(NOTE: A copy of this site and the data is available on the Minnesota Automated Plate Scanner website.)
History of the Asymmetric Thick Disk
In 1996, Jeff Larsen (then at U Minnesota) and Roberta Humphreys (U Minnesota) discovered "a large and significant asymmetry in the number of stars in the first quadrant I of the galaxy compared to complementary longitudes on the other side of the center-anticenter line." This
work, focusing on probable thick disk stars 30 degrees above the plane
of the galaxy showed a 30% excess in stars in the first quadrant versus
the corresponding field in the fourth quadrant covering 90
degrees of galactic longitude! (Larsen and Humphreys 1996)
Followup work by Jennifer Parker (then at U Minnesota), Roberta Humphreys, and Jeff Larsen showed that the asymmetry in star counts extended over an even larger part of the sky and "While the region of the asymmetric distribution is somewhat irregular in shape, it is also fairly uniform, stretching over several hundred square degrees on the sky. It is therefore a major substructure in the Galaxy due to more than small-scale clumpiness in the thick disk or inner halo" (Parker, Humphreys, and Larsen 2003).
Furthermore, they obtained spectra for 741 stars scattered across these
fields. Using the spectral analysis pipeline developed by their collaborator,
Tim Beers, they were able to use the metallicity estimates from the spectra
to show most of the stars in the sample were Thick Disk stars. Isolating
the thick disk stars, they demonstrated that the " VLSR velocities reveals a significant lag of approximately 80 to 90 km/s in the direction of Galactic rotation for the thick-disk stars in quadrant I, while in quadrant IV, the same population has only a approximately 20 km/s lag confirming the kinematic asymmetry between the two directions" (Parker, Humphreys, and Beers 2004).
So by 2004 it was clear there was a significant feature in the Thick Disk of the Milky Way that was causing an asymmetry in both the star counts and kinematics of the thick disk. At the end of Parker, Humphreys, and Beers (2004), three possibie explanations were proposed for the asymmetry:
- The asymmetry is due to a fossil remnant of a merger passng through Quadrant
- The thick disk and/or halo is triaxial with the major axis in Quadrant
I, thus they asymmetry just reflects that the long axis of the triaxial distribution
is nearer to us in Quadrant I than Quadrant IV.
- The asymmetry is due to a "gravitational wake" caused by the
interaction of the thick-disk/inner halo stars with the bar in the disk,
which is in Quadrant I, but lies a few kpc beyond the thick disk stars in
The "Mapping the Asymmetric Thick Disk" Project
Figure 1 [Click Image for Full
of excess for faint "Blue" stars with (top) 500 pc < Z < 1500 pc, (middle)
1500 pc < Z < 2500 pc, and (bottom) 2500 pc < Z < 4000 pc shown overlaid on
the density contours of the bar in the Disk as traced by IRAS AGB stars from
Weinberg (1992). All figures are in galactocentric
Cartesian coordinates. (Figure 11 from Larsen, Cabanela, and Humphreys 2011
In 2006, Roberta Humphreys (U Minnesota), Jeff Larsen (US Naval Academy),
and Juan Cabanela (then at St. Cloud State University, currently at Minnesota
State University Moorhead) proposed to NSF for a collaborative project to determine
the spatial extent of the asymmetry both in star counts and kinematics and
to better constrain the origin of the spatial and kinematic asymmetry. This
project focused on obtaining (1) multibandpass CCD observations of 63 roughly
1 degree square fields to extend the star counts to deeper magnitudes and (2)
obtaining high resolution spectra of several thousands stars in these fields
to allow a more extensive investigation of the kinematics of this feature.
By 2008, it was clear the stellar asymmetry had been confirmed by Juric
et al (2008) using the Sloan Digital Sky Survey (SDSS). However, their
interpretation of the excess in the star counts as a ringlike structure is
not supported by critical complementary data in the fourth quadrant, which
is not covered by the SDSS. Therefore, we published a short letter (Larsen,
Humphreys, and Cabanela 2008) to present stellar density maps from the
Minnesota Automated Plate Scanner Catalog of the POSS I showing that the
over density does not extend into the fourth quadrant and therefore the over
density is most probably not a ring. The asymmetry feature was
named the Hercules Thick-Disk Cloud.
Our initial analysis of the star count data focused on searching for a clear signature of triaxiality, an asymmetry in the star counts that extended to higher and higher Galactic longitude. However we found no evidence for an excess of faint blue stars at l≥55° including the faintest magnitude interval. This demonstrated the asymmetry's spatial limits and ruled out a triaxial thick disk as a likely explanation of the excess of Thick Disk stars in Quadrant I (Larsen et al. 2010).
By 2010, we had obtained multi-color UBVR photometry for 1.2 million stars
in 63 fields approximately 1 square degree each. This extensive star count
dataset allowed us to determine the spatial extent of the over density across
and along the line of sight, and estimate the size and mass of the Hercules
Thick Disk Cloud. Using photometric parallaxes we determined the stars responsible
for the excess are between 1 and 6 kiloparsecs from the Sun, 0.5 – 4
kpc above the Galactic plane, and extend approximately 3-4 kiloparsecs across
our line of sight (See Figure 1). This is a major substructure
in the Galaxy. The
distribution of the excess along our sight lines corresponds with the density
contours of the bar in the Disk, and its most distant stars are directly over
the bar. We also see through the Cloud to its far side. Over the
entire 500 square degrees of sky containing the Cloud, we estimate more than
5.6 million stars and 1.9 million solar masses of material. If the over density
is associated with the bar, it would exceed 1.4 billion stars and more than
than 50 million solar masses. (Larsen,
Cabanela, and Humphreys 2011)
By the end of this project we had also obtained radial velocities
and derived metallicity parameters for over 4000 Thick Disk-candidate stars
in Quadrant I (hereafter Q1), above and below the plane and in Quadrant IV
(hereafter Q4) above the plane. Using these spectroscopic observations, we
have confirmed the corresponding kinematic asymmetry first reported by Parker
et al. (2004), extended to greater distances and with more spatial coverage.
The metallicity parameters allowed us to separate the stars by population type:
Halo, Thick Disk, Metal-Weak Thick Disk, and (Thin) Disk stars. The Thick Disk
stars in Q1 have a rotational lag of 60 – 70
km/s relative to circular rotation, and the Metal-Weak Thick Disk stars have
an even greater lag of 100 km/s. Both lag their corresponding populations in
Q4 by about 30 km/s. Interestingly, the Disk stars in Q1 also appear to participate
in the rotational lag by about 30 km/s. The enhanced rotational lag for the
Thick Disk in Q1 extends to 4 kpc or more from the Sun. At 3 to 4 kpc, our
sight lines extend above the density contours on the near side of the bar,
and as our lines of sight pass directly over the bar the rotational lag appears
to decrease (See Figure 2).This is consistent
with a "gravitational wake" induced by the rotating bar in the Disk
which would trap and pile up stars behind it. (Humphreys,
Beers, Cabanela, Grammer, Davidson, Lee, and Larsen 2011)
Summary of Our Conclusions:
When we started this project in 2004, there
were three proposed explanations for the asymmetry in the star counts and kinematics
of the Thick Disk stars: (1) a fossil remnant, (2) a triaxial Thick Disk or
Halo, or (3) a dynamical interaction of the Thick Disk stars with the stellar
bar. Our new deeper star counts across 63 fields extending to higher Galactic
latitude and longitude than previous plate based work, eliminated a Triaxial
Thick Disk as an explanation of the observed star count asymmetry. Our spectroscopic
observations of over 4000 Thick Disk-candidate stars showed the kinematics
of these stars in the asymmetry were strongly tied to their position relative
to the stellar bar of the Galaxy in a way that is consistent with a "gravitational
wake" induced by the stellar bar in the Disk. Based
on our observational evidence, we conclude the best explanation of the Hercules
Thick Disk Cloud is that it is the result of the dynamical interaction of the
Thick Disk with the stellar bar.
Data Products produced by the "Mapping the Asymmetric Thick Disk" Project
We are in the process of placing the final complete data tables from this roject online and expect to have them up in the next few weeks (as of March 21, 2011):
- Photometric Data: All photometric data associated with Paper II except for data in three fields H300-20, H330-20, H315-20, and H295+31
which we are finalizing the U band data on. The U band data was NOT used in Paper II, but we want to make sure it is acceptible before posting the final version of the tables online.
- Table 3 from Paper II: Electronic Catalog of the Thick Disk Asymmetry Project [gzipped ASCII Data File (17537 KB)]
This catalog is also available on a field by field basis:
- Kinematic Data: All kinematic data associated with Paper III (which includes the 741 stars from Parker, Humphreys, and Beers (2004), reprocessed with the same n-SSPP pipeline used for the more modern spectra):
- Table A1 from Paper III: Catalog of Velocities, Metallicity, and Atmospheric Parameters (CCD Photometry) [ASCII Data File (250 KB)]
- Table A2 from Paper III: Catalog of Velocities, Metallicity, and Atmospheric Parameters (MAPS Photometry) [ASCII Data File (201 KB)]
Asymmetric Thick Disk Papers
The following papers are papers directly related to the asymetric thick disk produced by members of this project.
- "A Large Asymmetry in the Galactic Distribution of Faint Halo/Thick Disk Stars"
Larsen, J.A. and Humphreys, R.M. 1996, ApJL, 486, 99L. (ADS abstract page) (PDF)
- "The Asymmetric Thick Disk: A Star-Count and Kinematic Analysis. I. The Star Counts"
Parker, J.E., Humphreys, R.M., and Larsen, J.A. 2003, AJ, 126, 1346. (ADS abstract page) (PDF)
- "The Asymmetric Thick Disk: A Star-Count and Kinematic Analysis. II. The Kinematics"
Parker, J.E., Humphreys, R.M., and Beers, T.C. 2004, AJ, 127, 1567. (ADS abstract page) (PDF)
- "Mapping the Asymmetric Thick Disk: The Hercules Thick-Disk Cloud"
Humphreys, R.M., and Cabanela, J.E. 2008, ApJL, 687, 17L. (ADS abstract page) (ArXiv) (PDF)
- "Mapping the Asymmetric Thick Disk. I. A Search for Triaxiality" (aka Paper I)
Cabanela, J.E., Humphreys, R.M., and Haviland, A.P. 2010, AJ, 139, 348. (ADS abstract page) (ArXiv) (PDF)
- "Mapping the Asymmetric Thick Disk: II Distance, Size and Mass of the Hercules Thick Disk Cloud" (aka Paper II)
Cabanela, J.E., and Humphreys, R.M. 2011, AJ, 141, 130. (ADS abstract page) (ArXiv) (PDF)
- "Mapping the Asymmetric Thick Disk: III. The Kinematics and Interaction with the Galactic Bar" (aka Paper III)
Humphreys, R.M., Beers, T.C., Cabanela, J.E., Grammer, S., Davidson K., Lee, Y.S., and Larsen, J.A..2011 AJ, 141, 131. (ADS abstract page) (ArXiv) (PDF)
The following papers, while not producted by members of this collaboration for this project, provide some additional background information related to the paper.
- "The Automated Plate Scanner Catalog of the Palomar Observatory Sky Survey. II. The Archived Database"
Cabanela, J.E., Humphreys, R.M., Aldering, G., Larsen, J.A., Odewahn, S.C., Thurmes, P.M., and Cornuelle, C.S. 2003, PASP, 115, 837. (ADS abstract page) (PDF)
This paper describes the Minnesota Automated Plate Scanner (MAPS) Catalog, which was the catalog used for the first two papers in the list of "asymmetric thick disk papers."
- "The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars"
Allende Prieto, C., Sivarani, T., Beers, T. C,; Lee, Y.S., Koesterke, L., Shetrone, M., Sneden, C., Lambert, D.L., Wilhelm, R., Rockosi, C.M., Lai, D.K., Yanny, B., Ivans, I.I., Johnson, J.A., Aoki, W., Bailer-Jones, C.A.L, and Re Fiorentin, P. 2008, AJ, 136, 2070. (ADS abstract page) (ArXiv) (PDF)
We used the non-SEGUE Stellar Parameter Pipeline in
Humphreys et al. (2011) in order to obtain metallicity estimates for the stars in our sample. This pipeline uses the same basic algorithms as the SEGUE Stellar Parameter Pipeline, so it is worthwhile to review this paper to understand how the metallicity estimates are made.