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Register a new userInsights into formation scenarios of massive Early-Type galaxies from spatially resolved stellar population analysis in CALIFA
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Stefano Zibetti
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We perform spatially resolved stellar population analysis for a sample of 69 early-type galaxies (ETGs) from the CALIFA integral field spectroscopic survey, including 48 ellipticals and 21 S0s. We generate and quantitatively characterize profiles of light-weighted mean stellar age and metallicity within $lesssim 2R_e$, as a function of radius and stellar-mass surface density $mu_*$. We study in detail the dependence of profiles on galaxies global properties, including velocity dispersion $sigma_e$, stellar mass, morphology. ETGs are universally characterized by strong, negative metallicity gradients ($sim -0.3,text{dex}$ per $R_e$) within $1,R_e$, which flatten out moving towards larger radii. A quasi-universal local $mu_*$-metallicity relation emerges, which displays a residual systematic dependence on $sigma_e$, whereby higher $sigma_e$ implies higher metallicity at fixed $mu_*$. Age profiles are typically U-shaped, with minimum around $0.4,R_e$, asymptotic increase to maximum ages beyond $sim 1.5,R_e$, and an increase towards the centre. The depth of the minimum and the central increase anti-correlate with $sigma_e$. A possible qualitative interpretation of these observations is a two-phase scenario. In the first phase, dissipative collapse occurs in the inner $1,R_e$, establishing a negative metallicity gradient. The competition between the outside-in quenching due to feedback-driven winds and some form of inside-out quenching, possibly caused by central AGN feedback or dynamical heating, determines the U-shaped age profiles. In the second phase, the accretion of ex-situ stars from quenched and low-metallicity satellites shapes the flatter stellar population profiles in the outer regions.
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To investigate star formation and assembly processes of massive galaxies, we present here a spatially-resolved stellar populations analysis of a sample of 45 elliptical galaxies (Es) selected from the CALIFA survey. We find rather flat age and [Mg/Fe] radial gradients, weakly dependent on the effective velocity dispersion of the galaxy within half-light radius. However, our analysis shows that metallicity gradients become steeper with increasing galaxy velocity dispersion. In addition, we have homogeneously compared the stellar populations gradients of our sample of Es to a sample of nearby relic galaxies, i.e., local remnants of the high-z population of red nuggets. This comparison indicates that, first, the cores of present-day massive galaxies were likely formed in gas-rich, rapid star formation events at high redshift (z>2). This led to radial metallicity variations steeper than observed in the local Universe, and positive [Mg/Fe] gradients. Second, our analysis also suggests that a later sequence of minor dry mergers, populating the outskirts of early-type galaxies (ETGs), flattened the pristine [Mg/Fe] and metallicity gradients. Finally, we find a tight age-[Mg/Fe] relation, supporting that the duration of the star formation is the main driver of the [Mg/Fe] enhancement in massive ETGs. However, the star formation time-scale alone is not able to fully explain our [Mg/Fe] measurements. Interestingly, our results match the expected effect that a variable stellar initial mass function would have on the [Mg/Fe] ratio.
The Calar Alto Legacy Integral Field Area (CALIFA) is an ongoing 3D spectroscopic survey of 600 nearby galaxies of all kinds. This pioneer survey is providing valuable clues on how galaxies form and evolve. Processed through spectral synthesis techniques, CALIFA datacubes allow us to, for the first time, spatially resolve the star formation history of galaxies spread across the color-magnitude diagram. The richness of this approach is already evident from the results obtained for the first 107 galaxies. Here we show how the different galactic spatial sub-components (bulge and disk) grow their stellar mass over time. We explore the results stacking galaxies in mass bins, finding that, except at the lowest masses, galaxies grow inside-out, and that the growth rate depends on a galaxys mass. The growth rate of inner and outer regions differ maximally at intermediate masses. We also find a good correlation between the age radial gradient and the stellar mass density, suggesting that the local density is a main driver of galaxy evolution.
Physical and chemical properties of the interstellar medium (ISM) at sub-galactic ($sim$kpc) scales play an indispensable role in controlling the ability of gas to form stars. As part of the SMAUG (Simulating Multiscale Astrophysics to Understand Galaxies) project, in this paper, we use the TNG50 cosmological simulation to explore the physical parameter space of 8 resolved ISM properties in star-forming regions to constrain the areas of this hyperspace over which most star-forming environments exist. We deconstruct our simulated galaxies spanning a wide range of mass (M$_star = 10^{7-11}$ M$_odot$) and redshift ($0 leq z leq 3$) into kpc-sized regions, and statistically analyze the gas/stellar surface densities, gas metallicity, vertical stellar velocity dispersion, epicyclic frequency and dark-matter volumetric density representative of each region in the context of their star formation activity and galactic environment (radial galactocentric location). By examining the star formation rate (SFR) weighted distributions of these properties, we show that stars primarily form in two spatially distinct environmental regimes, which are brought about by an underlying bi-component radial SFR surface density profile in galaxies. We examine how the relative prominence of these two regimes depends on host galaxy mass and cosmic time. We also compare our findings with those from integral field spectroscopy observations and achieve a good overall agreement. Further, using dimensionality reduction, we characterise the aforementioned hyperspace to reveal a high-degree of multicollinearity in relationships amongst ISM properties that drive the distribution of star formation at kpc-scales. Based on this, we show that a reduced 3D representation underpinned by a multi-variate radius relationship is sufficient to capture most of the variance in the original 8D space.
The star formation main sequence (SFMS) is a tight relation between the galaxy star formation rate (SFR) and its total stellar mass ($M_star$). Early-type galaxies (ETGs) are often considered as low-SFR outliers of this relation. We study, for the first time, the separated distribution in the SFR vs. $M_star$ of bulges and discs of 49 ETGs from the CALIFA survey. This is achieved using C2D, a new code to perform spectro-photometric decompositions of integral field spectroscopy datacubes. Our results reflect that: i) star formation always occurs in the disc component and not in bulges; ii) star-forming discs in our ETGs are compatible with the SFMS defined by star forming galaxies at $z sim 0$; iii) the star formation is not confined to the outskirts of discs, but it is present at all radii (even where the bulge dominates the light); iv) for a given mass, bulges exhibit lower sSFR than discs at all radii; and v) we do not find a deficit of molecular gas in bulges with respect to discs for a given mass in our ETGs. We speculate our results favour a morphological quenching scenario for ETGs.
We present radial stellar population parameters for a subsample of 12 galaxies from the 36 isolated early-type galaxies of Reda et al. Using new long-slit spectra, central values and radial gradients for the stellar age, metallicity [Z/H] and alpha-element abundance [E/Fe] are measured. Similarly, the central stellar population parameters are derived for a further 5 isolated early-type galaxies using their Lick indices from the literature. On average, the seventeen isolated galaxies have mean central [Z/H]o and [E/Fe]o of 0.29+/-0.03 and 0.17+/-0.03 respectively and span a wide range of ages from 1.7 to 15 Gyrs. We find that isolated galaxies follow similar scaling relations between central stellar population parameters and galaxy velocity dispersion to their counterparts in high density environments. However, we note a tendency for isolated galaxies to have slightly younger ages, higher [Z/H] and lower [E/Fe]. Such properties are qualitatively consistent with the expectation of an extended star formation history for galaxies in lower density environments. Generally we measure constant age and [E/Fe] radial gradients. We find that the age gradients anti-correlate with the central galaxy age. Metallicity gradients range from near zero to strongly negative. For our high mass galaxies metallicity gradients are shallower with increasing mass. Such behaviour is not predicted in dissipational collapse models but might be expected in multiple mergers. The metallicity gradients correlate with the central age and metallicity, as well as to the age gradients. In conclusion, our stellar population data for isolated galaxies are more compatible with an extended merger/accretion history than early dissipative collapse.
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