Abstract
We explore constraints on the joint photometric and morphological evolution of typical low redshift galaxies as they move from the blue cloud through the green valley and on to the red ...sequence. We select Galaxy And Mass Assembly (GAMA) survey galaxies with 10.25 < log(M*/M⊙) < 10.75 and z < 0.2 classified according to their intrinsic u* − r* colour. From single component Sérsic fits, we find that the stellar mass-sensitive K-band profiles of red and green galaxy populations are very similar while g-band profiles indicate more disc-like morphologies for the green galaxies: apparent (optical) morphological differences arise primarily from radial mass-to-light ratio variations. Two-component fits show that most green galaxies have significant bulge and disc components and that the blue to red evolution is driven by colour change in the disc. Together, these strongly suggest that galaxies evolve from blue to red through secular disc fading and that a strong bulge is present prior to any decline in star formation. The relative abundance of the green population implies a typical time-scale for traversing the green valley ∼1–2 Gyr and is independent of environment, unlike that of the red and blue populations. While environment likely plays a rôle in triggering the passage across the green valley, it appears to have little effect on time taken. These results are consistent with a green valley population dominated by (early type) disc galaxies that are insufficiently supplied with gas to maintain previous levels of disc star formation, eventually attaining passive colours. No single event is needed to quench their star formation.
ABSTRACT We present 947 radial velocities of RR Lyrae variable stars in four fields located toward the Galactic bulge, observed within the data from the ongoing Bulge RR Lyrae Radial Velocity Assay ...(BRAVA-RR). We show that these RR Lyrae stars (RRLs) exhibit hot kinematics and null or negligible rotation and are therefore members of a separate population from the bar/pseudobulge that currently dominates the mass and luminosity of the inner Galaxy. Our RRLs predate these structures and have metallicities, kinematics, and spatial distribution that are consistent with a "classical" bulge, although we cannot yet completely rule out the possibility that they are the metal-poor tail of a more metal-rich ( Fe / H ∼ − 1 dex) halo-bulge population. The complete catalog of radial velocities for the BRAVA-RR stars is also published electronically.
We use a highly complete subset of the Galaxy And Mass Assembly II (GAMA-II) redshift sample to fully describe the stellar mass dependence of close pairs and mergers between 108 and 1012 M⊙. Using ...the analytic form of this fit we investigate the total stellar mass accreting on to more massive galaxies across all mass ratios. Depending on how conservatively we select our robust merging systems, the fraction of mass merging on to more massive companions is 2.0–5.6 per cent. Using the GAMA-II data we see no significant evidence for a change in the close pair fraction between redshift z = 0.05 and 0.2. However, we find a systematically higher fraction of galaxies in similar mass close pairs compared to published results over a similar redshift baseline. Using a compendium of data and the function γ
M
= A(1 + z)
m
to predict the major close pair fraction, we find fitting parameters of A = 0.021 ± 0.001 and m = 1.53 ± 0.08, which represents a higher low-redshift normalization and shallower power-law slope than recent literature values. We find that the relative importance of in situ star formation versus galaxy merging is inversely correlated, with star formation dominating the addition of stellar material below
$\mathcal {M}^*$
and merger accretion events dominating beyond
$\mathcal {M}^*$
. We find mergers have a measurable impact on the whole extent of the galaxy stellar mass function (GSMF), manifest as a deepening of the ‘dip’ in the GSMF over the next ∼Gyr and an increase in
$\mathcal {M}^*$
by as much as 0.01–0.05 dex.
ABSTRACT
We derive the evolution of the ultraviolet upturn colour from a sample of field luminous red galaxies at 0.3 < z < 0.7 with −24 < Mr < −21.5. No individual objects are securely detected, so ...we stack several hundred galaxies within absolute magnitude and redshift intervals. We find that the colour of the ultraviolet upturn (in observed NUV − i which is approximately equivalent to the classical FUV − V at the redshifts of our targets) does not change strongly with redshift to z = 0.7. This behaviour is similar to that observed in cluster ellipticals over this same mass range and at similar redshifts, and we speculate that the processes involved in the origin of the UV upturn are the same. The observations are most consistent with spectral synthesis models containing a fraction of a helium rich stellar population with abundances between 37 per cent and 42 per cent, although we cannot formally exclude a contribution due to residual star formation at the $\sim 0.5\, \mathrm{ per\,cent}$ level (however, this appears unlikely for cluster galaxies that are believed to be more quenched). This suggests that the ultraviolet upturn is a primordial characteristic of early-type galaxies at all redshifts and that an unexpected nucleosynthesis channel may lead to nearly complete chemical evolution at early times.
The modification of star formation (SF) in galaxy interactions is a complex process, with SF observed to be both enhanced in major mergers and suppressed in minor pair interactions. Such changes ...likely to arise on short time-scales and be directly related to the galaxy–galaxy interaction time. Here we investigate the link between dynamical phase and direct measures of SF on different time-scales for pair galaxies, targeting numerous star- formation rate (SFR) indicators and comparing to pair separation, individual galaxy mass and pair mass ratio. We split our sample into the higher (primary) and lower (secondary) mass galaxies in each pair and find that SF is indeed enhanced in all primary galaxies but suppressed in secondaries of minor mergers. We find that changes in SF of primaries are consistent in both major and minor mergers, suggesting that SF in the more massive galaxy is agnostic to pair mass ratio. We also find that SF is enhanced/suppressed more strongly for short-duration SFR indicators (e.g. Hα), highlighting recent changes to SF in these galaxies, which are likely to be induced by the interaction. We propose a scenario where the lower mass galaxy has its SF suppressed by gas heating or stripping, while the higher mass galaxy has its SF enhanced, potentially by tidal gas turbulence and shocks. This is consistent with the seemingly contradictory observations for both SF suppression and enhancement in close pairs.
We derive deep I-band luminosity functions and colour-magnitude diagrams from Hubble Space Telescope imaging for eleven 0.2 < z < 0.6 clusters observed at various stages of merging, and a comparison ...sample of five more relaxed clusters at similar redshifts. The characteristic magnitude M* evolves passively out to z = 0.6, while the faint-end slope of the luminosity function is α ∼ −1 at all redshifts. Cluster galaxies must have been completely assembled down to M
I
∼ −18 out to z = 0.6. We observe tight colour-magnitude relations over a luminosity range of up to 8 mag., consistent with the passive evolution of ancient stellar populations. This is found in all clusters, irrespective of their dynamical status (involved in a collision or not, or even within subclusters for the same object), and suggests that environment does not have a strong influence on galaxy properties. A red sequence luminosity function can be followed to the limits of our photometry: we see no evidence of a weakening of the red sequence to z = 0.6. The blue galaxy fraction rises with redshift, especially at fainter absolute magnitudes. We observe bright blue galaxies in clusters at z > 0.4 that are not encountered locally. Surface brightness selection effects preferentially influence the detectability of faint red galaxies, accounting for claims of evolution at the faint end.
Abstract
Using GALEX, Ultraviolet Optical Telescope (UVOT), and optical photometry, we explore the prevalence and strength of the Ultraviolet (UV) upturn in the spectra of quiescent early-type ...galaxies in several nearby clusters. Even for galaxies with completely passive optical colours, there is a large spread in vacuum UV colour consistent with almost all having some UV upturn component. Combining GALEX and UVOT data below 3000 Å, we generate for the first time comparatively detailed UV spectral energy distributions for Coma cluster galaxies. Fitting the UV upturn component with a blackbody, 26 of these show a range of characteristic temperatures (10 000–21 000K) for the UV upturn population. Assuming a single temperature to explain GALEX-optical colours could underestimate the fraction of galaxies with UV upturns and mis-classify some as systems with residual star formation. The UV upturn phenomenon is not an exclusive feature found only in giant galaxies; we identify galaxies with similar (or even bluer) FUV − V colours to the giants with upturns over a range of fainter luminosities. The temperature and strength of the UV upturn are correlated with galaxy mass. Under the plausible hypothesis that the sources of the UV upturn are blue horizontal branch stars, the most likely mechanism for this is the presence of a substantial (between 4 per cent and 20 per cent) Helium-rich (Y > 0.3) population of stars in these galaxies, potentially formed at z ∼ 4 and certainly at z > 2; this plausibly sets a lower limit of ${\sim } {\rm 0.3{\rm -} 0.8} \times 10^{10}$ M⊙ to the in situ stellar mass of ∼L* galaxies at this redshift.
We present spectrum synthesis fits to 808 K+A galaxies selected from the Sloan Digital Sky Survey (SDSS) and population synthesis of their spectral energy distributions, extending from the far-UV ...(0.15 μm) to the mid-IR (22 μm), based on the results of STARLIGHT code fitting to the SDSS spectra. Our modelling shows that K+A galaxies have undergone a large starburst, involving a median 50 per cent of their present stellar masses, superposed over an older stellar population. The metal abundance of the intermediate-age stars shows that star formation did not take place in pristine gas, but was part of a dramatic increase in the star formation rates for originally gas-rich objects. We find no evidence for ongoing QSO activity in the UV, which is well modelled by the emission of intermediate-age stars. We use K+A galaxies as local counterparts of high-redshift objects to test for the presence of thermally pulsing asymptotic giant branch (TP-AGB) stars in similarly aged populations and find no excess in the infrared due to emission from such stars, arguing that more distant galaxies are indeed old and massive at their redshift. All of our galaxies show significant excesses in the mid-IR compared to the light from their stars. We fit this ad hoc with a 300 K blackbody. Possible sources include TP-AGB stars, obscured young star clusters and hidden AGNs, heating a significant dust component.
ABSTRACT
We derived stellar ages and metallicities Z/H for ∼70 passive early-type galaxies (ETGs) selected from VANDELS survey over the redshift range 1.0 < z < 1.4 and stellar mass range 10 < ...log(M*/M⊙) < 11.6. We find significant systematics in their estimates depending on models and wavelength ranges considered. Using the full-spectrum fitting technique, we find that both Z/H and age increase with mass as for local ETGs. Age and metallicity sensitive spectral indices independently confirm these trends. According to EMILES models, for 67 per cent of the galaxies we find Z/H > 0.0, a percentage which rises to ∼90 per cent for log(M*/M⊙) > 11 where the mean metallicity is Z/H = 0.17 ± 0.1. A comparison with homogeneous measurements at similar and lower redshift does not show any metallicity evolution over the redshift range 0.0 < z < 1.4. The derived star formation (SF) histories show that the stellar mass fraction formed at early epoch increases with the mass of the galaxy. Galaxies with log(M*/M⊙) > 11.0 host stellar populations with Z/H > 0.05, formed over short time-scales (Δt50 < 1 Gyr) at early epochs (tform < 2 Gyr), implying high star formation rates (SFR > 100 M⊙ yr−1) in high-mass density regions (log(Σ1kpc) > 10 M⊙/kpc2). This sharp picture tends to blur at lower masses: log(M*/M⊙) ∼ 10.6 galaxies can host either old stars with Z/H < 0.0 or younger stars with Z/H > 0.0, depending on the duration (Δt50) of the SF. The relations between galaxy mass, age, and metallicities are therefore largely set up ab initio as part of the galaxy formation process. Mass, SFR, and SF time-scale all contribute to shape up the stellar mass–metallicity relation with the mass that modulates metals retention.
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