Thiago S. Goncalves

The GALEX Arecibo SDSS Survey - I. Gas fraction scaling relations of massive galaxies and first data release

We introduce the GALEX Arecibo SDSS Survey (GASS), an on-going large programme that is gathering high quality H i-line spectra using the Arecibo radio telescope for an unbiased sample of ~1000 galaxies with stellar masses greater than 10^(10) M_⊙ and redshifts 0.025 < z < 0.05 , selected from the Sloan Digital Sky Survey (SDSS) spectroscopic and Galaxy Evolution Explorer (GALEX) imaging surveys. The galaxies are observed until detected or until a low gas mass fraction limit (1.5–5 per cent) is reached. This paper presents the first Data Release, consisting of ~20 per cent of the final GASS sample. We use this data set to explore the main scaling relations of the H i gas fraction with galaxy structure and NUV−r colour. A large fraction (~60 per cent) of the galaxies in our sample are detected in H i. Even at stellar masses above 10^(11) M_⊙, the detected fraction does not fall below ~40 per cent. We find that the atomic gas fraction M_(HI)/M★ decreases strongly with stellar mass, stellar surface mass density and NUV−r colour, but is only weakly correlated with the galaxy bulge-to-disc ratio (as measured by the concentration index of the r-band light). We also find that the fraction of galaxies with significant (more than a few per cent) H I decreases sharply above a characteristic stellar surface mass density of 10^(8.5) M_⊙ kpc^(−2). The fraction of gas-rich galaxies decreases much more smoothly with stellar mass. One of the key goals of GASS is to identify and quantify the incidence of galaxies that are transitioning between the blue, star-forming cloud and the red sequence of passively evolving galaxies. Likely transition candidates can be identified as outliers from the mean scaling relations between M_(HI)/M★ and other galaxy properties. We have fitted a plane to the two-dimensional relation between the H I mass fraction, stellar surface mass density and NUV−r colour. Interesting outliers from this plane include gas-rich red sequence galaxies that may be in the process of regrowing their discs, as well as blue, but gas-poor spirals.

Luminous Thermal Flares from Quiescent Supermassive Black Holes

A dormant supermassive black hole lurking in the center of a galaxy will be revealed when a star passes close enough to be torn apart by tidal forces, and a flare of electromagnetic radiation is emitted when the bound fraction of the stellar debris falls back onto the black hole and is accreted. Although the tidal disruption of a star is a rare event in a galaxy,≈10^(–4) yr^(–1), observational candidates have emerged in all-sky X-ray and deep ultraviolet (UV) surveys in the form of luminous UV/X-ray flares from otherwise quiescent galaxies. Here we present the third candidate tidal disruption event discovered in the Galaxy Evolution Explorer (GALEX) Deep Imaging Survey: a 1.6 × 10^(43) erg s^(–1) UV/optical flare from a star-forming galaxy at z = 0.1855. The UV/optical spectral energy distribution (SED) during the peak of the flare measured by GALEX and Palomar Large Field Camera imaging can be modeled as a single temperature blackbody with T_(bb) = 1.7 × 10^5 K and a bolometric luminosity of 3 × 10^(45) erg s^(–1), assuming an internal extinction with E(B – V)_(gas) = 0.3. The Chandra upper limit on the X-ray luminosity during the peak of the flare, L_X (2 – 10 keV) M_g > – 18.9) to predict the detection capabilities of upcoming optical synoptic surveys.

LOCAL LYMAN BREAK GALAXY ANALOGS: THE IMPACT OF MASSIVE STAR-FORMING CLUMPS ON THE INTERSTELLAR MEDIUM AND THE GLOBAL STRUCTURE OF YOUNG, FORMING GALAXIES

We report on the results of Hubble Space Telescope optical and UV imaging, Spitzer mid-IR photometry, and optical spectroscopy of a sample of 30 low-redshift (z ~ 0.1 to 0.3) galaxies chosen from the Sloan Digital Sky Survey and Galaxy Evolution Explorer surveys to be accurate local analogs of the high-redshift Lyman break galaxies. The Lyman break analogs (LBAs) are similar in stellar mass, metallicity, dust extinction, star formation rate (SFR), physical size, and gas velocity dispersion, thus enabling a detailed investigation of many processes that are important in star-forming galaxies at high redshift. The main optical emission-line properties of LBAs, including evidence for outflows, are also similar to those typically found at high redshift. This indicates that the conditions in their interstellar medium are comparable. In the UV, LBAs are characterized by complexes of massive clumps of star formation, while in the optical they most often show evidence for (post-)mergers and interactions. In six cases, we find a single extremely massive (up to several ×10^9 M_☉) compact (radius ~10^2 pc) dominant central object (DCO). The DCOs are preferentially found in LBAs with the highest mid-IR luminosities (L_(24 μm) = 10^(10.3)-10^(11.2) L_☉) and correspondingly high SFRs (15-100 M_☉ yr^(–1)). We show that the massive star-forming clumps (including the DCOs) have masses much larger than the nuclear super star clusters seen in normal late-type galaxies. However, the DCOs do have masses, sizes, and densities similar to the excess light/central cusps seen in typical elliptical galaxies with masses similar to the LBA galaxies. We suggest that the DCOs form in the present-day examples of the dissipative mergers at high redshift that are believed to have produced the central cusps in local ellipticals (consistent with the disturbed optical morphologies of the LBAs). More generally, the properties of the LBAs are consistent with the idea that instabilities in a gas-rich disk lead to very massive star-forming clumps that eventually coalesce to form a spheroid. Finally, we comment on the apparent lack of energetically significant active galactic nuclei in the DCOs. We speculate that the DCOs are too young at present to grow a supermassive black hole because they are still in a supernova-dominated outflow phase (age less than 50 Myr).

The GALEX Arecibo SDSS Survey II: The Star Formation Efficiency of Massive Galaxies

We use measurements of the H I content, stellar mass and star formation rates (SFRs) in ~190 massive galaxies with M_★ > 10^(10) M_⊙, obtained from the GALEX (Galaxy Evolution Explorer) Arecibo SDSS (Sloan Digital Sky Survey) survey described in Paper I to explore the global scaling relations associated with the bin-averaged ratio of the SFR over the H I mass (i.e. ΣSFR/ΣM_(HI)), which we call the H I-based star formation efficiency (SFE). Unlike the mean specific star formation rate (sSFR), which decreases with stellar mass and stellar mass surface density, the SFE remains relatively constant across the sample with a value close to SFE = 10^(−9.5) yr^(−1) (or an equivalent gas consumption time-scale of ~3 × 10^9 yr). Specifically, we find little variation in SFE with stellar mass, stellar mass surface density, NUV −r colour and concentration (R_(90)/R_(50)). We interpret these results as an indication that external processes or feedback mechanisms that control the gas supply are important for regulating star formation in massive galaxies. An investigation into the detailed distribution of SFEs reveals that approximately 5 per cent of the sample shows high efficiencies with SFE > 10^(−9) yr^(−1), and we suggest that this is very likely due to a deficiency of cold gas rather than an excess SFR. Conversely, we also find a similar fraction of galaxies that appear to be gas-rich for their given sSFR, although these galaxies show both a higher than average gas fraction and lower than average sSFR. Both of these populations are plausible candidates for ‘transition’ galaxies, showing potential for a change (either decrease or increase) in their sSFR in the near future. We also find that 36 ± 5 per cent of the total H I mass density and 47 ± 5 per cent of the total SFR density are found in galaxies with M_★ > 10^(10) M_⊙.

THE KINEMATICS OF IONIZED GAS IN LYMAN-BREAK ANALOGS AT z ∼ 0.2

We present results for 19 “Lyman-break analogs” observed with Keck/OSIRIS with an adaptive-optics-assisted spatial resolution of less than 200 pc. We detect satellites/companions, diffuse emission, and velocity shear, all with high signal-to-noise ratios. These galaxies present remarkably high velocity dispersion along the line of sight (~70 km s^(−1)), much higher than standard star-forming spirals in the low-redshift universe. We artificially redshift our data to z ~ 2.2 to allow for a direct comparison with observations of high-z Lyman-break galaxies and find striking similarities between both samples. This suggests that either similar physical processes are responsible for their observed properties, or, alternatively, that it is very difficult to distinguish between different mechanisms operating in the low- versus high-redshift starburst galaxies based on the available data. The comparison between morphologies in the UV/optical continuum and our kinemetry analysis often shows that neither is by itself sufficient to confirm or completely rule out the contribution from recent merger events. We find a correlation between the kinematic properties and stellar mass, in that more massive galaxies show stronger evidence for a disk-like structure. This suggests a co-evolutionary process between the stellar mass buildup and the formation of morphological and dynamical substructure within the galaxy.

QUENCHING STAR FORMATION AT INTERMEDIATE REDSHIFTS: DOWNSIZING OF THE MASS FLUX DENSITY IN THE GREEN VALLEY

The bimodality in galaxy properties has been observed at low and high redshifts, with a clear distinction between star-forming galaxies in the blue cloud and passively evolving objects in the red sequence; the absence of galaxies with intermediate properties indicates that the quenching of star formation and subsequent transition between populations must happen rapidly. In this paper, we present a study of over 100 transiting galaxies in the so-called green valley at intermediate redshifts (z ~ 0.8). By using very deep spectroscopy with the DEIMOS instrument at the Keck telescope we are able to infer the star formation histories of these objects and measure the stellar mass flux density transiting from the blue cloud to the red sequence when the universe was half its current age. Our results indicate that the process happened more rapidly and for more massive galaxies in the past, suggesting a top-down scenario in which the massive end of the red sequence is forming first. This represents another aspect of downsizing, with the mass flux density moving toward smaller galaxies in recent times.

AN OSIRIS STUDY OF THE GAS KINEMATICS IN A SAMPLE OF UV-SELECTED GALAXIES: EVIDENCE OF “HOT AND BOTHERED” STARBURSTS IN THE LOCAL UNIVERSE

We present data from Integral Field Spectroscopy for three supercompact UV-Luminous Galaxies (ScUVLGs). As nearby (z ~ 0.2) compact (R_(50) ~ 1-2 kpc) bright Paschen-α sources, with unusually high star formation rates (SFR = 3-100 M_☉ yr^(–1)), ScUVLGs are an ideal population for studying detailed kinematics and dynamics in actively star-forming galaxies. In addition, ScUVLGs appear to be excellent analogs to high-redshift Lyman Break Galaxies (LBGs), and our results may offer additional insight into the dynamics of LBGs. Previous work by our team has shown that the morphologies of these galaxies exhibit tidal features and companions, and in this study we find that the dynamics of ScUVLGs are dominated by disturbed kinematics of the emission line gas—suggesting that these galaxies have undergone recent feedback, interactions, or mergers. While two of the three galaxies do display rotation, v/σ<1—suggesting dispersion-dominated kinematics rather than smooth rotation. We also simulate how these observations would appear at z ~ 2. Lower resolution and loss of low surface brightness features cause some apparent discrepancies between the low-z (observed) and high-z (simulated) interpretations and quantitatively gives different values for v/σ, yet simulations of these low-z analogs manage to detect the brightest regions well and resemble actual high-z observations of LBGs.

Quenching or Bursting: Star Formation Acceleration—A New Methodology for Tracing Galaxy Evolution

We introduce a new methodology for the direct extraction of galaxy physical parameters from multi-wavelength photometry and spectroscopy. We use semi-analytic models that describe galaxy evolution in the context of large scale cosmological simulation to provide a catalog of galaxies, star formation histories, and physical parameters. We then apply stellar population synthesis models and a simple extinction model to calculate the observable broad-band fluxes and spectral indices for these galaxies. We use a linear regression analysis to relate physical parameters to observed colors and spectral indices. The result is a set of coefficients that can be used to translate observed colors and indices into stellar mass, star formation rate, and many other parameters, including the instantaneous time derivative of the star formation rate which we denote the {\it Star Formation Acceleration (SFA)}, We apply the method to a test sample of galaxies with GALEX photometry and SDSS spectroscopy, deriving relationships between stellar mass, specific star formation rate, and star formation acceleration. We find evidence for a mass-dependent SFA in the green valley, with low mass galaxies showing greater quenching and higher mass galaxies greater bursting. We also find evidence for an increase in average quenching in galaxies hosting AGN. A simple scenario in which lower mass galaxies accrete and become satellite galaxies, having their star forming gas tidally and/or ram-pressure stripped, while higher mass galaxies receive this gas and react with new star formation can qualitatively explain our results.

Lyman Break Analogs: Constraints on the Formation of Extreme Starbursts at Low and High Redshift

Lyman Break Analogs (LBAs), characterized by high far-UV luminosities and surface brightnesses as detected by GALEX, are intensely star-forming galaxies in the low-redshift universe (z approximately equal to 0.2), with star formation rates reaching up to 50 times that of the Milky Way. These objects present metallicities, morphologies and other physical properties similar to higher redshift Lyman Break Galaxies (LBGs), motivating the detailed study of LBAs as local laboratories of this high-redshift galaxy population. We present results from our recent integral-field spectroscopy survey of LBAs with Keck/OSIRIS, which shows that these galaxies have the same nebular gas kinematic properties as high-redshift LBGs. We argue that such kinematic studies alone are not an appropriate diagnostic to rule out merger events as the trigger for the observed starburst. Comparison between the kinematic analysis and morphological indices from HST imaging illustrates the difficulties of properly identifying (minor or major) merger events, with no clear correlation between the results using either of the two methods. Artificial redshifting of our data indicates that this problem becomes even worse at high redshift due to surface brightness dimming and resolution loss. Whether mergers could generate the observed kinematic properties is strongly dependent on gas fractions in these galaxies. We present preliminary results of a CARMA survey for LBAs and discuss the implications of the inferred molecular gas masses for formation models.

Quenching Star Formation in the Green Valley: The Mass Flux at Intermediate Redshifts

We have obtained several hundred very deep spectra with DEIMOS/Keck in order to estimate the galactic mass flux density at intermediate redshifts (0.6 < z < 0.9) from the ”blue cloud” to the red sequence across the so-called ”green valley”, the intermediate region in the color-magnitude plot between those two populations. We use spectral indices (specifically D_n (4000) and H_(δ,A)) to determine star formation histories. Together with an independent measurement of number density of galaxies in each bin of the color-magnitude plot, one can infer the rate at which galaxies from a given sample are transiting through that bin. Measuring this value for all magnitude values, studies at lower redshift determined that the mass flux density in the green valley is comparable to both the mass build-up rate of the red sequence and the mass loss rate from the blue cloud. We show preliminary results for our intermediate redshift sample.