Starburst Galaxies Encyclopedia of Astronomy & Astrophysics

eaa.iop.org DOI: 10.1888/0333750888/1583

Starburst Galaxies T Heckman

From Encyclopedia of Astronomy & Astrophysics P. Murdin

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.

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. Dust grains are made of of star formation cannot be significantly less than the time elements heavier than H or He (‘metals’). Thus, the for gas on one side of the starburst to ‘communicate’ dust-content of starbursts correlates well with chemical with gas on the other side. Such ‘signals’ will travel at composition: typically only a few per cent of ultraviolet speed of sound in the gas or the velocity induced by the radiation escapes ‘metal-rich’ starbursts (having chemical gravitational field in the starburst. These velocities will composition like the Sun) while the majority of the be of order 102 km s−1,sot > 107 yr for a starburst ultraviolet light escapes the most ‘metal-poor’ starbursts. with a diameter of 1 kpc. The most powerful and intense Metal-poor starbursts tend to be less powerful and to occur starbursts seem to turning gas into stars as fast as allowed in smaller and less massive galaxies (as a result in part of by causality. the well-known mass–metallicity relation for galaxies).

How do we find starburst galaxies? What causes a starburst? The massive stars that power starbursts are mostly very From a purely empirical point of view, the causes of star- hot (T ∼ 20 000–50 000 K) and emit most of their radiation bursts become increasingly clear for starbursts of greater in the ultraviolet between 912 Å and roughly 2000 Å. and greater luminosity. The most luminous starbursts Thus, one technique for finding starbursts is to search for in the local universe are the so-called ‘ultra-luminous in- galaxies that are unusually bright in the ultraviolet. The frared galaxies’, which have bolometric luminosities of spectral region below about 3200 Å is inaccessible from roughly 1012L (nearly all of which is emitted in the mid- the surface of the Earth, so most ultraviolet surveys for and far-infrared). The power source is deeply buried in- starbursts in the local universe have been conducted in the side a dense, dusty region of molecular gas only a few near-ultraviolet region just longward of the atmospheric hundred parsecs in size and may consist of a combination cut-off. This will change in the near future as space- of a starburst and a dust-enshrouded QUASAR. The mass based ultraviolet imaging surveys are conducted at shorter of molecular gas (∼1010M) is comparable with the entire wavelengths. mass of the interstellar medium in a big SPIRAL GALAXY. Starbursts are rich in interstellar gas and dust (the raw These ultraluminous galaxies almost invariably material for star formation). The primary radiative output have highly disturbed morphologies that are strongly from the massive stars (ultraviolet radiation) is absorbed suggestive of the ongoing or recently completed merger of by this interstellar material which then re-radiates the two large DISK GALAXIES. Specific morphological structures energy in other forms. The gas absorbs nearly all the indicative of mergers include long narrow ‘tidal tails’

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 2 Starburst Galaxies E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS of stars and gas (the remnants of the outer disks of Thus, starbursts are indeed highly significant components the merging galaxies) and double nuclei. For less of the universe, past and present. powerful starbursts the evidence suggests that more mild gravitational interactions between galaxies are an Starbursts as analogs to high-redshift galaxies important triggering mechanism (e.g. the close passage of As described above, starbursts in the local universe are two galaxies, without their subsequent merger). often selected by either their ultraviolet or their far- During the close passage of two galaxies, tidal stresses infrared continuum emission or by visible-band emission act to strongly perturb the orbits of the stars and gas in lines. At high redshifts (z>2) the rest-frame ultraviolet, the galaxy disk. The dissipation of kinetic energy as gas visible, and far-infrared emission from a star-forming collides with gas allows the gas to become sufficiently galaxy will be observed in the visible near-infrared, and displaced from the stars that gravitational torques act submillimeter spectral regions respectively (see GALAXIES between the stars and gas to transfer significant amounts of AT HIGH REDSHIFT). With the Hubble Space Telescope and angular momentum from the gas to the stars. The gas can modern 10 m class ground-based telescopes operating thereby flow into the center of the galaxy, where it can fuel in the visible and near infrared, and rapid advances in a starburst. If the passage of the two galaxies is slow and submillimeter astronomy, it is now possible to detect and interpenetrating enough, dynamical friction can transfer study such high-redshift galaxies. enough kinetic energy from the stars to the galaxy dark- The ultraviolet-selected galaxies at high redshift matter halos to allow the two galaxies to merge into a single strongly resemble similarly selected local starbursts: they galaxy. Such mergers or strong interactions should take a have similar values for SFR, similar ultraviolet colors few times the galaxy rotation period (about 109 yr), with (suggesting a similar amount of reddening due to dust the intense starburst phase being significantly shorter (see absorption), and their ultraviolet and visible spectra GALAXIES: INTERACTIONS AND MERGERS). These timescales are show that they have similar stellar populations and gas loosely consistent with independent estimates of starburst dynamics. One important difference is that the regions lifetimes discussed above. of star formation in the high-redshift galaxies are typically However, many starbursts are not found in obviously larger and more luminous than in ultraviolet-selected local interacting systems. In such cases, a stellar bar is often starbursts. Depending on the uncertain corrections for present in the inner disk of the starburst galaxy. A bar dust extinction, the ultraviolet luminosities of the most can act to rob gas of its angular momentum, and transfer powerful high-redshift galaxies imply star-formation rates gas into the center of the galaxy where it can fuel a that can reach several hundred M per year over a region a starburst. The mechanism for triggering starbursts is most few kpc in size. To date, less is known about the nature of uncertain in the lowest-power starbursts, which occur in the submillimeter-selected galaxies at high redshift. The DWARF GALAXIES. In some cases it appears that the dwarf may available information suggests that these objects resemble have recently collided with an extragalactic gas cloud, but the local ultraluminous galaxies described above. in most cases there is no clear evidence for any type of It appears that the physical processes that we can interaction or stellar bar. study in considerable detail in local starbursts are directly applicable to high-redshift galaxies. Starbursts from a cosmological perspective Starbursts in context Galactic ’superwinds’ How important are starbursts? The far-infrared IRAS One of the most important processes that has been survey produced the best-studied sample and shows that observed in both local starbursts and star-forming galaxies starbursts provide about 10% of the bolometric luminosity at high redshift is the bulk outflow of warm and of the entire local universe. Using the luminosities of the hot gas at velocities close to or even exceeding the H recombination lines to estimate the star-formation rate escape velocity from the galactic gravitational potential (as described above), it appears that roughly 25% of all the well (a phenomenon sometimes called a ‘superwind’). massive stars in the local universe are formed in starbursts, Superwinds are driven by the collective effect of the kinetic while the rest are formed in the many-kpc-sized disks of energy that is deposited in the interstellar medium by normal galaxies like our own. While the measurements stellar winds and supernova explosions. It is believed that are difficult, and their interpretation is uncertain, a similar this kinetic energy is converted (via shocks) into thermal situation seems to hold out to a REDSHIFT of about 1 (over energy inside the starburst. The resulting hot gas has a half the way back to the big bang). pressure much greater than its surroundings, and so it will At still higher redshifts, it becomes almost impossible expand most rapidly along the direction of the steepest to measure the amount of star formation in the large-scale pressure gradient in the interstellar medium (e.g. along disks of normal galaxies. The great distances and redshift- the minor axis of the galaxy’s gas disk). This leads to a dimming of the light mean that only intense and luminous poorly collimated bipolar outflow. regions of star formation can be readily detected. The In local starbursts, there are a variety of probes of detected objects appear rather similar to local starbursts superwind physics. The hot outflowing gas produces (see below), and by themselves can plausibly account thermal x-ray emission, and spectroscopy of this gas for much of the early star formation in the universe. implies temperatures of 3–10 million K. This hot gas can

T Heckman