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Register a new userThe Palomar/MSU Nearby Star Spectroscopic Survey III: Chromospheric Activity, M-dwarf Ages and the Local Star Formation History
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We present high-resolution echelle spectroscopy of 676 nearby M dwarfs. Our measurements include radial velocities, equivalent widths of important chromospheric emission lines, and rotational velocities for rapidly rotating stars. We identify several distinct groups by their H alpha properties, and investigate variations in chromospheric activity amongst early (M0-M2.5) and mid (M3-M6) dwarfs. Using a volume-limited sample together with a relationship between age and chromospheric activity, we show that the rate of star formation in the immediate Solar Neighbourhood has been relatively constant over the last 4 Gyr. In particular our results are inconsistent with recent large bursts of star formation. We use the correlation between H alpha activity and age as a function of colour to set constraints on the properties of L and T dwarf secondary components in binary systems. We also identify a number of interesting stars, including rapid rotators, radial velocity variables, and spectroscopic binaries.
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We investigate the relationship between age and chromospheric activity for 139 M dwarf stars in wide binary systems with white dwarf companions. The age of each system is determined from the cooling age of its white dwarf component. The current limit for activity-age relations found for M dwarfs in open clusters is 4 Gyr. Our unique approach to finding ages for M stars allows for the exploration of this relationship at ages older than 4 Gyr. The general trend of stars remaining active for a longer time at later spectral type is confirmed. However, our larger sample and greater age range reveals additional complexity in assigning age based on activity alone. We find that M dwarfs in wide binaries older than 4 Gyr depart from the log-linear relation for clusters and are found to have activity at magnitudes, colors and masses which are brighter, bluer and more massive than predicted by the cluster relation. In addition to our activity-age results, we present the measured radial velocities and complete space motions for 161 white dwarf stars in wide binaries.
The Local Group (LG) hosts many dwarf galaxies with diverse physical characteristics in terms of morphology, mass, star formation, and metallicity. To this end, LG can offer a unique site to tackle questions about the formation and evolution of galaxies by providing detailed information. While large telescopes are often the first choices for such studies, small telescope surveys that perform dedicated observations are still important, particularly in studying bright objects in the nearby universe. In this regard, we conducted a nine epoch survey of 55 dwarf galaxies called the Local Group dwarf galaxies survey using the 2.5m Isaac Newton Telescope (INT) in La Palma to identify Long-Period Variable (LPV) stars, namely Asymptotic Giant Branch (AGB) and Red Super Giant (RSG) stars. AGB stars formed at different times and studying their radial distribution and mass-loss rate can shed light on the structure formation in galaxies. To further investigate the evolutionary path of these galaxies, we construct their star formation history (SFH) using the LPV stars, which are at the final stages of their evolution and therefore experience brightness fluctuations on the timescales between hundred to thousand days. In this paper, we present some of the results of the Local Group dwarf galaxies survey.
We present multi-wavelength global star formation rate (SFR) estimates for 326 galaxies from the Star Formation Reference Survey (SFRS) in order to determine the mutual scatter and range of validity of different indicators. The widely used empirical SFR recipes based on 1.4 GHz continuum, 8.0 $mu$m polycyclic aromatic hydrocarbons (PAH), and a combination of far-infrared (FIR) plus ultraviolet (UV) emission are mutually consistent with scatter of $raise{-0.8ex}stackrel{textstyle <}{sim }$0.3 dex. The scatter is even smaller, $raise{-0.8ex}stackrel{textstyle <}{sim }$0.24 dex, in the intermediate luminosity range 9.3<log(L(60 $mu$m/L$_odot$)<10.7. The data prefer a non-linear relation between 1.4 GHz luminosity and other SFR measures. PAH luminosity underestimates SFR for galaxies with strong UV emission. A bolometric extinction correction to far-ultraviolet luminosity yields SFR within 0.2 dex of the total SFR estimate, but extinction corrections based on UV spectral slope or nuclear Balmer decrement give SFRs that may differ from the total SFR by up to 2 dex. However, for the minority of galaxies with UV luminosity ${>}5times10^9$ L$_{odot}$ or with implied far-UV extinction <1 mag, the UV spectral slope gives extinction corrections with 0.22~dex uncertainty.
We have obtained deep images of the highly isolated (d = 1 Mpc) Aquarius dwarf irregular galaxy (DDO 210) with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The resulting color-magnitude diagram (CMD) reaches more than a magnitude below the oldest main-sequence turnoff, allowing us to derive the star formation history (SFH) over the entire lifetime of the galaxy with a timing precision of ~10% of the lookback time. Using a maximum likelihood fit to the CMD we find that only ~10% of all star formation in Aquarius took place more than 10 Gyr ago (lookback time equivalent to redshift z ~2). The star formation rate increased dramatically ~6-8 Gyr ago (z ~ 0.7-1.1) and then declined until the present time. The only known galaxy with a more extreme confirmed delay in star formation is Leo A, a galaxy of similar M(HI)/M(stellar), dynamical mass, mean metallicity, and degree of isolation. The delayed stellar mass growth in these galaxies does not track the mean dark matter accretion rate from CDM simulations. The similarities between Leo A and Aquarius suggest that if gas is not removed from dwarf galaxies by interactions or feedback, it can linger for several gigayears without cooling in sufficient quantity to form stars efficiently. We discuss possible causes for the delay in star formation including suppression by reionization and late-time mergers. We find reasonable agreement between our measured SFHs and select cosmological simulations of isolated dwarfs. Because star formation and merger processes are both stochastic in nature, delayed star formation in various degees is predicted to be a characteristic (but not a universal) feature of isolated small galaxies.
We have determined the O/H and N/O of a sample of 122751 SFGs from the DR7 of the SDSS. For all these galaxies we have also determined their morphology and their SFH using the code STARLIGHT. The comparison of the chemical abundance with the SFH allows us to describe the chemical evolution in the nearby universe (z < 0.25) in a manner which is consistent with the formation of their stellar populations and morphologies. A 45% of the SFGs in our sample show an excess of abundance in nitrogen relative to their metallicity. We also find this excess to be accompanied by a deficiency of oxygen, which suggests that this could be the result of effective starburst winds. However, we find no difference in the mode of star formation of the nitrogen rich and nitrogen poor SFGs. Our analysis suggests they all form their stars through a succession of bursts of star formation extended over a few Gyr period. What produces the chemical differences between these galaxies seems therefore to be the intensity of the bursts: the galaxies with an excess of nitrogen are those that are presently experiencing more intense bursts, or have experienced more intense bursts in their past. We also find evidence relating the chemical evolution process to the formation of the galaxies: the galaxies with an excess of nitrogen are more massive, have more massive bulges and earlier morphologies than those showing no excess. As a possible explanation we propose that the lost of metals consistent with starburst winds took place during the formation of the galaxies, when their potential wells were still building up, and consequently were weaker than today, making starburst winds more efficient and independent of the final mass of the galaxies. In good agreement with this interpretation, we also find evidence consistent with downsizing, according to which the more massive SFGs formed before the less massive ones.
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