Frank Bigiel

THE STAR FORMATION EFFICIENCY IN NEARBY GALAXIES: MEASURING WHERE GAS FORMS STARS EFFECTIVELY

We measure the star formation efficiency (SFE), the star formation rate (SFR) per unit of gas, in 23 nearby galaxies and compare it with expectations from proposed star formation laws and thresholds. We use H I maps from The H I Nearby Galaxy Survey (THINGS) and derive H2 maps of CO measured by HERA CO-Line Extragalactic Survey and Berkeley-Illinois-Maryland Association Survey of Nearby Galaxies. We estimate the SFR by combining Galaxy Evolution Explorer (GALEX) far-ultraviolet maps and the Spitzer Infrared Nearby Galaxies Survey (SINGS) 24 ?m maps, infer stellar surface density profiles from SINGS 3.6 ?m data, and use kinematics from THINGS. We measure the SFE as a function of the free fall and orbital timescales, midplane gas pressure, stability of the gas disk to collapse (including the effects of stars), the ability of perturbations to grow despite shear, and the ability of a cold phase to form. In spirals, the SFE of H2 alone is nearly constant at (5.25 ? 2.5) ? 10?10 yr?1 (equivalent to an H2 depletion time of 1.9 ? 109 yr) as a function of all of these variables at our 800 pc resolution. Where the interstellar medium (ISM) is mostly H I, however, the SFE decreases with increasing radius in both spiral and dwarf galaxies, a decline reasonably described by an exponential with scale length 0.2r 25-0.25r 25. We interpret this decline as a strong dependence of giant molecular cloud (GMC) formation on environment. The ratio of molecular-to-atomic gas appears to be a smooth function of radius, stellar surface density, and pressure spanning from the H2-dominated to H I-dominated ISM. The radial decline in SFE is too steep to be reproduced only by increases in the free-fall time or orbital time. Thresholds for large-scale instability suggest that our disks are stable or marginally stable and do not show a clear link to the declining SFE. We suggest that ISM physics below the scales that we observe?phase balance in the H I, H2 formation and destruction, and stellar feedback?governs the formation of GMCs from H I.

The star formation law in nearby galaxies on sub-kpc scales

We present a comprehensive analysis of the relationship between star formation rate surface density, ΣSFR, and gas surface density, Σgas, at sub-kpc resolution in a sample of 18 nearby galaxies. We use high-resolution H I data from The H I Nearby Galaxy Survey, CO data from HERACLES and the BIMA Survey of Nearby Galaxies, 24 μm data from the Spitzer Space Telescope, and UV data from the Galaxy Evolution Explorer. We target seven spiral galaxies and 11 late-type/dwarf galaxies and investigate how the star formation law differs between the H2 dominated centers of spiral galaxies, their H I dominated outskirts and the H I rich late-type/dwarf galaxies. We find that a Schmidt-type power law with index N = 1.0 ± 0.2 relates ΣSFR and ΣH2 across our sample of spiral galaxies, i.e., that H2 forms stars at a constant efficiency in spirals. The average molecular gas depletion time is ~2 × 109 years. The range of ΣH2 over which we measure this relation is ~3-50 M ☉ pc–2, significantly lower than in starburst environments. We find the same results when performing a pixel-by-pixel analysis, averaging in radial bins, or when varying the star formation tracer used. We interpret the linear relation and constant depletion time as evidence that stars are forming in giant molecular clouds with approximately uniform properties and that ΣH2 may be more a measure of the filling fraction of giant molecular clouds than changing conditions in the molecular gas. The relationship between total gas surface density (Σgas) and ΣSFR varies dramatically among and within spiral galaxies. Most galaxies show little or no correlation between ΣHI and ΣSFR. As a result, the star formation efficiency (SFE), ΣSFR/Σgas, varies strongly across our sample and within individual galaxies. We show that this variation is systematic and consistent with the SFE being set by local environmental factors: in spirals the SFE is a clear function of radius, while the dwarf galaxies in our sample display SFEs similar to those found in the outer optical disks of the spirals. We attribute the similarity to common environments (low density, low metallicity, H I dominated) and argue that shear (which is typically absent in dwarfs) cannot drive the SFE. In addition to a molecular Schmidt law, the other general feature of our sample is a sharp saturation of H I surface densities at ΣHI ≈ 9 M ☉ pc–2 in both the spiral and dwarf galaxies. In the case of the spirals, we observe gas in excess of this limit to be molecular.

Heracles: The HERA CO Line Extragalactic Survey

Original article can be found at: http://www.iop.org/EJ/journal/1538-3881 Copyright American Astronomical Society. DOI: 10.1088/0004-6256/137/6/4670 [Full text of this article is not available in the UHRA]

The CO-to-H2 Conversion Factor and Dust-to-gas Ratio on Kiloparsec Scales in Nearby Galaxies

We present ~kiloparsec spatial resolution maps of the CO-to-H_2 conversion factor (α_(CO)) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for α_(CO) and the DGR by assuming that the DGR is approximately constant on kiloparsec scales. With this assumption, we can combine maps of dust mass surface density, CO-integrated intensity, and H I column density to solve for both αCO and the DGR with no assumptions about their value or dependence on metallicity or other parameters. Such a study has just become possible with the availability of high-resolution far-IR maps from the Herschel key program KINGFISH, ^(12)CO J = (2-1) maps from the IRAM 30 m large program HERACLES, and H I 21 cm line maps from THINGS. We use a fixed ratio between the (2-1) and (1-0) lines to present our α_(CO) results on the more typically used ^(12)CO J = (1-0) scale and show using literature measurements that variations in the line ratio do not affect our results. In total, we derive 782 individual solutions for α_(CO) and the DGR. On average, α_(CO) = 3.1 M_☉ pc^(–2) (K km s^(–1))^(–1) for our sample with a standard deviation of 0.3 dex. Within galaxies, we observe a generally flat profile of α_(CO) as a function of galactocentric radius. However, most galaxies exhibit a lower α_(CO) value in the central kiloparsec—a factor of ~2 below the galaxy mean, on average. In some cases, the central α_(CO) value can be factors of 5-10 below the standard Milky Way (MW) value of α_(CO,MW) = 4.4 M_☉ pc^(–2) (K km s^(–1))^(–1). While for α_(CO) we find only weak correlations with metallicity, the DGR is well-correlated with metallicity, with an approximately linear slope. Finally, we present several recommendations for choosing an appropriate α_(CO) for studies of nearby galaxies.

EXTREMELY INEFFICIENT STAR FORMATION IN THE OUTER DISKS OF NEARBY GALAXIES

We combine data from The Hi Nearby Galaxy Survey and the GALEX Nearby Galaxy Survey to study the relationship between atomic hydrogen (Hi) and far-ultraviolet (FUV) emission outside the optical radius (r25 )i n 17 spiral and 5 dwarf galaxies. In this regime, Hi 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 Hi and FUV intensities both decline with increasing radius in this regime, with the scale length of the FUV profile typically half that of the Hi profile. Despite the mismatch in profiles, there is a significant spatial correlation (at 15 �� resolution) between local FUV and Hi intensities; near r25 this correlation is quite strong, in fact stronger than anywhere inside r25 (where Hi 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/Hi 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 (∼10 11 yr). We observe a clear relationship between FUV/Hi and Hi column in the outer disks, with the SFE increasing with increasing Hi column. Despite observing systematic variations in FUV/Hi, we find no clear evidence for stepfunction-type star formation thresholds, though we emphasize that it may not be realistic to expect them. When compared with results from insider25, 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 Hi column is one of the key environmental factors—perhaps the key factor—in setting the SFR in outer galaxy disks.

Dust and Atomic Gas in Dwarf Irregular Galaxies of the M81 Group: The SINGS and THINGS View

We present observations of the dust and atomic gas phase in seven dwarf irregular galaxies of the M81 group from the Spitzer SINGS and VLA THINGS surveys. The Spitzer observations provide a first glimpse of the nature of the nonatomic ISM in these metal-poor (Z ~ 0.1 Z_☉), quiescent (SFR ~ 0.001-0.1 M_☉ yr^(-1)) dwarf galaxies. Most detected dust emission is restricted to H I column densities >1 × 10^(21) cm^(-2), and almost all regions of high H I column density (>2.5 × 10^(21) cm^(-2)) have associated dust emission. Spitzer spectroscopy of two regions in the brightest galaxies (IC 2574 and Holmberg II) show distinctly different spectral shapes and aromatic features, although the galaxies have comparable gas-phase metallicities. This result emphasizes that the strength of the aromatic features is not a simple linear function of metallicity. We estimate dust masses of ~10^(4)-10^(6) M_☉ for the M81 dwarf galaxies, resulting in an average dust-to-gas ratio (M_(dust)/M_(H I)) of ~3 × 10^(-4) (1.5 × 10^(-3) if only the H I that is associated with dust emission is considered); this is an order of magnitude lower than the typical value derived for the SINGS spirals. The dwarf galaxies are underluminous per unit star formation rate at 70 μm as compared to the more massive galaxies in SINGS by a factor of ~2. However, the average 70/160 μm ratio in the sample dwarf galaxies is higher than what is found in the other galaxies of the SINGS sample. This can be explained by a combination of a lower dust content in conjunction with a higher dust temperature in the dwarfs.

The Herschel Dwarf Galaxy Survey - I. Properties of the low-metallicity ISM from PACS spectroscopy

Context. The far-infrared (FIR) lines are important tracers of the cooling and physical conditions of the interstellar medium (ISM) and are rapidly becoming workhorse diagnostics for galaxies throughout the universe. There are clear indications of a different behavior of these lines at low metallicity that needs to be explored. Aims. Our goal is to explain the main differences and trends observed in the FIR line emission of dwarf galaxies compared to more metal-rich galaxies, and how this translates in ISM properties. Methods. We present Herschel/PACS spectroscopic observations of the [C ii] 157 μm, [O i] 63 and 145 μm, [O iii] 88 μm, [N ii] 122 and 205 μm, and [N iii] 57 μm fine-structure cooling lines in a sample of 48 low-metallicity star-forming galaxies of the guaranteed time key program Dwarf Galaxy Survey. We correlate PACS line ratios and line-to-LTIR ratios with LTIR, LTIR/LB, metallicity, and FIR color, and interpret the observed trends in terms of ISM conditions and phase filling factors with Cloudy radiative transfer models. Results. We find that the FIR lines together account for up to 3 percent of LTIR and that star-forming regions dominate the overall emission in dwarf galaxies. Compared to metal-rich galaxies, the ratios of [O iii]88/[N ii]122 and [N iii]57/[N ii]122 are high, indicative of hard radiation fields. In the photodissociation region (PDR), the [C ii]157/[O i]63 ratio is slightly higher than in metal-rich galaxies, with a small increase with metallicity, and the [O i]145/[O i]63 ratio is generally lower than 0.1, demonstrating that optical depth effects should be small on the scales probed. The [O iii]88/[O i]63 ratio can be used as an indicator of the ionized gas/PDR filling factor, and is found to be ~4 times higher in the dwarfs than in metal-rich galaxies. The high [C ii]/LTIR, [O i]/LTIR, and [O iii]/LTIR ratios, which decrease with increasing LTIR and LTIR/LB, are interpreted as a combination of moderate far-UV fields and a low PDR covering factor. Harboring compact phases of a low filling factor and a large volume filling factor of diffuse gas, the ISM of low-metallicity dwarf galaxies has a more porous structure than that of metal-rich galaxies.

Evidence for a non-universal Kennicutt-Schmidt relationship using hierarchical Bayesian linear regression

For investigating the relationship between the star formation rate and gas surface density, we develop a Bayesian linear regression method that rigorously treats measurement uncertainties and accounts for hierarchical data structure. The hierarchical Bayesian method simultaneously estimates the intercept, slope, and scatter about the regression line of each individual subject (e.g. a galaxy) and the population (e.g. an ensemble of galaxies). Using synthetic datasets, we demonstrate that the method accurately recovers the underlying parameters of both the individuals and the population, especially when compared to commonly employed ordinary least squares techniques, such as the bisector fit. We apply the hierarchical Bayesian method to estimate the Kennicutt-Schmidt (KS) parameters of a sample of spiral galaxies compiled by Bigiel et al. (2008). We find significant variation in the KS parameters, indicating that no single KS relationship holds for all galaxies. This suggests that the relationship between molecular gas and star formation differs from galaxy to galaxy, possibly due to the influence of other physical properties within a given galaxy, such as metallicity, molecular gas fraction, stellar mass, and/or magnetic fields. In four of the seven galaxies the slope estimates are sub-linear, especially for M51, where unity is excluded at the 2� level. We estimate the mean index of the KS relationship for the population to be 0.84, with 2� range [0.63, 1.0]. For the galaxies with sub-linear KS relationships, a possible interpretation is that CO emission is tracing some molecular gas that is not directly associated with star formation. Equivalently, a sub-linear KS relationship may be indicative of an increasing gas depletion time at higher surface densities, as traced by CO emission. The hierarchical Bayesian method can account for all sources of uncertainties, including variations in the conversion of observed intensities to star formation rates and gas surface densities (e.g. the XCO factor), and is therefore well suited for a thorough statistical analysis of the KS relationship.

TIGHTLY CORRELATED H I AND FUV EMISSION IN THE OUTSKIRTS OF M83

We compare sensitive H I data from The H I Nearby Galaxy Survey and deep far-ultraviolet (FUV) data from the Galaxy Evolution Explorer in the outer disk of M83. The FUV and H I maps show a stunning spatial correlation out to almost 4 optical radii (r 25), roughly the extent of our maps. This underscores that H I traces the gas reservoir for outer-disk star formation (SF), and it implies that massive (at least low level) SF proceeds almost everywhere that H I is observed. Whereas the average FUV intensity steadily decreases with increasing radius before leveling off at ~1.7 r 25, the decline in H I surface density is more subtle. Low H I columns (2 M ? pc?2) contribute most of the mass in the outer disk, which is not the case within r 25. The time for SF to consume the available H I, inferred from the ratio of H I to FUV intensity, rises with increasing radius before leveling off at ~100?Gyr, i.e., many Hubble times, near ~1.7 r 25. Assuming that the relatively short H2 depletion times observed in the inner parts of galaxies hold in outer disks, the conversion of H I into bound, molecular clouds seems to limit SF in outer galaxy disks. The long consumption times suggest that most of the extended H I observed in M83 will not be consumed by in situ SF. However, even these low SF rates are enough for moderate chemical enrichment in a closed outer disk to be expected.

Variations in the Star Formation Efficiency of the Dense Molecular Gas across the Disks of Star-Forming Galaxies

We present a new survey of HCN(1-0) emission, a tracer of dense molecular gas, focused on the little-explored regime of normal star-forming galaxy disks. Combining HCN, CO, and infrared (IR) emission, we investigate the role of dense gas in Star Formation (SF), finding systematic variations in both the apparent dense gas fraction and the apparent SF efficiency (SFE) of dense gas. The latter may be unexpected, given the popularity of gas density threshold models to explain SF scaling relations. We used the IRAM 30-m telescope to observe HCN(1-0) across 29 nearby disk galaxies whose CO(2-1) emission has previously been mapped by the HERACLES survey. Because our observations span a range of galactocentric radii, we are able to investigate the properties of the dense gas as a function of local conditions. We focus on how the IR/CO, HCN/CO, and IR/HCN ratios (observational cognates of the SFE, dense gas fraction, and dense gas SFE) depend on the stellar surface density and the molecular/atomic ratio. The HCN/CO ratio correlates tightly with these two parameters across a range of 2.1 dex and increases in the high surface density parts of galaxies. Simultaneously, the IR/HCN ratio decreases systematically with these same parameters and is ~6-8 times lower near galaxy centers than in the outer regions. For fixed line-mass conversion factors, these results are incompatible with a simple model in which SF depends only on the gas mass above some density threshold. Only a specific set of environment-dependent conversion factors can render our observations compatible with such a model. Whole cloud models, such as the theory of turbulence regulated SF, do a better job of matching our data. We explore one such model in which variations in the Mach number and in the mean density would respectively drive the trends within galaxy disks and the differences between disk and merging galaxies (abridged).