What makes up an elliptical galaxy

Elliptical galaxies are usually found in the most violent places in the Universe, like at the heart of galaxy clusters and in compact groups of galaxies. In these places, elliptical galaxies have had an accelerated life, with many galaxy mergers and several periods of star formation.

These constant mergers and collisions increased their size and used up all the gas available for star formation. The smallest dwarf elliptical galaxies are no larger than a globular cluster and can contain a mere 10 million stars. While spiral galaxies are bright, elliptical galaxies are dim.

Spiral galaxies are hotbeds of star formation, but elliptical galaxies aren't nearly as prolific because they contain less gas and dust, which means fewer new and brighter stars are born. The existing stars inside an elliptical galaxy tend to be older, giving off more red light than younger stars. So, why do astronomers think elliptical galaxies dominate the sky? Because when specific regions of the sky are studied in depth, more elliptical galaxies appear.

Astronomers think such counts are consistent throughout the universe. Because elliptical galaxies contain older stars and less gas, scientists think that they are nearing the end of the evolutionary line for galaxies.

The universe is a violent place, and collisions between galaxies are frequent — indeed, the Milky Way is due to crash into the Andromeda Galaxy in a few billion years. When two spirals collide, they lose their familiar shape, morphing into the less-structured elliptical galaxies.

A supermassive black hole is thought to lie at the center of these ancient galaxies. These gluttonous giants consume gas and dust, and may play a role in the slower growth of elliptical galaxies.

We here show the distribution of the assembly redshifts for the same galaxies that we analysed in Fig. We define the assembly time as the redshift when 50 per cent or 80 per cent of the final stellar mass is already contained in a single object. The assembly history of ellipticals hence parallels the hierarchical growth of dark matter haloes, in contrast to the formation history of the stars themselves. Figs 4 and 5 imply that a significant fraction of present elliptical galaxies has assembled relatively recently through purely stellar mergers.

This finding agrees with recent observational results Bell et al. We define the assembly redshift as the time when 50 per cent 80 per cent of the stars that make up the galaxy at redshift zero are already assembled in one single object.

Note that more massive ellipticals typically assemble their stars later cf. Table 1 lists, for different mass bins, medians and upper and lower quartiles of the distributions of lookback times corresponding to the formation and assembly redshifts defined above. Formation times and assembly times for model elliptical galaxies in different mass bins.

The first two columns indicate the extrema of the mass bins. The next columns list the lower quartile, the median and upper quartile for each of the formation and assembly times defined in the text. Tf50, Tf80 represent the lookback times corresponding to the redshifts when 50 or 80 per cent of the stars were first formed. Ta50 and Ta80 represent the lookback times corresponding to the redshifts when 50 or 80 per cent of the mass was first assembled in a single object.

All times are in Gyr. The large volume of our simulation allows us to study how properties of model elliptical galaxies depend on their stellar mass and on the environment. In each panel, filled circles represent the median of the distributions, while the error bars mark the upper and lower quartiles. In the upper panel, the empty circles show the lookback times corresponding to the formation redshifts as defined in the upper panel of Fig.

It is interesting to note that the lookback time corresponding to the redshift when half of the stars had formed is a very good approximation to the luminosity-weighted age over the full range of masses shown. Note that the scatter in these quantities particularly for the colour and metallicity is very small, indicating that the main driver of these trends is the stellar mass, as also reflected in Fig.

The results shown in Fig. We plan to investigate the relative contribution of age and metallicity in shaping the observed colour-magnitude relation in a future paper. Symbols indicate the median value of the distributions at each mass, while the error bars link the upper and lower quartiles. The open symbols in the top panel correspond to the lookback time of the upper panel in Fig. For each halo, we compute the mass-weighted average age, metallicity and colour.

Elliptical galaxies in high density environments are on average older, more metal rich and redder than isolated elliptical galaxies. The median stellar metallicities of galaxies in our clusters are higher than the corresponding values for the field.

The same quantities as in Fig. We can also investigate how the properties of model elliptical galaxies depend on cluster-centric distance. This is shown in Fig. We find in total 51 clusters in the whole simulation box with virial mass larger than this value. The bottom panel of Fig. A radial dependence of galaxy properties is, however, also a natural consequence of the fact that mixing of the galaxy population is incomplete during cluster assembly.

This implies that the cluster-centric distance of the galaxies is correlated with the time they were accreted onto a larger structure Diaferio et al.

The scatter shown in Fig. Symbols and lines have the same meaning as in Fig. Filled circles represent the median of the distributions in our default model while empty circles represent the median of the distributions in a model where bulge growth through disc instability is switched off.

Interestingly, bulge growth through disc instability seems to be an efficient process for intermediate mass ellipticals but rather ineffective for the most massive ellipticals in our sample. As expected, more massive galaxies are made up of more pieces. We recall that these most massive elliptical galaxies are, however, also the ones with the oldest stellar populations. Effective number N eff of progenitors as a function of galaxy stellar mass.

Filled circles represent the median of the distribution in our default model, while the error bars indicate the upper and lower quartiles. Empty circles and the corresponding error bars are for a model where bulge formation through disc instability is switched off. The vertical dashed line corresponds to the limit above which our morphological-type determination is robust see Section 3. We have combined a large high-resolution cosmological N -body simulation with semi-analytic techniques to investigate the formation and evolution of elliptical galaxies in a hierarchical merger model.

Understanding the formation and the evolution of these systems represents an issue of fundamental interest as 50 per cent or more of the stellar mass in the local Universe appears to be in early-type systems and bulges Bell et al. In this paper, we have focused on the dependence of the star formation histories, ages and metallicities on environment and on galaxy stellar mass. There is also a clear trend for increasing ages and metallicities, and for redder colours, with decreasing cluster-centric distance.

This can again be viewed as a natural expectation of hierarchical models where the distance of the galaxies from the cluster centre is correlated with the time they were accreted onto the larger system. When this infall happens, we assume that the galaxy is stripped of its hot gas reservoir so it is no longer able to accrete fresh material for star formation.

The galaxy then rapidly consumes its cold gas moving towards the red sequence. We have also investigated how the properties of model elliptical galaxies change as a function of the stellar mass. We have shown — and this is perhaps the most important result of our study — that in our model the most massive elliptical galaxies have the oldest and most metal rich stellar populations, in agreement with observational results see, e.

Nelan et al. In addition, they are also characterized by the shortest formation time-scales, in qualitative agreement with the recently established down-sizing scenario Cowie et al. However, these old ages are in marked contrast to the late assembly times we find for these galaxies.

In fact, our results show that massive ellipticals are predicted to be assembled later than their lower mass counterparts, and that they have a larger effective number of progenitor systems. This is a key difference between the hierarchical scenario and the traditional monolithic collapse picture. Our results disagree with previous semi-analytic models that found a trend for more massive ellipticals to be younger than less massive ones Baugh et al. In order to understand the origin of this discrepancy we have re-run our model with different assumptions.

In the middle panel, we show the same results but for a model in which no AGN feedback and no artificial cooling cut-off is included. In the middle panel, results are shown for a model without AGN feedback and without any cooling cut-off. Different line styles have the same meaning as in Fig. Too many massive systems are, however, produced at redshift zero, at odds with observations. Late mergers and late accretion, which still involve a substantial amount of gas in this model, cause the formation of luminous and young bulge stars.

An artificial cut-off of the gas condensation, similar to that employed in previous models, produces results that are qualitatively similar see also De Lucia to those obtained with the more physically motivated AGN model introduced by Croton et al. The figure also indicates, however, that the model does not produce a monotonic behaviour as a function of stellar mass. This is better seen in Fig. Filled circles show the result for the model with AGN feedback, open circles show the result for the model without AGN feedback and without any artificial cut-off and filled triangles show the result for the model with the artificial cooling cut-off.

For larger masses, the median age stays almost constant for the model with AGN feedback, decreases for the model without suppression of the cooling flows and shows a non-monotonic behaviour for the model with an artificial cooling cut-off. We note, however, that the differences between this scheme and a model with AGN feedback are small. In our analysis, a cooling flow cut-off is hence able to approximately produce the same result as AGN feedback, which is still different from many earlier results.

Median age of model elliptical galaxies as a function of galaxy stellar mass. Filled circles show results for our default model. Empty circles are for a model without AGN feedback and without any cooling cut-off.

We note that the earlier semi-analytic results were based on Monte Carlo merger trees constructed with the extended Press-Schechter formalism, and not by measuring merger trees directly from high-resolution numerical simulations as we have done here. We believe this reversal to be a combination of the change in the physical model and in the cosmology. This leaves room for late gaseous mergers and gas accretion that can substantially rejuvenate the stellar population of elliptical galaxies.

Another important difference between our model and previous ones is that we explicitly follow dark matter substructures within each halo, even after their progenitor haloes have been accreted by larger structures.

Springel et al. If too many of these satellite galaxies are assumed to merge on too short a time-scale, excessively bright and blue central galaxies result. The short formation time-scales we find for very massive ellipticals are qualitatively in agreement with those required by Thomas et al. The detailed census of ages and metallicities for stars in our model elliptical galaxies depends on details of our feedback model and chemical enrichment scheme.

We plan to come back to these issues in future work. GDL would like to thank M. Pannella and V. Strazzullo for intense and provocative discussions on our poor knowledge of galaxy formation and evolution. We thank B. Irregular galaxies—such as the Large and Small Magellanic Clouds that flank our Milky Way—appear misshapen and lack a distinct form, often because they are within the gravitational influence of other galaxies close by.

They are full of gas and dust, which makes them great nurseries for forming new stars. Some galaxies occur alone or in pairs, but they are more often parts of larger associations known as groups, clusters, and superclusters. Our Milky Way, for instance, is in the Local Group , a galaxy group about 10 million light-years across that also includes the Andromeda galaxy and its satellites. The Local Group and its neighbor galaxy cluster, the Virgo Cluster , both lie within the larger Virgo Supercluster , a concentration of galaxies that stretches about million light-years across.

The Virgo Supercluster, in turn, is a limb of Laniakea, an even bigger supercluster of , galaxies that astronomers defined in Galaxies in clusters often interact and even merge together in a dynamic cosmic dance of interacting gravity. When two galaxies collide and intermingle, gases can flow towards the galactic center, which can trigger phenomena like rapid star formation.

Our own Milky Way will merge with the Andromeda galaxy in about 4. Because elliptical galaxies contain older stars and less gas than spiral galaxies, it seems that the galaxy types represent part of a natural evolution: As spiral galaxies age, interact, and merge, they lose their familiar shapes and become elliptical galaxies. But astronomers are still working out the specifics, such as why elliptical galaxies follow certain patterns in brightness, size, and chemical composition.

The universe's first stars ignited some million years after the big bang, the explosive moment Gravity had sculpted the first galaxies into shape by the time the universe turned million years old , or less than 3 percent of its current age. Astronomers now think that nearly all galaxies— with possible exceptions —are embedded in huge haloes of dark matter.

Theoretical models also suggest that in the early universe, vast tendrils of dark matter provided normal matter the gravitational scaffold it needed to coalesce into the first galaxies. But there are still open questions about how galaxies form. Some believe that galaxies formed from smaller clusters of about one million stars, known as globular clusters , while others hold that galaxies formed first, and later birthed globular clusters.

It's also difficult to figure out how many of a given galaxy's stars formed in situ from its own gas , versus forming in another galaxy and joining the party later. By letting astronomers peer into the universe's farthest reaches—and earliest moments—instruments such as NASA's James Webb Space Telescope should help resolve lingering questions.