And the Rest: The Stellar Archeological Record of M82 Outside the Central Starburst

Abstract

Deep images obtained with MegaCam and WIRCam on the Canada-France-Hawaii Telescope are used to probe the stellar content outside of the central star-forming regions of M82. Stars evolving on the asymptotic giant branch (AGB) are traced along the major axis out to projected distances of 12 kpc, which corresponds to 13 disk scale lengths. The numbers of red supergiants (RSGs) and AGB stars normalized to local surface brightness (the "specific frequency" or SF) is constant when R GC > 4 kpc, indicating that RSGs and AGB stars are well mixed throughout the disk. Moreover, the SF of bright AGB stars in the outer disks of M82 and the Sc galaxy NGC 2403 are identical, suggesting that the specific star-formation rates (SFRs) in these galaxies during intermediate epochs were similar. This similarity in stellar content, coupled with the presence of an extended stellar disk, is consistent with M82 having been a late-type disk galaxy prior to interacting with M81. Still, there is a paucity of RSGs in the outer disk of M82 when compared with NGC 2403, indicating that the SFR in the outer regions of M82 during the past ∼0.1 Gyr has declined dramatically with respect to that in isolated late-type galaxies. The stellar content of the M82 disk plane is also investigated. A mixture of bright main-sequence stars, RSGs, and AGB stars is detected out to minor-axis distances of 7 kpc. These stars, which span a range of ages, are concentrated along the outflow. The brightest extraplanar AGB stars define a system with an exponential scale height of 1.8 ± 0.2 kpc, as measured along the minor axis. It is suggested that the young-and intermediate-aged stars in the extraplanar regions formed in structures similar to M82 South, and that these were subsequently disrupted by the tidal action of M82.

Resolving the Stellar Outskirts of M81: Evidence for a Faint, Extended Structural Component

“…However, the ages and metallicities were very uncertain because of the unknown star formation histories (SFHs), the shallow depth of the nearinfrared images, and the inherent difficulties of sky-subtracting and flat-fielding in the infrared. Further analysis by Yoachim & Dalcanton (2008b, 2008a has indeed revealed a variety of thick disk kinematics and compositions amongst the low-mass galaxies in the Dalcanton & Bernstein (2002) sample. The properties of the higher-mass galaxies' thick disks were more difficult to constrain because their thin disk light was too dominant in the regions with adequate signal.…”

“…This means going beyond the Local Group, a task that has only recently become possible thanks to the advent of wide-field imagers on large telescopes (e.g., BlandHawthorn et al 2005;Davidge 2008aDavidge , 2008b. At a distance of 3.6 Mpc, corresponding to (m − M) 0 = 27.78 (Freedman et al 1994), M81 is the nearest massive spiral like the MW and M31, yet it is still close enough to resolve its low-mass giant stars.…”

The apparent “reversed” motion of gas and stars in M 82

“…1b), but the high velocities of the x 2 -orbits are not seen in their observation 2 . As also noticed by Davidge (2008), the flat or gradually increasing stellar radial velocity curve shown in Fig. 1b may contradict the declining H i and CO velocities derived by Sofue (1998), although the spatial coverage of the near-IR observation is too small to arrive at a firm conclusion.…”

“…However, the opinion that the CO and H i at large radial distances may not yield true rotational velocities was, for instance, brought up by Whitmore et al (1984), Davidge (2008), and Mayya & Carrasco (2009), with the authors pointing out that the true tangential velocities representing the kinematically dominant mass of the bar, bulge, and stellar disk may only be measurable in stellar absorption lines. Since the Balmer stellar absorption lines may be influenced by ionized gas Balmer emission lines, an uncontaminated measurement exploring the rotation of the outer region of the disk should be based on the near-IR stellar Ca ii absorption lines or, with some reservation because of interfering interstellar absorption (Table A.1), on the blue Ca ii stellar absorption lines.…”

“…Despite this encounter, on deep images and photometric tracings (e.g. Devine & Bally 1999;Ohyama et al 2002;Mayya et al 2005;Ichikawa et al 1995) the nearly edge-on stellar body of M 82 extends ∼10 kpc and ∼3 kpc along the major and minor axes, respectively, with a surprisingly regular IR light distribution of the stars, with no warps (Davidge 2008) and tidal extensions seen in H i and CO. Rotation velocity studies of the ionized gas and stars at optical and near-IR wavelengths extend along the major axis only to ∼120−160 (∼2−3 kpc) 1 from the center so that the rotation of the intermediate and outer regions of M 82 is poorly known, while outside the nuclear region the H i and CO gas shows a highly disturbed distribution and motion (Yun et al 1993(Yun et al , 1994Brouillet et al 1991;Taylor et al 2001;Walter et al 2002) that may not be representative of the post-encounter bulge and disk.…”