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

We present far-infrared (FIR) photometry at 150 and 205 micron(s) of eight low-redshift starburst galaxies obtained with the Infrared Space Observatory (ISO) ISOPHOT. Five of the eight galaxies are detected in both wave bands, and these data are used, in conjunction with IRAS archival photometry, to model the dust emission at lambda approximately greater than 40 microns. The FIR spectral energy distributions (SEDs) are best fitted by a combination of two modified Planck functions, with T approx. 40 - 55 K (warm dust) and T approx. 20-23 K (cool dust) and with a dust emissivity index epsilon = 2. The cool dust can be a major contributor to the FIR emission of starburst galaxies, representing up to 60% of the total flux. This component is heated not only by the general interstellar radiation field, but also by the starburst itself. The cool dust mass is up to approx. 150 times larger than the warm dust mass, bringing the gas-to-dust ratios of the starbursts in our sample close to Milky Way values, once resealed for the appropriate metallicity. The ratio between the total dust FIR emission in the range 1-1000 microns and the IRAS FIR emission in the range 40 - 120 microns is approx. 1.75, with small variations from galaxy to galaxy. This ratio is about 40% larger than previously inferred from data at millimeter wavelengths. Although the galaxies in our sample are generally classified as "UV bright," for four of them the UV energy emerging shortward of 0.2 microns is less than 15% of the FIR energy. On average, about 30% of the bolometric flux is coming out in the UV-to-near-IR wavelength range; the rest is emitted in the FIR. Energy balance calculations show that the FIR emission predicted by the dust reddening of the UV-to-near-IR stellar emission is within a factor of approx. 2 of the observed value in individual galaxies and within 20% when averaged over a large sample. If our sample of local starbursts is representative of high-redshift (z approx. greater than 1), UV - bright star-forming galaxies, these galaxies' FIR emission will be generally undetected in submillimeter surveys, unless: (1) their bolometric luminosity is comparable to or larger than that of ultraluminous FIR galaxies and (2) their FIR SED contains a cool dust component.

/pdf/the-dust-content-and-opacity-of-actively-star-forming-3rvrlvzkyq.pdf

The Dust Content and Opacity of Actively Star-Forming Galaxies

Over the past two decades, an avalanche of data from multiwavelength imaging and spectroscopic surveys has revolutionized our view of galaxy formation and evolution. Here we review the range of complementary techniques and theoretical tools that allow astronomers to map the cosmic history of star formation, heavy element production, and reionization of the Universe from the cosmic "dark ages" to the present epoch. A consistent picture is emerging, whereby the star-formation rate density peaked approximately 3.5 Gyr after the Big Bang, at z~1.9, and declined exponentially at later times, with an e-folding timescale of 3.9 Gyr. Half of the stellar mass observed today was formed before a redshift z = 1.3. About 25% formed before the peak of the cosmic star-formation rate density, and another 25% formed after z = 0.7. Less than ~1% of today's stars formed during the epoch of reionization. Under the assumption of a universal initial mass function, the global stellar mass density inferred at any epoch matches reasonably well the time integral of all the preceding star-formation activity. The comoving rates of star formation and central black hole accretion follow a similar rise and fall, offering evidence for co-evolution of black holes and their host galaxies. The rise of the mean metallicity of the Universe to about 0.001 solar by z = 6, one Gyr after the Big Bang, appears to have been accompanied by the production of fewer than ten hydrogen Lyman-continuum photons per baryon, a rather tight budget for cosmological reionization.

/pdf/cosmic-star-formation-history-5ce1d74cmt.pdf

Cosmic Star-Formation History

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

▪ Abstract The Universe is in transition. At early times, galactic evolution was dominated by hierarchical clustering and merging, processes that are violent and rapid. In the far future, evolution will mostly be secular—the slow rearrangement of energy and mass that results from interactions involving collective phenomena such as bars, oval disks, spiral structure, and triaxial dark halos. Both processes are important now. This review discusses internal secular evolution, concentrating on one important consequence, the buildup of dense central components in disk galaxies that look like classical, merger-built bulges but that were made slowly out of disk gas. We call these pseudobulges. We begin with an “existence proof”—a review of how bars rearrange disk gas into outer rings, inner rings, and stuff dumped onto the center. The results of numerical simulations correspond closely to the morphology of barred galaxies. In the simulations, gas is transported to small radii, where it reaches high densities and...

Secular Evolution and the Formation of Pseudobulges in Disk Galaxies

SN 2001ja was observed twice in three months using the Chandra X-Ray Observatory. The X-ray flux could be due to interaction with the circumstellar medium, perhaps dominated by the reverse shock heated thermal plasma, or from inverse Compton scattering at the forward shock. In both cases, for a steady wind-like circumstellar density profile, the X-ray flux is expected to fall off as a power law or faster. But the flux from the position of SN 2011ja, increased by a factor of three between these observations. In this presentation, we investigated possible reasons, including contamination from other astrophysical sources such as a X-Ray Binary, within the Chandra's resolution, in the host galaxy using our observations, modelling and pre-explosion Chandra/XMM data.

The strange case of SN 2011ja and its host

Radio Continuum Emission in the Resonance-Ring Galaxies NGC 1433 and NGC 6300

Fundamental Properties of a Large, Unbiased Sample of Massive, Young, Embedded Star Clusters in the Milky Way

Abstract SN 1978K is the oldest-known Type IIn supernova, and one of the closest. We report new radio observations at high frequency and spatial resolution. SN 1978K has been detected at 34 and 94 GHz with the Australia Telescope Compact Array, while Very Long Baseline Interferometry at 8.4 GHz has allowed us to derive the past average expansion velocity, which indicates significant deceleration as the blast wave interacts with the dense circumstellar medium.

/pdf/new-radio-observations-of-the-type-iin-supernova-1978k-47yihtggft.pdf

New radio observations of the Type IIn Supernova 1978K

During a supernova event, a massive star explodes, having exhausted its readily available supply of fuel for nuclear fusion. The explosion releases newly synthesized heavier elements from inside the stellar core, making these available to the cosmos for subsequent generations of star (and planet) formation. However, while these events put out more light than their entire host galaxy for a few days, optical surveys miss up to half of the supernovae predicted to be occurring.1 The problem is even worse when galaxies collide in what are called Luminous IR Galaxies (LIRGs), which then form stars at a prodigious rate, yet appear to yield hardly any supernovae. LIRGs are sufficiently cocooned in dust that their light is all but extinguished, like being in a smoke-filled room. As with firefighters, we can switch to using IR cameras on our telescope to see the heat radiation from the supernova, which can penetrate the dust more easily. However, that still is not enough, as LIRGs are clumpy and also far away, making it challenging to spot a new supernova in the act. Using the Hubble Space Telescope (HST) we can overcome the blurring effect of Earth’s atmosphere. But getting time on HST to observe a LIRG once is hard enough, let alone observing a LIRG every few months to see if a supernova has occurred. Fortunately, we can now produce IR images from the ground that are as sharp as those from HST, using the technique of laser-guide-star adaptive optics. With a laser more than 1000 brighter than a laser pointer, we create an artificial star high in the atmosphere (see Figure 1). By adjusting a small, flexible mirror inside the telescope hundreds of times a second we restore the blurred laser image to a crisp point, and in so doing sharpen up the view of any astronomical object in the background. Between 2008 and 2011 we used such a system on the Gemini North 8m telescope on Mauna Kea in Hawaii to monitor eight LIRGs for supernovae. We found six supernovae, four of Figure 1. Laser-guide-star propagation at the Gemini North 8m telescope in Hawaii, which helps to sharpen the view for instruments attached to the back of the telescope. (Reprinted with permission from the Gemini Observatory/AURA.)

Stuart D. Ryder

Papers

The strange case of SN 2011ja and its host

Radio Continuum Emission in the Resonance-Ring Galaxies NGC 1433 and NGC 6300

Fundamental Properties of a Large, Unbiased Sample of Massive, Young, Embedded Star Clusters in the Milky Way

New radio observations of the Type IIn Supernova 1978K

Seeking supernovae with laser-guide-star adaptive optics