arXiv subjects

P. Côté

Publications and source records attributed to P. Côté.

29 records · Page 2Linked to original sources

arXiv2018

Ultra-Violet Imaging Telescope Observations of the Star-Forming Ring in NGC7252: Evidence of Possible AGN Feedback Suppressing Central Star Formation

Some post-merger galaxies are known to undergo a starburst phase that quickly depletes the gas reservoir and turns it into a red-sequence galaxy, though the details are still unclear. Here we explore the pattern of recent star formation in the central region of the post-merger galaxy NGC7252 using high resolution UV images from the UVIT on ASTROSAT. The UVIT images with 1.2 and 1.4 arcsec resolution in the FUV and NUV are used to construct a FUV-NUV colour map of the central region. The FUV-NUV pixel colour map for this canonical post-merger galaxy reveals a blue circumnuclear ring of diameter $\sim$ 10 " (3.2 kpc) with bluer patches located over the ring. Based on a comparison to single stellar population models, we show that the ring is comprised of stellar populations with ages $\lesssim$ 300 Myr, with embedded star-forming clumps of younger age ($\lesssim$ 150Myr). The suppressed star formation in the central region, along with the recent finding of a large amount of ionised gas, leads us to speculate that this ring may be connected to past feedback from a central super-massive black hole that has ionised the hydrogen gas in the central $\sim$ 4" $\sim$ 1.3 kpc.

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arXiv2018

The Next Generation Fornax Survey (NGFS): IV. Mass and Age Bimodality of Nuclear Clusters in the Fornax Core Region

We present the analysis of 61 nucleated dwarf galaxies in the central regions ($<R_{\rm vir}/4$) of the Fornax galaxy cluster. The galaxies and their nuclei are studied as part of the Next Generation Fornax Survey (NGFS) using optical imaging obtained with the Dark Energy Camera (DECam) mounted at Blanco/CTIO and near-infrared data obtained with VIRCam at VISTA/ESO. We decompose the nucleated dwarfs in nucleus and spheroid, after subtracting the surface brightness profile of the spheroid component and studying the nucleus using PSF photometry.In general, nuclei are consistent with colors of confirmed metal-poor globular clusters, but with significantly smaller dispersion than other confirmed compact stellar systems in Fornax. We find a bimodal nucleus mass distribution with peaks located at $\log({\cal M_*}/M_\odot)\!\simeq\!5.4$ and $\sim\,6.3$. These two nucleus sub-populations have different stellar population properties, the more massive nuclei are older than $\sim\!2$ Gyr and have metal-poor stellar populations ($Z\leq0.02\,Z_\odot$), while the less massive nuclei are younger than $\sim\!2$ Gyr with metallicities in the range $0.02\!<\!Z/Z_\odot\!\leq\!1$. We find that the nucleus mass (${\cal M}_{\rm nuc}$) vs. galaxy mass (${\cal M}_{\rm gal}$) relation becomes shallower for less massive galaxies starting around $10^8\,M_\odot$ and the mass ratio $η_n\!=\!{\cal M}_{\rm nuc}/{\cal M}_{\rm gal}$ shows a clear anti-correlation with ${\cal M}_{\rm gal}$ for the lowest masses, reaching $10\%$. We test current theoretical models of nuclear cluster formation and find that they cannot fully reproduce the observed trends. A likely mixture of in-situ star formation and star-cluster mergers seem to be acting during nucleus growth over cosmic time.

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arXiv2018

Dissecting star-formation in the "Atoms-for-Peace" galaxy: Ultra-Violet Imaging Telescope observations of the post-merger galaxy NGC7252

The tidal tails of post-merger galaxies exhibit ongoing star formation far from their disks. The study of such systems can be useful for our understanding of gas condensation in diverse environments. The ongoing star formation in the tidal tails of post-merger galaxies can be directly studied from ultraviolet (UV) imaging observations. The post merger galaxy NGC7252 ("Atoms-for-Peace" galaxy) is observed with the Astrosat UV imaging telescope (UVIT) in broadband NUV and FUV filters to isolate the star forming regions in the tidal tails and study the spatial variation in star formation rates. Based on ultraviolet imaging observations, we discuss star formation regions of ages $<$ 200 Myrs in the tidal tails. We measure star formation rates in these regions and in the main body of the galaxy. The integrated star formation rate of NGC7252 (i.e., that in the galaxy and tidal tails combined) without correcting for extinction is found to be 0.81 $\pm$ 0.01 M$_{\odot}$/yr. We show that the integrated star formation rate can change by an order of magnitude if the extinction correction used in star formation rates derived from other proxies are taken into consideration. The star formation rates in the associated tidal dwarf galaxies (NGC7252E, SFR=0.02 M$_{\odot}$/yr and NGC7252NW, SFR=0.03 M$_{\odot}$/yr) are typical of dwarf galaxies in the local Universe. The spatial resolution of the UV images reveals a gradient in star formation within the tidal dwarf galaxy. The star formation rates show a dependence on the distance from the centre of the galaxy. This can be due to the different initial conditions responsible for the triggering of star formation in the gas reservoir that was expelled during the recent merger in NGC7252.

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arXiv2017

The Pristine survey III: Spectroscopic confirmation of an efficient search for extremely metal-poor stars

The Pristine survey is a narrow-band, photometric survey focused around the wavelength region of the Ca II H & K absorption lines, designed to efficiently search for extremely metal-poor stars. In this work, we use the first results of a medium-resolution spectroscopic follow-up to refine the selection criteria for finding extremely metal-poor stars ($\textrm{[Fe/H]} \leq -3.0$) in the Pristine survey. We consider methods by which stars can be selected from available broad-band and infrared photometry plus the additional Pristine narrow-band photometry. The spectroscopic sample presented in this paper consists of 205 stars in the magnitude range $14 < V < 18$. Applying the photometric selection criteria cuts the sample down to 149 stars, and from these we report a success rate of 70% for finding stars with $\textrm{[Fe/H]} \leq -2.5$ and 22% for finding stars with $\textrm{[Fe/H]} \leq -3.0$. These statistics compare favourably with other surveys that search for extremely metal-poor stars, namely an improvement by a factor of $\sim 4-5$ for recovering stars with $\textrm{[Fe/H]} \leq -3.0$. In addition, Pristine covers a fainter magnitude range than its predecessors, and can thus probe deeper into the Galactic halo.

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arXiv2015

NGC4370: a case study for testing our ability to infer dust distribution and mass in nearby galaxies

A fraction of the early-type galaxy population hosts a prominent dust lane. Methods to quantify the dust content of these systems based on optical imaging data usually yield dust masses which are an order of magnitude lower than dust masses derived from the observed FIR emission. High-quality optical data from the Next Generation Virgo cluster Survey (NGVS) and FIR/submm observations from the Herschel Virgo Cluster Survey (HeViCS) allow us to revisit previous methods to determine the dust content in galaxies and explore new ones. We aim to derive the dust mass in NGC 4370 from both optical and FIR data, and investigate the need to invoke a putative diffuse dust component. We create color and attenuation maps, which are converted to approximate dust mass maps based on simple dust geometries. Dust masses are also derived from SED fits to FIR/submm observations. Finally, inverse radiative transfer fitting is performed to investigate more complex dust geometries. The empirical methods applied to the optical data yield lower limits of 3.4e5 solar masses, an order of magnitude below the total dust masses derived from SED fitting. In contrast, radiative transfer models yield dust masses which are slightly lower, but fully consistent with the FIR-derived mass. Dust is more likely to be distributed in a ring around the centre of NGC 4370 as opposed to an exponential disc or a simple foreground screen. Moreover, using inverse radiative transfer fitting, we are able to constrain most of the parameters describing these geometries. The resulting dust masses are high enough to account for the dust observed at FIR/submm wavelengths, so that no diffuse dust component needs to be invoked. We furthermore caution for the interpretation of dust masses and optical depths based on optical data alone, using overly simplistic star-dust geometries and the neglect of scattering effects. [ABRIDGED]

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arXiv2014

The Next Generation Virgo Cluster Survey. XV. The photometric redshift estimation for background sources

The Next Generation Virgo Cluster Survey is an optical imaging survey covering 104 deg^2 centered on the Virgo cluster. Currently, the complete survey area has been observed in the u*giz-bands and one third in the r-band. We present the photometric redshift estimation for the NGVS background sources. After a dedicated data reduction, we perform accurate photometry, with special attention to precise color measurements through point spread function-homogenization. We then estimate the photometric redshifts with the Le Phare and BPZ codes. We add a new prior which extends to iAB = 12.5 mag. When using the u*griz-bands, our photometric redshifts for 15.5 \le i \lesssim 23 mag or zphot \lesssim 1 galaxies have a bias |Δz| < 0.02, less than 5% outliers, and a scatter σ_{outl.rej.} and an individual error on zphot that increase with magnitude (from 0.02 to 0.05 and from 0.03 to 0.10, respectively). When using the u*giz-bands over the same magnitude and redshift range, the lack of the r-band increases the uncertainties in the 0.3 \lesssim zphot \lesssim 0.8 range (-0.05 < Δz < -0.02, σ_{outl.rej} ~ 0.06, 10-15% outliers, and zphot.err. ~ 0.15). We also present a joint analysis of the photometric redshift accuracy as a function of redshift and magnitude. We assess the quality of our photometric redshifts by comparison to spectroscopic samples and by verifying that the angular auto- and cross-correlation function w(θ) of the entire NGVS photometric redshift sample across redshift bins is in agreement with the expectations.

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arXiv2013

The Rich Globular Cluster System of Abell 1689 and the Radial Dependence of the Globular Cluster Formation Efficiency

We study the rich globular cluster (GC) system in the center of the massive cluster of galaxies Abell 1689 (z=0.18), one of the most powerful gravitational lenses known. With 28 HST/ACS orbits in the F814W bandpass, we reach magnitude I_814=29 with >90% completeness and sample the brightest ~5% of the GC system. Assuming the well-known Gaussian form of the GC luminosity function (GCLF), we estimate a total population of N(GC_total) = 162,850 GCs within a projected radius of 400kpc. As many as half may comprise an intracluster component. Even with the sizable uncertainties, which mainly result from the uncertain GCLF parameters, this is by far the largest GC system studied to date. The specific frequency S_N is high, but not uncommon for central galaxies in massive clusters, rising from S_N~5 near the center to ~12 at large radii. Passive galaxy fading would increase S_N by ~20% at z=0. We construct the radial mass profiles of the GCs, stars, intracluster gas, and lensing-derived total mass, and we compare the mass fractions as a function of radius. The estimated mass in GCs, M(GC_total)=3.9x10^10 Msun, is comparable to ~80% of the total stellar mass of the Milky Way. The shape of the GC mass profile appears intermediate between those of the stellar light and total cluster mass. Despite the extreme nature of this system, the ratios of the GC mass to the baryonic and total masses, and thus the GC formation efficiency, are typical of those in other rich clusters when comparing at the same physical radii. The GC formation efficiency is not constant, but varies with radius, in a manner that appears similar for different clusters; we speculate on the reasons for this similarity in profile.

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arXiv2013

Kinematics of Outer Halo Globular Clusters in M31

We present the first kinematic analysis of the far outer halo globular cluster (GC) population in the Local Group galaxy M31. Our sample contains 53 objects with projected radii of ~20-130 kpc, of which 44 have no previous spectroscopic information. GCs with projected radii >30 kpc are found to exhibit net rotation around the minor axis of M31, in the same sense as the inner GCs, albeit with a smaller amplitude of 79 +/-19 km/s. The rotation-corrected velocity dispersion of the full halo GC sample is 106 +/-12 km/s, which we observe to decrease with increasing projected radius. We find compelling evidence for kinematic-coherence amongst GCs which project on top of halo substructure, including a clear signature of infall for GCs lying along the North-West stream. Using the tracer mass estimator, we estimate the dynamical mass of M31 within 200 kpc to be M_M31 = (1.2-1.5) +/- 0.2 x 10^12 M_sun. This value is highly dependent on the chosen model and assumptions within.

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arXiv2012

Globular cluster systems in fossil groups: NGC6482, NGC1132 and ESO306-017

We study the globular cluster (GC) systems in three representative fossil group galaxies: the nearest (NGC6482), the prototype (NGC1132) and the most massive known to date (ESO306-017). This is the first systematic study of GC systems in fossil groups. Using data obtained with the Hubble Space Telescope Advanced Camera for Surveys in the F475W and F850LP filters, we determine the GC color and magnitude distributions, surface number density profiles, and specific frequencies. In all three systems, the GC color distribution is bimodal, the GCs are spatially more extended than the starlight, and the red population is more concentrated than the blue. The specific frequencies seem to scale with the optical luminosities of the central galaxy and span a range similar to that of the normal bright elliptical galaxies in rich environments. We also analyze the galaxy surface brightness distributions to look for deviations from the best-fit Sérsic profiles; we find evidence of recent dynamical interaction in all three fossil group galaxies. Using X-ray data from the literature, we find that luminosity and metallicity appear to correlate with the number of GCs and their mean color, respectively. Interestingly, although NGC6482 has the lowest mass and luminosity in our sample, its GC system has the reddest mean color, and the surrounding X-ray gas has the highest metallicity.

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arXiv2009

Testing Fundamental Physics with Distant Star Clusters: Analysis of Observational Data on Palomar 14

We use the distant outer halo globular cluster Palomar 14 as a test case for classical vs. modified Newtonian dynamics (MOND). Previous theoretical calculations have shown that the line-of-sight velocity dispersion predicted by these theories can differ by up to a factor of three for such sparse, remote clusters like Pal 14. We determine the line-of-sight velocity dispersion of Palomar 14 by measuring radial velocities of 17 red giant cluster members obtained using the Very Large Telescope (VLT) and Keck telescope. The systemic velocity of Palomar 14 is (72.28+-0.12)km/s. The derived velocity dispersion of (0.38+-0.12)km/s of the 16 definite member stars is in agreement with the theoretical prediction for the classical Newtonian case according to Baumgardt et al. (2005). In order to exclude the possibility that a peculiar mass function might have influenced our measurements, we derived the cluster's main sequence mass function down to 0.53Msolar using archival images obtained with the Hubble Space Telescope. We found a mass function slope of 1.27+-0.44, which is, compared to the canonical mass function, a significantly shallower slope. The derived lower limit on the cluster's mass is higher than the theoretically predicted mass in case of MOND. Our data are consistent with a central density of 0.1 Msolar pc^-3. We need no dark matter in Palomar 14. If the cluster is on a circular orbit, our spectroscopic and photometric results argue against MOND, unless this cluster experienced significant mass loss.

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arXiv1993

Dynamics of the Globular Cluster NGC 362

In this paper we have examined the internal dynamics of the globular cluster NGC 362. A V band surface brightness profile (SBP) was constructed from CCD images, and, after it was determined that the cluster is not post core-collapse, fit with single- and multi-mass King-Michie (KM) models. The total cluster luminosity is 1.70 $\pm 0.1 \times 10^5$ L$_V$\sol. A total of 285 stellar spectra were obtained of 215 stars for radial velocity determinations. Four stars showed strong evidence for radial velocity variations and are probably members of binary systems. The true cluster binary fraction was determined from simulations to be 0.15 for circular orbits or 0.27 for orbits with a distribution function $f(e) = e$ ($e$ is eccentricity). This relatively high binary detection frequency may indicate that NGC 362 is overabundant in binaries compared to other clusters. The 208 remaining stars showed no sign of rotation. The best agreement with both the kinematic data and the SBP were for shallow mass functions $x = 0.0 - 0.5$ and intermediate amounts of anisotropy in the velocity dispersion tensor. The cluster mass is M = $2.5 - 3.5 \times 10^5$ M\sol\ for a global mass-to-light ratio of M/L$_V$ = 1.5 - 2.0 M\sol/L$_V$\sol. This low value for $x$ is in disagreement with the correlation between $x$ and the height above the Galactic disk seen for a sample of other clusters. The results are also different from the sharp turn-up in the low mass end of mass functions derived from some deep luminosity functions of three other globular clusters.

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