We report measurements of the mass density, Omega_M, and
cosmological-constant energy density, Omega_Lambda, of the universe based on
the analysis of 42 Type Ia supernovae discovered by the Supernova Cosmology
Project. The magnitude-redshift data for these SNe, at redshifts between 0.18
and 0.83, are fit jointly with a set of SNe from the Calan/Tololo Supernova
Survey, at redshifts below 0.1, to yield values for the cosmological
parameters. All SN peak magnitudes are standardized using a SN Ia lightcurve
width-luminosity relation. The measurement yields a joint probability
distribution of the cosmological parameters that is approximated by the
relation 0.8 Omega_M - 0.6 Omega_Lambda ~= -0.2 +/- 0.1 in the region of
interest (Omega_M <~ 1.5). For a flat (Omega_M + Omega_Lambda = 1) cosmology we
find Omega_M = 0.28{+0.09,-0.08} (1 sigma statistical) {+0.05,-0.04}
(identified systematics). The data are strongly inconsistent with a Lambda = 0
flat cosmology, the simplest inflationary universe model. An open, Lambda = 0
cosmology also does not fit the data well: the data indicate that the
cosmological constant is non-zero and positive, with a confidence of P(Lambda >
0) = 99%, including the identified systematic uncertainties. The best-fit age
of the universe relative to the Hubble time is t_0 = 14.9{+1.4,-1.1} (0.63/h)
Gyr for a flat cosmology. The size of our sample allows us to perform a variety
of statistical tests to check for possible systematic errors and biases. We
find no significant differences in either the host reddening distribution or
Malmquist bias between the low-redshift Calan/Tololo sample and our
high-redshift sample. The conclusions are robust whether or not a
width-luminosity relation is used to standardize the SN peak magnitudes.

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Measurements of Omega and Lambda from 42 High-Redshift Supernovae

A universal initial mass function (IMF) is not intuitive, but so far no convincing evidence for a variable IMF exists. The detection of systematic variations of the IMF with star-forming conditions would be the Rosetta Stone for star formation.



In this contribution an average or Galactic-field IMF is defined, stressing that there is evidence for a change in the power-law index at only two masses: near 0.5 M⊙ and near 0.08 M⊙. Using this supposed universal IMF, the uncertainty inherent in any observational estimate of the IMF is investigated by studying the scatter introduced by Poisson noise and the dynamical evolution of star clusters. It is found that this apparent scatter reproduces quite well the observed scatter in power-law index determinations, thus defining the fundamental limit within which any true variation becomes undetectable. The absence of evidence for a variable IMF means that any true variation of the IMF in well-studied populations must be smaller than this scatter.



Determinations of the power-law indices α are subject to systematic errors arising mostly from unresolved binaries. The systematic bias is quantified here, with the result that the single-star IMFs for young star clusters are systematically steeper by Δα≈0.5 between 0.1 and 1 M⊙ than the Galactic-field IMF, which is populated by, on average, about 5-Gyr-old stars. The MFs in globular clusters appear to be, on average, systematically flatter than the Galactic-field IMF (Piotto & Zoccali; Paresce & De Marchi), and the recent detection of ancient white-dwarf candidates in the Galactic halo and the absence of associated low-mass stars (Ibata et al.; Mendez & Minniti) suggest a radically different IMF for this ancient population. Star formation in higher metallicity environments thus appears to produce relatively more low-mass stars. While still tentative, this is an interesting trend, being consistent with a systematic variation of the IMF as expected from theoretical arguments.

/pdf/on-the-variation-of-the-initial-mass-function-30qbs7h433.pdf

On the variation of the initial mass function

Observations of star formation rates (SFRs) in galaxies provide vital clues to the physical nature of the Hubble sequence and are key probes of the evolutionary histories of galaxies. The focus of this review is on the broad patterns in the star formation properties of galaxies along the Hubble sequence and their implications for understanding galaxy evolution and the physical processes that drive the evolution. Star formation in the disks and nuclear regions of galaxies are reviewed separately, then discussed within a common interpretive framework. The diagnostic methods used to measure SFRs are also reviewed, and a self-consistent set of SFR calibrations is presented as an aid to workers in the field. One of the most recognizable features of galaxies along the Hubble sequence is the wide range in young stellar content and star formation activity. This variation in stellar content is part of the basis of the Hubble classification itself (Hubble 1926), and understanding its physical nature and origins is fundamental to understanding galaxy evolution in its broader context. This review deals with the global star formation properties of galaxies, the systematics of those properties along the Hubble sequence, and their implications for galactic evolution. I interpret “Hubble sequence” in this context very loosely, to encompass not only morphological type but other properties such as gas content, mass, bar structure, and dynamical environment, which can strongly influence the largescale star formation rate (SFR).

/pdf/star-formation-in-galaxies-along-the-hubble-sequence-31q0r4zp4u.pdf

Star formation in galaxies along the hubble sequence

▪ Abstract At luminosities above 1011 , infrared galaxies become the dominant population of extragalactic objects in the local Universe (z ≲ 0.3), being more numerous than optically selected starburst and Seyfert galaxies and quasi-stellar objects at comparable bolometric luminosity. The trigger for the intense infrared emission appears to be the strong interaction/merger of molecular gas-rich spirals, and the bulk of the infrared luminosity for all but the most luminous objects is due to dust heating from an intense starburst within giant molecular clouds. At the highest luminosities (Lir > 1012 ), nearly all objects appear to be advanced mergers powered by a mixture of circumnuclear starburst and active galactic nucleus energy sources, both of which are fueled by an enormous concentration of molecular gas that has been funneled into the merger nucleus. These ultraluminous infrared galaxies may represent an important stage in the formation of quasi-stellar objects and powerful radio galaxies. They may al...

/pdf/luminous-infrared-galaxies-zoi3y3hutv.pdf

Luminous infrared galaxies

We review progress over the past decade in observations of large-scale star formation, with a focus on the interface between extragalactic and Galactic studies. Methods of measuring gas contents and star-formation rates are discussed, and updated prescriptions for calculating star-formation rates are provided. We review relations between star formation and gas on scales ranging from entire galaxies to individual molecular clouds.

Star Formation in the Milky Way and Nearby Galaxies

Results are reported from new observations of large-scale H-alpha filamentary structures lying outside of classical H II regions in Magellanic irregular galaxies. These structures (froth) are seen as arcs, loops, and filaments, some of which are centered on OB associations and some of which extend away from giant H II regions into regions of lower H I column density. The data show the following: (1) the level of ionization of the froth is lower than in H II regions in the same galaxies; (2) frothy features have large line velocity widths that often exceed those found in classical giant H II regions in the same galaxies; (3) the froth has H-alpha surface brightnesses of about 100 Rayleighs, 10-100 times brighter than typical diffuse Galactic emission; and (4) dimensions are large, a typical filament is 700 x 50 pc. 94 refs.

Properties of ionized interstellar froth in irregular galaxies

H I spatial power spectra were determined for a sample of 24 nearby dwarf irregular galaxies selected from the Local Irregulars That Trace Luminosity Extremes-The H I Nearby Galaxy Survey sample. The two-dimensional power spectral indices asymptotically become a constant for each galaxy when a significant part of the line profile is integrated. For narrow channel maps, the power spectra become shallower as the channel width decreases, and this shallowing trend continues to our single channel maps. This implies that even the highest velocity resolution of 1.8 kms(-1) is not smaller than the thermal dispersion of the coolest, widespread H I component. The one-dimensional power spectra of azimuthal profiles at different radii suggest that the shallower power spectra for narrower channel width is mainly contributed by the inner disks, which indicates that the inner disks have proportionally more cooler H I than the outer disks. Galaxies with lower luminosity (M-B > -14.5 mag) and star formation rate (SFR, log(SFR (M-circle dot yr(-1))) < -2.1) tend to have steeper power spectra, which implies that the H I line-of-sight depths can be comparable with the radial length scales in low-mass galaxies. A lack of a correlation between the inertial-range spectral indices and SFR surface density implies that either non-stellar power sources are playing a fundamental role in driving the interstellar medium turbulent structure or the nonlinear development of turbulent structures has little to do with the driving sources.

https://iopscience.iop.org/article/10.1088/0004-637X/754/1/29/pdf

H I Power Spectra and the Turbulent Interstellar Medium of Dwarf Irregular Galaxies

We present low- and high-dispersion emission-line observations of kiloparsec-sized, low surface brightness filaments of ionized gas in four irregular galaxies. These four galaxies span a range in sizes and star formation rates. In Sextans A, deep Hα images show that the filaments are part of a complicated shell of radius 600 pc around the eastern HII complex. In NGC 2366 there are several filaments emanating from the supershell around the giant HII region NGC 2363, extending up to 1.3 kpc from its center. In NGC 1569 we examine the brightest filament which is 800 pc in length. In NGC 4449 several filaments in the outer part of the galaxy appear to form a coherent structure of radius 1.7 kpc. However, part of the filaments fall along the inner edge of a 2 kpc diameter H I supershell. At the base of this structure near the center of the galaxy ionized gas is expanding at 30 km s-1. A comparison with models for the formation of large shells suggests that the filament in NGC 1569 and the shell in Sextans A are fit reasonably with standard stellar-powered supershell models of order 10 Myr old. The filaments in NGC 2366, on the other hand, are sufficiently far from the giant H II region NGC 2363 that they would have to be older than the age of the massive stars in the H II region and so probably are the result of a previous star-forming event. In NGC 4449 we are probably seeing several supershell events, one being the H I supershell with the filaments as the ionized inner edge and the other seen only in Hα. The structures in NGC 4449 may be too large to be entirely in the plane of the galaxy. The FWHM of the velocity profiles of the filaments are surprisingly high and are similar to those seen in normal H II regions in the same galaxies. Emission-line ratios show that the shells and filaments of all size scales are primarily photoionized. Since the supergiant filaments are located large distances from concentrations of massive stars, this implies that ionizing photons are able to travel distances of order kiloparsecs or that a diffuse source of ionizing photons exists, even in the outer parts of galaxies. Large distance propagation of ionizing radiation can most readily occur in cases such as NGC 1569 where filaments are above the Galactic plane, but long path lengths are still required. The Hα luminosities imply that the percentage of photons that must escape H II regions is about 25%. The large velocity FWHM of the emission-line profiles of supergiant filaments are understood in a photoionization model as a result of the high velocity dispersion in the neutral gas. These data show that energy input from newly created massive stars plays an important role in modifying the ISM of small galaxies and that these effects include both organized gas flows and photoionization over kiloparsec-sized distances. This process is expected to be important in regulating star formation in small, low-mass irregular galaxies.

http://iopscience.iop.org/article/10.1086/303520/pdf

An Emission-line Study of Supergiant Ionized Filaments in Irregular Galaxies*

We present UBVJHKHalpha and HI data of the irregular galaxy NGC 2366. It is a normal boxy-shaped disk seen at high inclination angle. We do not see any unambiguous observational signature of a bar. There is an asymmetrical extension of stars along one end of the major axis of the galaxy, and this is where the furthest star-forming regions are found, at 1.3R_Holmberg. The HI is normal in many respects but shows some anomalies: 1) The integrated HI shows two ridges running parallel to the major axis that deproject to a large ring. 2) The velocity field exhibits several large-scale anomalies superposed on a rotating disk. 3) The inclination and position angles derived from the kinematics differ from those dervied from the optical and HI mor- phology. 4) There are regions in the HI of unusually high velocity dispersion that correlate with deficits of HI emission in a manner suggestive of long-range, turbulent pressure equilibrium. Star-forming regions are found where the gas densities locally exceed 6 Msolar/pc^2. NGC 2366, like other irregulars, has low gas densities relative to the critical gas densities of gravitational instability models. Because of the lack of shear in the optical galaxy, there is little competition to the slow gravitational contraction that follows energy dissipation. However, the peak gas densities in the star-forming regions are equal to the local tidal densities for gravitational self-binding of a rotating cloud. Evidently the large scale gas concentrations are marginally bound against background galactic tidal forces. This condition for self-binding may be more fundamental than the instability condition because it is local, three-dimensional, and does not involve spiral arm generation as an intermediate step toward star formation.

https://arxiv.org/pdf/astro-ph/0104091.pdf

Neutral Hydrogen and Star Formation in the Irregular Galaxy NGC 2366

Radial surface brightness profiles of spiral galaxies are classified into three types: (I) single exponential, or the light falls off with one exponential to a break before falling off (II) more steeply, or (III) less steeply. Profile breaks are also found in dwarf disks, but some dwarf Type IIs are flat or increasing out to a break before falling off. Here we re-examine the stellar disk profiles of 141 dwarfs: 96 dwarf irregulars (dIms), 26 Blue Compact Dwarfs (BCDs), and 19 Magellanic-type spirals (Sms). We fit single, double, or even triple exponential profiles in up to 11 passbands: GALEX FUV and NUV, ground-based UBVJHK and Hα, and Spitzer 3.6 and 4.5 μm. We find that more luminous galaxies have brighter centers, larger inner and outer scale lengths, and breaks at larger radii; dwarf trends with MB extend to spirals. However, the V-band break surface brightness is independent of break type, MB , and Hubble type. Dwarf Type II and III profiles fall off similarly beyond the breaks but have different interiors and IIs break ~twice as far as IIIs. Outer Type II and III scale lengths may have weak trends with wavelength, but pure Type II inner scale lengths clearly decrease from the FUV to visible bands whereas Type III inner scale lengths increase with redder bands. This suggests the influence of different star formation histories on profile type, but nonetheless the break location is approximately the same in all passbands. Dwarfs continue trends between profile and Hubble types such that later-type galaxies have more Type II but fewer Type I and III profiles than early-type spirals. BCDs and Sms are over-represented as Types III and II, respectively, compared to dIms.

http://iopscience.iop.org/article/10.1088/0004-6256/146/5/104/pdf

Deidre A. Hunter

Papers

Properties of ionized interstellar froth in irregular galaxies

H I Power Spectra and the Turbulent Interstellar Medium of Dwarf Irregular Galaxies

An Emission-line Study of Supergiant Ionized Filaments in Irregular Galaxies*

Neutral Hydrogen and Star Formation in the Irregular Galaxy NGC 2366

Surface brightness profiles of dwarf galaxies. i. profiles and statistics