to constrain the beam-diluted density and temperature of the molecular gas. We analyzed the MUSE spectrum using Starburst99 to characterize the young stellar population associated with the HâŻII region.Results. The unusually bright headlight cloud is massive (1â
ââ
2â
Ăâ
107âMâ), with a beam-diluted density of nH2â=â5â
Ăâ
104 cmâ3 based on LVG modeling. It has a low virial parameter, suggesting that the CO emission associated with this cloud may be overluminous due to heating by the HâŻII region. A young (2â
ââ
4 Myr) stellar population with mass 3â
Ăâ
105âMâ is associated.Conclusions. We argue that the headlight cloud is currently being destroyed by feedback from young massive stars. Due to the large mass of the cloud, this phase of the its evolution is long enough for the impact of feedback on the excitation of the gas to be observed. The high mass of the headlight cloud may be related to its location at a spiral co-rotation radius, where gas experiences reduced galactic shear compared to other regions of the disk and receives a sustained inflow of gas that can promote the mass growth of the cloud