Summary

ticals exhibit continuing star formation from z <1. Thus, the pattern of downsizing in both star formation and morphology is gradual and appears to operate over a large range in mass and extend to the lowest redshifts.

The growing evidence for downsizing and its morphological extension raises many questions. While mergers offer a natural explanation for the link between the con- tinuing star formation and morphological transformations we present in this work, it is not clear what is driving this mass-dependent downsizing behavior. Most likely several competing physical processes, including mass-dependent galaxy mergers, are responsible for shaping the Hubble Sequence.

Our main result is the type-dependent galaxy stellar mass functions over three redshift intervals spanning the range 0.2 < z < 1.4. The morphological breakdown of the most massive galaxies (M∗ ≈10¹¹ M⊙) changes significantly with redshift. At z ∼1, Ellipticals, Spirals, and Peculiars are present in similar numbers. By a redshift of 0.3, Ellipticals dominate the high-mass population, suggesting that merging or some other transformation process is active.

At all redshifts in our sample, Spirals and Peculiars dominate at lower masses while E/S0’s become prominent at higher masses. The observed transition mass, Mtr = 2–3×10¹⁰ M⊙, is similar to that apparent in lower redshift studies. There is evidence that Mtr was higher at early times, suggesting a morphological extension of the “downsizing” pattern observed in the star formation rate. Just as the most massive galaxies emerge from a phase of rapid star formation at the earliest times, massive galaxies are also the first to evolve into predominantly early-type morpholo- gies. This morphological transformation is completed 1–2 Gyr after the galaxies leave their bursting phase.

Finally, we derive the integrated stellar mass densities of the three populations and find similar results as Brinchmann & Ellis (2000). We find further evidence for the transformation of Peculiars as well as Spirals into early-type galaxies as a function of time. Based on the observed mass functions, this transformation process appears to be more important at lower masses (M∗ <

∼ 10¹¹ M⊙) because the most massive E/S0’s are already in place at z ∼1.

In the future it will be possible to extend this kind of study with the primary aim of reducing statistical uncertainty and the effects of cosmic variance. Large galaxy surveys like DEEP2 (Davis et al. 2003) and COMBO-17 (Rix et al. 2004) are promising in this regard because they contain tens of thousands of galaxies spread over a wide area, although we note that stellar mass studies benefit greatly from spectroscopic redshifts. Extending the combined mass function to lower masses may help reveal the nature of star formation fromz ∼1 toz = 0. At the same time, reducing cosmic variance will allow for more detailed studies on the type-dependent evolution of the mass function and its relation to merging and star formation.

Acknowledgments

We wish to thank the referee for very valuable comments, Dr. Tommaso Treu for help developing the morphological classification scheme and for useful discussions, and Dr. Jarle Brinchmann for advice on the stellar mass estimator. We also thank Dr. Avishai Dekel for helpful discussions.

Supported by NSF grant AST-0307859 and NASA STScI grant HST-AR-09920.01- A.

Chapter 4

The Mass Assembly History of Field Galaxies: Detection of an Evolving Mass Limit for

Star-Forming Galaxies ¹

We characterize the mass-dependent evolution of galaxies in a large sample of more than 8,000 galaxies using spectroscopic redshifts drawn from the DEEP2 Galaxy Redshift Survey in the range 0.4 < z < 1.4 and stellar masses calculated from K- band photometry obtained at Palomar Observatory. This sample spans more than 1.5 square degrees in four independent fields. Using restframe (U−B) color and [OII]

line widths, we distinguish star-forming from passive populations in order to explore the nature of “downsizing”—a pattern in which the sites of active star formation shift from high mass galaxies at early times to lower mass systems at later epochs. Over the redshift range probed, we identify a mass limit, MQ, above which star formation appears to be quenched. The physical mechanisms responsible for downsizing can thus be empirically quantified by charting the evolution in this threshold mass. We find thatMQ decreases with time by a factor of≈5 across the redshift range sampled according to MQ ∝ (1 + z)^4.5. We demonstrate that this behavior is quite robust to the use of various indicators of star formation activity, including morphological type. To further constrain possible quenching mechanisms, we investigate how this downsizing signal depends on the local galaxy environment using the projected 3^rd-

1Much of this chapter has been previously published as Bundy et al. (2005b)

nearest-neighbor statisticDp,3, which is particularly well-suited for large spectroscopic samples. For the majority of galaxies in regions near the median density, there is no significant correlation between downsizing and environment. However, a weak trend is observed in the comparison between more extreme environments that are more than 3 times overdense or underdense relative to the median. Here, we find that downsizing is accelerated in overdense regions which host higher numbers of massive, early-type galaxies and fewer late-types as compared to the underdense regions. Our results significantly constrain recent suggestions for the origin of downsizing and indicate that the process for quenching star formation must, primarily, be internally driven, with little dependence on large scale structure. By quantifying both the time and density dependence of downsizing, our survey provides a valuable benchmark for galaxy models incorporating baryon physics.

4.1 Introduction

The redshift interval from z ≈1 to z = 0 accounts for roughly half of the age of the universe and provides a valuable baseline over which to study the final stages of galaxy assembly. From many surveys spanning this redshift range, it is now well-established that the global star formation rate (SFR) declines by an order of magnitude (e.g., Broadhurst et al. 1992; Lilly et al. 1996; Cowie et al. 1999; Flores et al. 1999; Wilson et al. 2002). An interesting characteristic of this evolution in SFR is the fact that sites of active star formation shift from including high mass galaxies atz >∼1 to only lower mass galaxies subsequently. This pattern, referred to by Cowie et al. (1996) as “downsizing,” seems contrary to the precepts of hierarchical structure formation, and so understanding the physical processes that drive it is an important problem in galaxy formation.

The observational evidence for downsizing is now quite extensive. The primary evidence comes from field surveys encompassing all classes of galaxies to z ≈ 1 and beyond (Brinchmann & Ellis 2000; Bell et al. 2005b; Bauer et al. 2005; Juneau et al.

2005; Faber et al. 2005). However, the trends are also seen in specific populations such

as field spheroidals, both in their stellar mass functions (Fontana et al. 2004; Bundy et al. 2005a) and in more precise fundamental plane studies (Treu et al. 2005a; van der Wel et al. 2005) which track the evolving mass/light ratio as a function of dynamical mass. The latter studies find that massive spheroidals completed the bulk of their star formation to within a few percent prior to z ≃1, whereas lower mass ellipticals continue to grow by as much as 50% in terms of stellar mass after z ∼ 1. Finally, detailed analyses of the spectra of nearby galaxies suggest similar trends (Heavens et al. 2004; Jimenez et al. 2005).

Downsizing is important to understand as it signifies the role that feedback plays in the mass-dependent evolution of galaxies. As a consequence, its physical origin has received much attention theoretically. Recent analytic work by Dekel & Birnboim (2004), for instance, suggests that the distinction between star-forming and passive systems can be understood via several characteristic mass thresholds governed by the physics of clustering, shock heating, and various feedback processes. Some of these processes have been implemented in numerical and semi-analytic models, including mass-dependent star formation rates (Menci et al. 2005), regulation through feedback by supernovae (e.g., Cole et al. 2000; Benson et al. 2003; Nagashima & Yoshii 2004), and active galactic nuclei (AGN) (e.g., Silk & Rees 1998; Granato et al. 2004; Dekel

& Birnboim 2004; Hopkins et al. 2005a; Croton et al. 2005; De Lucia et al. 2005;

Scannapieco et al. 2005). However, most models have, until now, primarily addressed the mass distinction between star-forming and quiescent galaxies as defined at the present epoch (e.g., Kauffmann et al. 2003b). Quantitative observational measures of the evolving mass dependencevia higher redshift data have not been available.

This work is concerned with undertaking a systematic study of how the mass- dependent growth of galaxies progresses over a wide range of epochs. The goal is to quantify the patterns by which assembly proceeds as a basis for further model- ing. Does downsizing result largely from the assembly history of massive early-type galaxies or is there a decline in the fraction of massive star-forming systems? In the quenching of star formation, what are the primary processes responsible and how are they related to the hierarchical framework of structure assembly as envisioned in the

CDM paradigm? Does downsizing ultimately result from internal physical processes localized within galaxies such as star formation and AGN feedback (Croton et al.

2005; De Lucia et al. 2005), or is it caused by external effects related to the immedi- ate environment, such as ram pressure stripping and encounters with nearby galaxies in groups and clusters?

In this study we combine the large spectroscopic sample contained in the DEEP2 Galaxy Redshift Survey (Davis et al. 2003) with stellar masses based on extensive near-infrared imaging conducted at Palomar Observatory to characterize the assembly history and evolution of galaxies since z ≈ 1.2. Our primary goal is to quantify the downsizing signal in physical terms and test its environmental dependence so that it is possible to constrain the mechanisms responsible. A plan of this chapter is as follows. Section §4.2 presents the observations and characterizes the sample, while§4.3 describes our methods for measuring stellar masses, star formation activity, and environmental density. We discuss how we estimate errors in the derived mass functions in §4.4 and present our results in §4.5. We discuss our interpretations of the results in §4.6 and conclude in §4.7. Where necessary, we assume a standard cosmological model with ΩM = 0.3, ΩΛ= 0.7,H0 = 100hkm s⁻¹ Mpc⁻¹ and h= 0.7.