on a small sample from the second data release of the COLD GASS survey (Saintonge et al 2011) shows encouraging gas depletion signatures (§5.4). A more detailed study with better statistics is needed.
Environmental effects on Hi and star formation properties of massive galax-ies –Chapter 6
For the first time, we studied the Hi content of massive galaxies as a continuous function of local density, filling the observational gap currently existing between fields and clusters.
By comparing the effects of environment at different density scales on global and inner specific SFR (§6.3) and Hi content (§6.4), we showed clear signs of cold gas stripping in galaxies with 10 <Log M∗[M] <10.5, outside clusters. Once the first order dependence of galaxy properties on stellar mass had been removed, in fact, we measured a gradual in-crease in the suppression of star formation from the inner to the outer regions, and an even stronger Hi deficiency as a function of local density. All these effects can be explained by a mechanism acting on the disk from the outside-in, like the ram-pressure of the cold ISM (§6.5). Finally, a comparison with mock catalogs extracted from one of the most recent semi-analytic models (Guo et al. 2011), which includes only starvation through removal of the hot gas, showed how models underestimate the environmental effects, especially on the cold gas component of galaxies (§6.5).
7.2 Outlook
In our work, we investigated the connection between atomic hydrogen and star formation with the purpose of testing quenching mechanisms, using for the first time homogeneous data for a large sample not biased towards any morphology or star formation regime. As already underlined in the Introduction, this work is statistical in nature and does not aim at ruling out any of the suggested mechanisms acting in individual galaxies. Rather, we wanted to understand their “statistical efficiency” in producing the observed bimodality of massive galaxies in the local Universe. We proved that bulge-dominated galaxies are not less efficient in forming stars than disk-dominated ones: once a galaxy has a gas disk component, star formation will eventually take place in it. Therefore, in isolated galaxies there must be additional mechanisms at work which heat or deplete the cold gas, and/or prevent hot gas from cooling onto a disk. The main mechanism currently suggested is feedback from AGN, in both its radio (mechanical) and quasar (radiative) mode. Our results show no signature of negative feedback on the atomic reservoir of
massive galaxies, but rather suggest a co-evolution of AGN and star formation, both fed by the gas available. Observations of galaxy colours becoming redder when galaxies host an AGN (e.g. Kauffmann et al. 2003a; Heckman et al. 2004; Schawinski et al. 2007), which have been considered as supporting evidence of an AGN-driven galaxy evolution, could be explained by the following: when a galaxy starts to consume its atomic reservoir, there is no material for either accretion onto black hole or conversion into molecular gas and subsequent star formation. Of course, we cannot and we do not exclude that radiative feedback is efficient at higher redshifts, when AGN were more efficiently accreting. And we do not exclude that the radio-mode, for which we do not have a significant sample in our data, plays a major role. There is some evidence of atomic and molecular hydrogen outflows driven by radio jets at low and high redshifts (Nesvadba et al. 2008; Guillard et al.
2012), or quasar feedback through AGN winds at high redshifts (Feruglio et al. 2010), but to date they are based on a very limited number of galaxies so that their incidence is not constrained.
While we did not observe both bulges and AGN to have a significant influence on the global cold gas content of massive galaxies, we actually proved that the environment plays a major role in quenching their star formation. In fact, we measured signatures of atomic hydrogen stripping in intermediate mass galaxies located in environments less dense than clusters. Environmental effects on real galaxies are actually stronger than currently assumed in models: we clearly showed that models that include only hot gas removal are not able to reproduce observations. We therefore suggest that, in order to improve our understanding of galaxy evolution, models should implement additional environmental effects on the cold ISM of galaxies, which could solve part of the puzzle in the formation of massive and passive systems.
7.2.1 Future prospects
We showed how the study of the atomic gas phase in massive galaxies on statistically significant, unbiased samples is necessary and effective to properly constrain the dominant mechanisms creating the observed galaxy bimodality. The most promising mechanisms, which both models and observations should investigate, are environmental ones. From the observational side there are in fact some aspects that need to be better investigated. First, we need to extend the mass range considered in this work to lower mass galaxies: if we are observing signatures of cold gas stripping, we should measure an even stronger effect on less massive galaxies, which are on average gas richer and in which the gas is less bound.
Second, we should increase the statistics for the more massive systems and expand the
7.2 Outlook 119 local density range studied, in order to identify the threshold in both M? and environment at which stripping of the cold gas starts/ceases to be effective. For this latter investigation, a larger volume is necessary. The paucity of massive systems (Log M?> 1011M) in our sample is in fact a consequence of cosmic variance; the small number of large groups and the lack of clusters in our sample is also a consequence of the limited volume sampled.
Finally, spatially resolved data showing the outside-in stripping effect on atomic gas which is suggested by our trends would represent the “smocking gun” evidence for our conclusions.
To improve the analysis performed here, therefore, new data and samples should fulfill the following characteristics. (i)Observations should spatially resolve the gas distribution. (ii) Samples should include data about additional hydrogen phases, first of all the one directly coupled with star formation on short time scales, i.e. H2. This will further clarify the connection ISM-star formation in massive galaxies, as Hi constitutes only the first phase in the process leading to star formation in galaxies. (iii) Samples should cover larger volumes over broader redshift ranges. Quantifying how the relative importance of the studied processes and their efficiency evolves with redshift is key to understand the main drivers of galaxy evolution. Recent studies have started to assemble statistical samples (Tacconi et al. 2010, Förster Schreiber et al. 2011) to constrain the dependence of star formation on molecular gas content and how this dependence varies with environment at intermediate to high redshift (1 < z < 3), but samples are still small, biased toward the luminous end of the galaxy population and without Hi data available.
In the next decade, new radio facilities are going to see the light, and fulfill the listed requirements. New telescopes and new receivers are under development and are specifically designed to achieve the characteristics required for the undertaking of major Hi surveys, with better sensitivity and angular resolution, and larger redshift ranges probed. The exploitation of new technologies has already started on the existing facilities, like the VLA telescope which is currently being upgraded to the new Expanded VLA (EVLA1).
It will be particularly useful to spatially resolve the Hi in galaxies in environments from groups to clusters up to z ∼0.53, and to map the intragroup medium. Similarly, the Westerbork Synthesis Radio Telescope (WRST) is going to be equipped with APERTIF (Verheijen et al. 2008), a new focal plane array which provides several beams on the sky increasing the survey speed of the telescope by a factor of 20. One of the major programme planned is an Hi survey of the northern sky that overlaps with SDSS out to z ∼0.15, to study the evolution of the gaseous content of ∼ 2×105 galaxies and its role in galaxy evolution. With such survey, we will be able to compare the efficiency of both local and
1http://www.aoc.nrao.edu/evla
global environments of galaxies in regulating their gas content. Its data could detect, thus confirm, stripping signatures on the atomic gas. We could study the role of Hi when galaxies evolve from the blue cloud to the red sequence, and which is the gas content of massive, red galaxies at earlier stage of their evolution than the ones probed here. In the far future, the Square Kilometer Array (SKA2) will be an aperture synthesis radio telescope with a collecting area of up to one million square meters spread over at least 3000 km. SKA science cases are similar to the ones addressed by APERTIF, but the telescope will push observations out to z∼3, performing Hiline surveys of a billion galaxies. In the meanwhile, two of its pathfinders, namely MeerKAT (Booth et al. 2009) and the Australian Square Kilometre Array Pathfinder (ASKAP Johnston et al. 2009), are under construction to test new technologies.
From the molecular gas side, the Atacama Large Millimeter/Submillimeter Array (ALMA3) should see complete light in 2013, counting 66 antennas operating at wavelengths of 0.3 to 9.6 mm (thus suitable for indirect measurements of H2). Although the telescope is not designed as a survey instrument, it will allow the study of the star formation - ISM - galaxy connection out to z ∼ 3. In addition, ALMA resolution will be for example suitable to study the dynamics of the molecular gas in the few tens of parsec around accreting AGN, thus giving better insight into their physics.
To conclude, upcoming data will be of primary importance to expand the analysis of this thesis, because they will show how and when gas is converted into stars, and with which efficiency, up to the epoch when massive galaxies had the bulk of their star formation.
At the same time, our stacking approach will be crucial for the exploitation of upcoming surveys. The sensitivity limits that we face now in the local Universe, in fact, will appear in deeper data at higher redshift, where the numbers of individual detections will be limited by flux dilution.
2http://www.skatelescope.org/
3http://www.almaobservatory.org/
A
Further Stacking tests
In this Appendix we present further tests that we performed before and after the develop-ment of the stacking tool in order to verify the reliability of the whole process.
In the first part (A.1), we describe a simple simulation which we completed before building the actual stacking tool. The goal was to mimic the same steps that we would then have to apply to the ALFALFA data, but implemented for a sample of model Hi spectra for which we knew a priori both noises and signals. In the second part (A.2), we present the comparison between two different possible stacking procedures, to support the choice of the one used in this work. We co-add “gas-fraction” spectra, as explained in Section 3.2.3, but here we show that different approaches lead to compatible results.