Galaxy formation and evolution

Abstract:

We present a stellar population analysis of the absorption line strength maps for 48 early-type galaxies from the SAURON sample. Using the line strength index maps of Hβ, Fe5015 and Mg b, measured in the Lick/IDS system and spatially binned to a constant signal-to-noise ratio, together with predictions from up-to-date stellar population models, we estimate the simple stellar population-equivalent (SSP-equivalent) age, metallicity and abundance ratio [alpha;/Fe] over a two-dimensional field extending up to approximately one effective radius. A discussion of calibrations and differences between model predictions is given. Maps of SSP-equivalent age, metallicity and abundance ratio [alpha;/Fe] are presented for each galaxy. We find a large range of SSP-equivalent ages in our sample, of which ~40 per cent of the galaxies show signs of a contribution from a young stellar population. The most extreme cases of post-starburst galaxies, with SSP-equivalent ages of ≤3 Gyr observed over the full field-of-view, and sometimes even showing signs of residual star formation, are restricted to low-mass systems (σe≤ 100 km s-1 or ~2 × 1010 M.). Spatially restricted cases of young stellar populations in circumnuclear regions can almost exclusively be linked to the presence of star formation in a thin, dusty disc/ring, also seen in the near-UV or mid-IR on top of an older underlying stellar population.The flattened components with disc-like kinematics previously identified in all fast rotators are shown to be connected to regions of distinct stellar populations. These range from the young, still star-forming circumnuclear discs and rings with increased metallicity preferentially found in intermediate-mass fast rotators, to apparently old structures with extended disc-like kinematics, which are observed to have an increased metallicity and mildly depressed [alpha;/Fe] ratio compared to the main body of the galaxy. The slow rotators, often harbouring kinematically decoupled components (KDC) in their central regions, generally show no stellar population signatures over and above the well-known metallicity gradients in early-type galaxies and are largely consistent with old (≥10 Gyr) stellar populations.Using radially averaged stellar population gradients we find in agreement with Spolaor et al. a mass-metallicity gradient relation where low-mass fast rotators form a sequence of increasing metallicity gradient with increasing mass. For more massive systems (above ~3.5 × 1010 M.) there is an overall downturn such that metallicity gradients become shallower with increased scatter at a given mass leading to the most massive systems being slow rotators with relatively shallow metallicity gradients. The observed shallower metallicity gradients and increased scatter could be a consequence of the competition between different star formation and assembly scenarios following a general trend of diminishing gas fractions and more equal-mass mergers with increasing mass, leading to the most massive systems being devoid of ordered motion and signs of recent star formation. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

We demonstrate that the comparison of Tully-Fisher relations (TFRs) derived from global H-i linewidths to TFRs derived from the circular-velocity profiles of dynamical models (or stellar kinematic observations corrected for asymmetric drift) is vulnerable to systematic and uncertain biases introduced by the different measures of rotation used. We therefore argue that to constrain the relative locations of the TFRs of spiral and S0 galaxies, the same tracer and measure must be used for both samples. Using detailed near-infrared imaging and the circular velocities of axisymmetric Jeans models of 14 nearby edge-on Sa-Sb spirals and 14 nearby edge-on S0s drawn from a range of environments, we find that S0s lie on a TFR with the same slope as the spirals, but are on average 0.53 ± 0.15-mag fainter at KS band at a given rotational velocity. This is a significantly smaller offset than that measured in earlier studies of the S0 TFR, which we attribute to our elimination of the bias associated with using different rotation measures and our use of earlier-type spirals as a reference. Since our measurement of the offset avoids systematic biases, it should be preferred to previous estimates. A spiral stellar population in which star formation is truncated would take ≈1-Gyr to fade by 0.53-mag at KS band. If S0s are the products of a simple truncation of star formation in spirals, then this finding is difficult to reconcile with the observed evolution of the spiral/S0 fraction with redshift. Recent star formation could explain the observed lack of fading in S0s, but the offset of the S0 TFR persists as a function of both stellar and dynamical mass. We show that the offset of the S0 TFR could therefore be explained by a systematic difference between the total mass distributions of S0s and spirals, in the sense that S0s need to be smaller or more concentrated than spirals. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

We have searched for star-forming galaxies at z- 7-10 by applying the Lyman-break technique to newly released Y-, J- and H-band images (1.1, 1.25 and 1.6 μm) from Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. By comparing these images of the Hubble Ultra Deep Field with the Advanced Camera for Surveys (ACS) z'-band (0.85 μm) images, we identify objects with red colours, (z'-Y)AB > 1.3, consistent with the Lyman α forest absorption at z≈ 6.7-8.8. We identify 12 of these z'-drops down to a limiting magnitude YAB < 28.5 (equivalent to a star formation rate of 1.3-M--yr-1 at z= 7.1), all of which are undetected in the other ACS filters. We use the WFC3 J-band image to eliminate contaminant low-mass Galactic stars, which typically have redder colours than z≈ 7 galaxies. One of our z'-drops is probably a T-dwarf star. The z≈ 7-z'-drops appear to have much bluer spectral slopes than Lyman-break galaxies at lower redshift. Our brightest z'-drop is not present in the NICMOS J-band image of the same field taken 5 years before, and is a possible transient object. From the 10 remaining z≈ 7 candidates we determine a lower limit on the star formation rate density of 0.0017-M--yr-1-Mpc-3 for a Salpeter initial mass function, which rises to 0.0025-0.004-M--yr-1-Mpc-3 after correction for luminosity bias. The star formation rate density is a factor of ≈10 less than that of Lyman-break galaxies at z= 3-4, and is about half the value at z≈ 6. We also present the discovery of seven Y-drop objects with (Y-J)AB > 1.0 and JAB < 28.5 which are candidate star-forming galaxies at higher redshifts (z≈ 8-9). We find no robust J-drop candidates at z≈ 10. While based on a single deep field, our results suggest that this star formation rate density would produce insufficient Lyman continuum photons to reionize the Universe unless the escape fraction of these photons is extremely high (fesc > 0.5), and the clumping factor of the Universe is low. Even then, we need to invoke a large contribution from galaxies below our detection limit (a steep faint-end slope). The apparent shortfall in ionizing photons might be alleviated if stellar populations at high redshift are of low metallicity or have a top-heavy initial mass function. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

The Herschel ATLAS is the largest open-time key project that will be carried out on the Herschel Space Observatory. It will survey 570 deg 2 of the extragalactic sky, 4 times larger than all the other Herschel extragalactic surveys combined, in five far-infrared and submillimeter bands. We describe the survey, the complementary multiwavelength data sets that will be combined with the Herschel data, and the six major science programs we are undertaking. Using new models based on a previous submillimeter survey of galaxies, we present predictions of the properties of the ATLAS sources in other wave bands. © 2010. The Astronomical Society of the Pacific. All rights reserved.

Abstract:

We present a range of new observations of the 'broad absorption-line radio galaxy' 4C +72.26 (z≈ 3.5), including sensitive rest-frame ultraviolet integral field spectroscopy using the Gemini/GMOS-N instrument and Subaru/CISCO K-band imaging and spectroscopy. We show that 4C +72.26 is a system of two vigorously star-forming galaxies superimposed along the line of sight separated by ∼1300 ± 200 km s-1 in velocity, with each demonstrating spectroscopically resolved absorption lines. The most active star-forming galaxy also hosts the accreting supermassive black hole which powers the extended radio source. We conclude that the star formation is unlikely to have been induced by a shock caused by the passage of the radio jet, and instead propose that a collision is a more probable trigger for the star formation. Despite the massive starburst, the ultraviolet-mid-infrared spectral energy distribution suggests that the pre-existing stellar population comprises ∼1012 M⊙ of stellar mass, with the current burst only contributing a further ∼2 per cent, suggesting that 4C +72.26 has already assembled most of its final stellar mass. © 2010 The Authors. Journal compilation © 2010 RAS.