Broadband photometry of galaxies measures an unresolved mix of complex stellar populations, gas, and dust. Interpreting these data is a challenge for models: many studies have shown that properties ...derived from modeling galaxy photometry are uncertain by a factor of two or more, and yet answering key questions in the field now requires higher accuracy than this. Here, we present a new model framework specifically designed for these complexities. Our model, Prospector- , includes dust attenuation and re-radiation, a flexible attenuation curve, nebular emission, stellar metallicity, and a six-component nonparametric star formation history. The flexibility and range of the parameter space, coupled with Monte Carlo Markov chain sampling within the Prospector inference framework, is designed to provide unbiased parameters and realistic error bars. We assess the accuracy of the model with aperture-matched optical spectroscopy, which was excluded from the fits. We compare spectral features predicted solely from fits to the broadband photometry to the observed spectral features. Our model predicts H luminosities with a scatter of ∼0.18 dex and an offset of ∼0.1 dex across a wide range of morphological types and stellar masses. This agreement is remarkable, as the H luminosity is dependent on accurate star formation rates, dust attenuation, and stellar metallicities. The model also accurately predicts dust-sensitive Balmer decrements, spectroscopic stellar metallicities, polycyclic aromatic hydrocarbon mass fractions, and the age- and metallicity-sensitive features Dn4000 and Hδ. Although the model passes all these tests, we caution that we have not yet assessed its performance at higher redshift or the accuracy of recovered stellar masses.
We present updated stellar population models appropriate for old ages (>1 Gyr) and covering a wide range in metallicities (−1.5 Fe/H 0.3). These models predict the full spectral variation associated ...with individual element abundance variation as a function of metallicity and age. The models span the optical-NIR wavelength range (0.37-2.4 m), include a range of initial mass functions, and contain the flexibility to vary 18 individual elements including C, N, O, Mg, Si, Ca, Ti, and Fe. To test the fidelity of the models, we fit them to integrated light optical spectra of 41 Galactic globular clusters (GCs). The value of testing models against GCs is that their ages, metallicities, and detailed abundance patterns have been derived from the Hertzsprung-Russell diagram in combination with high-resolution spectroscopy of individual stars. We determine stellar population parameters from fits to all wavelengths simultaneously ("full spectrum fitting"), and demonstrate explicitly with mock tests that this approach produces smaller uncertainties at fixed signal-to-noise ratio than fitting a standard set of 14 line indices. Comparison of our integrated-light results to literature values reveals good agreement in metallicity, Fe/H. When restricting to GCs without prominent blue horizontal branch populations, we also find good agreement with literature values for ages, Mg/Fe, Si/Fe, and Ti/Fe.
The spectral absorption lines in early-type galaxies contain a wealth of information regarding the detailed abundance pattern, star formation history, and stellar initial mass function (IMF) of the ...underlying stellar population. Using our new population synthesis model that accounts for the effect of variable abundance ratios of 11 elements, we analyze very high quality absorption line spectra of 38 early-type galaxies and the nuclear bulge of M31. These data extend to 1 mu m and they therefore include the IMF-sensitive spectral features Na I, Ca II, and FeH at 0.82 mu m, 0.86 mu m, and 0.99 mu m, respectively. The models fit the data well, with typical rms residuals <, ~1%. Strong constraints on the IMF and therefore the stellar mass-to-light ratio, (M/L) sub(stars), are derived for individual galaxies. We find that the IMF becomes increasingly bottom-heavy with increasing velocity dispersion and Mg/Fe, At the lowest dispersions and Mg/Fe values the derived IMF is consistent with the Milky Way (MW) IMF, while at the highest dispersions and Mg/Fe values the derived IMF contains more low-mass stars (is more bottom-heavy) than even a Salpeter IMF. Our best-fit (M/L) sub(stars) values do not exceed dynamically based M/L values. We also apply our models to stacked spectra of four metal-rich globular clusters in M31 and find an (M/L) sub(stars) that implies fewer low-mass stars than a MW IMF, again agreeing with dynamical constraints. We discuss other possible explanations for the observed trends and conclude that variation in the IMF is the simplest and most plausible.
We propose that cold filamentary accretion in massive galaxies at high redshifts can lead to the formation of star-forming clumps in the halos of these galaxies without dark matter substructure. In ...certain cases, these clumps can be the birthplaces of metal-poor globular clusters (MP GCs). Using cosmological simulations, we show that narrow streams of dense gas feeding massive galaxies from the cosmic web can fragment, producing star-forming clumps. We then derive an analytical model for the properties of streams as a function of halo mass and redshift, and assess when these are gravitationally unstable, when this can lead to collapse and star formation in the halo, and when it may result in the formation of MP GCs. For stream metallicities 0.01 Z , this is likely to occur at z > 4.5 . At z ∼ 6 , the collapsing clouds have masses of ∼ ( 5 - 10 ) × 10 7 M , and the average stream pressure is ∼ 10 6 cm − 3 K . The conditions for GC formation are met in the extremely turbulent "eyewall" at ∼ 0.3 R v , where counter-rotating streams can collide, driving very large densities. Our scenario can account for the observed kinematics and spatial distribution of MP GCs, the correlation between their mass and metallicity, and the mass ratio between the GC system and the host halo. For MW-mass halos, we infer that ∼ 30 % of MP GCs could have formed in this way, with the remainder likely accreted in mergers. Our predictions for GC formation along circumgalactic filaments at high redshift are testable with JWST.
ABSTRACT Galaxies are thought to grow through accretion; as less massive galaxies are disrupted and merge over time, their debris results in diffuse, clumpy stellar halos enveloping the central ...galaxy. Here we present a study of the variation in the stellar halos of galaxies, using data from the Dragonfly Nearby Galaxies Survey (DNGS). The survey consists of wide field, deep ( mag arcsec−2) optical imaging of nearby galaxies using the Dragonfly Telephoto Array. Our sample includes eight spiral galaxies with stellar masses similar to that of the Milky Way, inclinations of 16-19 degrees and distances between 7-18 Mpc. We construct stellar mass surface density profiles from the observed g-band surface brightness in combination with the g − r color as a function of radius, and compute the halo fractions from the excess stellar mass (relative to a disk+bulge fit) beyond 5 half-mass radii. We find a mean halo fraction of 0.009 0.005 and a large rms scatter of dex. The peak-to-peak scatter of the halo fraction is a factor of -while some galaxies feature strongly structured halos resembling that of M31, three of the eight have halos that are completely undetected in our data. We conclude that spiral galaxies as a class exhibit a rich variety in stellar halo properties, implying that their assembly histories have been highly non-uniform. We find no convincing evidence for an environmental or stellar mass dependence of the halo fraction in the sample.
The stellar initial mass function (IMF) describes the mass distribution of stars at the time of their formation and is of fundamental importance for many areas of astrophysics. The IMF is reasonably ...well constrained in the disk of the Milky Way but we have very little direct information on the form of the IMF in other galaxies and at earlier cosmic epochs. Here we report observations of the Na i doublet and the Wing-Ford molecular FeH band in the spectra of elliptical galaxies. These lines are strong in stars with masses less than 0.3M (where M is the mass of the Sun) and are weak or absent in all other types of stars. We unambiguously detect both signatures, consistent with previous studies that were based on data of lower signal-to-noise ratio. The direct detection of the light of low-mass stars implies that they are very abundant in elliptical galaxies, making up over 80% of the total number of stars and contributing more than 60% of the total stellar mass. We infer that the IMF in massive star-forming galaxies in the early Universe produced many more low-mass stars than the IMF in the Milky Way disk, and was probably slightly steeper than the Salpeter form in the mass range 0.1M to 1M .
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
It is now well-established that the stellar initial mass function (IMF) can be determined from the absorption line spectra of old stellar systems, and this has been used to measure the IMF and its ...variation across the early-type galaxy population. Previous work focused on measuring the slope of the IMF over one or more stellar mass intervals, implicitly assuming that this is a good description of the IMF and that the IMF has a universal low-mass cutoff. In this work we consider more flexible IMFs, including two-component power laws with a variable low-mass cutoff and a general non-parametric model. We demonstrate with mock spectra that the detailed shape of the IMF can be accurately recovered as long as the data quality is high (S/N 300 −1) and cover a wide wavelength range (0.4-1.0 m). We apply these flexible IMF models to a high S/N spectrum of the center of the massive elliptical galaxy NGC 1407. Fitting the spectrum with non-parametric IMFs, we find that the IMF in the center shows a continuous rise extending toward the hydrogen-burning limit, with a behavior that is well-approximated by a power law with an index of −2.7. These results provide strong evidence for the existence of extreme (super-Salpeter) IMFs in the cores of massive galaxies.
In this paper we present results based on modeling stacked spectra of early-type galaxies drawn from the Sloan Digital Sky Survey as a function of velocity dispersion, sigma, from 90 kms super(-1) to ...300 km s super(-1). Our population synthesis model includes variation in 16 elements from C to Ba, a two-component star formation history, the shift in effective temperature, Delta T sub(eff), of the stars with respect to a solar metallicity isochrone, and the stellar initial mass function, among other parameters. In our approach we fit the full optical spectra rather than a select number of spectral indices and are able to, for the first time, measure the abundances of the elements V, Cr, Mn, Co, and Ni from the integrated light of distant galaxies. A variety of tests reveal that the systematic uncertainties in our measurements are probably 0.05 dex or less. Our derived Mg/Fe and fO/Fe abundance ratios are 0.05-0.1 dex lower than most previous determinations.
ABSTRACT Recently a population of large, very low surface brightness, spheroidal galaxies was identified in the Coma cluster. The apparent survival of these ultra-diffuse galaxies (UDGs) in a rich ...cluster suggests that they have very high masses. Here, we present the stellar kinematics of Dragonfly 44, one of the largest Coma UDGs, using a 33.5 hr integration with DEIMOS on the Keck II telescope. We find a velocity dispersion of = 47 − 6 + 8 km s − 1 , which implies a dynamical mass of M dyn ( < r 1 / 2 ) = 0.7 − 0.2 + 0.3 × 10 10 M within its deprojected half-light radius of r 1 / 2 = 4.6 0.2 kpc . The mass-to-light ratio is M / L I ( < r 1 / 2 ) = 48 − 14 + 21 M / L , and the dark matter fraction is 98% within r 1 / 2 . The high mass of Dragonfly 44 is accompanied by a large globular cluster population. From deep Gemini imaging taken in 0 4 seeing we infer that Dragonfly 44 has 94 − 20 + 25 globular clusters, similar to the counts for other galaxies in this mass range. Our results add to other recent evidence that many UDGs are "failed" galaxies, with the sizes, dark matter content, and globular cluster systems of much more luminous objects. We estimate the total dark halo mass of Dragonfly 44 by comparing the amount of dark matter within r = 4.6 kpc to enclosed mass profiles of NFW halos. The enclosed mass suggests a total mass of ∼ 10 12 M , similar to the mass of the Milky Way. The existence of nearly dark objects with this mass is unexpected, as galaxy formation is thought to be maximally efficient in this regime.
We report the discovery of 47 low surface brightness objects in deep images of a 3degrees x 3degrees field centered on the Coma cluster, obtained with the Dragonfly Telephoto Array. The objects have ...central surface brightness mu (g, 0) ranging from 24-26 mag arcsec super(-2) and effective radii r sub(eff) = 3"-10", as measured from archival Canada-France-Hawaii Telescope images. From their spatial distribution we infer that most or all of the objects are galaxies in the Coma cluster. This relatively large distance is surprising as it implies that the galaxies are very large: with r sub(eff) = 1.5-4.6 kpc their sizes are similar to those of Llow * galaxies even though their median stellar mass is only ~6 x 10 super(7) M sub(middot in circle). The galaxies are relatively red and round, with left angle bracketg - iright angle bracket = 0.8 and left angle bracketb/aright angle bracket = 0.74. One of the 47 galaxies is fortuitously covered by a deep Hubble Space Telescope Advanced Camera for Surveys (ACS) observation. The ACS imaging shows a large spheroidal object with a central surface brightness 74475 = 25.8 mag arcsec super(-2), a Sersic index n = 0.6, and an effective radius of 7", corresponding to 3.4kpc at the distance of Coma. The galaxy is not resolved into stars, consistent with expectations for a Coma cluster object. We speculate that these "ultra-diffuse galaxies" may have lost their gas supply at early times, possibly resulting in very high dark matter fractions.
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