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Register a new userDark Matter Fraction in Lens Galaxies: New Estimates from Microlensing
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Jorge Jim\\'enez-Vicente
Publication date
2014
fields
Physics
and research's language is
English
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We present a joint estimate of the stellar/dark matter mass fraction in lens galaxies and the average size of the accretion disk of lensed quasars from microlensing measurements of 27 quasar image pairs seen through 19 lens galaxies. The Bayesian estimate for the fraction of the surface mass density in the form of stars is $alpha=0.21pm0.14$ near the Einstein radius of the lenses ($sim 1 - 2$ effective radii). The estimate for the average accretion disk size is $R_{1/2}=7.9^{+3.8}_{-2.6}sqrt{M/0.3M_sun}$ light days. The fraction of mass in stars at these radii is significantly larger than previous estimates from microlensing studies assuming quasars were point-like. The corresponding local dark matter fraction of 79% is in good agreement with other estimates based on strong lensing or kinematics. The size of the accretion disk inferred in the present study is slightly larger than previous estimates.
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We use the IllustrisTNG (TNG) cosmological simulations to provide theoretical expectations for the dark matter mass fractions (DMFs) and circular velocity profiles of galaxies. TNG predicts flat circular velocity curves for $z = 0$ Milky Way (MW)-like galaxies beyond a few kpc from the galaxy centre, in better agreement with observational constraints than its predecessor, Illustris. TNG also predicts an enhancement of the dark matter mass within the 3D stellar half-mass radius ($r_mathrm{half}$; $M_mathrm{200c} = 10^{10}-10^{13}mathrm{M}_{odot}$, $z le2$) compared to its dark matter only and Illustris counterparts. This enhancement leads TNG present-day galaxies to be dominated by dark matter within their inner regions, with $f_mathrm{DM}(<r_mathrm{half})gtrsim0.5$ at all masses and with a minimum for MW-mass galaxies. The 1$sigma$ scatter is $lesssim$ 10~per~cent at all apertures, which is smaller than that inferred by some observational datasets, e.g. 40 per cent from the SLUGGS survey. TNG agrees with the majority of the observationally inferred values for elliptical galaxies once a consistent IMF is adopted (Chabrier) and the DMFs are measured within the same apertures. The DMFs measured within $r_mathrm{half}$ increase towards lower redshifts: this evolution is dominated by the increase in galaxy size with time. At $zsim2$, the DMF in disc-like TNG galaxies decreases with increasing galaxy mass, with $f_mathrm{DM}(<r_mathrm{half}) sim 0.10-0.65$ for $10^{10} lesssim M_{rm stars}/mathrm{M}_{odot} lesssim 10^{12}$, and are two times higher than if TNG galaxies resided in Navarro-Frenk-White dark matter haloes unaffected by baryonic physics. It remains to be properly assessed whether recent observational estimates of the DMFs at $zsim2$ rule out the contraction of the dark matter haloes predicted by the TNG model.
We present a observational study of the dark matter fraction in 225 rotation supported star-forming galaxies at $zapprox 0.9$ having stellar mass range: $ 9.0 leq log(M_* mathrm{M_odot}) leq 11.0$ and star formation rate: $0.49 leq log left(SFR mathrm{[M_{odot} yr^{-1}]} right) leq 1.77$. This is a sub sample of KMOS redshift one spectroscopic survey (KROSS) previously studied by citet{GS20}. The stellar masses ($M_*$) of these objects were previously estimated using mass-to-light ratios derived from fitting the spectral energy distribution of the galaxies. Star formation rates were derived from the H$_alpha$ luminosities. The total gas masses ($M_{gas}$) are determined by scaling relations of molecular and atomic gas citep[][respectively] {Tacconi2018, Lagos2011}. The dynamical masses ($M_{dyn}$) are directly derived from the rotation curves (RCs) at different scale lengths (effective radius: $R_e$, $sim 2 R_e$ and $sim 3 R_e$) and then the dark matter fractions ($f_{ DM }=1-M_{bar}/M_{dyn}$) at these radii are calculated. We report that at $zsim 1$ only a small fraction ($sim 5%$) of our sample has a low ($< 20%$) DM fraction within $sim$ 2-3 $R_e$. The majority ($> 72%$) of SFGs in our sample have dark matter dominated outer disks ($sim 5-10$ kpc) in agreement with local SFGs. Moreover, we find a large scatter in the fraction of dark matter at a given stellar mass (or circular velocity) with respect to local SFGs, suggesting that galaxies at $z sim 1$, a) span a wide range of stages in the formation of stellar disks, b) have diverse DM halo properties coupled with baryons.
We present a new approach in the study of the Initial Mass function (IMF) in external galaxies based on quasar microlensing observations. We use measurements of quasar microlensing magnifications in 24 lensed quasars to estimate the average mass of the stellar population in the lens galaxies without any a priori assumption on the shape of the IMF. The estimated mean mass of the stars is $langle M rangle =0.16^{+0.05}_{-0.08} M_odot$ (at 68% confidence level). We use this average mass to put constraints into two important parameters characterizing the IMF of lens galaxies: the low-mass slope, $alpha_2$, and the low-mass cutoff, $M_{low}$. Combining these constraints with prior information based on lensing, stellar dynamics, and absorption spectral feature analysis, we calculate the posterior probability distribution for the parameters $M_{low}$ and $alpha_2$. We estimate values for the low-mass end slope of the IMF $langle alpha_2rangle=-2.6pm 0.9$ (heavier than that of the Milky Way) and for the low-mass cutoff $langle M_{low}rangle=0.13pm0.07$. These results are in good agreement with previous studies on these parameters and remain stable against the choice of different suitable priors.
We use X-ray and optical microlensing measurements to study the shape of the dark matter density profile in the lens galaxies and the size of the (soft) X-ray emission region. We show that single epoch X-ray microlensing is sensitive to the source size. Our results, in good agreement with previous estimates, show that the size of the X-ray emission region scales roughly linearly with the black hole mass, with a half light radius of $R_{1/2}simeq(24pm14) r_g$ where $r_g=GM_{BH}/c^2$. This corresponds to a size of $log(R_{1/2}/cm)=15.6^{+0.3}_{-0.3}$ or $sim$ 1 light day for a black hole mass of $M_{BH}=10^9 M_sun$. We simultaneously estimated the fraction of the local surface mass density in stars, finding that the stellar mass fraction is $alpha=0.20pm0.05$ at an average radius of $sim 1.9 R_{e}$, where $R_e$ is the effective radius of the lens. This stellar mass fraction is insensitive to the X-ray source size and in excellent agreement with our earlier results based on optical data. By combining X-ray and optical microlensing data, we can divide this larger sample into two radial bins. We find that the surface mass density in the form of stars is $alpha=0.31pm0.15$ and $alpha=0.13pm0.05$ at $(1.3pm0.3) R_{e}$ and $(2.3pm0.3) R_{e}$, respectively, in good agreement with expectations and some previous results.
Large galaxies may contain an atmosphere of hot interstellar X-ray gas, and the temperature and radial density profile of this gas can be used to measure the total mass of the galaxy contained within a given radius r. We use this technique for 102 early-type galaxies (ETGs) with stellar masses M_* > 10^10 M_Sun, to evaluate the mass fraction of dark matter (DM) within the fiducial radius r = 5 r_e, denoted f_5 = f_{DM}(5r_e). On average, these systems have a median f_5 = 0.8 - 0.9 with a typical galaxy-to-galaxy scatter +-0.15. Comparisons with mass estimates made through the alternative techniques of satellite dynamics (e.g. velocity distributions of globular clusters, planetary nebulae, satellite dwarfs) as well as strong lensing show encouraging consistency over the same range of stellar mass. We find that many of the disk galaxies (S0/SA0/SB0) have a significantly higher mean $f_5$ than do the pure ellipticals, by Delta f_5 = 0.1. We suggest that this higher level may be a consequence of sparse stellar haloes and quieter histories with fewer major episodes of feedback or mergers. Comparisons are made with the Magneticum Pathfinder suite of simulations for both normal and centrally dominant Brightest Cluster galaxies. Though the observed data exhibit somewhat larger scatter at a given galaxy mass than do the simulations, the mean level of DM mass fraction for all classes of galaxies is in good first-order agreement with the simulations. Lastly, we find that the group galaxies with stellar masses near M_* ~ 10^11 M_Sun have relatively more outliers at low $f_5$ than in other mass ranges, possibly the result of especially effective AGN feedback in that mass range leading to expansion of their dark matter halos.
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