Steeper, Flatter, or Just "Salpeter"? Evidence from Galaxy Evolution and Galaxy Clusters

A single-slope "Salpeter" IMF overpredicts the stellar M/L ratio of local elliptical galaxies by about a factor of 2, which requires the IMF to be flatter below about one solar mass. On the other hand a Salpeter IMF for stars more massive than the sun predicts an evolution with redshift of the fundamental plane of ellipticals in clusters which is in agreement with the observations and a formation at z>~3 for these galaxies. A "Salpeter" IMF for 1<M<40 solar masses also predicts the observed amount of heavy elements (oxygen and silicon) in clusters of galaxies.Comment: 10 pages, 7 figures, to appear on "IMF@50: The Initial Mass Function 50 Years Later", ed. E. Corbelli, F. Palla, & H. Zinnecker (Dordrecht: Kluwer), in press. Invited talk at the International Workshop held in Abbazia di Spineto, Tuscany, Italy -- May 16-20, 200

Frequency dependent relaxation rate in the superconducting YBa2Cu3O{6+delta}

The submillimeter-wave 3 cm{-1} < nu < 35 cm{-1} complex conductivity of the reduced YBa2Cu3O{6+delta} film (Tc=56.5 K) was investigated for temperatures 4 K < T < 300 K and compared to the properties of the same film in the optimally doped state. The frequency dependence of the effective quasiparticle scattering rate 1/tau*(nu) was extracted from the spectra. 1/tau*(nu) is shown to be frequency independent at low frequencies and high temperatures. A gradual change to 1/tau*~nu^{1.5} law is observed as temperature decreases. In order to explain the observed temperature dependence of the low frequency spectral weight above Tc, the quasiparticle effective mass is supposed to be temperature dependent for T>Tc.Comment: 4 pages, 4 figure

The Far-Infrared, UV and Molecular Gas Relation in Galaxies up to z=2.5

We use the infrared excess (IRX) FIR/UV luminosity ratio to study the relation between the effective UV attenuation (A_IRX) and the UV spectral slope (beta) in a sample of 450 1<z<2.5 galaxies. The FIR data is from very deep Herschel observations in the GOODS fields that allow us to detect galaxies with SFRs typical of galaxies with log(M)>9.3. Thus, we are able to study galaxies on and even below the main SFR-stellar mass relation (main sequence). We find that main sequence galaxies form a tight sequence in the IRX--beta plane, which has a flatter slope than commonly used relations. This slope favors a SMC-like UV extinction curve, though the interpretation is model dependent. The scatter in the IRX-beta plane, correlates with the position of the galaxies in the SFR-M plane. Using a smaller sample of galaxies with CO gas masses, we study the relation between the UV attenuation and the molecular gas content. We find a very tight relation between the scatter in the IRX-beta plane and the specific attenuation (S_A), a quantity that represents the attenuation contributed by the molecular gas mass per young star. S_A is sensitive to both the geometrical arrangement of stars and dust, and to the compactness of the star forming regions. We use this empirical relation to derive a method for estimating molecular gas masses using only widely available integrated rest-frame UV and FIR photometry. The method produces gas masses with an accuracy between 0.12-0.16 dex in samples of normal galaxies between z~0 and z~1.5. Major mergers and sub-millimeter galaxies follow a different S_A relation.Comment: 11 pages, 6 pages appendix, 11 figures, accepted to Ap

A Thirty Kiloparsec Chain of "Beads on a String" Star Formation Between Two Merging Early Type Galaxies in the Core of a Strong-Lensing Galaxy Cluster

New Hubble Space Telescope ultraviolet and optical imaging of the strong-lensing galaxy cluster SDSS J1531+3414 (z=0.335) reveals two centrally dominant elliptical galaxies participating in an ongoing major merger. The interaction is at least somewhat rich in cool gas, as the merger is associated with a complex network of nineteen massive superclusters of young stars (or small tidal dwarf galaxies) separated by ~1 kpc in projection from one another, combining to an estimated total star formation rate of ~5 solar masses per year. The resolved young stellar superclusters are threaded by narrow H-alpha, [O II], and blue excess filaments arranged in a network spanning ~27 kpc across the two merging galaxies. This morphology is strongly reminiscent of the well-known "beads on a string" mode of star formation observed on kpc-scales in the arms of spiral galaxies, resonance rings, and in tidal tails between interacting galaxies. Nevertheless, the arrangement of this star formation relative to the nuclei of the two galaxies is difficult to interpret in a dynamical sense, as no known "beads on a string" systems associated with kpc-scale tidal interactions exhibit such lopsided morphology relative to the merger participants. In this Letter we present the images and follow-up spectroscopy, and discuss possible physical interpretations for the unique arrangement of the young stellar clusters. While we suggest that this morphology is likely to be dynamically short-lived, a more quantitative understanding awaits necessary multiwavelength follow-up, including optical integral field spectroscopy, ALMA sub-mm interferometry, and Chandra X-ray imaging.Comment: 7 pages, 4 figures, accepted for publication in ApJ Letters. High resolution images of the cluster can be found at http://hubblesite.org/news/2014/2

Sub-millimeter galaxies as progenitors of compact quiescent galaxies

Three billion years after the big bang (at redshift z=2), half of the most massive galaxies were already old, quiescent systems with little to no residual star formation and extremely compact with stellar mass densities at least an order of magnitude larger than in low redshift ellipticals, their descendants. Little is known about how they formed, but their evolved, dense stellar populations suggest formation within intense, compact starbursts 1-2 Gyr earlier (at 3<z<6). Simulations show that gas-rich major mergers can give rise to such starbursts which produce dense remnants. Sub-millimeter selected galaxies (SMGs) are prime examples of intense, gas-rich, starbursts. With a new, representative spectroscopic sample of compact quiescent galaxies at z=2 and a statistically well-understood sample of SMGs, we show that z=3-6 SMGs are consistent with being the progenitors of z=2 quiescent galaxies, matching their formation redshifts and their distributions of sizes, stellar masses and internal velocities. Assuming an evolutionary connection, their space densities also match if the mean duty cycle of SMG starbursts is 42 (+40/-29) Myr (consistent with independent estimates), which indicates that the bulk of stars in these massive galaxies were formed in a major, early surge of star-formation. These results suggests a coherent picture of the formation history of the most massive galaxies in the universe, from their initial burst of violent star-formation through their appearance as high stellar-density galaxy cores and to their ultimate fate as giant ellipticals.Comment: ApJ (in press

PHIBSS: molecular gas content and scaling relations in z~1-3 normal star forming galaxies

We present PHIBSS, the IRAM Plateau de Bure high-z blue sequence CO 3-2 survey of the molecular gas properties in normal star forming galaxies (SFGs) near the cosmic star formation peak. PHIBSS provides 52 CO detections in two redshift slices at z~1.2 and 2.2, with log(M*(M_solar))>10.4 and log(SFR(M_solar/yr))>1.5. Including a correction for the incomplete coverage of the M*-SFR plane, we infer average gas fractions of ~0.33 at z~1.2 and ~0.47 at z~2.2. Gas fractions drop with stellar mass, in agreement with cosmological simulations including strong star formation feedback. Most of the z~1-3 SFGs are rotationally supported turbulent disks. The sizes of CO and UV/optical emission are comparable. The molecular gas - star formation relation for the z=1-3 SFGs is near-linear, with a ~0.7 Gyrs gas depletion timescale; changes in depletion time are only a secondary effect. Since this timescale is much less than the Hubble time in all SFGs between z~0 and 2, fresh gas must be supplied with a fairly high duty cycle over several billion years. At given z and M*, gas fractions correlate strongly with the specific star formation rate. The variation of specific star formation rate between z~0 and 3 is mainly controlled by the fraction of baryonic mass that resides in cold gas.Comment: Submitted to the Astrophysical Journal; 14 figure

The lesser role of starbursts for star formation at z=2

Two main modes of star formation are know to control the growth of galaxies: a relatively steady one in disk-like galaxies, defining a tight star formation rate (SFR)-stellar mass sequence, and a starburst mode in outliers to such a sequence which is generally interpreted as driven by merging. Such starburst galaxies are rare but have much higher SFRs, and it is of interest to establish the relative importance of these two modes. PACS/Herschel observations over the whole COSMOS and GOODS-South fields, in conjunction with previous optical/near-IR data, have allowed us to accurately quantify for the first time the relative contribution of the two modes to the global SFR density in the redshift interval 1.5<z<2.5, i.e., at the cosmic peak of the star formation activity. The logarithmic distributions of galaxy SFRs at fixed stellar mass are well described by Gaussians, with starburst galaxies representing only a relatively minor deviation that becomes apparent for SFRs more than 4 times higher than on the main sequence. Such starburst galaxies represent only 2% of mass-selected star forming galaxies and account for only 10% of the cosmic SFR density at z~2. Only when limited to SFR>1000M(sun)/yr, off-sequence sources significantly contribute to the SFR density (46+/-20%). We conclude that merger-driven starbursts play a relatively minor role for the formation of stars in galaxies, whereas they may represent a critical phase towards the quenching of star formation and morphological transformation in galaxies.Comment: Accepted for publication in ApJ Letter

Structure and star formation in galaxies out to z=3: evidence for surface density dependent evolution and upsizing

We present an analysis of galaxies in the CDF-South. We find a tight relation to z=3 between color and size at a given mass, with red galaxies being small, and blue galaxies being large. We show that the relation is driven by stellar surface density or inferred velocity dispersion: galaxies with high surface density are red and have low specific star formation rates, and galaxies with low surface density are blue and have high specific star formation rates. Surface density and inferred velocity dispersion are better correlated with specific star formation rate and color than stellar mass. Hence stellar mass by itself is not a good predictor of the star formation history of galaxies. In general, galaxies at a given surface density have higher specific star formation rates at higher redshift. Specifically, galaxies with a surface density of 1-3 10^9 Msun/kpc^2 are "red and dead" at low redshift, approximately 50% are forming stars at z=1, and almost all are forming stars by z=2. This provides direct additional evidence for the late evolution of galaxies onto the red sequence. The sizes of galaxies at a given mass evolve like 1/(1+z)^(0.59 +- 0.10). Hence galaxies undergo significant upsizing in their history. The size evolution is fastest for the highest mass galaxies, and quiescent galaxies. The persistence of the structural relations from z=0 to z=2.5, and the upsizing of galaxies imply that a relation analogous to the Hubble sequence exists out to z=2.5, and possibly beyond. The star forming galaxies at z >= 1.5 are quite different from star forming galaxies at z=0, as they have likely very high gas fractions, and star formation time scales comparable to the orbital time.Comment: 20 pages, accepted for publication in ApJ, 2008, 68