eaa.iop.org DOI: 10.1888/0333750888/1583 Starburst Galaxies T Heckman From Encyclopedia of Astronomy & Astrophysics P. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888 Institute of Physics Publishing Bristol and Philadelphia Downloaded on Thu Mar 02 23:48:03 GMT 2006 [131.215.103.76] Terms and Conditions Starburst Galaxies E N C Y C LO P E D IA O F A S TRONOMY AND A STROPHYSICS Starburst Galaxies hydrogen). The production of these ionizing photons is dominated by the most massive, shortest-lived stars A starburst galaxy is one undergoing a brief episode (M>25M and lifetimes less than 7 million years). of intense STAR FORMATION, usually in its central region. Measurements of Q are based on measurements of the The massive stars in the burst generate most of the rate at which H ions and electrons recombine, which total luminosity of the entire galaxy. Starburst galaxies in turn is measured by the luminosity of the so-called are fascinating objects in their own right and are the recombination emission lines of hydrogen (after the sites where roughly 25% of all the massive stars in the luminosity is corrected for absorption due to dust inside local universe are being formed. They offer unique the starburst). L Q laboratories for the study of the formation and evolution Given a measurement of bol or , we can estimate of massive stars, the effects of massive stars on the the mass of the population of massive stars required to interstellar medium and the physical processes that were produce this quantity. If we divide this mass by the lifetime important in building galaxies and chemically enriching of the appropriately massive stars we obtain an estimate the INTERGALACTIC MEDIUM. of the mean formation rate of the massive stars. This technique tells us almost nothing about the rate at which What is a starburst? low-mass stars (like our Sun) are being formed, since these Overview L Q stars contribute only a negligible amount to bol or . There is no rigorous definition of a starburst galaxy, but Indeed, it is entirely possible that starbursts form only several different criteria are often used. These are clear massive stars (unlike the mode of star formation in normal conceptually, but each is subject to uncertainties in its galaxies like our own). If we assume that starbursts form application. a normal complement of low-mass stars, the implied star- − Starbursts are usually located in the centers of formation rates usually range from 1M to 100M yr 1. galaxies, and have typical radii of 100–1000 pc (1–10% of the size of their ‘host’ galaxy). Despite their small size, The burst intensity they are converting gas into massive stars at a rate that A useful way to define a starburst is to consider the burst exceeds that found throughout the rest of their host galaxy. intensity—the rate of star formation per unit area ( — The starburst is ‘fueled’ by a copious supply of interstellar SFR typically given in units of M per year per kiloparsec2). gas (primarily in the form of molecular hydrogen) that has In normal star-forming galaxies like our own Milky Way, been accumulated in the center of the galaxy. The available the star-formation rate is a few M per year throughout a gas supply is sufficient to sustain the current rate of star 8 galactic disk with a radius of about 10 kpc (e.g. SFR has a formation for only of order 10 yr (∼1% of the age of the − − − typical value of 10 2M yr 1 kpc 2). In a typical starburst universe). The dust grains associated with the molecular − galaxy, the star-formation rate would be 10M yr 1 in a gas usually absorb most of the radiation produced by the region with a radius of 0.5 kpc. The implied SFR is then burst’s stars. This can make it difficult to determine many − − 10M yr 1 kpc 2,or103 times greater than in a normal of the basic properties of starbursts. galaxy. Star-formation rates It is important to emphasize that there is no particular We do not directly measure the star-formation rate in ‘magic’ value for SFR that separates normal and starburst a starburst but must infer it from measurements of the galaxies. A continuum of values is observed, spanning at starburst’s luminous output. Two techniques are routinely least 6 orders of magnitude from the most quiescent star- used. forming normal galaxies to the most intense starbursts. The first is to use the bolometric luminosity of the Although calculating a global value for SFR is a useful L way to quantify a starburst, star formation is not uniformly starburst ( bol), which is the luminosity integrated over L the entire electromagnetic spectrum. bol for a starburst is distributed throughout the bursting region. Instead, the dominated by the emission from hot, massive, short-lived star formation occurs both in compact (few parsec scale) stars. These have masses greater than about 8 times the star clusters and in a more smoothly distributed mode. mass of the Sun (M>8M) with lifetimes of less than The most massive CLUSTERS (the ‘super star clusters’) have 5 6 M about 40 million years. The bulk of the radiation from estimated masses of (10 –10 ) and may be close analogs these stars is emitted in the ultraviolet part of the spectrum. to young GLOBULAR CLUSTERS. Dust grains in interstellar medium of the starburst absorb this radiation, are heated, and cool by emitting far-infrared The burst duration L radiation. Thus, a good approximation of bol is given A starburst is by definition a transient event. The duration by the sum of the starburst’s ultraviolet and infrared of a starburst (t) must be much smaller than the age of luminosities. the galaxy in which it occurs (t 1010 yr). Another The second technique uses measurements of the way of stating this requirement is that the present rate of amount of ionizing radiation produced by the starburst star formation must greatly exceed the past rate averaged (often denoted by Q, the total number of photons over the age of the galaxy. Unfortunately, it is difficult to produced per second that are capable of ionizing atomic accurately determine how long starbursts last. Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 1 Starburst Galaxies E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS Perhaps the most commonly used technique to stellar photons with energies sufficient to photoionize estimate t is to calculate the gas-depletion time: the mass neutral hydrogen (E>13.6eV,orλ<912 Å). of interstellar gas in the starburst divided by the present The subsequent recombination of hydrogen or helium rate of star formation. This is then a rough estimate of ions and resulting radiative cascade produce H and He how much longer the starburst can be sustained before recombination emission lines. The free thermal electrons running out of gas. Gas-depletion times in starbursts in the gas can also collisionally excite ions, whose radiative are usually of order 108 yr, but they are highly uncertain decay produces emission lines. Many of the strongest for many reasons. For one thing, the mass of molecular lines of both types are in the visible part of the spectrum, gas is difficult to determine to better than a factor of a and together constitute a few per cent of the bolometric few since the determination relies on indirect arguments luminosity of the starburst. Thus, another way to find and observations of a trace molecule (usually CO). The starbursts is to search for galaxies with unusually bright estimated molecular gas masses range from 108M to emission lines. 1010M, increasing as a function of the luminosity of the The dust grains in the starburst are effective at starburst. In addition, the star-formation rate itself is quite absorbing ultraviolet photons of all wavelengths. The uncertain, primarily because we do not know the rate at grains are heated by this radiation and cool by emitting which low-mass stars are being formed (see above). radiation. The equilibrium temperatures that result from More sophisticated estimates of the duration of balancing heating and cooling rates are usually in the Q L starbursts utilize measurements of , bol and the range 10–100 K, and the emitted radiation therefore lies in starburst mass plus detailed information on the stellar the mid- and far-infared spectral region (λ ∼ 30–300 µm). population (for example the relative numbers of massive The survey by the INFRARED ASTRONOMY SATELLITE (IRAS) in main sequence and post-main-sequence stars such as red the 1980s (which surveyed nearly the entire sky in the mid supergiants and Wolf–Rayet stars). These measurements and far infrared) has produced the most extensive and are then compared with models for the evolution of best-studied sample of starbursts. a population of massive stars (see STELLAR EVOLUTION). These three types of surveys select samples of Resulting estimates for burst duration range from a few starbursts that overlap one another but nevertheless million years for the smallest and least powerful starbursts have important systematic differences. The most direct (which occur in dwarf galaxies), up to 107 or 108 yr for difference is that dusty starbursts are preferentially powerful starbursts. detected in the far-infared surveys, while the less dusty More generally, the minimum possible duration of a starbursts are preferentially found by the ultraviolet starburst is set by considerations of causality: the duration and emission-line surveys.