The Universe of Galaxies: from large to small

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The Universe of Galaxies:

from large to small

Physics of Galaxies 2011 part 1 – introduction

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Galaxies lie at the

crossroads of astronomy

The study of galaxies brings together nearly all astronomical disciplines:

stellar astronomy: the formation and evolution of stars in galaxies

“gastrophysics”: the behavior of and the interaction between gas in and between galaxies

high and low energy processes: from dust to AGN cosmology: the formation and evolution of galaxies

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And uses nearly all observational techniques...

from low-frequency radio observations (LOFAR)

through the radio, mm, sub-mm, infrared, optical, and UV bands

to the X-ray and γ-ray bands

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Messier 51: UV, optical, and NIR

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Arp 85

NGC 5195 SB0 pec LINER

M51

SA(s)bc pec Sy 2.5 NGC 5194

Messier 51:

Radio (HI and

CO), NIR, mid-IR, and X-ray

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Galaxies span a huge range in sizes and masses!

From giant elliptical galaxies with masses of 10

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times the mass of our own Sun, typically living in large clusters of galaxies...

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M86

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...to spiral galaxies like our own Milky Way, with masses of 10

11

times that of our own Sun, usually living alone

or with one or two similarly-sized companions...

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M101

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...to dwarf galaxies with masses of only a million or so solar masses, which (as far as we can tell) are always the satellites of bigger galaxies

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Draco

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Hercules

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Galaxy mass functions 949

Figure 4. Bivariate distribution for SB versus mass. The contours represent the volume-corrected number densities from the sample: logarithmically spaced with four contours per factor of 10. The lowest dashed contour corresponds to 10−5 Mpc−3 per 0.5 × 0.5 bin, while the lowest solid contour corresponds to 5.6 × 10−5 Mpc−3. The grey dashed-line regions represent areas of low completeness (70 per cent or lower as estimated by Blanton et al. 2005a). The diamonds with error bars represent the median and 1σ ranges over certain masses with a straight-line fit shown by the middle dotted line. The outer dotted lines represent ±1 σ .

survey (Cross & Driver 2002). From the tests of Blanton et al.

(2005a), as shown in fig. 3 of that paper, the SDSS main galaxy sample has 70 per cent or greater completeness in the range 18–

23 mag arcsec−2 for the effective SB µR50,r.

In order to identify at what point the GSMF becomes incomplete because of the SB limit, we computed the bivariate distribution in SB versus stellar mass. Fig. 4 shows this distribution represented by solid and dashed contours (1/Vmax and 1/n LSS corrected). There is a relationship between peak SB and log M, which is approximately linear in the range 108.5 to 1011 M#. At lower masses, the distri- bution is clearly affected by the low-SB incompleteness at µR50,r

> 23 mag arcsec−2. Therefore, any GSMF values for lower masses should be regarded as lower limits if there are no corrections for SB completeness.

The other important consideration is the fact that the r-band selection is not identical to the mass selection required for the GSMF. This is nominally corrected for by 1/Vmax but it should be noted that galaxies with high M/L at a given mass are viewed over significantly smaller volumes than those with low M/L. Fig. 5 shows the bivariate distribution in M/L versus mass. The limits at various redshifts for the SDSS main galaxy sample are also identified. For example, galaxies with M < 108 M# and log (M/Lr) > 0.1 are only in the sample at z < 0.008. At these low redshifts, the stellar mass and Vmax depend significantly on the Hubble-flow corrections.

However, it does appear that the SB limit affects the completeness of GSMF values at higher masses than the M/L limits. At M <

108.5 M#, the SB limit becomes significant, while at M < 108 M#, the GSMF is affected both by the constrained volume for high M/L galaxies and, more severely, by the SB incompleteness.

3.2 Corrected GSMF with lower limits at the faint end

Fig. 6 shows the results of the GSMF determination. The binned GSMF is represented by points with Poisson error bars, with lower

Figure 5. Bivariate distribution for M/L versus mass. The contours rep- resent the volume-corrected number densities: logarithmically spaced with four contours per factor of 10. The lowest dashed contour corresponds to 10−4 Mpc−3 per 0.5 × 0.2 bin. The dotted lines represent the observable limits for an r < 17.8 mag limit and different redshift limits (ignoring k-corrections). The grey dashed line region represents galaxies that can only be observed at z < 0.008 where Hubble-flow corrections are significant (cz < 2400 km s−1).

Figure 6. GSMF extending down to 107 M# determined from the NYU- VAGC. The points represent the non-parametric GSMF with Poisson error bars; at M < 108.5 M# the data are shown as lower limits because of the SB incompleteness (Fig. 4). The dashed line represents a double-Schechter function extrapolated from a fit to the M > 108 M# data points. The dotted line shows the same type of function with a faint-end slope of α2 = −1.8 (fitted to M > 108.5 M# data). The dash–dotted line represents a power- law slope of −2.0. The shaded region shows the range in the GSMF from varying the stellar mass used and changing the redshift range.

limits represented by arrows. The GSMF has been corrected for volume (1/Vmax) and LSS (1/n) effects.9 The masses used were

9 We compared the GSMF computed using 1/n(z) correction for LSS vari- ations with the stepwise maximum-likelihood method (Efstathiou et al.

1988). There was good agreement between the two methods after matching

$C 2008 The Authors. Journal compilation $C 2008 RAS, MNRAS 388, 945–959

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Galaxy mass functions 949

Figure 4. Bivariate distribution for SB versus mass. The contours represent the volume-corrected number densities from the sample: logarithmically spaced with four contours per factor of 10. The lowest dashed contour corresponds to 10−5 Mpc−3 per 0.5 × 0.5 bin, while the lowest solid contour corresponds to 5.6 × 10−5 Mpc−3. The grey dashed-line regions represent areas of low completeness (70 per cent or lower as estimated by Blanton et al. 2005a). The diamonds with error bars represent the median and 1σ ranges over certain masses with a straight-line fit shown by the middle dotted line. The outer dotted lines represent ±1 σ .

survey (Cross & Driver 2002). From the tests of Blanton et al.

(2005a), as shown in fig. 3 of that paper, the SDSS main galaxy sample has 70 per cent or greater completeness in the range 18–

23 mag arcsec−2 for the effective SB µR50,r.

In order to identify at what point the GSMF becomes incomplete because of the SB limit, we computed the bivariate distribution in SB versus stellar mass. Fig. 4 shows this distribution represented by solid and dashed contours (1/Vmax and 1/n LSS corrected). There is a relationship between peak SB and log M, which is approximately linear in the range 108.5 to 1011 M#. At lower masses, the distri- bution is clearly affected by the low-SB incompleteness at µR50,r

> 23 mag arcsec−2. Therefore, any GSMF values for lower masses should be regarded as lower limits if there are no corrections for SB completeness.

The other important consideration is the fact that the r-band selection is not identical to the mass selection required for the GSMF. This is nominally corrected for by 1/Vmax but it should be noted that galaxies with high M/L at a given mass are viewed over significantly smaller volumes than those with low M/L. Fig. 5 shows the bivariate distribution in M/L versus mass. The limits at various redshifts for the SDSS main galaxy sample are also identified. For example, galaxies with M < 108 M# and log (M/Lr) > 0.1 are only in the sample at z < 0.008. At these low redshifts, the stellar mass and Vmax depend significantly on the Hubble-flow corrections.

However, it does appear that the SB limit affects the completeness of GSMF values at higher masses than the M/L limits. At M <

108.5 M#, the SB limit becomes significant, while at M < 108 M#, the GSMF is affected both by the constrained volume for high M/L galaxies and, more severely, by the SB incompleteness.

3.2 Corrected GSMF with lower limits at the faint end

Fig. 6 shows the results of the GSMF determination. The binned GSMF is represented by points with Poisson error bars, with lower

Figure 5. Bivariate distribution for M/L versus mass. The contours rep- resent the volume-corrected number densities: logarithmically spaced with four contours per factor of 10. The lowest dashed contour corresponds to 10−4 Mpc−3 per 0.5 × 0.2 bin. The dotted lines represent the observable limits for an r < 17.8 mag limit and different redshift limits (ignoring k-corrections). The grey dashed line region represents galaxies that can only be observed at z < 0.008 where Hubble-flow corrections are significant (cz < 2400 km s−1).

Figure 6. GSMF extending down to 107 M# determined from the NYU- VAGC. The points represent the non-parametric GSMF with Poisson error bars; at M < 108.5 M# the data are shown as lower limits because of the SB incompleteness (Fig. 4). The dashed line represents a double-Schechter function extrapolated from a fit to the M > 108 M# data points. The dotted line shows the same type of function with a faint-end slope of α2 = −1.8 (fitted to M > 108.5 M# data). The dash–dotted line represents a power- law slope of −2.0. The shaded region shows the range in the GSMF from varying the stellar mass used and changing the redshift range.

limits represented by arrows. The GSMF has been corrected for volume (1/Vmax) and LSS (1/n) effects.9 The masses used were

9 We compared the GSMF computed using 1/n(z) correction for LSS vari- ations with the stepwise maximum-likelihood method (Efstathiou et al.

1988). There was good agreement between the two methods after matching

$C 2008 The Authors. Journal compilation $C 2008 RAS, MNRAS 388, 945–959

ellipticals

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Galaxy mass functions 949

Figure 4. Bivariate distribution for SB versus mass. The contours represent the volume-corrected number densities from the sample: logarithmically spaced with four contours per factor of 10. The lowest dashed contour corresponds to 10−5 Mpc−3 per 0.5 × 0.5 bin, while the lowest solid contour corresponds to 5.6 × 10−5 Mpc−3. The grey dashed-line regions represent areas of low completeness (70 per cent or lower as estimated by Blanton et al. 2005a). The diamonds with error bars represent the median and 1σ ranges over certain masses with a straight-line fit shown by the middle dotted line. The outer dotted lines represent ±1 σ .

survey (Cross & Driver 2002). From the tests of Blanton et al.

(2005a), as shown in fig. 3 of that paper, the SDSS main galaxy sample has 70 per cent or greater completeness in the range 18–

23 mag arcsec−2 for the effective SB µR50,r.

In order to identify at what point the GSMF becomes incomplete because of the SB limit, we computed the bivariate distribution in SB versus stellar mass. Fig. 4 shows this distribution represented by solid and dashed contours (1/Vmax and 1/n LSS corrected). There is a relationship between peak SB and log M, which is approximately linear in the range 108.5 to 1011 M#. At lower masses, the distri- bution is clearly affected by the low-SB incompleteness at µR50,r

> 23 mag arcsec−2. Therefore, any GSMF values for lower masses should be regarded as lower limits if there are no corrections for SB completeness.

The other important consideration is the fact that the r-band selection is not identical to the mass selection required for the GSMF. This is nominally corrected for by 1/Vmax but it should be noted that galaxies with high M/L at a given mass are viewed over significantly smaller volumes than those with low M/L. Fig. 5 shows the bivariate distribution in M/L versus mass. The limits at various redshifts for the SDSS main galaxy sample are also identified. For example, galaxies with M < 108 M# and log (M/Lr) > 0.1 are only in the sample at z < 0.008. At these low redshifts, the stellar mass and Vmax depend significantly on the Hubble-flow corrections.

However, it does appear that the SB limit affects the completeness of GSMF values at higher masses than the M/L limits. At M <

108.5 M#, the SB limit becomes significant, while at M < 108 M#, the GSMF is affected both by the constrained volume for high M/L galaxies and, more severely, by the SB incompleteness.

3.2 Corrected GSMF with lower limits at the faint end

Fig. 6 shows the results of the GSMF determination. The binned GSMF is represented by points with Poisson error bars, with lower

Figure 5. Bivariate distribution for M/L versus mass. The contours rep- resent the volume-corrected number densities: logarithmically spaced with four contours per factor of 10. The lowest dashed contour corresponds to 10−4 Mpc−3 per 0.5 × 0.2 bin. The dotted lines represent the observable limits for an r < 17.8 mag limit and different redshift limits (ignoring k-corrections). The grey dashed line region represents galaxies that can only be observed at z < 0.008 where Hubble-flow corrections are significant (cz < 2400 km s−1).

Figure 6. GSMF extending down to 107 M# determined from the NYU- VAGC. The points represent the non-parametric GSMF with Poisson error bars; at M < 108.5 M# the data are shown as lower limits because of the SB incompleteness (Fig. 4). The dashed line represents a double-Schechter function extrapolated from a fit to the M > 108 M# data points. The dotted line shows the same type of function with a faint-end slope of α2 = −1.8 (fitted to M > 108.5 M# data). The dash–dotted line represents a power- law slope of −2.0. The shaded region shows the range in the GSMF from varying the stellar mass used and changing the redshift range.

limits represented by arrows. The GSMF has been corrected for volume (1/Vmax) and LSS (1/n) effects.9 The masses used were

9 We compared the GSMF computed using 1/n(z) correction for LSS vari- ations with the stepwise maximum-likelihood method (Efstathiou et al.

1988). There was good agreement between the two methods after matching

$C 2008 The Authors. Journal compilation $C 2008 RAS, MNRAS 388, 945–959

ellipticals spirals

15

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Galaxy mass functions 949

Figure 4. Bivariate distribution for SB versus mass. The contours represent the volume-corrected number densities from the sample: logarithmically spaced with four contours per factor of 10. The lowest dashed contour corresponds to 10−5 Mpc−3 per 0.5 × 0.5 bin, while the lowest solid contour corresponds to 5.6 × 10−5 Mpc−3. The grey dashed-line regions represent areas of low completeness (70 per cent or lower as estimated by Blanton et al. 2005a). The diamonds with error bars represent the median and 1σ ranges over certain masses with a straight-line fit shown by the middle dotted line. The outer dotted lines represent ±1 σ .

survey (Cross & Driver 2002). From the tests of Blanton et al.

(2005a), as shown in fig. 3 of that paper, the SDSS main galaxy sample has 70 per cent or greater completeness in the range 18–

23 mag arcsec−2 for the effective SB µR50,r.

In order to identify at what point the GSMF becomes incomplete because of the SB limit, we computed the bivariate distribution in SB versus stellar mass. Fig. 4 shows this distribution represented by solid and dashed contours (1/Vmax and 1/n LSS corrected). There is a relationship between peak SB and log M, which is approximately linear in the range 108.5 to 1011 M#. At lower masses, the distri- bution is clearly affected by the low-SB incompleteness at µR50,r

> 23 mag arcsec−2. Therefore, any GSMF values for lower masses should be regarded as lower limits if there are no corrections for SB completeness.

The other important consideration is the fact that the r-band selection is not identical to the mass selection required for the GSMF. This is nominally corrected for by 1/Vmax but it should be noted that galaxies with high M/L at a given mass are viewed over significantly smaller volumes than those with low M/L. Fig. 5 shows the bivariate distribution in M/L versus mass. The limits at various redshifts for the SDSS main galaxy sample are also identified. For example, galaxies with M < 108 M# and log (M/Lr) > 0.1 are only in the sample at z < 0.008. At these low redshifts, the stellar mass and Vmax depend significantly on the Hubble-flow corrections.

However, it does appear that the SB limit affects the completeness of GSMF values at higher masses than the M/L limits. At M <

108.5 M#, the SB limit becomes significant, while at M < 108 M#, the GSMF is affected both by the constrained volume for high M/L galaxies and, more severely, by the SB incompleteness.

3.2 Corrected GSMF with lower limits at the faint end

Fig. 6 shows the results of the GSMF determination. The binned GSMF is represented by points with Poisson error bars, with lower

Figure 5. Bivariate distribution for M/L versus mass. The contours rep- resent the volume-corrected number densities: logarithmically spaced with four contours per factor of 10. The lowest dashed contour corresponds to 10−4 Mpc−3 per 0.5 × 0.2 bin. The dotted lines represent the observable limits for an r < 17.8 mag limit and different redshift limits (ignoring k-corrections). The grey dashed line region represents galaxies that can only be observed at z < 0.008 where Hubble-flow corrections are significant (cz < 2400 km s−1).

Figure 6. GSMF extending down to 107 M# determined from the NYU- VAGC. The points represent the non-parametric GSMF with Poisson error bars; at M < 108.5 M# the data are shown as lower limits because of the SB incompleteness (Fig. 4). The dashed line represents a double-Schechter function extrapolated from a fit to the M > 108 M# data points. The dotted line shows the same type of function with a faint-end slope of α2 = −1.8 (fitted to M > 108.5 M# data). The dash–dotted line represents a power- law slope of −2.0. The shaded region shows the range in the GSMF from varying the stellar mass used and changing the redshift range.

limits represented by arrows. The GSMF has been corrected for volume (1/Vmax) and LSS (1/n) effects.9 The masses used were

9 We compared the GSMF computed using 1/n(z) correction for LSS vari- ations with the stepwise maximum-likelihood method (Efstathiou et al.

1988). There was good agreement between the two methods after matching

$C 2008 The Authors. Journal compilation $C 2008 RAS, MNRAS 388, 945–959

ellipticals spirals

dwarfs

15

(19)

Galaxy mass functions 949

Figure 4. Bivariate distribution for SB versus mass. The contours represent the volume-corrected number densities from the sample: logarithmically spaced with four contours per factor of 10. The lowest dashed contour corresponds to 10−5 Mpc−3 per 0.5 × 0.5 bin, while the lowest solid contour corresponds to 5.6 × 10−5 Mpc−3. The grey dashed-line regions represent areas of low completeness (70 per cent or lower as estimated by Blanton et al. 2005a). The diamonds with error bars represent the median and 1σ ranges over certain masses with a straight-line fit shown by the middle dotted line. The outer dotted lines represent ±1 σ .

survey (Cross & Driver 2002). From the tests of Blanton et al.

(2005a), as shown in fig. 3 of that paper, the SDSS main galaxy sample has 70 per cent or greater completeness in the range 18–

23 mag arcsec−2 for the effective SB µR50,r.

In order to identify at what point the GSMF becomes incomplete because of the SB limit, we computed the bivariate distribution in SB versus stellar mass. Fig. 4 shows this distribution represented by solid and dashed contours (1/Vmax and 1/n LSS corrected). There is a relationship between peak SB and log M, which is approximately linear in the range 108.5 to 1011 M#. At lower masses, the distri- bution is clearly affected by the low-SB incompleteness at µR50,r

> 23 mag arcsec−2. Therefore, any GSMF values for lower masses should be regarded as lower limits if there are no corrections for SB completeness.

The other important consideration is the fact that the r-band selection is not identical to the mass selection required for the GSMF. This is nominally corrected for by 1/Vmax but it should be noted that galaxies with high M/L at a given mass are viewed over significantly smaller volumes than those with low M/L. Fig. 5 shows the bivariate distribution in M/L versus mass. The limits at various redshifts for the SDSS main galaxy sample are also identified. For example, galaxies with M < 108 M# and log (M/Lr) > 0.1 are only in the sample at z < 0.008. At these low redshifts, the stellar mass and Vmax depend significantly on the Hubble-flow corrections.

However, it does appear that the SB limit affects the completeness of GSMF values at higher masses than the M/L limits. At M <

108.5 M#, the SB limit becomes significant, while at M < 108 M#, the GSMF is affected both by the constrained volume for high M/L galaxies and, more severely, by the SB incompleteness.

3.2 Corrected GSMF with lower limits at the faint end

Fig. 6 shows the results of the GSMF determination. The binned GSMF is represented by points with Poisson error bars, with lower

Figure 5. Bivariate distribution for M/L versus mass. The contours rep- resent the volume-corrected number densities: logarithmically spaced with four contours per factor of 10. The lowest dashed contour corresponds to 10−4 Mpc−3 per 0.5 × 0.2 bin. The dotted lines represent the observable limits for an r < 17.8 mag limit and different redshift limits (ignoring k-corrections). The grey dashed line region represents galaxies that can only be observed at z < 0.008 where Hubble-flow corrections are significant (cz < 2400 km s−1).

Figure 6. GSMF extending down to 107 M# determined from the NYU- VAGC. The points represent the non-parametric GSMF with Poisson error bars; at M < 108.5 M# the data are shown as lower limits because of the SB incompleteness (Fig. 4). The dashed line represents a double-Schechter function extrapolated from a fit to the M > 108 M# data points. The dotted line shows the same type of function with a faint-end slope of α2 = −1.8 (fitted to M > 108.5 M# data). The dash–dotted line represents a power- law slope of −2.0. The shaded region shows the range in the GSMF from varying the stellar mass used and changing the redshift range.

limits represented by arrows. The GSMF has been corrected for volume (1/Vmax) and LSS (1/n) effects.9 The masses used were

9 We compared the GSMF computed using 1/n(z) correction for LSS vari- ations with the stepwise maximum-likelihood method (Efstathiou et al.

1988). There was good agreement between the two methods after matching

$C 2008 The Authors. Journal compilation $C 2008 RAS, MNRAS 388, 945–959

ellipticals spirals

dwarfs

most of the mass is here

15

(20)

Galaxy mass functions 949

Figure 4. Bivariate distribution for SB versus mass. The contours represent the volume-corrected number densities from the sample: logarithmically spaced with four contours per factor of 10. The lowest dashed contour corresponds to 10−5 Mpc−3 per 0.5 × 0.5 bin, while the lowest solid contour corresponds to 5.6 × 10−5 Mpc−3. The grey dashed-line regions represent areas of low completeness (70 per cent or lower as estimated by Blanton et al. 2005a). The diamonds with error bars represent the median and 1σ ranges over certain masses with a straight-line fit shown by the middle dotted line. The outer dotted lines represent ±1 σ .

survey (Cross & Driver 2002). From the tests of Blanton et al.

(2005a), as shown in fig. 3 of that paper, the SDSS main galaxy sample has 70 per cent or greater completeness in the range 18–

23 mag arcsec−2 for the effective SB µR50,r.

In order to identify at what point the GSMF becomes incomplete because of the SB limit, we computed the bivariate distribution in SB versus stellar mass. Fig. 4 shows this distribution represented by solid and dashed contours (1/Vmax and 1/n LSS corrected). There is a relationship between peak SB and log M, which is approximately linear in the range 108.5 to 1011 M#. At lower masses, the distri- bution is clearly affected by the low-SB incompleteness at µR50,r

> 23 mag arcsec−2. Therefore, any GSMF values for lower masses should be regarded as lower limits if there are no corrections for SB completeness.

The other important consideration is the fact that the r-band selection is not identical to the mass selection required for the GSMF. This is nominally corrected for by 1/Vmax but it should be noted that galaxies with high M/L at a given mass are viewed over significantly smaller volumes than those with low M/L. Fig. 5 shows the bivariate distribution in M/L versus mass. The limits at various redshifts for the SDSS main galaxy sample are also identified. For example, galaxies with M < 108 M# and log (M/Lr) > 0.1 are only in the sample at z < 0.008. At these low redshifts, the stellar mass and Vmax depend significantly on the Hubble-flow corrections.

However, it does appear that the SB limit affects the completeness of GSMF values at higher masses than the M/L limits. At M <

108.5 M#, the SB limit becomes significant, while at M < 108 M#, the GSMF is affected both by the constrained volume for high M/L galaxies and, more severely, by the SB incompleteness.

3.2 Corrected GSMF with lower limits at the faint end

Fig. 6 shows the results of the GSMF determination. The binned GSMF is represented by points with Poisson error bars, with lower

Figure 5. Bivariate distribution for M/L versus mass. The contours rep- resent the volume-corrected number densities: logarithmically spaced with four contours per factor of 10. The lowest dashed contour corresponds to 10−4 Mpc−3 per 0.5 × 0.2 bin. The dotted lines represent the observable limits for an r < 17.8 mag limit and different redshift limits (ignoring k-corrections). The grey dashed line region represents galaxies that can only be observed at z < 0.008 where Hubble-flow corrections are significant (cz < 2400 km s−1).

Figure 6. GSMF extending down to 107 M# determined from the NYU- VAGC. The points represent the non-parametric GSMF with Poisson error bars; at M < 108.5 M# the data are shown as lower limits because of the SB incompleteness (Fig. 4). The dashed line represents a double-Schechter function extrapolated from a fit to the M > 108 M# data points. The dotted line shows the same type of function with a faint-end slope of α2 = −1.8 (fitted to M > 108.5 M# data). The dash–dotted line represents a power- law slope of −2.0. The shaded region shows the range in the GSMF from varying the stellar mass used and changing the redshift range.

limits represented by arrows. The GSMF has been corrected for volume (1/Vmax) and LSS (1/n) effects.9 The masses used were

9 We compared the GSMF computed using 1/n(z) correction for LSS vari- ations with the stepwise maximum-likelihood method (Efstathiou et al.

1988). There was good agreement between the two methods after matching

$C 2008 The Authors. Journal compilation $C 2008 RAS, MNRAS 388, 945–959

ellipticals spirals

dwarfs

most of the mass is here most of the

galaxies are here

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How does this come about?

Why are there many more small galaxies than big galaxies?

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Galaxies are regular

Galaxies follow, by and large, scaling relations (or laws)

like stars --- the HR diagram is a kind of scaling law, in log L vs. log Teff

the HR diagram arises from the fact that stars

“forget” their initial conditions on short

(hydrodynamic) timescales in stars, evolution is clean;

formation is messy

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Galaxies, however, do not forget their initial conditions

scaling relations tell us

about the initial conditions of galaxy formation as well as subsequent processes for galaxies, neither

formation nor evolution are clean!

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Galaxies are composed of two types of matter:

Baryonic matter---the stuff we’re all made of---

which composes roughly 15% of the matter (but only 4.4% of the energy density) in the Universe Dark matter---the vast majority of which is not baryonic---which composes the other 85% of the mass of the Universe (but only 24% of the energy density)

Structure and morphology of galaxies

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The ingredients of the Universe

0.5%3.5%

24.0%

72.0%

Dark energy Dark matter Gas Stars

Energy

Mass

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These types of matter have different radial distributions:

Baryons are concentrated

(primarily) to the inner tens of kpc;

Dark matter can extend to hundreds of kpc

Why?

Dissipation: baryons can lose energy through radiation, but DM can’t

DM: 85% of mass Baryons

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Galaxy formation with cold dark matter

In the very early Universe, small lumps (“fluctuations”) in the nearly uniform density of dark and normal matter

provide “seeds” for the growth of structure

Since only gravity acts on dark matter, the dark

matter collapses into halos that trap the gas (normal matter or “baryons”) inside

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Galaxy formation with cold dark matter

Because the dark matter moves slowly (remember that it’s “cold”), small halos form first...

...and then merge to make larger halos

this is called “hierarchical galaxy formation”:

small objects form early and merge to make bigger objects

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Galaxy formation with cold dark matter

As the gas settles into the dark matter halos, it will cool by radiation – that is, giving off light – which will allow it to condense even more than the dark matter

Eventually, stars form when the gas is cool and dense enough

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So what happens next?

As stars form from the gas inside the halos, they create

“metals” through nuclear reactions inside the stars By tracing the evolution of the elements, we can infer when and how long it took for stars to form

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The evolution of the elements

After the Big Bang, only H, He, and Li are made in the Universe

All heavier elements are made in stars

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Elements made up of even numbers of protons – like Mg, Ca, Na, Si, S – are called α-elements

This is because these are made of adding He nuclei (“α particles”) to lighter elements

These are made only in the explosions of high-mass stars: “type II supernovae”

Elements near iron (Fe) are made in either the

explosions of high-mass stars or in the explosions of low(er)-mass stars: “type Ia supernovae”

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It is important to realize that stars live a time inversely proportional to their mass

the higher the mass of a star, the shorter its lifetime!

This means that the elements are produced on different timescales

α-elements are produced quickly (high-mass stars!) Fe elements take longer (low-mass stars!)

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Since [Fe/H] – a measure of the number of iron atoms per hydrogen atom – increases with time as more stars form and die, it is a (very rough!) clock

As time passes and [Fe/H] increases, eventually the low-mass stars with explode and the ratio of α-

elements to iron elements ([α/Fe]) will start to decrease

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Plots of this trend of

[α/Fe] vs. [Fe/H]

thus give a very

powerful view into how long stars

formed in galaxies

P1: ARK/vks P2: MBL/plb QC: MBL/tkj T1: MBL

July 2, 1997 16:55 Annual Reviews AR037-13

506 MCWILLIAM

Figure 1 A schematic diagram of the trend of α-element abundance with metallicity. Increased initial mass function and star formation rate affect the trend in the directions indicated. The knee in the diagram is thought to be due to the onset of type Ia supernovae (SN Ia).

initially above solar owing to nucleosynthesis by SN II, but as time continues after the burst (with no new star formation) the SN II diminish, only SN Ia enrich the gas; ultimately subsolar [O/Fe] ratios occur. Wyse & Gilmore (1991) claimed that the composition of the LMC is fit by this model.

Elements like C, O, and those in the iron-peak, thought to be produced in stars from the original hydrogen, are sometimes labeled as “primary.” The label “secondary” is reserved for elements thought to be produced from pre- existing seed nuclei, such as N and s-process heavy elements. The abundance of a primary element is expected to increase in proportion to the metallicity, thus [M/Fe] is approximately constant. For a secondary element, [M/Fe] is expected to increase linearly with [Fe/H] because the yield is proportional to the abundance of preexisting seed nuclei. One difficulty is that N and the s- process elements (both secondary) do not show the expected dependence on metallicity.

THE SOLAR IRON ABUNDANCE

It is sobering, and somewhat embarrassing, that the solar iron abundance is in dispute at the level of 0.15 dex. This discrepancy comes in spite of the fact that more than 2000 solar iron lines, with reasonably accurate g f values, are available for abundance analysis; that the solar spectrum is measured with much higher S/N and dispersion than for any other star; that LTE corrections to Fe I abundance are small, at only +0.03 dex (Holweger et al 1991); and

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If we find that stars in the dwarf satellites of the Milky Way have the same elemental abundance pattern as

the Milky Way itself, we gain confidence that the Milky Way’s halo can be built from similar dwarfs in the past

...and that the current dwarfs are the “remnants” of the galaxy formation process

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So the idea is then that small halos host dwarf galaxies which then merge to make bigger halos hosting bigger galaxies

Where are the “leftovers” from this process?

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Figure

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References

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