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Galaxies and the Universe - Starburst Galaxies

Galaxies and the Universe - Starburst Galaxies - Gala ie...

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Unformatted text preview: 1/15/12 Gala ie and he Uni e e - S a b Sab S e ga a ie , hei c ei, h 50. S f hi a be Gala ie Ga a ie e ide ce f a ece a d a ie i c ea e i SFR b a ch a a fac f a ge Ba e - i e i i a d e i ae id h high L(IR)/LB a id ga c i i e ca e M(ga )/SFR a g adi c i eii The b a be ga a - ide c fi ed a a egi ab he c e Ma (b b ea a ) a e a cia ed i h i e ac i g e gi g ga a ie e e e ed i f - i i ed a e f UV- b igh ga a ie ( he Ma a ia ca a ica ec a e e b h e f H II egi , i h a b e e a c i a he i eg a ed ec f he e NGC 7714 f he Ke ic ec (fe h d ed a ec ca e ). .Sab ae g g) IR- b igh e . Thei d g i e e i i , a ee i aaa: The e f edde i g a d b c a i i a b ic e , a d a e de ai ed i e e a i f hei ci a de i i -i e e ie c ica ed. Ca e i e a . (1994 A J 429, 582) de i ed a effec i e edde i g a , i c di g effec f ca e i g a d he i f a a d d , hich cha ge e a ica de e di g he e a ici f he e a d he fa - IR f ac i . The e i e ide ce ha he edde i g f he ga ad a i e a ica diffe e , e ha d e d a cia ed ecifica i h he ga e i i egi , a d .a . a.ed /keel/gala ie / a b .h ml 1/7 1/15/12 Gala ie and he Uni e e - S a b Gala ie , . M ( , 108 , ), ; 30 D 357: M LMC , HST UV .S I .T M , W , .I ( M " W " , ). T ' , MV=- 15 ( .T NGC 4569 .O .E - 19 H II , H II ), , (30 D ) (NGC 604) . Sab me chani m : M , , .O ' CO , ' .H ( " T "). .W 10% ), .A .a . a.ed /keel/gala ie / a b ( .h ml .T 2/7 1/15/12 Gala ie and he Uni e e - S a b i a i ai e e ie ce a i c ea e i SFR ag i de. The b i f e c fi ed a fe c . Thi efe e ce f di bed ga a ie e ha ce e (a d h a ea c ib e Gala ie ica 30%, hi e a fe e e ie ce i c ea e f a de h d ed a ec ea he c e , a h gh di - ide b ha ed a a ge f ec a i ha ca e he ab ). f ae Clo d colli ion in a pe be d di k. If ga c d i a di ha e bi ha a c e e a he , a ea i e i e ba i he e ia c d ca e c i i ha d he i e a e ace. U de he ide ead a i ha c d c i i a e i i g ie f a f a i , hi c d ead a e e i i e de e de ce f SFR e ba i (Li , P i g e, a d Ree 1988 A J 328, 103). S c - Ma ce a d Sca (1987 A JS 64, 39) fi d ha he a e f c i i de e d e ii e he a i f i e ca e be ee he ife i e f a c d a d he ea c i i i e a , a d ha he SFR h d de g a ge e c i ab e a d be he ea . Colli ion be e e n clo d o iginall be longing o diffe e n gala ie . Si i a , d i g i e e a a i g ec e e ge , c d igh c ide a a e f h ica e a f di . The e i ece a i a ag a e ba ie he e, a ide adia a ge f ca i c d be affec ed. C i i e ci ie c d bec e i e high, i hich ca e h c i i a i di cia i f ec a a e ia d be i a . M de b O a d K a (1990 A J 349, 480) gge ha e f ac i f high- e ci c d c ii be ca ab e f ie di g efficie a f a i , a id c e e di i f ec a a e ia ih f i g a . Ba , incl ding idall ind ce d ba , and adial ga mo ion . If a ida di bed e ha a a ge e gh egi i h a id- b d - i e a i c e, he c a i ca i d ce a ba ha a ch ge ha he e c e i e f. N g chi (1988 A&A 203, 459) ha h ha hi ca ead b a ia cha e i g f ga i he c ea egi , e ha eadi g a c ea a b . I hi ca e, i e ac i - i d ced a f a i igh ie bi h gica e ide ce f a e c e , e ce f he e e ce f he ba . Tidall ind ce d de n i a e . Tida e ba i ca d ce e ced i a de i a e ,a i he e - died ca e f M51 (T ea dT e 1972 A J 178, 623, H a d a d B d 1990 AJ 99, 1798). The e e ia i i a ide fa ab e i e f acc ai fi e e a a e a d a f a i , i h he e ha ce e bei g i he f ac i f he di cc ied b g de i ea a he ha a e ce f igge i g a f a i . Di k in abili ie p od ce d b pe ba ion in he po e n ial. The e i a e a ab e ag ee e be ee egi f i a i hich a f a i i b e ed, a d egi i hich ga i ab e b he T e (1964 A J 139, 1217) a d Q i (1972 A JL 176, L9) d a ica c i e i (Za a d Si a 1988 A f, 29, 190; Ke ic 1989 A J 344, 685). A c a i c d di he a i c e (a d ca e ic c ic f e e c ) b e gh e de addi i a ga i he e di ce ab e c a e ( e ha ia a ha e cha ge i ec a a e ia ). Thi ee ha e bee fi gge ed b E. La i ai e . U e ed i e i c de he he he i e ac i a d c a e i e ca e a e c a ib e, a d he a ia di ib i f e ig a f ai . D mping of ga in o E/S0 e m . O he i e ga ga a ie c d ac i e ig ifica a e ia f a f a i if h ica a fe f ga a e ace d i g a e c e i h a ga - ich e . S i a (1988 A f. 28, 495) f d ha he e c di i f ga a fe a e i i a ig ifica f ac i fc e ec e be ee a ia e ga a e , b i i c ea f e i i - i e a i ic h f e hi igh a e ace. C i , he e c ea ee ch a d i g a e ace, he ga a ecei i g he ga a ha e de ec ab e a f a i , e ha ea i g ha a c i ica a ( face de i ) i eeded igge a b ief ab af e a ged acc e i e i de. Di e c impac of ga - ich d a f a e lli e in o di k : Thi i a a ge- ca e a ia f he ic e f c d c i i be ee ga a ie . I hi ca e, he b i i e ac i i h a b igh c ai i he e .a . a.ed /keel/gala ie / a b .h ml 3/7 1/15/12 Gala ie and he Uni e e - S a b Gala ie causing the fireworks. Statistics of faint companions are not yet well enough determined to tell how important this process might be. From general considerations, some of the induced star formation must be triggered by processes not requiring direct contact of disk material from different galaxies; some objects with high SFR are too far apart, and relatively undisturbed, so that internal effects of tidal stress must be responsible. Detailed modelling is thwarted by the great range of relevant physical scales in some of these cases. In testing these proposals, studies of the ISM in interacting systems, and understanding their dynamics, are crucial. For example, H2 masses in combination with SFR estimates can suggest whether the SFR goes up because of creation or accumulation of new molecular gas (and normal accompanying star formation), or via an enhancement of the "efficiency" of star formation. A survey of 13 merger candidates by Young et al. (1986 ApJL 311, L17) suggested that the SFR reflects large molecular gas content; more recent results (Young, IAU Symp. 146) extend this by suggesting that the H2/H I mass ratio is systematically larger in interacting systems than in normal spirals. CO surveys of complete and well- understood sets of both interacting and non- interacting galaxies are urgently needed (and in progress). For very luminous galaxies which are dusty enough that most of their power emerges in the far- IR (once known as IRAS galaxies, now sharing such acronyms as LIRG, ULIRGs, PIGs, or ELFs), it can be subtle to tell whether the dominant energy source is a starburst or AGN. Compact, flat- spectrum radio sources indicate an AGN, but more diffuse radio emission can come from star- forming nuclei as well. Condon and Broderick (1988 AJ 96, 30) have introduced a ratio of radio and far- IR flux densities as a discriminant, based on the empirical relation found for star- forming regions and the fact that powerful AGN are usually more radio- loud. Mid- IR spectra have proven to be very useful, since these photons emerge through the surrounding dust. High- ionization species indicate an AGN, while their lack and strong PAH features (destroyed by the intense hard radiation from an AGN) suggest a starburst. Laurent et al. (2000 A&A 359, 887) find that while the "unidentified" PAH band at 6.2 microns occurs only in starbursts, there is dust continuum emission from 3- 10 microns which is characteristic only ofthe very hot region around an AGN. Of course, a relatively unobscured nucleus can be classified from its optical spectrum. Some nuclei of both flavors are so dusty that opacity effects control what we see in the visible range. As examples, I'll point out the dusty nuclei of NGC 253 and 2903, in which the dust blocks most of the star formation, looks chaotic, and shows streamers probably associated with global winds. To stress how powerful these differential opacity effects are, the nucleus of NGC 1614 has a large Balmer decrement (Hα/ H = 10) but a flat UV spectrum and detectable Lyman α emission, so that we are seeing different regions at different wavelengths. Here's the optical image of the center of NGC 253, from the Hubble Heritage collection: .a . a.ed /keel/gala ie / a b .h ml 4/7 1/15/12 Gala ie and he Uni e e - S a b Gala ie The high energy densities, both in starlight and mechanical input through stellar winds and supernovae, can actually unbind the ISM from starburst galaxies. The heated ISM can set up a global (or super) wind, detetcable in optical line emission, scattered starlight, and soft X- rays (most prominently from the interface at the edge of the roughly conical outflow). Most of the escaping matter can be so hot that we don't even see it in X- rays, cooling only at the interface with less disturbed ISM. This wind may be important in forming early- type galaxies, since one has to sweep the gas out of a merger product if it's going to end up as an elliptical. Something like this seems to have happened early in the history of clusters and groups, since intracluster X- ray gas shows chemical traces of having been processed by massive stars. The best- known example of a starburst wind is blowing out of M82, as shown in this image with H emission coded red. Compare the Chandra image (somewhat rotated) to see how even the part of the wind that does show up can dominate the X- ray emission. Winds are also often seen via P Cygni profiles of some absorption lines - Na D has been used for optical surveys, and these lines are so strong in high- redshift Lyman- break galaxies that they make it difficult to get an accurate redshift for just the stars, much less see stellar absorption lines from many atomic species. .a . a.ed /keel/gala ie / a b .h ml 5/7 1/15/12 Gala ie and he Uni e e - S a b Gala ie Starbursts may be the best local analogs to galaxies during their formation, with large amounts of both gas and stellar energy input present. Indeed, many high- redshift galaxies shows the characteristic UV spectra of very young stellar populations. The implications of this are not really straightforward, though because of selection effects in both UV flux (the kind we see when redshifted at =4 or so) and surface brightness (so that starforming objects and regions within them are the easiest things to identify at large redshift). A cosmology with expanding spacetime gives a surface- brightness dimming going as 1/(1+z)4, which is very substantial for cosmologically interesting redshifts. Of special interest is Lyman emission, which generally traces a wind (radiative- transfer effects make this the easiest geometry for the line photons to escape without being converted into something else during resonant scattering - see a spectral example). Starburst galaxies have calculated star- forming rates as high as hundreds of solar masses per year (exhaustion timescales of order 108 years), and correspondingly high expected supoernova rates. Searches for the expected supernovae have had mixed results. High- resolution radio observations of M82 and NGC 253 shows rich .a . a.ed /keel/gala ie / a b .h ml 6/7 1/15/12 Gala ie and he Uni e e - S a b Gala ie collections of small (sometimes fading and expanding) sources that are just right to be radio- bright supernova remnants, so that part checks out. Looking for the supernovae themselves has been less successful, with only a handful seen in starburst nuclei (against formidable background and confusion problems). There has been a better track record in near- IR monitoring, such as finding an obscured SN in NGC 3690 within a fairly short time. However, this becomes a very intense use of telescope time, so it has yet to be pursued on an appropriately large scale. What do fading starbursts look like? Stellar evolutionary models lead us to expect galaxies that are fairly blue (but rapidly reddening with time unless the burst was of large relative mass amplitude), whose spectral features are dominated by either supergiants or the upper main sequennce. This would account for the "E+A" galaxies which show a mixture of old and intermediate- age spectral features, and for the small population of cluster members with anomalously strong H absorption, since this line will be the most prominent unconfused feature against an older background after ~109 years. It is still unclear whether the relative numbers of starbursts and post- starbursts are right to conclude that we understand the connection, since such different technques are use to recognize them. « Star formation in galaxies | Environmental effects on galaxies » Course Home | Bill Keel's Home Page | Image Usage and Copyright Info | UA Astronomy k [email protected] Ls cags 920 at hne: /09 .a . a.ed /keel/gala ie / a b .h ml . a.ed 20-09 0020 7/7 ...
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