Star Formation in Galaxies Along the Hubble Sequence

TL;DR

Kennicutt reviews how star formation rates vary along the Hubble sequence, separating disk and circumnuclear star formation and cataloguing SFR diagnostics from UV to FIR. It demonstrates that disk SFRs follow a Schmidt law with N ~ 1.4 linked to gas content, while circumnuclear bursts are driven by gas inflows and dynamical processes, often reaching extreme efficiencies in luminous IR systems. The work interprets these trends within an evolutionary framework, highlighting the central roles of gas supply, bars, and interactions in shaping SF histories and the Hubble sequence, and discusses the uncertainties in calibrations and the need for spatially-resolved studies. Looking ahead, it outlines how future facilities will refine SFR measurements across cosmic time and enable detailed tests of the physical regulation of star formation in galaxies.

Abstract

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 properties 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.

Figures (9)

• Figure 1: Integrated spectra of elliptical, spiral, and irregular galaxies, from Kennicutt (1992b). The fluxes have been normalized to unity at 5500 Å.
• Figure 2: Relationship between SFR per unit broadband luminosity in the $UBV$ passbands and integrated color, from the evolutionary synthesis models of Kennicutt et al (1994). The models are for 10-billion-year-old disks, a Salpeter IMF, and exponential star formation histories. The $U$, $B$, and $V$ luminosities are normalized to those of the Sun in the respective bandpasses.
• Figure 3: Distribution of integrated H$\alpha$+[NII] emission-line equivalent widths for a large sample of nearby spiral galaxies, subdivided by Hubble type and bar morphology. The right axis scale shows corresponding values of the stellar birthrate parameter $b$, which is the ratio of the present SFR to that averaged over the past, as described in Section 5.1.
• Figure 4: Distributions of 40- to 120-$\mu$m infrared luminosity for nearby galaxies, normalized to near-infrared $H$ luminosity, as a function of Hubble type. Adapted from Devereux & Hameed (1997), with elliptical and irregular galaxies excluded.
• Figure 5: Correlation between disk-averaged SFR per unit area and average gas surface density, for 61 normal disk galaxies. Symbols are coded by Hubble type: Sa--Sab (open triangles); Sb--Sbc (open circles); Sc--Sd (solid points); Irr (cross). The dashed and dotted lines show lines of constant global star formation efficiency. The error bars indicate the typical uncertainties for a given galaxy, including systematic errors.