Can galaxy growth be sustained through HI-rich minor mergers?

Local galaxies with specific star-formation rates (star-formation rate per unit mass; sSFR~0.2-10/Gyr) as high as distant galaxies (z~1-3), are very rich in HI. Those with low stellar masses, log M_star (M_sun)=8-9, for example, have M_HI/M_star~5-30. Using continuity arguments of Peng et al. (2014), whereby the specific merger rate is hypothesized to be proportional to the specific star-formation rate, and HI gas mass measurements for local galaxies with high sSFR, we estimate that moderate mass galaxies, log M_star (M_sun)=9-10.5, can acquire sufficient gas through minor mergers (stellar mass ratios ~4-100) to sustain their star formation rates at z~2. The relative fraction of the gas accreted through minor mergers declines with increasing stellar mass and for the most massive galaxies considered, log M_star (M_sun)=10.5-11, this accretion rate is insufficient to sustain their star formation. We checked our minor merger hypothesis at z=0 using the same methodology but now with relations for local normal galaxies and find that minor mergers cannot account for their specific growth rates, in agreement with observations of HI-rich satellites around nearby spirals. We discuss a number of attractive features, like a natural down-sizing effect, in using minor mergers with extended HI disks to support star formation at high redshift. The answer to the question posed by the title, "Can galaxy growth be sustained through \HI-rich minor mergers?", is maybe, but only for relatively low mass galaxies and at high redshift.Comment: 6 pages, 3 figures; in final acceptance by A&

On the Structure and Morphology of the `Diffuse Ionized Medium' in Star-Forming Galaxies

Deep H$\alpha$ images of a sample of nearby late-type spiral galaxies have been analyzed to characterize the morphology and energetic significance of the ``Diffuse Ionized Medium'' (DIM). We find that the DIM properties can be reasonably unified as a function of relative surface brightness, by using a new method to quantify the DIM importance in galaxies. This new approach is more consistent with the fundamentally morphological definition of the DIM as being `Diffuse', compared to the traditional way adopted in previous studies that could only isolate the DIM based on an absolute surface brightness criterion. Our results suggest that the variation of the DIM's significance among the galaxies is small enough so that the fractional contribution of the DIM to the global H$\alpha$ luminosity in the galaxies is fairly constant, as has been observed. We found a smooth structural transition from HII regions to the DIM, suggesting that the ionizing energy for the DIM mainly comes from HII regions.Comment: 30 pages, 12 figures, AASTeX styl

Dissecting the complex environment of a distant quasar with MUSE

High redshift quasars can be used to trace the early growth of massive galaxies and may be triggered by galaxy-galaxy interactions. We present MUSE science verification data on one such interacting system consisting of the well-studied z=3.2 PKS1614+051 quasar, its AGN companion galaxy and bridge of material radiating in Lyalpha between the quasar and its companion. We find a total of four companion galaxies (at least two galaxies are new discoveries), three of which reside within the likely virial radius of the quasar host, suggesting that the system will evolve into a massive elliptical galaxy by the present day. The MUSE data are of sufficient quality to split the extended Lyalpha emission line into narrow velocity channels. In these the gas can be seen extending towards each of the three neighbouring galaxies suggesting that the emission-line gas originates in a gravitational interaction between the galaxies and the quasar host. The photoionization source of this gas is less clear but is probably dominated by the two AGN. The quasar's Lyalpha emission spectrum is double-peaked, likely due to absorbing neutral material at the quasar's systemic redshift with a low column density as no damping wings are present. The spectral profiles of the AGN and bridge's Lyalpha emission are also consistent with absorption at the same redshift indicating this neutral material may extend over > 50 kpc. The fact that the neutral material is seen in the line of sight to the quasar and transverse to it, and the fact that we see the quasar and it also illuminates the emission-line bridge, suggests the quasar radiates isotropically and any obscuring torus is small. These results demonstrate the power of MUSE for investigating the dynamics of interacting systems at high redshift.Comment: 9 pages, 6 figures, published in MNRA

The age structure of stellar populations in the solar vicinity. Clues of a two-phase formation history of the Milky Way disk

We analyze high quality abundances data of solar neighborhood stars and show that there are two distinct regimes of [alpha/Fe] versus age which we identify as the epochs of the thick and thin disk formation. A tight correlation between metallicity and [alpha/Fe] versus age is clearly identifiable on thick disk stars, implying that this population formed from a well mixed ISM, over a time scale of 4-5 Gyr. Thick disk stars vertical velocity dispersion correlate with age, with the youngest objects having as small scale heights as those of thin disk stars. A natural consequence of these two results is that a vertical metallicity gradient is expected in this population. We suggest that the thick disk set the initial conditions for the formation of the inner thin disk. This provides also an explanation of the apparent coincidence between the step in metallicity at 7-10 kpc in the thin disk and the confinment of the thick disk at about R<10 kpc. We suggest that the outer thin disk developped outside the influence of the thick disk, but also that the high alpha-enrichment of the outer regions may originate from a primordial pollution by the gas expelled from the thick disk. Local metal-poor thin disk stars, whose properties are best explained by an origin in the outer disk, are shown to be as old as the youngest thick disk (9-10 Gyr), implying that the outer thin disk started to form while the thick disk formation was still on-going in the inner Galaxy. We point out that, given the tight age-abundance relations in the thick disk, an inside-out process would give rise to a radial gradient in abundances in this population which is not observed. Finally, we argue that the data discussed here leave little room for radial migration, either to have contaminated the solar vicinity, or, to have redistributed stars in significant proportion across the solar annulus.Comment: Accepted in A&A, Revised version with new figures and extended discussio

The cluster environments of radio loud quasars

We have carried out multi-colour imaging of the fields of a statistically complete sample of low-frequency selected radio loud quasars at 0.6<z<1.1, in order to determine the characteristics of their environments. The largest radio sources are located in the field, and smaller steep-spectrum sources are more likely to be found in richer environments, from compact groups through to clusters. This radio-based selection (including source size) of high redshift groups and clusters is a highly efficient method of detecting rich environments at these redshifts. Although our single filter clustering measures agree with those of other workers, we show that these statistics cannot be used reliably on fields individually, colour information is required for this.Comment: 5 pages, 3 figures, contribution to "Tracing Cosmic Evolution with Galaxy Clusters" (Sesto 2001), ASP Conference Serie

The detection of FIR emission from high redshift star-forming galaxies in the ECDF-S

ABRIDGED: We have used the LABOCA Survey of the ECDF-S (LESS) to investigate rest-frame FIR emission from typical SF systems (LBGs) at redshift 3, 4, and 5. We initially concentrate on LBGs at z~3 and select three subsamples on stellar mass, extinction corrected SF and rest-frame UV-magnitude. We produce composite 870micron images of the typical source in our subsamples, obtaining ~4sigma detections and suggesting a correlation between FIR luminosity and stellar mass. We apply a similar procedure to our full samples at z~3, 4, 4.5 and 5 and do not obtain detections - consistent with a simple scaling between FIR luminosity and stellar mass. In order to constrain the FIR SED of these systems we explore their emission at multiple wavelengths spanning the peak of dust emission at z~3 using the Herschel SPIRE observations of the field. We obtain detections at multiple wavelengths for both our stellar mass and UV-magnitude selected samples, and find a best-fit SED with T_dust in the ~33-41K range. We calculate L_FIR, obscured SFRs and M_dust, and find that a significant fraction of SF in these systems is obscured. Interestingly, our extinction corrected SFR sample does not display the large FIR fluxes predicted from its red UV-spectral slope. This suggests that the method of assuming an intrinsic UV-slope and correcting for dust attenuation may be invalid for this sample - and that these are not in fact the most actively SF systems. All of our z~3 samples fall on the `main sequence' of SF galaxies at z~3 and our detected subsamples are likely to represent the high obscuration end of LBGs at their epoch. We compare the FIR properties of our subsamples with various other populations, finding that our stellar mass selected sample shows similar FIR characteristics to SMGs at the same epoch and therefore potentially represents the low L_FIR end of the high redshift FIR luminosity function.Comment: 18 pages, 10 figure, MNRAS accepted, corrected typos, acknowledgements adde

When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy

Quenching, the cessation of star formation, is one of the most significant events in the life cycle of galaxies. We show here the first evidence that the Milky Way experienced a generalised quenching of its star formation at the end of its thick disk formation $\sim$9 Gyr ago. Elemental abundances of stars studied as part of the APOGEE survey reveal indeed that in less than $\sim$2 Gyr the star formation rate in our Galaxy dropped by an order-of-magnitude. Because of the tight correlation between age and alpha abundance, this event reflects in the dearth of stars along the inner disk sequence in the [Fe/H]-[$\alpha$/Fe] plane. Before this phase, which lasted about 1.5 Gyr, the Milky Way was actively forming stars. Afterwards, the star formation resumed at a much lower level to form the thin disk. These events are very well matched by the latest observation of MW-type progenitors at high redshifts. In late type galaxies, quenching is believed to be related to a long and secular exhaustion of gas. In our Galaxy, it occurred on a much shorter time scale, while the chemical continuity before and after the quenching indicates that it was not due to the exhaustion of the gas. While quenching is generally associated with spheroids, our results show that it also occurs in galaxies like the Milky Way, possibly when they are undergoing a morphological transition from thick to thin disks. Given the demographics of late type galaxies in the local universe, in which classical bulges are rare, we suggest further that this may hold true generally in galaxies with mass lower than or approximately $M^*$, where quenching could be directly a consequence of thick disk formation. We emphasize that the quenching phase in the Milky Way could be contemporaneous with, and related to, the formation of the bar. We sketch a scenario on how a strong bar may inhibit star formation.Comment: 17 pages, 8 figures. Published versio

The Physical Properties of LBGs at z>5: Outflows and the "pre-enrichment problem"

We discuss the properties of Lyman Break galaxies (LBGs) at z>5 as determined from disparate fields covering approximately 500 sq. arcmin. While the broad characteristics of the LBG population has been discussed extensively in the literature, such as luminosity functions and clustering amplitude, we focus on the detailed physical properties of the sources in this large survey (>100 with spectroscopic redshifts). Specifically, we discuss ensemble mass estimates, stellar mass surface densities, core phase space densities, star-formation intensities, characteristics of their stellar populations, etc as obtained from multi-wavelength data (rest-frame UV through optical) for a subsample of these galaxies. In particular, we focus on evidence that these galaxies drive vigorous outflows and speculate that this population may solve the so-called ``pre-enrichment problem''. The general picture that emerges from these studies is that these galaxies, observed about 1 Gyr after the Big Bang, have properties consistent with being the progenitors of the densest stellar systems in the local Universe -- the centers of old bulges and early type galaxies.Comment: 4 pages, to appear in "Pathways Through an Eclectic Universe", J. H. Knappen, T. J. Mahoney, and A. Vazedekis (Eds.), ASP Conf. Ser., 200