EXTREMELY INEFFICIENT STAR FORMATION IN THE OUTER DISKS OF NEARBY GALAXIES

By F. Bigiel, A. Leroy, F. Walter, L. Blitz, E. Brinks, W. D. de Blok, B. Berkeley, Mpia Heidelberg, Nrao, U. Hertfordshire and 2 more. 2010

303 citations recorded by the source corpus.

We combine data from The H i Nearby Galaxy Survey and the GALEX Nearby Galaxy Survey to study the relationship between atomic hydrogen (H i) and far-ultraviolet (FUV) emission outside the optical radius (r25) in 17 spiral and 5 dwarf galaxies. In this regime, H i is likely to represent most of the interstellar medium (ISM) and FUV emission to trace recent star formation with little bias due to extinction, so that the two quantities closely trace the underlying relationship between gas and star formation rate (SFR). The azimuthally averaged H i and FUV intensities both decline with increasing radius in this regime, with the scale length of the FUV profile typically half that of the H i profile. Despite the mismatch in profiles, there is a significant spatial correlation (at 15″ resolution) between local FUV and H i intensities; near r25 this correlation is quite strong, in fact stronger than anywhere inside r25 (where H i is not a good tracer for the bulk of the ISM), and shows a decline toward larger radii. The star formation efficiency (SFE)—defined as the ratio of FUV/H i and thus the inverse of the gas depletion time—decreases with galactocentric radius across the outer disks, though much shallower than across the optical disks. On average, we find the gas depletion times to be well above a Hubble time (∼1011 yr). We observe a clear relationship between FUV/H i and H i column in the outer disks, with the SFE increasing with increasing H i column. Despite observing systematic variations in FUV/H i, we find no clear evidence for step-function-type star formation thresholds, though we emphasize that it may not be realistic to expect them. When compared with results from inside r25, we find outer disk star formation to be distinct in several ways: it is extremely inefficient (depletion times of many Hubble times which are also long compared to either the free fall or orbital timescale) with column densities and SFRs lower than found anywhere inside the optical disks. It appears that the H i column is one of the key environmental factors—perhaps the key factor—in setting the SFR in outer galaxy disks.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/b086449d9caecc5503e3275ae75812a392393f40

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Cold gas properties of the Herschel Reference Survey - II. Molecular and total gas scaling relations (2014, background): “The same THINGS team has recently made several efforts to extend their analysis to different physical conditions, such as those encountered in the outskirt of extended discs (Bigiel et al. 2010), in Hi-dominated regions (Schruba et al. 2012), or in metal-poor regions (Bolatto et al. 2011).”
ESTIMATING THE STAR FORMATION RATE AT 1 kpc SCALES IN NEARBY GALAXIES (2012, methodology): “For a steeper relationship like the ΣSFR ∼ Σ2HI found in the outer parts of galaxies (Bigiel et al. 2010) the implied uncertainty is ∼ 0.3.”
A CONSTANT MOLECULAR GAS DEPLETION TIME IN NEARBY DISK GALAXIES (2011, background): “Based on detailed studies of Local Group galaxies (e.g. Blitz et al. 2007; Bolatto et al. 2008; Bigiel et al. 2010a; Fukui & Kawamura 2010), they speculated that the ap- 14 We normalize the fit at ΣH2 = 10 M⊙ pc −2 following B08. proximately linear ΣSFR-ΣH2 relation arises because star formation in…”
The dust scaling relations of the Herschel Reference Survey (2012, background): “10), the Hi profile is constant (Bigiel et al. 2010) and extends up to 8 times the scalelengthof the dust disk.”
Molecular hydrogen abundances of galaxies in the EAGLE simulations (2015, background): “For example, on kiloparsec E-mail: claudia.lagos@icrar.org scales, observations show a nearly linear correlation between the star formation rate (SFR) surface density and the surface density of H2 (Bigiel et al. 2008, 2010; Leroy et al. 2008; Schruba et al. 2011).”
BARYONS MATTER: WHY LUMINOUS SATELLITE GALAXIES HAVE REDUCED CENTRAL MASSES (2012, result): “…formation in the new model is tied directly to the local H2 abundance, which is regulated by the gas metallicity and the ability of the gas to self-shield, in accordance with observational results (Leroy et al. 2008; Bigiel et al. 2008; Blanc et al. 2009; Bigiel et al. 2010; Schruba et al. 2011).”
THE IMPACT OF INTERACTIONS, BARS, BULGES, AND ACTIVE GALACTIC NUCLEI ON STAR FORMATION EFFICIENCY IN LOCAL MASSIVE GALAXIES (2012, background): “Since star formation is known to be very inefficient in these outer regions (tdep(HI)> tHubble, Bigiel et al. 2010), the relative contributions of outer disk star formation to the total value may be responsible for some of the galaxy-to-galaxy star formation efficiency variations, and the…”
THE ARECIBO LEGACY FAST ALFA SURVEY: THE GALAXY POPULATION DETECTED BY ALFALFA (2012, background): “In the THINGS dwarf galaxies, the contribution of H2 to the total gas budget is generally small even in the inner disks, also corresponding to a low SFE (Bigiel et al. 2010a).”
An uncertainty principle for star formation - I. Why galactic star formation relations break down below a certain spatial scale (2014, background): “This is why discsdo not show the same truncation in the UV (Thilker et al. 2007) as when observed in Hα (Martin & Kennicutt 2001; also see Bigiel et al. 2010).”
The baryonic assembly of dark matter haloes (2011, background): “At the low densities of outer discs, star formation is observed to become very inefficient, with depletion time-scales up to ∼1011 yr (e.g. Bigiel et al. 2010).”
Star Formation in the Milky Way and Nearby Galaxies (2012, background): “Recent analyses have become available (Kennicutt et al. 2007; Bigiel et al. 2008, 2010, 2011; Leroy et al. 2008; Blanc et al. 2009; Eales et al. 2010; Verley et al. 2010; Liu et al. 2011; Rahman et al. 2011, 2012; Schruba et al. 2011), and several other major studies are ongoing.”
Implementing molecular hydrogen in hydrodynamic simulations of galaxy formation (2012, methodology): “Bigiel et al. (2008, 2010) created a spatially resolved K-S relation using data from THINGS and related surveys by finding the SFR surface density as a functionof the gas surface density on sub-kpc scales.”
The lifecycle of molecular clouds in nearby star-forming disc galaxies (2019, background): “…et al. 2010; Leroy et al. 2013; Kreckel et al. 2018; Kruijssen et al. 2019) as well as in low-density environments, such as in low surface brightness galaxies or in galaxy outskirts (e.g. Kennicutt 1989; Martin & Kennicutt 2001; Boissier et al. 2003; Bigiel et al. 2010; Goddard et al. 2010).”
A MOLECULAR STAR FORMATION LAW IN THE ATOMIC-GAS-DOMINATED REGIME IN NEARBY GALAXIES (2011, background): “H i does not correlate significantly with star formation in the H2–dominated inner parts of galaxy disks, though the correlation between H i and recent star formation becomes stronger in the H i–dominated outer parts of galaxies (and further increases at even larger radii; see Bigiel et al. 2010b).”
The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “Conversely, Figure 2 shows that H2-poor regions can have depletion times for the neutral ISM as a whole (i.e., including the H i as well as molecular gas) up to ∼ 100 Gyr (Wyder et al. 2009; Bigiel et al. 2010; Rafelski et al. 2011; Bolatto et al. 2011; Cantalupo et al. 7 2012; Huang et al. 2012).”
THE STATE OF THE GAS AND THE RELATION BETWEEN GAS AND STAR FORMATION AT LOW METALLICITY: THE SMALL MAGELLANIC CLOUD (2011, background): “The τmol dep in the SMC is consistent with the range observed in normal disks for Σmol derived from dust modeling. yond the Local Group, the molecular depletion time is log[τmoldep /(1Gyr)] ∼ 0.2± 0.3, or τ mol dep ≈ 1.6 Gyr.”
A unified model for galactic discs: star formation, turbulence driving, and mass transport (2017, background): “…distribution of individual pixels in the g − ̇∗ plane for inner galaxies (blue; Leroy et al. 2013; L10 in the legend) and outer galaxies and dwarfs (red; Bigiel et al. 2010; B10 in the legend); red circles with error bars show the median and scatter of the outer galaxy data. describing the data…”
METALLICITY-DEPENDENT QUENCHING OF STAR FORMATION AT HIGH REDSHIFT IN SMALL GALAXIES (2011, background): “In gas where, averaged over kpc scales, the dominant chemical state is H i rather than H2, the star formation rate is more than an order of magnitude lower than in molecular gas of comparable surface density (Bigiel et al. 2010).”
MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES (2013, methodology): “Kinematic analysis (e.g., Tan 2000) and the inclusion of outer disks (e.g., Bigiel et al. 2010) should yield the lever arms to better understand the impact of local conditions on star formation in molecular gas.”
What controls star formation in the central 500 pc of the Galaxy (2013, background): “It has been shown thatΣSFR drops below the relations of equations (1) and (2) beyond a certain galactocentric radius (e.g. Martin & Kennicutt 2001; Bigiel et al. 2010) – the straightforward detection of cutoff radii is well-suited for testing star formation thresholds.”