We explore the dependence of galaxy stellar population properties that are derived from broad-band spectral energy distribution fitting - such as age, stellar mass, dust reddening, etc. - on a ...variety of parameters, such as star formation histories, age grid, metallicity, initial mass function (IMF), dust reddening and reddening law, filter setup and wavelength coverage. Mock galaxies are used as test particles. We confirm our earlier results based on real z= 2 galaxies, that usually adopted τ-models lead to overestimate the star formation rate and to underestimate the stellar mass. Here, we show that - for star-forming galaxies - galaxy ages, masses and reddening can be well determined simultaneously only when the correct star formation history is identified. This is the case for inverted-τ models at high-z, for which we find that the mass recovery (at fixed IMF) is as good as ∼0.04 dex. However, since the right star formation history is usually unknown, we quantify the offsets generated by adopting standard fitting setups. Stellar masses are generally underestimated, which results from underestimating the age. For mixed fitting setups with a variety of star formation histories the median mass recovery at z∼ 2-3 is as decent as ∼0.1 dex (at fixed IMF), albeit with large scatter. The situation worsens towards lower redshifts, because of the variety of possible star formation histories and ages. At z∼ 0.5 the stellar mass can be underestimated by as much as ∼0.6 dex (at fixed IMF). A practical trick to improve upon this figure is to exclude reddening from the fitting parameters, as this helps to avoid unrealistically young and dusty solutions. Stellar masses are underestimated by a smaller amount (∼0.3 dex at z∼ 0.5). Reddening and the star formation rate should then be determined via a separate fitting. As expected, the recovery of properties is better for passive galaxies, for which e.g. the mass can be fully recovered (within ∼0.01 dex at fixed IMF) when using a fitting setup including metallicity effects. In both cases of star-forming as well as passive galaxies, the recovery of physical parameters is dependent on the spectral range involved in the fitting. We find that a coverage from the rest-frame ultraviolet to the rest-frame near-infrared appears to be optimal. We also quantify the effect of narrowing the wavelength coverage or adding and removing filter bands, which can be useful for planning observational surveys. Finally, we provide scaling relations that allow the transformation of stellar masses obtained using different template fitting setups and stellar population models.
Abstract
We present a new spectral fitting code, firefly, for deriving the stellar population properties of stellar systems. firefly is a chi-squared minimization fitting code that fits combinations ...of single-burst stellar population models to spectroscopic data, following an iterative best-fitting process controlled by the Bayesian information criterion. No priors are applied, rather all solutions within a statistical cut are retained with their weight. Moreover, no additive or multiplicative polynomials are employed to adjust the spectral shape. This fitting freedom is envisaged in order to map out the effect of intrinsic spectral energy distribution degeneracies, such as age, metallicity, dust reddening on galaxy properties, and to quantify the effect of varying input model components on such properties. Dust attenuation is included using a new procedure, which was tested on Integral Field Spectroscopic data in a previous paper. The fitting method is extensively tested with a comprehensive suite of mock galaxies, real galaxies from the Sloan Digital Sky Survey and Milky Way globular clusters. We also assess the robustness of the derived properties as a function of signal-to-noise ratio (S/N) and adopted wavelength range. We show that firefly is able to recover age, metallicity, stellar mass, and even the star formation history remarkably well down to an S/N ∼ 5, for moderately dusty systems. Code and results are publicly available.1
We discuss chemical enrichments of ∼4000 Sloan Digital Sky Survey early-type galaxies using as tracers a large variety of element abundance ratios, namely C/Fe, N/Fe, O/Fe, Mg/Fe, Ca/Fe and Ti/Fe. We ...utilize the stellar population models of absorption line indices from Thomas, Maraston & Johansson which are based on the MILES stellar library. We confirm previous results of increasing age, Z/H and O/Fe ratios (most often represented by α/Fe in the literature) with velocity dispersion. We further derive identical correlations with velocity dispersion for the abundance ratios O/Fe, Mg/Fe and C/Fe, implying that C/Mg and C/O are close to solar values. This sets a lower limit on the formation time-scales and starburst components of early-type galaxies to ∼0.4 Gyr, which is the lifetime of a 3 M⊙ star, since the full C enrichment must be reached. N/Fe correlates with velocity dispersion, but offset to lower values and with a steeper slope compared to the other element ratios. We do not find any environmental dependencies for the abundances of C and N, contrary to previous reports in the literature. Fe/H does not correlate with velocity dispersion over the entire parameter range covered, but for fixed age we find a steep trend for the Fe/H-σ relation. This trend is weaker than the analogous for total metallicity (which also shows steeper trends at fixed age) owing to the lower Fe contribution from Type Ia supernova (SN Ia) for more massive early-type galaxies. We find Ca/Fe ratios that are close to solar values over the entire velocity dispersion range covered. Tentative, due to large scatter, the results for Ti/Fe indicate that Ti follows the trends of Ca. This implies a significant contribution from SN Ia to the enrichment of heavy α-elements and puts strong constraints on supernova nucleosynthesis and models of galactic chemical evolution.
Fitting synthetic spectral energy distributions (SEDs) to the multiband photometry of galaxies to derive their star formation rates (SFRs), stellar masses, ages, etc. requires making a priori ...assumptions about their star formation histories (SFHs). A widely adopted parametrization of the SFH, the so-called τ models where SFR ∝ e−t/τ is shown to lead to unrealistically low ages when applied to a sample of actively star-forming galaxies at z∼ 2, a problem shared by other SFHs when the age is left as a free parameter in the fitting procedure. This happens because the SED of such galaxies, at all wavelengths, is dominated by their youngest stellar populations, which outshine the older ones. Thus, the SED of such galaxies conveys little information on the beginning of star formation (SF), i.e. on the age of their oldest stellar populations. To cope with this problem, besides τ models (hereafter called direct-τ models), we explore a variety of SFHs, such as constant SFR and inverted-τ models (with SFR ∝ e+t/τ), along with various priors on age, including assuming that SF started at high redshift in all the galaxies in the test sample. We find that inverted-τ models with such latter assumption give SFRs and extinctions in excellent agreement with the values derived using only the UV part of the SED, which is the one most sensitive to ongoing SF and reddening. These models are also shown to accurately recover the SFRs and masses of mock galaxies at z∼ 2 constructed from semi-analytic models, which we use as a further test. All other explored SFH templates do not fulfil these two tests as well as inverted-τ models do. In particular, direct-τ models with unconstrained age in the fitting procedure overestimate SFRs and underestimate stellar mass, and would exacerbate an apparent mismatch between the cosmic evolution of the volume densities of SFR and stellar mass. We conclude that for high-redshift star-forming galaxies an exponentially increasing SFR with a high formation redshift is preferable to other forms of the SFH so far adopted in the literature.
The Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey is currently acquiring integral-field spectroscopy for the largest sample of galaxies to date. By 2020, the MaNGA Survey-which ...is one of three core programs in the fourth-generation Sloan Digital Sky Survey (SDSS-IV)-will have observed a statistically representative sample of 104 galaxies in the local universe (z 0.15). In addition to a robust data-reduction pipeline (DRP), MaNGA has developed a data-analysis pipeline (DAP) that provides higher-level data products. To accompany the first public release of its code base and data products, we provide an overview of the MaNGA DAP, including its software design, workflow, measurement procedures and algorithms, performance, and output data model. In conjunction with our companion paper (Belfiore et al.), we also assess the DAP output provided for 4718 observations of 4648 unique galaxies in the recent SDSS Data Release 15 (DR15). These analysis products focus on measurements that are close to the data and require minimal model-based assumptions. Namely, we provide stellar kinematics (velocity and velocity dispersion), emission-line properties (kinematics, fluxes, and equivalent widths), and spectral indices (e.g., D4000 and the Lick indices). We find that the DAP provides robust measurements and errors for the vast majority (>99%) of analyzed spectra. We summarize assessments of the precision and accuracy of our measurements as a function of signal-to-noise. We also provide specific guidance to users regarding the limitations of the data. The MaNGA DAP software is publicly available and we encourage community involvement in its development.
The aim of this paper is to set constraints on the epochs of early-type galaxy formation through the "archaeology" of the stellar populations in local galaxies. Using our models of absorption-line ...indices that account for variable abundance ratios, we derive ages, total metallicities, and element ratios of 124 early-type galaxies in high- and low-density environments. The data are analyzed by comparison with mock galaxy samples created through Monte Carlo simulations taking the typical average observational errors into account, in order to eliminate artifacts caused by correlated errors. We find that all three parameters, age, metallicity, and a/Fe ratio, are correlated with velocity dispersion. We show that these results are robust against recent revisions of the local abundance pattern at high metallicities. To recover the observed scatter we need to assume an intrinsic scatter of about 20% in age, 0.08 dex in Z/H, and 0.05 dex in a/Fe. All low-mass objects with M* 10 super(10) M sub( )(s 130 km s super(-1)) show evidence for the presence of intermediate-age stellar populations with low a/Fe ratios. About 20% of the intermediate-mass objects with 10 super(10) M*/M sub( )10 super(11) 110 s/(km s super(-1)) 230; both elliptical and lenticular galaxies must have either a young subpopulation or a blue horizontal branch. On the basis of the above relationships, valid for the bulk of the sample, we show that the Mg-s relation is mainly driven by metallicity, with similar contributions from the a/Fe ratio (23%) and age (17%). We further find evidence for an influence of the environment on the stellar population properties. Massive early-type galaxies in low-density environments seem on average 62 Gyr younger and slightly (60.05-0.1 dex) more metal-rich than their counterparts in high-density environments. No offsets in the a/Fe ratios are instead detected. With the aid of a simple chemical evolution model, we translate the derived ages and a/Fe ratios into star formation histories. We show that most star formation activity in early-type galaxies is expected to have happened between redshifts 63 and 5 in high-density environments and between redshifts 1 and 2 in low-density environments. We conclude that at least 50% of the total stellar mass density must have already formed at z 6 1, in good agreement with observational estimates of the total stellar mass density as a function of redshift. Our results suggest that significant mass growth in the early-type galaxy population below z 6 1 must be restricted to less massive objects, and a significant increase of the stellar mass density between redshifts 1 and 2 should be present, caused mainly by the field galaxy population. The results of this paper further imply the presence of vigorous star formation episodes in massive objects at z 6 2-5 and evolved elliptical galaxies around z 6 1, both observationally identified as SCUBA galaxies and extremely red objects, respectively.
We measure and analyse the clustering of the Baryon Oscillation Spectroscopic Survey (BOSS) relative to the line of sight (LOS), for LOWZ and CMASS galaxy samples drawn from the final Data Release ...12. The LOWZ sample contains 361 762 galaxies with an effective redshift of z
lowz = 0.32, and the CMASS sample 777 202 galaxies with an effective redshift of z
cmass = 0.57. From the power spectrum monopole and quadrupole moments around the LOS, we measure the growth of structure parameter f times the amplitude of dark matter density fluctuations σ8 by modelling the redshift-space distortion signal. When the geometrical Alcock–Paczynski effect is also constrained from the same data, we find joint constraints on fσ8, the product of the Hubble constant and the comoving sound horizon at the baryon-drag epoch H(z)r
s(z
d), and the angular distance parameter divided by the sound horizon D
A(z)/r
s(z
d). We find f(z
lowz)σ8(z
lowz) = 0.394 ± 0.062, D
A(z
lowz)/r
s(z
d) = 6.35 ± 0.19, H(z
lowz)r
s(z
d) = (11.41 ± 0.56) 103 km s− 1 for the LOWZ sample, and f(z
cmass)σ8(z
cmass) = 0.444 ± 0.038, D
A(z
cmass)/r
s(z
d) = 9.42 ± 0.15, H(z
cmass)r
s(z
d) = (13.92 ± 0.44) 103 km s− 1 for the CMASS sample. We find general agreement with previous BOSS DR11 measurements. Assuming the Hubble parameter and angular distance parameter are fixed at fiducial Λcold dark matter values, we find f(z
lowz)σ8(z
lowz) = 0.485 ± 0.044 and f(z
cmass)σ8(z
cmass) = 0.436 ± 0.022 for the LOWZ and CMASS samples, respectively.
A new tool for the evolutionary synthesis of stellar populations is presented, which is based on three independent matrices, giving respectively (1) the fuel consumption during each evolutionary ...phase as a function of stellar mass, (2) the typical temperatures and gravities during such phases, and (3) the colours and bolometric corrections as functions of gravity and temperature. The modular structure of the code allows one easily to assess the impact on the synthetic spectral energy distribution of the various assumptions and model ingredients, such as, for example, uncertainties in stellar evolutionary models, the mixing length, the temperature distribution of horizontal branch stars, asymptotic giant branch mass loss, and colour–temperature transformations. The so‐called ‘AGB phase transition’ in Magellanic Cloud clusters is used to calibrate the contribution of the thermally pulsing asymptotic giant branch phase to the synthetic integrated luminosity. As an illustrative example, solar‐metallicity (Y = 0.27, Z = 0.02) models, with ages ranging between 30 Myr and 15 Gyr and various choices for the slope of the initial mass function, are presented. Synthetic broad‐band colours and the luminosity contributions of the various evolutionary stages are compared with Large Magellanic Cloud and Galactic globular cluster data. In all these cases, a good agreement is found. Finally, the evolution is presented of stellar mass‐to‐light ratios in the bolometric and UBVRK passbands, in which the contribution of stellar remnants is accounted for.
Abstract
We study the prospects for the detection of continuous gravitational signals from normal Galactic neutron stars, i.e., nonrecycled stars. We use a synthetic population generated by evolving ...stellar remnants in time, according to several models. We consider the most recent constraints set by all-sky searches for continuous gravitational waves and use them for our detectability criteria. We discuss the detection prospects for the current and the next generation of gravitational-wave detectors. We find that neutron stars whose ellipticity is solely caused by magnetic deformations cannot produce any detectable signal, not even by third-generation detectors. The currently detectable sources all have
B
≲ 10
12
G and deformations that are not solely due to the magnetic field. For these, we find in fact that the larger the magnetic field, the higher the ellipticity required for the signal to be detectable, and this ellipticity is well above the value induced by the magnetic field. Third-generation detectors such as the Einstein Telescope and Cosmic Explorer will be able to detect up to ≈250 more sources than current detectors. We briefly treat the case of recycled neutron stars with a simplified model. We find that continuous gravitational waves from these objects will likely remain elusive to detection by current detectors, but should be detectable with the next generation of detectors.
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