Star Counts Redivivus. IV. Density Laws through Photometric Parallaxes

By Michael H. Siegel, S. Majewski, I. Reid, I. Thompson. 2002

187 citations recorded by the source corpus.

In an effort to more precisely define the spatial distribution of Galactic field stars, we present an analysis of the photometric parallaxes of stars in seven Kapteyn selected areas. Our photometry database covers ~14.9 deg2 and contains over 130,000 stars, of which approximately 70,000 are in a color range (0.4 ≤ R-I ≤ 1.5) for which relatively unambiguous photometric parallaxes can be derived. We discuss our photometry pipeline, our method of determining photometric parallaxes, and our analysis efforts. We also address the affects of Malmquist bias, subgiant/giant contamination, metallicity, and binary stars upon the derived density laws. The affect of binary stars is the most significant of these biases—a binary star fraction of 50% could result in derived scale heights that are 80% of the actual values. We find that while the disklike populations of the Milky Way are easily constrained in a simultaneous analysis of all seven fields, no good simultaneous solution for the halo is found. We have applied halo density laws taken from other studies and find that the Besançon flattened power-law halo model (c/a = 0.6,ρ ∝ r-2.75) produces the best fit to our data. With this halo, the thick disk has a scale height of 750 pc with an 8.5% normalization to the old disk. The old-disk scale height is ~280-300 pc for our early-type (5.8 ≤ MR < 6.8) dwarfs and rises to ~ 350 pc for our late-type (8.8 ≤ MR ≤ 10.2) dwarf stars. Corrected for a binary fraction of 50%, these scale heights are 940 and 350-375 pc, respectively. Even with this model, there are systematic discrepancies between the observed and predicted density distributions—discrepancies apparent only at the faint magnitudes reached by our survey. Specifically, our model produces density overpredictions in the inner Galaxy and density underpredictions in the outer Galaxy. A possible escape from this dilemma is offered by modeling the stellar halo as a two-component system, as favored by studies of blue horizontal branch/RR Lyrae stars and recent analyses of the kinematics of metal-poor stars. In this paradigm, the halo has a flattened inner distribution and a roughly spherical but substructured outer distribution. Further reconciliation could be provided by a flared thick disk, a structure consistent with a merger origin for that population.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/d3e79f78f23806970896428f22e6e5ebb5e62967

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Citations in the corpus

A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. I. Morphology of the Sagittarius Core and Tidal Arms (2003, result): “This result is at odds with (a) star count studies, which typically find c=a 0:6 0:8 (e.g., Robin, Reylé, & Crézé 2000; Siegel et al. 2002; Reid & Majewski 1993 and references therein); (b) dynamical studies of halo tracers (Binney, May, & Ostriker 1987; Amendt & Cuddeford 1994; see also van der…”
The Fundamental Scaling Relations of Elliptical Galaxies (2005, methodology): “In all cases, the stellar disk scale height hd ¼ 0:2rd , similar to the Milky Way (cf. Siegel et al. 2002).”
MAPPING THE STELLAR STRUCTURE OF THE MILKY WAY THICK DISK AND HALO USING SEGUE PHOTOMETRY (2009, background): “The role of the thin disk is limited by only looking at the area at |Z| >1.0 kpc, and we fix its parameters at Lthin=2.6 kpc and Hthin=0.25 kpc, following Jurić et al. (2008) and in line with earlier star count results (e.g Siegel et al. 2002).”
The Evolution of the MBH-σ Relation (2005, methodology): “The vertical scale heights of the stellar disks are set to 0.2Rd, similar to the MilkyWay scaling (Siegel et al. 2002).”
THE GENESIS OF THE MILKY WAY'S THICK DISK VIA STELLAR MIGRATION (2010, background): “…to previous work (e.g. Bahcall & Soneira 1980) which identifies a clear change of slope in the counts of disk stars as a function of distance from the Galactic plane; this change in slope is usually interpreted as the transition from the thin to thick disk (Gilmore & Reid 1983; Siegel et al. 2002).”
A Spectroscopic Study of the Ancient Milky Way: F- and G-Type Stars in the Third Data Release of the Sloan Digital Sky Survey (2005, background): “…constrained, but it extends to distances in excess of 100 kpc from the Galactic plane, and its density in the plane is likely a fraction of a percent of the thin-disk density (Norris 1999; Chen et al. 2001; Siegel et al. 2002; Larsen & Humphreys 2003; Robin et al. 2003; Cabrera-Lavers et al. 2005).”
KINEMATICAL AND CHEMICAL VERTICAL STRUCTURE OF THE GALACTIC THICK DISK. II. A LACK OF DARK MATTER IN THE SOLAR NEIGHBORHOOD, (2012, background): “…closer to a sech2 function than an exponential (Camm 1950, 1952), but observational evidences confirm that the Galactic thick disk, object of our investigation, is well described by the double exponential law of Equation (9) (e.g., Hammersley et al. 1999; Siegel et al. 2002; Jurić et al. 2008).”
Merger Histories of Galaxy Halos and Implications for Disk Survival (2007, background): “The thin disk has vertical scale height that is just ∼ 10% of its radial scale length (Siegel et al. 2002; Juric et al. 2005; Newberg et al. 2006), and contains stars as old as ∼ 10 Gyr (Nordström et al. 2004).”
Preliminary Parallaxes of 40 L and T Dwarfs from the US Naval Observatory Infrared Astrometry Program (2004, methodology): “Column 7 of Table 1 gives the codes H, S, or IR indicating whether the Hawley et al. (2002), Siegel et al. (2002), and/or infrared calibrations were employed.”
Oxygen abundances in nearby stars - Clues to the formation and evolution of the Galactic disk (2007, methodology): “6.1 in Siegel et al. 2002).”
Improving PARSEC models for very low mass stars (2014, background): “…appear along well–defined colour–magnitude relations, as indicated by stars in open clusters, that made them amenable for the distance derivations via photometric parallaxes, and hence valuable probes of the Milky Way structure (Siegel et al. 2002; Jurić et al. 2008; Ivezić, Beers & Jurić 2012).”
The stellar halo of the Galaxy (2008, background): “Siegel et al. (2002) observed seven Kapteyn selected areas (14.9 deg2) (hence ensuring uniform photometry) and preferred n = −2.75 and q = 0.6.”
Mass models of the Milky Way (2011, methodology): “It is therefore also larger than the values in the range 2 to 2.5 kpc found by some other recent studies (Ojha et al. 1999; Chen et al. 1999; Siegel et al. 2002), and the value used for the old (age > 0.15Gyr) disc in the Besançon Galaxy model (2.53 kpc, Robin et al. 2003).”
The Palomar/MSU Nearby Star Spectroscopic Survey. IV. The Luminosity Function in the Solar Neighborhood and M Dwarf Kinematics (2002, background): “…particularly the abundance and age distribution, remain uncertain, but star counts at z > 1000 pc demonstrate there are few, if any, stars younger than a few gigayears, and that the vertical density distribution has a scale height 3–4 times that of MV > 4 disk stars (Siegel et al. 2002).”
Photometry and the Metallicity Distribution of the Outer Halo of M31. II. The 30 Kiloparsec Field (2001, background): “Morrison et al. 2000; Yanny et al. 2000; Chen et al. 2001; Siegel et al. 2002, and references within).”
The Haunted Halos of Andromeda and Triangulum: A Panorama of Galaxy Formation in Action (2007, background): “There is broad agreement that the stellar halo is flattened with b/a ∼ 0.6 (e.g., Morrison et al. 2000; Yanny et al. 2000; Chen et al. 2001; Siegel et al. 2002), though there are indications that the distribution becomes spherical beyond 15 – 20 kpc (Chiba & Beers 2000).”
Shredded Galaxies as the Source of Diffuse Intrahalo Light on Varying Scales (2007, background): “The stellar halo of the Milky Way contains fIHL 1% of the Galaxy’s total luminosity (Morrison 1993; Wetterer & McGraw 1996; Morrison et al. 2000; Chiba & Beers 2000; Yanny et al. 2000; Ivezić et al. 2000; Siegel et al. 2002).”