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Astronomía Extragaláctica y Cosmología Observacional

Depto. de Astronomía (UGto). Astronomía Extragaláctica y Cosmología Observacional. Lecture 2 Properties of Galaxies. Observational Properties Photometric Properties magnitudes and luminosities colors (and color gradients) surface brightness (and photometric diameters) K-corrections

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Astronomía Extragaláctica y Cosmología Observacional

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  1. Depto. de Astronomía (UGto) Astronomía Extragaláctica y Cosmología Observacional Lecture 2 Properties of Galaxies • Observational Properties • Photometric Properties • magnitudes and luminosities • colors (and color gradients) • surface brightness (and photometric diameters) • K-corrections • brightness profiles • Spectroscopic Properties • Physical Properties • the real shape • mass, luminosity and diameter • gas fraction • stellar populations • chemical composition • Correlations of Structural Parameters • the Faber-Jackson relation (E) • the Tully-Fisher relation (S)

  2. Apparent magnitudes: m1 – m2 = – 2.5 log10 (f1/f2) m = – 2.5 log10 f + cte Absolute magnitudes: f = (D/d)2 F m – M = – 2.5 log10 (f/F) m – M = 5 log10 d[pc] – 5 + A m – M = 5 log10 d[kpc] + 10 + A m – M = 5 log10 d[Mpc] + 25 + A + K • Photometric properties: magnitudes and luminosities f = ∫0∞ fT F R d T → transmission of atm. F→ transmission of filter R→ efficiency of telescope system • Luminosity • L = Pot = E / t • Flux • F = Pot / A = E / t.A • L = A.F = 4r2 T4

  3. Photometric properties: filters and photometric systems u g r i z SDSS system [Fukugita et al. 1996, AJ 111, 1748] UBVRIJHKLM Johnson-Cousins-Glass system [Johnson & Morgan 1953, ApJ 117, 313 Cousins 1976, MNRAS 81, 25 Glass 1974, MNASSA 33, 53]

  4. V -25 -24 -23 -22 -21 -20 -19 -18 -17 -16 -15 -14 -13 -12 -11 -10 -9 -8 D/cD E S0 Sa Sb Sc Sd/Sm Im/Irr LSB dE dSph BCD • Photometric properties: absolute magnitudes of galaxies Correlates poorly with Hubble type [Roberts & Haynes 1994, ARAA 32, 115]

  5. main sequence of stars • Photometric properties: colors • Mean colors: • galaxy colors are related to mean stellar population content, metallicities, ages, SF and internal dust extinction • galaxy colors correlate with type: E, composed almost entirely of red old stars,are redder, while Irr, the least evolved systems, containing a lot of gas, are bluer • however,there are great variations in color from one galaxy to another for the same type (dispersion) Correlates well with Hubble type [Roberts & Haynes 1994, ARAA 32, 115] 

  6. NGC 4494 (E) • Photometric properties: color gradients • Color gradients: • usually E are redder in their nucleus than in their outer regions – since E are basically formed by a single pop of old stars (same age and almost no gas and no SF), the (WEAK) color gradient may be due to a gradient of metallicity! (bluer when lower metal abundance) • in S, color change globally from red to blue: from bulge to disc (STRONG color gradients) • If we consider only the disc, color indices do not seem to vary significantly – this imply a • practically constant SF (continuous inflow of gas from exterior to disc?) • Small gradients on discs due to metallicity gradients, different • degree of internal extinction by dust, or gradients on the mean • ages of stars are possible (Sbc) [Peletier et al. 1990, A&A 233, 62] [Gadotti & Dos Anjos 2001, AJ 122, 1298]

  7. Photometric properties: color gradients • Color gradients: • most E/S0 are concentrated on a spot, presenting a weak • but definite color gradient, while about 10% are bluer • in the center (emission lines, SF burst in the center) • late-type galaxies are spread on a branch below the E/S0 • (stronger gradients) that goes to positive gradients for • very late galaxies (with SF bursts in their centers) (g–i) = (g–i)R<0.5Rpet – (g–i)R>0.5Rpet [Park & Choi 2005, ApJ 635, L29] 

  8. Surface brightness: Σ = – 2.5 log10 (f/Ω) + cte • Photometric properties: surface brightness • to first approximation, the Σof an extended object is • independent of its distance: f falls of as 1/d2 and also • does the Ω in the same proportion • is measured in mag arcsec-2 (or μm, where m is the magnitude band) • Isophotes: • curves that encircle areas of certain Σ (lines of equal Σ) • Isophotal (or photometric) diameter: • diameter at which some particular Σ level is reached (semimajor axis of the corresponding isophote) Holmberg radius → half diameter of galaxy at Σ = 26.5 μpg de Vaucouleurs radius → half diameter of galaxy at Σ = 25 μB, corrected to face-on and of dust obscuration (called D25) [de Vaucouleurs et al. 1991, 3rd Reference Catalogue of Bright Galaxies]

  9. Photometric properties: surface brightness • Corrections to surface brightness measurements: • sky brightness – at a good site, on a moonless night, the blank sky typically has a Σ23 μB, due to: • air glow (photochemical processes in the upper atmosphere + Hg and Na lines radiation from • street lamps of nearby cities) • zodiacal light (sunlight scattered off particulate matter in the SS) • MW background (diffuse light from faint and unresolved stars in the Galaxy) • extragalactic background (diffuse light from distant, faint, unresolved galaxies) • seeing – the effect of seeing introduces an apparent core (PSF) into a surface brightness profile • deprojection – correction of inclination in relation to the plane of sky (to face-on) • obscuration by internal dust – for the case of late-type galaxies, dust can absorb or scatter their own light – the apparent lum. of a transparent galaxy is independent of its orientation to the LOS (stars emit light isotropically), but peak Σ increases as galaxy is tipped from face-on to edge-on (since at edge-on orientation the same lum. comes from a smaller area than at face-on) – but a galaxy filled with dust will appear to be less luminous when seen edge-on than face-on (light passes a longer column of ISM) – blue light is more strongly absorbed and scattered than red light μUμBμVμRμI 22.0 22.7 21.8 20.9 19.9 [see Binney & Merrifield 1998, Galactic Astronomy, cap. 4.2 and 4.4]

  10. Photometric properties: K-corrections [Pence 1976, ApJ 203, 39; Hogg et al. 2002, astro-ph 0210394]  • K-correction (redshift dimming): • since we do not cover the entire spectral range of galaxies we observe, to compare the measurements of galaxies at different z we must put them at a same reference frame • (normally a standard measure at z = 0) • so, the correction, called k-correction, depends on bands (filters) of observations, shapes of galaxies’s SEDs and z • must be applied to magnitudes and Σ

  11. Photometric properties: K-corrections K-correction (redshift dimming):

  12. Photometric properties: surface brightness profile for spheroids • Radial profiles of Ellipticals: • E have a pronounced maximum of Σat their centre, and a rapid and uniform decrease with radial distance, following a quasi power law • similar profile to bulges of S0s and spirals • cDs have envelope and deviate in large r Hubble’s profile [Hubble 1930] Σ(r) = Σ0 (r/Rc + 1) –2 Σ0 = Σ(r=0) Rc is the core radius (where the surface brightness is Σ0/2) de Vaucouleurs’ profile [de Vaucouleurs 1948] log[Σ(r)/Σe] = –3.3307 [(r/Re)1/4 –1] Σ(r) = Σe exp{– 7.6692 [(r/Re)1/4 – 1]} Re = (ae be)1/2, is the radius containing half of total light (effective radius) Σe= Σ(Re) Σ0103.33Σe 2141 Σe Ltot = 7.215 πΣe Re2 (b/a) < Σ >e = 3.6072 Σe

  13. Photometric properties: surface brightness profile for spheroids

  14. Early-type’s profile: peaked or shallow cusps? • Center brightness of early-type galaxies: • bright E, S0 and BCGs have a bimodal distribution of central cusp slope: brighter galaxies have • core centers while less bright ones have peaked centers (HST sample). [Lauer et al. 2007, ApJ 664, 226]

  15. Photometric properties: surface brightness profile for discs • Radial profile of Spirals and Lenticulars: • Since S are much more complex systems (bulge, bar, disc, spiral arms, rings, dust, ...), their profiles also show a considerable variety and individuality of form. • their profile can be decomposed on two main compenents: bulge (that closely resemble the profile of ellipticals) and disc (that has an exponential profile) Exponential profile [Freeman 1970] Σ(r) = Σ0 e-r/h Σ0 = Σ(r=0), extrapolated h is the disc scale length Ldisc = 4π h2Σ0 < Σ >0 = 1.9016 Σ0 Bulge fraction (T = total luminosity) B/T = Re2Σe / (Re2Σe + 0.28 h2Σ0) D/B = (B/T)1  1

  16. Photometric properties: surface brightness profile for discs

  17. Spirals profile’s: constant central surface brightness? • Freeman’s Law: • Freeman (1970) noted that, although the disc components of large disc galaxies (from S0 to Im) have a wide range of lum., there is a remarkably little scatter in their value of Σ0: 21.7  0.3 μB • Disney (1976) proposed that this result could be influenced by selection effects (brightest and largest disc galaxies were naturally the first that could be measured). • Van der Kruit (1989) suggested that this law is valid for “non-dwarf galaxies”. • LSB galaxies, more carefully studied from 90’s, also do not fit to the Freeman’s Law. In fact, they are usually defined as galaxies with Σ0 fainter than 23 μB, and are found with Σ0 as faint as 25 μB.

  18. Photometric properties: general surface brightness profile Sersic’s generalized profile [Sersic 1968] Σ(r) = Σe exp{–bn [(r/Re)1/n – 1]} bn = 2n – 0.324 [Trujillo, Graham & Caon 2001] For a regular elliptical/bulge profile n = 4  b4 = 7.67 Σ(r) = Σe exp{–7.67 [(r/Re)1/4 – 1]} For an exponential disc profile n = 1  b1 = 1.68 Σ(r) = Σe exp{–1.68 [(r/Re)– 1]} = exp{1.68} Σe exp{–1.68 r/Re} = 5.36 Σe exp{–r/(Re/1.68)} = Σ0 exp{–r/h} Σ0 = 5.3567 Σe h = Re/1.6783 [Pannella et al. 2006, ApJ 639, L1]

  19. Spectroscopic properties • Galaxy spectra: • integrated spectra of galaxies give us information mainly on the stellar populations that compose the galaxy and on star formation (SF) • classes of galaxy spectra are correlated with morphological classification • population synthesis is the construction of a galaxy spectra from the combination of specific proportions of different types of stellar spectra. [Humason 1936] – first attempt to classify galaxy spectra: E (spectra close to G3.6) Sc (resembles F8.8) • Absorption spectra (early-type): • stellar population: old stars • no recent SF (very low cold gas) • Balmer break (λ < 4000) – opacity of stellar • photosphere increases fastly below this λ • (presence of metals in different degrees of • ionization) • absorption lines (produced on the atm. of cold • red giants): CaII K (λ3934), CaII H (λ3969), • G (λ4304), Mgb (λ5175), Ca+Fe (λ5269), • Na D (λ5893), etc • Emission spectra (late-type): • stellar population: rich in young OB stars • continuous SF (early S – decreases w/ time) • (very gas rich) • high UV continuum (indicative of SF) • emission lines (produced by the gas, • photoionized by energy from massive OB • stars): OII (λ3727), Hβ(λ4861), OIII (λ4959, • λ5007), Hα (λ6563), NII (λ6548, λ6584), • SII (λ6717, λ6731), etc

  20. H CaFe Na CaFe H H Na Mg Na H OII Mg CaFe Mg G G G K H H K H K H OIII OII H NII H SII OIII OII NII OIII H Na H Mg OIII G NII H SII K H OII SII • Spectroscopic properties [Kennicutt 1992, ApJS 79, 255] 

  21. CaFe H Na Mg G H K Absorption spectra (early-type) stellar spectra for comparison

  22. OII H OIII OIII H NII SII Emission spectra (late-type) stellar spectra for comparison

  23. Spectroscopic classification [Dressler et al. 1999, ApJS 122, 51]

  24. i a i b • Physical properties: the real shape of galaxies • Elipticals • ellipsoids spheroids a = b > c  oblate a > b = c  prolate triaxial a > b > c • Spirals • disks (flat oblate spheroids) plane of sky inclination angle: i = arc cos (b/a) edge-on i = 90° face-on i = 0° cos(i) = b/a

  25. Property E S Irr Total mass (M) 105 – 1013 108 – 1012 107 – 1010 Total luminosity (L) 105 – 1011 109 – 1011 107 – 1010 Diameter (MW) 0.01 – 5 0.02 – 1.5 0.05 – 0.25 • Physical properties: masses Correlates poorly with Hubble type Total masses: M/L ratio: [Roberts & Haynes 1994, ARAA 32, 115]

  26. Physical properties: gas (HI + H2) fraction Gas fraction: Correlates well with Hubble type [Roberts & Haynes 1994, ARAA 32, 115] 

  27. Physical Properties: stellar content [Baade 1940] – introduced the idea of stellar populations (discs  ellipticals/bulges) • Population I: • discs (particularly spiral arms) • hot blue supergiants • accompained by gas and dust • Open Clusters • young (CM diagram) • higher metal abundances • Population II: • E, bulges, halos • cool red giants • gas and dust free • Globular Clusters • very old (CM diagram) • deficient in metals • Population III: • zero metal

  28. Physical Properties: stellar populations Modern stellar populations: Population typical stars velocity shape of system metal abundance dispersion (respect to H) Halo pop. GC, red giants 130 spherical 0.003 Intermediate pop. II high vel. stars 50 intermediate 0.01 Disc pop. weak line stars 30 intermediate 0.02 Intermediate pop. I strong line stars 20 intermediate 0.03 Extreme pop. I blue supergiants 10 flat 0.04

  29. Physical Properties: chemical composition Metallicity: LMC X  H Y  He Z  “metals” SMC halo [Fe/H] = log(Fe/H) – log(Fe/H) {12 + log(O/H)} = 8.91 Mg2 BCD [Kunth & Östlin 2000, AeAR 10, 1] 

  30. Physical Properties: chemical composition Galaxies • Gas metallicity • O, N, S, Ne, Ar, Fe in optical emission lines • of HII regions or PNe • Fe in the X-ray spectra of hot ICM of clusters • absorption lines of QSO Lyα systems • up-to-date measure, but subjected to gradients and • contamination (winds from young massive stars) [Golev & Prugniel 1998, AeAS 132, 255] Glob. Cl. [O/H] R23=(OII+OIII)/Hβ • Stellar metallicity • absorption lines in the spectra of individual • stars (nearby galaxies) • Mg2 in the integrated spectra of distant • early-type galaxies • more mixed, but metallicity at the time of • formation and depends on population [Mg/Fe]

  31. Physical Properties: chemical composition Metallicity: (Z/Z) mean disc bulge halo Ellipticals 0.3 – 2 Spirals --0.1 – 1 3 > 0.0001 LSB 0.02 – 0.1 dE0.01 – 0.2 dIrr0.02 – 0.1 BCD 0.005  0.5 dSph0.006  0.2 High-z abs-line systems0.001 – 0.3 But. correlates well with mass (mass-mettalicity relation) [Terlevich et al 1991, AeAS 91, 285; Mateo 1998, ARAA 36,435; Kunth & Östlin 2000, AeAR 10, 1] • Objects with high and low metallicity are found at all z’s! • Objects that in the Local Univ appear as metal deficient are expected to be even more • deficient at high-z, if we could observe their precursors • Metallicity may be determined by: star formation history (SFH), outflows/inflows, • mergers/interactions, and mixing.

  32. Physical Properties: chemical composition • Mass-Metallicity relation: • less massive galaxies are less able to retain the gas and stellar ejecta, loosing the freshly • produced metals in the form of galactic outflows [Tremonti et al. 2004, ApJ 613, 898] • high mass stars (that produce more and fastly metals) are preferencially produced in high • SF epochs or sites (variable integrated stellar IMF) [Köppen et al. 2007, MNRAS 375, 673]

  33. Physical Properties: chemical composition Galaxy formation and evolution models: hierarchical [Ogando et al. 2005, ApJ 632, L61] monolithic Primordial He abundance: [Izotov & Thuan 1998, ApJ 500, 188] [see also, Peimbert et al. 2007, ApJ 666, 636]

  34. Correlations of physical properties: the Faber-Jackson relation [Faber & Jackson 1976] – discovered, for E, a relation between the lum. (L) and the central stellar velocity dispersion (σ0): Le σ04 [Djorgovski & Davies 1987 and Dressler et al. 1987] – introduced the concept of Fundamental Plane (L  σ0r) log Re= 0.36 <Σe> + 1.4 log σ0 Le σ08/3Σe-3/5 [Σ = f (L,r)] [Dressler et al. 1987] – also proposed the Dn–σ relation (incorporating the dependende of both L and Σe into a new variable Dn) σ0 Dn3/4 Dn → Σn = 20.75 μB • if σ0 can be measured for an E, its intrinsic L can be found by the relation, and hence, by measuring its flux, its distance can be found.

  35. Correlations of physical properties: the Fundamental Plane [Kormendy & Djorgovski 1989, ARAA 27, 235]

  36. Correlations of physical properties: Fundamental Plane for dwarves

  37. Correlations of physical properties: the Tully-Fisher relation [Tully & Fisher 1977] – discovered, for S, a relation between the lum. (L) and the width of the 21-cm HI line profile (that is related to the maximum rotation velocity – Doppler broadening). LB Wαα = 2.5 [Aaronson, Huchra & Mould 1979] – in the IR, the Tully-Fisher relation is more tightly correlated [Aaronson & Mould 1983] – α = 3.5 (B band) α = 4.3 (H band) • if W can be measured for an S, its intrinsic L can be found by the relation, and hence, by measuring its flux, its distance can be found.

  38. References: • Papers and books: • E.P. Hubble 1930, ApJ 71, 231 • M. Humason 1936, ApJ 83, 10 • W. Baade 1940, ApJ 100, 137 • G. De Vaucouleurs 1948, Ann. Astrophys. 11, 247 • J.–L. Sersic 1968, “Atlas de Galaxias Australes” (Cordoba, Obs. Astronomico) • K.C. Freeman 1970, ApJ 160, 811 • M. Disney 1976, Nature 263, 573 • S.M. Faber & R.E. Jackson 1976, ApJ 204, 668 • B. Tully & J.R. Fisher 1977, A&A 54, 661 • M. Aaronson, J. Huchra & J. Mould 1979, ApJ 229, 1 • M. Aaronson & J. Mould 1983, ApJ 265, 1 • S. Djorgovski & M. Davis 1987, ApJ 313, 59 • A. Dressler et al. 1987, ApJ 313, 42 • P.C. van der Kruit 1989, In: “The Milky Way as a Galaxy”, eds. R. Buser & I. King • I. Trujillo et al. 2001, MNRAS 326, 869 • K. Rakos et al. 2003, Ap&SS, 284, 803 • J. Moustakas & R. Kennicutt 2006, ApJ 651, 155

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