3. EVOLUCIÓN
3.3. El mandato en la época del derecho común
3.3.2. El derecho castellano
Although we now have a reliable picture of the evolution of massive quiescent systems (at least for
z <1.5), we still know very little about the manner in which they formed. Particularly, there are
two fundamental issues with important implications for models of galaxy formation: what are the progenitors of red nuggets? And what is the physical mechanism that shuts off star formation at these early cosmic epochs?
In a merger event, the size of the remnant is strongly influenced by the amount of dissipation involved (e.g., Khochfar & Silk, 2006; Hopkins et al., 2006). In order to produce compact remnants, the progenitors must be very rich in gas. Alternatively, it is also possible to produce a compact galaxy via violent disk instability, triggered by intense gas inflows (e.g., Dekel et al., 2009; Dekel & Burkert, 2014; Zolotov et al., 2015). Both major merger and disk instability activities need to be highly dissipational in order to leave a very compact remnant. This is in agreement with the observational fact that compact galaxies are only formed at high redshift, since the typical gas fraction strongly declines with cosmic time. Different formation mechanisms need not to be mutually exclusive; in
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log(M ) [M ]∗
⊙log(
σ
int) [km/s]
Compact SFGs X−ray AGN Extended SFGs Compact quiescent (vdS12, ) B14ab Erb+06 SINS (F−S+09) Maseda+13 Masters+14 8 9 10 11 12 1.6 1.8 2 2.2 2.4 2.6 2.8Figure 6.2 Distribution of velocity dispersions and stellar masses for star-forming galaxies atz∼2, from Barro et al. (2014b). The cyan points represent normal-size galaxies, while the blue points are compact galaxies (i.e., blue nuggets). Regular star-forming galaxies from other studies are shown in gray. The red points are quiescent galaxies (i.e., red nuggets), for which the velocity dispersions have been measured from absorption lines by van de Sande et al. (2013); Belli et al. (2014a,b). The distribution of the blue nuggets matches remarkably well the one of the red nuggets, and suggests a direct evolutionary link between these two populations.
fact, cosmological simulations suggest that there might be multiple channels for the formation of compact systems (Wellons et al., 2015).
Once a compact galaxy is formed via some gas-rich process, the star-formation needs to be shut off before a red nugget can be formed. The problem of galaxy quenching is therefore different from the issue of the compact sizes, but necessarily connected to it. The large amount of gas that collapses to the center might trigger intense star formation followed by supernova, stellar, or AGN feedback (Zolotov et al., 2015). Another possibility is that the fast growth of the bulge leads to gravitational quenching (e.g., Genzel et al., 2014b).
If the above scenario is correct, there is one straightforward prediction that is relatively easy to test: the existence of a population ofcompact star-forming galaxies atz∼2. These systems, which are the intermediate step between large gas-rich galaxies and compact quiescent objects, have been called blue nuggets (Barro et al., 2013). Such population has been identified in observations from wide surveys and, remarkably, its number density is approximately in agreement with the one found for red nuggets at a similar redshift (Barro et al., 2013, 2014a). Spectroscopic follow-up (Barro et al., 2014b; Nelson et al., 2014) yields velocity dispersions, as measured from emission lines, that
are much larger than the typical values for star-forming galaxies at z∼2, and match the velocity dispersions measured from absorption lines in quiescent compact galaxies, as shown in Figure 6.2. The agreement on the kinematics is an important additional clue towards the identification of blue nuggets as the immediate progenitors of red nuggets. However, comparing emission lines with the absorption lines of different systems might not give consistent results. Ideally, the velocity dispersion should be measured from both absorption and emission lines in the same system, which must be observed during the quenching phase. To date, only two such measurements have been performed (Barro et al., 2014b, 2015), and the results are still inconclusive. A different approach is to try to reconstruct the properties of the progenitor by observing the quenched population. As an illustration of this method, we showed that by accurately constraining the star formation history of a z ∼ 2 quiescent galaxy it is possible to estimate the star formation rate of its progenitor. We found a value of∼160M∗/yr, remarkably consistent with the typical star formation rates of blue nuggets.
Sub-millimeter galaxies are another population suggested as the progenitors of red nuggets. These high-redshift systems are characterized by very large star formation rates and have been shown to be related to gas-rich mergers (e.g., Tacconi et al., 2008). Their kinematics and sizes at 3< z <6 match the distributions found for massive quiescent galaxies atz∼2, and the number densities and star formation rates are also consistent with the expectations from simple models (Toft et al., 2014, see Figure 6.3). This possibility is not in contrast with the idea that blue nuggets are the progenitors of compact quiescent galaxies, as the two populations might represent different evolutionary phases and/or phases for objects of different masses, since the sub-millimeter galaxies have generally higher redshifts and masses compared to blue nuggets.
Although substantial uncertainty remains, particularly for the early phases, a coherent picture for the formation and evolution of massive galaxies is emerging from theoretical and observational studies. Massive systems form at very early cosmic times in gas-rich mergers or in disk instabilities fed by massive gas inflows; they go through a sub-millimeter and/or a blue nugget phase; they are quenched possibly because of supernova or AGN feedback, and they turn into compact quies- cent galaxies. After this point, their evolution is dominated by dry minor mergers, which cause a substantial size growth, until they appear atz∼0 as massive elliptical galaxies.
Understanding the formation of the earliest quiescent galaxies is also important for testing cur- rent models based on the ΛCDM cosmology. While the downsizing in star formation has been satisfactorily explained by recent models, a possible discrepancy about the downsizing in mass as- sembly remains. As quiescent massive objects are photometrically detected at higher and higher redshifts (Straatman et al., 2014; Spitler et al., 2014), it is not clear whether a hierarchical forma- tion of structure, in which the more massive dark matter halos are assembled last, can explain the observations (e.g., Steinhardt et al., 2015).
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Figure 6.3 Comparison of the star formation rate distribution for sub-millimeter galaxies and com- pact quiescent systems, from Toft et al. (2014). Assuming thatz∼2 red nuggets form in Eddington limited starbursts, it is possible to calculate the expected distribution for the star formation rate of their progenitors, which is shown in red. The blue curves are 1000 realizations of the observed star formation rate (derived from the infrared luminosity) for z > 3 sub-millimeter galaxies. The two distributions are in good agreement, consistent with the idea that sub-millimeter galaxies represent an evolutionary phase in the formation of compact quiescent galaxies.