Mergers. Mechanisms and Evolution

Transcription

1 Mergers Mechanisms and Evolution Simona Vegetti Dr. S.C. Trager

2 Clustering among the nebulae Galaxies do not live separately, but tend to cluster together forming different kinds of groups: Lundmark 1927: investigation of ~ 8000 NGC-objects, ~ 200 are double and multiple systems; Holmberg 1937: there is an unbroken line of transition: double galaxies, multiple galaxies, metagalactic clusters, metagalactic superclusters; the components may be elliptical, spiral or irregular galaxies; the formation of double is probably due by capture.

3 Clustering among the nebulae Edge to edge parabolic motion Tidal interaction is asymmetric in time and space and show up in the form of spiral arms. Clockwise Counterclockwise The loss of energy in tidal disturbances at a close parabolic encounter between two galaxies is large enough to effect a capture. Holmberg 1941

4 Arp s Catalog: peculiar galaxies Arp 243 Arp 240 Zwicky 1953,1956,1959 Bridges and tails in interacting galaxies are correlated with tidal disturbances...?

5 1972ApJ T Galactic Bridges and Tails Parabolic encounters, M = 1011M, Rmin = 25 kpc Spin plane Line of sight Orbit plane Free parameters: m1/m2 : m1= m2, m1<m2, m1>m2 inclination of the orbit, i < 180 argument of the pericentre ω 90 longitute λ tilt β Toomre &Toomre 1972

6 1972ApJ T Galactic Bridges and Tails i = 0 Centre of mass i = 180 Victim m1 = m2 λ = 0 ω = 0 β = 0 Toomre &Toomre 1972

7 1972ApJ. Galactic Bridges and Tails m2 = 4 m1 m2 = 0.25 m1 i = 0 λ=0 ω=0 β=0 Toomre &Toomre 1972

8 1972ApJ T 1972ApJ T Galactic Bridges and Tails i = 45 i =60 i = 75 m2 = 0.25m1 λ = 0 β = 0 Tilted orbits Inclined orbits i = 30 i = 60 m2 = m1 λ = 0,90 β = i,90

9 Galactic Bridges and Tails Main conclusions: Two-armed (temporary) spiral structures can results from the tidal action of a passing galaxy, provided the passage is close and direct; Good bridges (dense and narrow long surviving) arise if the satellite mass is smaller; it helps to have the orbit plane inclined to the spin plane; Good tails arise if the two masses are at least the same, or for a very close and slow encounter; Exceptional thin bridges and tails may simply be due to particular favorable viewing angles. Toomre &Toomre 1972

10 Galaxy Morphology 560 z = 4.00 M. Steinmetz, J.F. Navarro / New Astronomy 7 (2002) old stars young stars gas Fig. 1. The most massive progenitor at z 5 4 shown edge-on. Gas particles are shown in green, young (i.e. less than 200 Myr old) stars in blue and older stars in red. Horizontal bars in each panel 10 are 5 (physical) kpc long. 5 Kpc M = M v = 180 km/s r =3 kpc mixed violently on a short timescale (Toomre, 1977; well illustrated by the evolutionary sequence shown Barnes and Hernquist, 1992). Galaxy morphology in Figs These figures show, at various times, thus evolves continuously throughout a galaxy s the luminous (baryonic) component of a massive 12 lifetime, and is determined by a delicate balance dark halo ( M ( at z 5 0) formed in the between the mode of gas accretion and the detailed LCDM cosmogony (Bahcall et al., 1999). LCDM merger history of an individual galaxy. Here we assumes a low-density universe (V ), currently 12 present a cosmological gasdynamical simulation dominated by a cosmological constant (VL 5 0.7), Ω0 = 0.3, ΩΛconfirms = 0.7,thatσa8 single = 0.9, mgin= M, a baryonic density parameter Vb h 22, which galaxy the CDM with cosmogony run=through the whole Hubble and a Hubble parameter h 5 H0 /(100 km s 21 mdm= 1, Mmay 0.5 kpc, ξ sequence during its lifetime, validating previous Mpc 21 ) The LCDM power spectrum is2002 Steinmetz & Navarro theoretical expectation and illustrating the inextricnormalized so that the present-day rms mass fluctua- N-body/gas-dynamical simulation including star formation and feedback M 21

11 cosmogony may run through the whole Hubble sequence during its lifetime, validating previous theoretical expectation and illustrating the inextricable link between morphology and the hierarchical mode of galaxy formation. and a Hubble parameter h 5 H0 /(100 km s Mpc ) The LCDM power spectrum is normalized so that the present-day rms mass fluctua21 tions on spheres of radius 8 h Mpc is s The evolution of this system is characterized by episodes of smooth accretion punctuated by two major merger events, one at z 3.3 and the second at z 0.6. The simulations are performed with GRAPESPH (Steinmetz, 1996), a cosmological hydrodynamics code that includes the effects of gravity, gas dynamics, and radiative cooling and heating Galaxy Morphology 2. The simulation The continual metamorphosis of galaxy morphologies in hierarchically clustering universes is z = 3.46 z = 3.15 z = 2.80 z = 2.07 Fig. 2. The formation of a bulge and the rebirth of a disk. The sequence shows the formation of a bulge by the merger of two almost equal M. Steinmetz, J.F. Navarro / New Astronomy 7 (2002) mass pure disk systems at z 3. After z 5 3 smooth accretion of gas regenerates the disk component around the bulge. Blue is used in each panel to denote stars formed since the preceding frame (since z for the first panel), red for older stars. Horizontal bars in each panel are 5 (physical) kpc long. z = 1.85 old stars young stars gas Fig. 3. The appearance of the galaxy at z 5 1.8, seen edge-on. Green is used for the gas, blue for young stars (i.e. formed since z 5 3), red for older stars. At this time, the morphology of the galaxy is reminiscent of early type spirals, with a dense bulge surrounded by a disk of gas and young stars. Horizontal bars in each panel are 5 (physical) kpc long. Steinmetz & Navarro 2002

12 Fig. 3. The appearance of the galaxy at z 5 1.8, seen edge-on. Green is used for the gas, blue for young stars (i.e. formed since z 5 3), red for older stars. At this time, the morphology of the galaxy is reminiscent of early type spirals, with a dense bulge surrounded by a disk of gas and young stars. Horizontal bars in each panel are 5 (physical) kpc long. Galaxy Morphology Fig. 3. The appearance of the galaxy at z 5 1.8, seen edge-on. Green is used for the gas, blue for young stars (i.e. formed since z 5 3), red for older stars. At this time, the morphology of the galaxy is reminiscent of early type spirals, with a dense bulge surrounded by a disk of gas and young stars. Horizontal bars in each panel are 5 (physical) kpc long. z = 1.79 z = 1.20 z = 0.98 z = 0.79 Fig. 4. The tidal triggering of bar instability by a satellite. At z the disk develops a well defined, long-lasting bar pattern as a result of tidal forcing by the satellite shown in the z panel. The satellite is finally disrupted at z after 7 pericentric passages. The bar pattern, however, survives for several Gyr, as shown in the picture. Stars less than 1.5 Gyr old are shown in blue, older stars in red. Note that the bar is most prominent in the young component. Horizontal bars in each panel are 5 (physical) kpc long. Fig. 4. The tidal triggering of bar instability by a satellite. At z the disk develops a well defined, long-lasting bar pattern as a result of tidal forcing by the satellite shown in the z panel. The satellite is finally disrupted at z after 7 pericentric passages. The bar pattern, however, survives for several Gyr, as shown in the picture. Stars less than 1.5 Gyr old are shown in blue, older stars in red. Note that the bar is most prominent in the young component. Horizontal bars in each panel are 5 (physical) kpc long. z = 0.70 z = 0.65 z = 0.60 z = 0.27 Fig. 5. A major merger and the formation of an elliptical galaxy. The disk merges with another system of about half its mass at z to form a spheroidal system of stars resembling an elliptical galaxy. All remaining gas is driven to the center of the remnant and consumed in a burst of star formation lasting 600 Myr. Stars less than 1.5 Gyr old are shown in blue, older in red. A movie illustrating the whole evolution of the galaxy can be found in the supplemental material (Annex) associated with this manuscript. Steinmetz & Navarro 2002

13 Observations From a theoretical point of view, CDM simulations can accurately predict the redshift evolution of dark matter haloes. By an observational point of view the picture is unclear: we still lack an understanding of the astrophysical processes which produce galaxies as we observe them. There is considerable controversy over the significance of interaction in shaping galaxies, especially when it comes to ETGs.

14 Early-type Galaxies ETGs are less numerous than spirals, but they contain 20% of the present-day stellar mass of the Universe; Since z ~1 the stellar population contained in the red-sequence galaxy population is increasing; there is a growing number of L * ETGs; The bulk of their stellar population formed at z >2; Evidence for a younger component due to recent star formation;

15 Early-type Galaxies Low-luminosity: disky isophotes; more rotationally supported; power-law (Sérsic) light distribution all the way to the centre; High-luminosity: boxy isophotes; supported mostly by random motion; core in the light distribution. on average younger. If they both form through merger, differences may be due to different kind of progenitors

16 High-L Early-type Galaxies Gas-rich disky mergers cannot give birth to massive ETGs: not enough massive blue forming-star galaxies at that could fade into present day massive red galaxies; difficult to reproduce the observed properties: boxy isophotes, high central densities, low-scatter in the FP and in the colormagnitude relation, high frequency of GC. Solutions: they form via dry merger of less massive ETGs; they form via monolithic collapse.

17 Merging rates Measuring the merging rate: counts of the relicts of tidal interaction in the local Universe; counts of kinematically close pairs at higher z; via the clustering and two-point correlation function of ETGs; via the evolution of the LF with redshift. Complications: selection effects; cosmic variance;

18 rarese & Merritt 2000); and the presence of massive galaxies at high redshift (e.g., Franx et al. 2003; Daddi et al. 2004; Glazebrook et al. 2004) These apparently contradictory lines of evidence may be largely reconciled by postulating that most elliptical galaxies formed through (nearly) dissipationless (or dry ) mergers of red, bulge-dominated galaxies rather than mergers of spiral disks. Motivated by the comparatively red colors of ellipticals exhibiting fine-structure Schweizer & Seitzer (1992) discuss this possibility, but argue that mergers between early-type galaxies are statistically unlikely as the median field galaxy is an Sb spiral (see also Silva & Bothun 1998). However, most mergers likely occur in groups, where the early-type galaxy fraction is much higher than in the general field (e.g., Zabludoff & Mulchaey 1998). Furthermore, semianalyical models of galaxy formation have predicted that the most recent mergers of bright ellipticals were between gas-poor, bulge-dominated galaxies (Kauffmann & Haehnelt 2000; Khochfar & Burkert Mergers between gas-poor, bulge-dominated galaxies are generally more difficult to recognize than mergers between gasrich disks. Elliptical-elliptical mergers do not develop prominent tidal tails dotted with star forming regions, but are instead characterized by the ejection of broad fans of stars, and in certain cases an asymmetric deformation of the inner isophotes (e.g., Rix & White 1989; Balcells & Quinn 1990; Combes et al. 1995). Tails may develop if one of the progenitors rotates or has a disk component (Combes et al. 1995), but as there is no cold, young component these are expected to be more diffuse than those seen in encounters between late-type galaxies. These effects are illustrated in Fig. 1, which shows the evolution of an off-axis collision between two hot stellar systems. The simulation was performed with an implementation of the Barnes & Hut (1986) hierarchical tree code, using 65,536 particles. The galaxies have a 3:1 mass ratio and are represented by Plummer models. The simulation illustrates the rapid merging of the two bodies, the lack of tidal tails, and the development Dry Mergers: tidal debris Mergers between gas-poor bulgedominated galaxies are generally much more difficult to recognize: do not develop tidal tails with spots of forming-stars region; F IG. 1. Illustration of an off-axis collision between hot stellar systems, with mass ratio 3:1. The simulation was performed with an implementation of the Barnes & Hut (1986) tree code, using 65,536 particles. The galaxies merge quickly; after the merger the only morphological signature is the presence of a broad asymmetric fan of material, which rapidly becomes more diffuse. Note the lack of tidal tails, which only form if there is a significant cold component. Simulation of a off-axis collision between hot stellar systems with mass ratio 3:1 Dokkum et al characterized by ejection of broad fans of stars with very low brightness, and high M/L; visible for shorter period because of dynamical evolution.

19 Dry Mergers: tidal debris Dokkum et al Multi-Wavelength Survey by Yale-Chile + NOAO Deep Wide-Field Survey: 126 galaxies at z~ 0.1; 35% shows clear sign of past interaction; 18% the interaction is still in progress; (a) (b) 47% appears undisturbed 86 are bulge-dominated galaxies, 71% of which recently experienced merger with mean mass ratio 1:4; (c) (d) F IG. 4. Same as Fig. 3, but now highlighting faint surface brightness levels in the summed exposures. The four systems have extensive tidal debris, extending to 1 or more from the center. Features can be reliably detected down to 28 mag arcsec 2. Despite their large extent the features contain less than 10 % of the total light of the galaxies. dm/m = 0.09 ± 0.04 Gyr-1 The tidal features are almost always red. In many cases the features are only visible in the R and I frames despite the substantial depth of the blue exposures. They also appear smooth, showing no or very little evidence for clumps and condensations. In these respects the features are very different from the blue, clumpy tidal tails seen in spiral spiral interactions (e.g., Mirabel, Dottori, & Lutz 1992; Hunsberger, Charlton, & Zaritsky 1996), and from the sharp shells and ripples de- are not well suited to identify narrow features, as the FWHM resolution of our data is about 2 kpc at z 0 1. The remarkable nature of these red mergers and their remnants is illustrated in Fig. 3. The figure shows two examples of ongoing mergers, an example of a strongly disturbed merger remnant, and an example of a galaxy with more subtle distortions at faint surface brightness levels, arranged in a plausible red merger sequence. In all cases the tidal features are smooth 35% of bulge-dominated galaxies experienced major merger involving >20% of their final mass; Stellar mass density in red galaxies has increased of a factor 2 between 0 < z < 1

20 Dry Mergers: close pair Lin et al DEEP2 Redshift Survey: galaxies at z = 1.2; Measure of the pair fraction with a certain projected separation (10 h -1 kpc < r< rmax) and a relative velocity < 500 km/s; Nmg rmax = 30,50,100 h -1 kpc; Magnitude correction to ensure selection of same galaxies at different z: M(z) = M(0) - Qz; z Only 9% of the present-day L * galaxies have undergone major merger Nc (1+z) m, m=1.60 (Q = 0), m= 0.51(Q=1) for rmax = 50 h -1 kpc; Major merger rate: Nmg = 0.5 n(z) Nc(z) Cmg T -1 mg = h -3 Mpc -3 Gyr -1

21 IG. Dry Mergers: close pairs HST, GEMS+ COMBO-17: 379 ETGs between 0.1 < z < 0.7; fmg pair with projected separation < 20 kpc, V-band absolute magnitude difference 1.5 and photometric Δz < 0.1; Six dry mergers are observed, with luminosity ratio between 1:1 and 4:1; Present day luminous spheroidal galaxies have on average experienced between 0.5 and 0.2 dry mergers; z Semianalytic models grey line Lin et al. 2004: tringles 9. A comparison of the dry merger fraction with previous observans of merger fractions, and the predictions of a semi-analytic model. Gray Bell et al. 2006a

22 Dry merger rate by clustering The measure of the galaxy interaction rate by counting the incidence of strongly disturbed galaxies and close pairs has provided important constraints on the merging rate, but suffers from uncertainties: disturbed galaxies incidence: minor gas-rich interaction may produce much more spectacular results than a major dry merger, the time scale of visibility is short and highly depend on the orbits, both the gas and mass ratio are uncertain; galaxy pairs: it allows study of the progenitors and is straightforward to quantify, but is contaminated by projections, luminosity-boosts by star formation and is characterized by a small statistic. An alternative method is offered by the measure of galaxy clustering.

23 Dry merger rate by clustering Galaxy clustering is a tool for studying different kind of phenomena at different scales: large scale (100 Mpc) can be used to test cosmological hypothesis as the homogeneity or the flatness of the Universe; Intermediate scale ( Mpc): probe of the relation between galaxies and dark matter haloes, deviation from a power-law can be interpreted in the frame of HOD; Small scale (<1 Mpc) features in the correlation function are correlated with dynamical frictions, tidal interaction, stellar feedback and other dissipative processes: can be translated into number density of merger events in the near future.

24 Dry Mergers: clustering assive galaxies 3 The merger rate of massive galaxies functions and number densities as input, we find that 65% of projected close luminous pairs have real space separations < 30 kpc; the corresponding fraction for massive galaxies is 69%. It is important to note that this contamination is with galaxies which are correlated with the host (i.e., those which are primarily nearby with 30 < r/kpc! 1000), and would likely have very similar redshifts to the primary galaxy (i.e., much of contamination is suffered by spectroscopic close pair samples). It is important to note that the exact fraction depends on the detailed form of the correlation function and should not be blindly adopted by workers using rather different sample cuts: in particular, our estimate is slightly higher than the estimate of 50% from Patton et al. (2000), which was derived using a very similar approach with a less clustered lower-luminosity parent sample Comparison with published merger fraction determinations F IG. 1. Grey: Projected correlation function w(r p ) for the 0.4 < z < 0.8 volume-limited sample of luminous MB <!20 galaxies. The power law fit to w(r p ) is overplotted, and parameters given in Table 1. The vertical dashed line at 15 kpc shows the radius within which COMBO-17 s object detection pipeline no longer reliably separates nearly equal-luminosity close galaxy pairs: this corresponds to the radius at which the correlation function starts to deviate strongly from a power law. Black: The projected correlation function of the volume-limited 0.4 < z < 0.8 sample of massive M > M" galaxies. TABLE 1 R EAL SPACE CORRELATION FUNCTION PARAMETERS AND PAIR FRACTIONS Sample r0 /Mpc γ n/mpc3 CLOSE F IG. 3. The close pair fraction (with real space separations < 30 kpc) of luminous (MB <!20; left panel) and massive (M > M" ; right panel) galaxies. Left: Gray data points show the merger fraction of galaxies MB!!20, taken from a variety of sources (see 5.3 for details). The P(r < r f )with Somerville et al. model prediction of the major merger fraction for MB <!20 5 default magnitude range is!21 MB! 5 log10 h100!18; corresponding to!22! MB!!19 for H0 = 70 km s!1 Mpc!1. Using their table 3, one can convert their results to a narrower range in absolute magnitude!21 MB! 5 log10 h100!19, or!22! MB!!20 in our units, by dividing by 2.5. Accordingly, we adjust Patton et al. s values upwards by 4/3 (to account for r < 7 kpc pairs) and downwards by a factor of 2.5 (to account for the magnitude range). de Propris et al. (2005) adopt a magnitude range of!22 MB! 5 log10 h100!19, corresponding to!23! MB!!20 for our choice of H0, thus the only correction applied is 4/3, to account for missed r < 7 kpc pairs. At z > 0.3, we show the fraction of MB!!20 galaxies in close pairs with! 30 kpc separation (their 20h!1 100 kpc values) taken from Le Fèvre et al. (2000, open diamonds); we have adjusted the values downwards to 65% of their original values to account for projection within galaxy groups (as discussed above, as opposed to projection of random galaxies along the line of sight, which Le Fèvre et al. corrected for already). We do not show values from Bundy et al. (2004), whose optical pair statistics agree well with Le Fèvre et al. s values but who argue (based on near-infrared data) that many of the apparently luminous pairs are in fact minor mergers which have been boosted in rest-frame B-band luminosity by enhanced star formation2. We attempt also to include the estimates of Lin et al. 10 s!1 Mpc!1 for the purposes of (2004). They adopt H0 = 70 km quoting k-corrected magnitudes, thus their evolution-corrected!21 MBe!19 sample corresponds roughly to!22! MB!!20, remembering that the evolution correction is roughly 1 magnitude per unit redshift. They quote their pair fractions in terms of h100 = 1 (L. Lin, 2006, priv. comm.) thus their 10 < r/h!1 kpc < 30 bin corresponds to 15 < r < 42 kpc adopting our H0. Since we find that P(r < r f ) r f, their 27 kpc of coverage for their pair fraction should be approximately equal to the P(r < r f ) which we would calculate within 30 kpc. We do, however, apply a correction of 0.65 to their measurements, to account for projection at small radii (following 5.2). It is clear that the COMBO < z < 0.8 estimate is quantitatively consistent with these estimates, to within the combined uncertainties, with the advantage of robust projection correction, a volume-limited galaxy sample, large sample size, and COMBO-17: luminous galaxies between 0.4< z < 0.8; 4% +-1% are in close physical pairs; 50% of present-day galaxies with M* > 5 10 M have undergone a major merger since z = 0.81; Bell et al. 2006b

25 Dry Mergers: clustering w(θ) NOAO Deep Wide-Field Survey + Spitzer IRAC Shallow Survey: 3 samples with constant number of galaxies (1000) between 0.4 < z < 0.6, 0.6 < z <0.8, 0.8 < z <1.0; Passive evolution with no merger cannot reproduce the observed angular clustering of luminous (> 1.6 L * ) red galaxies. θ White et al. 2007

26 Dry Mergers: clustering Dark matter haloes evolution is well understood and well described by numerical simulations; Ngal with a model for halo occupation we can connect the results of N-body simulation with the galaxies we observe; Mass A passively evolving HOD produces too many galaxies in high-massive haloes at z = 0.5 when compared with the HOD that best fits the data; 1/3 of the most luminous satellites undergo merging or disruption between 0.5 < z < 0.9 White et al. 2007

27 Dry Mergers: clustering Sloan Digital Sky Survey: luminous red galaxies between 0.16 < z < 0.36; ɛ(r) Merging rate: Gyr -1 Mpc -3 r (h -1 Mpc) The probability that an ETG merge with another ETG in one Gyr is < 1%: strict upper limit to mergers. Correlation function on scale 0.1 /h - 8 /h Mpc Masjedi et al. 2006

28 Dry Mergers: ETGs fraction COSMOS Field: ETGs between 0.6 < z < 0.8; Φ[Mpc -3 Mag -1 ] MB Both the morphologically and photometrically selected samples show no evolution in the number density of bright (MB = -20.7); Allowing for different star formation histories and cosmic variance a 30% maximum decrease in the number density is estimated at redshift z. Scarlata et al. 2007

29 Conclusions Merging in general and dry merger in particular is still an open problem in modern cosmology: dry merging play an important role in shaping the galaxies: HOW IMPORTANT? How does the role played by merging evolve with redshift? Are dry mergers important at z = 1? Can we explain the observed red galaxy population with dry mergers?