arXiv subjects

Simon Zieleniewski

Publications and source records attributed to Simon Zieleniewski.

4 recordsLinked to original sources

arXiv2018

The stellar population and initial mass function of NGC 1399 with MUSE

We present spatially resolved measurements of the stellar initial mass function (IMF) in NGC 1399, the largest elliptical galaxy in the Fornax Cluster. Using data from the Multi Unit Spectroscopic Explorer (MUSE) and updated state-of-the-art stellar population synthesis models from Conroy et. al 2018, we use full spectral fitting to measure the low-mass IMF, as well as a number of individual elemental abundances, as a function of radius in this object. We find that the IMF in NGC 1399 is heavier than the Milky Way in its centre and remains radially constant at a super-salpeter slope out to 0.7 R$_{e}$. At radii larger than this, the IMF slope decreases to become marginally consistent with a Milky Way IMF just beyond R$_{e}$. The inferred central V-band M/L ratio is in excellent agreement with the previously reported dynamical M/L measurement from Houghton et. al 2006. The measured radial form of the M/L ratio may be evidence for a two-phase formation in this object, with the central regions forming differently to the outskirts. We also report measurements of a spatially resolved filament of ionised gas extending 4 arcseconds (404 pc at $D_{\mathrm{L}}=21.1$ Mpc) from the centre of NGC 1399, with very narrow equivalent width and low velocity dispersion ($65\pm14$ \kms). The location of the emission, combined with an analysis of the emission line ratios, leads us to conclude that NGC 1399's AGN is the source of ionising radiation.

astro-ph.GA↗

arXiv2018

Radial measurements of IMF-sensitive absorption features in two massive ETGs

We make radial measurements of stellar initial mass function (IMF) sensitive absorption features in the two massive early-type galaxies NGC 1277 and IC 843. Using the Oxford Short Wavelength Integral Field SpecTrogaph (SWIFT), we obtain resolved measurements of the NaI0.82 and FeH0.99 indices, among others, finding both galaxies show strong gradients in NaI absorption combined with flat FeH profiles at $\sim0.4$Å. We find these measurements may be explained by radial gradients in the IMF, appropriate abundance gradients in [Na/Fe] and [Fe/H], or a combination of the two, and our data is unable to break this degeneracy. We also use full spectral fitting to infer global properties from an integrated spectrum of each object, deriving a unimodal IMF slope consistent with Salpeter in IC 843 ($x=2.27\pm0.17$) but steeper than Salpeter in NGC 1277 ($x=2.69\pm0.11$), despite their similar FeH equivalent widths. Independently, we fit the strength of the FeH feature and compare to the E-MILES and CvD12 stellar population libraries, finding agreement between the models. The IMF values derived in this way are in close agreement with those from spectral fitting in NGC 1277 ($x_{\mathrm{CvD}}=2.59^{+0.25}_{-0.48}$ , $x_{\mathrm{E-MILES}}=2.77\pm0.31$), but are less consistent in IC 843, with the IMF derived from FeH alone leading to steeper slopes than when fitting the full spectrum ($x_{\mathrm{CvD}}=2.57^{+0.30}_{-0.41}$, $x_{\mathrm{E-MILES}}=2.72\pm0.25$). This work highlights the importance of a large wavelength coverage for breaking the degeneracy between abundance and IMF variations, and may bring into doubt the use of the Wing-Ford band as an IMF index if used without other spectral information.

astro-ph.GA↗

arXiv2016

Radial gradients in initial mass function sensitive absorption features in the Coma brightest cluster galaxies

Using the Oxford Short Wavelength Integral Field specTrograph (SWIFT), we trace radial variations of initial mass function (IMF) sensitive absorption features of three galaxies in the Coma cluster. We obtain resolved spectroscopy of the central 5kpc for the two central brightest-cluster galaxies (BCGs) NGC4889, NGC4874, and the BCG in the south-west group NGC4839, as well as unresolved data for NGC4873 as a low-$σ_*$ control. We present radial measurements of the IMF-sensitive features sodium NaI$_{\rm{SDSS}}$, calcium triplet CaT and iron-hydride FeH0.99, along with the magnesium MgI0.88 and titanium oxide TiO0.89 features. We employ two separate methods for both telluric correction and sky-subtraction around the faint FeH feature to verify our analysis. Within NGC4889 we find strong gradients of NaI$_{\rm{SDSS}}$ and CaT but a flat FeH profile, which from comparing to stellar population synthesis models, suggests an old, $α$-enhanced population with a Chabrier, or even bottom-light IMF. The age and abundance is in line with previous studies but the normal IMF is in contrast to recent results suggesting an increased IMF slope with increased velocity dispersion. We measure flat NaI$_{\rm{SDSS}}$ and FeH profiles within NGC4874 and determine an old, possibly slightly $α$-enhanced and Chabrier IMF population. We find an $α$-enhanced, Chabrier IMF population in NGC4873. Within NGC4839 we measure both strong NaI$_{\rm{SDSS}}$ and strong FeH, although with a large systematic uncertainty, suggesting a possible heavier IMF. The IMFs we infer for these galaxies are supported by published dynamical modelling. We stress that IMF constraints should be corroborated by further spectral coverage and independent methods on a galaxy-by-galaxy basis.

astro-ph.GA↗

arXiv2015

The initial mass functions of M31 and M32 through far red stellar absorption features

Using the Oxford Short Wavelength Integral Field specTrograph (SWIFT), we investigate radial variations of several initial mass function (IMF) dependent absorption features in M31 and M32. We obtain high signal-to-noise spectra at six pointings along the major axis of M31 out to ~ 700" (2.7 kpc) and a single pointing of the central 10 pc for M32. In M31 the sodium NaI λ8190 index shows a flat equivalent width profile at ~ 0.4 Å through the majority of the bulge, with a strong gradient up to 0.8 Å in the central 10" (38 pc); the Wing-Ford FeH λ9916 index is measured to be constant at 0.4 Å for all radii; and calcium triplet CaT λ8498, 8542, 8662 shows a gradual increase through the bulge towards the centre. M32 displays flat profiles for all three indices, with FeH at ~ 0.5 Å, very high CaT at ~ 0.8 Å and low NaI at ~ 0.1 Å. We analyse these data using stellar population models. We find that M31 is well described on all scales by a Chabrier IMF, with a gradient in sodium enhancement of [Na/Fe] ~ +0.3 dex in the outer bulge, rising within the central 10" to perhaps [Na/Fe] ~ +1.0 dex in the nuclear region. We find M32 is described by a Chabrier IMF and young stellar age in line with other studies. Models show that CaT is much more sensitive to metallicity and [α/Fe] than to IMF. We note that the centres of M31 and M32 have very high stellar densities and yet we measure Chabrier IMFs in these regions.

astro-ph.GA↗