The Low-Mass Initial Mass Function of Dwarf Galaxy Ursa Minor

Hubble Space Telescope Cycle 19 GO Proposal

Principal Investigator: Mr. Joel Leja Institution: Yale University Electronic Mail: [email protected] Scientific Category: RESOLVED STELLAR POPULATIONS Scientific Keywords: Dwarf Galaxies, Local Group Galaxies, Low-Mass And Cool Stars, Star Formation Instruments: ACS

Proprietary Period: 6

Orbit Request Prime Parallel Cycle 19 11 0

Abstract

IMF deviations are most likely present in extreme environments. The nearby dwarf galaxies are thus a natural laboratory for an IMF investigation. These galaxies are extremely dark-matter dominated (M/L~100) and metal- poor ([Fe/H]~-2), with total masses around 10^9 solar masses [Tolstoy et al., 2009]. Yet, unlike globular clusters, their relaxation timescale is greater than the present age of the universe, so their initial stellar population is preserved against mass segregation. Furthermore, while more extreme environments can be found farther away, dwarf galaxies are unique because they are close enough to resolve individual stars: for the closest few (75 kpc), stars are resolvable down to roughly 0.3 solar masses with Hubble Space Telescope data. This is crucial; the high-mass end of the IMF is well-characterized because bright stars are easy to measure, but the behavior of the low-mass end remains uncertain. These space-based data will allow direct measurement of the low-mass end of the IMF in Local Group dwarf galaxies, a unique and fruitful study that has not been successfully completed before now. Mr. Joel Leja : The Low-Mass Initial Mass Function of Dwarf Galaxy Ursa Minor Investigators:

Investigator Institution Country PI Mr. Joel Leja Yale University USA/CT Number of investigators: 1

Target Summary:

Target RA Dec Magnitude URSA MINOR 15 09 11.3400 +67 12 51.70 V~23.5, main sequence turn-off

Observing Summary:

Target Config Mode and Spectral Elements Flags Orbits URSA MINOR ACS/WFC Imaging F606W 5 URSA MINOR ACS/WFC Imaging F814W 6 Total prime orbits: 11 Scientiﬁc Justiﬁcation

All astronomical knowledge comes from the measurement of photons, and the primary source of these photons is stars. The theory of stellar evolution implies that most physical prop- erties of a star can be derived from its initial mass, which, for a single star, is usually an observable quantity. However, most extragalactic sources of interest are composed of unresolvable stellar populations. In this situation, it becomes necessary to use an initial mass function (IMF) to parametrize the number of stars as a function of mass. An IMF must be assumed before any conclusions may be drawn regarding the composition of unresolved stellar populations. The importance of the IMF cannot be understated– most problems in extragalactic astrophysics can be ”solved” by an appropriate choice of IMF (Bastian et al., 2010). There are strong theoretical reasons to believe the IMF should be a function of the local environment (metallicity, dark matter fraction, magnetic fields), yet strangely, there has been no significant deviation measured since the initial study fifty-five years ago [Salpeter, 1955] until very recently; van Dokkum and Conroy (2011) definitively measured a bottom-heavy IMF in bright elliptical galaxies. This motivates a search in the corresponding direction, the nearby dark-matter dominated dwarf galaxies.

The Perfect Target: There have been hints from ground-based data that the IMF in these dwarf galaxies is irregular. Early analysis of data from the ground-based 3.6m Canada- France-Hawaii Telescope (CFHT) data indicate that the IMF in the ultra-faint dwarf galaxy Coma Berenices diﬀers decisively from the Salpeter IMF (see ﬁgure 2); deeper imaging is required to prove this conclusively. An HST proposal submitted by Prof. Marla Geha is exploring this possibility. To perform a parallel analysis of the regular dwarf galaxies, Ursa Minor is the natural target, as it is the closest at 75 kpc. This proximity is crucial to probing the lower mass limits.

Past Attempts: The sole IMF study published on local dwarf galaxies of any type is Wyse et. al (2002), which compared the mass function of the dwarf galaxy Ursa Minor to two globular clusters using HST data and found no signiﬁcant diﬀerence. There are several issues with this older study that have risen with reexamination. First, the IMF was measured with WFPC2 data down to mF 814W =27.2; the ACS/WFC imaging will go considerably deeper. More importantly, authors assumed a distance of 65 kpc to Ursa Minor, whereas more recent studies of spectroscopic metallicity (Kirby et al., 2010) point towards a distance of 75 kpc. This is crucial because the study hinged on a K-S test between the populations of Ursa Minor and the globular clusters being compared, and the K-S test is highly sensitive to systematic errors in the distance. The authors noted if Ursa Minor was only slightly farther away, the results of this test would reverse- and indeed, this has turned out to be true. This also lowers the mass limit of the test to 0.4 M ,whichnolongerencompassesthelowmass ⊙ stars in question. This is strong motivation for another attempt at this study.

What is Better: There are multiple reasons to believe a second study will have both

1 more accuracy and increased longevity. The measured mass function will not be just compared with existing mass functions, but indeed, with various theoretical models for the IMF. This will rule out the possibility of bias due to mass segregation in the globular clusters. Due to WFC3’s increased sensitivity, the imaging will also deeper by 1.5 magnitudes, which will probe below the 0.4 M limit in question. This represents a substantial improvement ⊙ over the original study. Since a different field of Ursa Minor can be probed with the new pointing, data from the original WFPC2 image can also be scaled and combined with the new pointing in order to enhance the M =0.4-0.7 data. ⊙ Accounting for Binaries: Alargefraction,perhapsamajority,ofstarsareinbinaries. The effect of unresolvable binary companions on the derived luminosity function becomes larger toward lower stellar masses- the range examined in this study- and can alter the derived IMF by a factor of two at the lowest masses. It is possible to mitigate the effect of binaries on one’s conclusion in several ways. First, comparison of results to IMFs derived in other environments without binary corrections, such as globular clusters, is possible, and will result in unbiased results.. Second, one can correct for the effects of binaries making standard assumptions for the binary fraction and mass ratio distribution (Kroupa 1993) in order to infer the true IMF. The standard assumptions are likely to be reasonable even in this extreme environment, as there is little evidence that binary fraction depends on metallicity.

Larger Effect: Confirmed measurement of deviations in the IMF will have a substantial effect on the astronomical community. Outstanding problems in extragalactic astronomy re- quire an assumed IMF in order to even be defined; with a modified IMF, they might change or disappear entirely. For example, the quantity of dark matter inferred from dynamical studies depends upon the stellar mass-to-light ratio; one can ”hide” matter in low-mass stars. Ob- servational evidence for these deviations also open up all sorts of interesting questions in the theory of star formation. The physical origin of deviations will also need to be investigated, and this will reveal much about the physical processes involved in star formation itself. In addition, investigation of the IMF in ultra-faint dwarf galaxies will involve characterizing much about these objects that is currently unknown. Their star-formation history will be an ancillary outcome. Deeper imaging will help to constrain the mass, age, and metallicity of these tiny satellites, which in turn will lead to further confirmation of the existing ΛCDM cosmological model.

References

[1] N. Bastian, K. R. Covey, and M. R. Meyer. Looking for Systematic Variations in the Stellar Initial Mass Function. ArXiv e-prints, November 2010.

[2] E. N. Kirby, P. Guhathakurta, J. D. Simon, M. C. Geha, C. M. Rockosi, C. Sneden, J. G. Cohen, S. T. Sohn, S. R. Majewski, and M. Siegel. Multi-element Abundance

2 Figure 1: A color-magnitude diagram of Ursa Minor in F606W and F814W. The main-sequence cutoﬀis diﬃcult to discern, as many of the brighter stars have been excised; it is V 23.5. ∼

Measurements from Medium-resolution Spectra. II. Catalog of Stars in Milky Way Dwarf Satellite Galaxies. , 191:352–375, December 2010.

[3] P. Kroupa, C. A. Tout, and G. Gilmore. The distribution of low-mass stars in the Galactic disc. , 262:545–587, June 1993.

[4] E. E. Salpeter. The Luminosity Function and Stellar Evolution. , 121:161–+, January 1955.

[5] P. van Dokkum and C. Conroy. Conﬁrmation of Enhanced Dwarf-sensitive Absorption Features in the Spectra of Massive Elliptical Galaxies: Further Evidence for a Non- universal Initial Mass Function. ArXiv e-prints,February2011.

[6] R. F. G. Wyse, G. Gilmore, M. L. Houdashelt, S. Feltzing, L. Hebb, J. S. Gallagher, III, and T. A. Smecker-Hane. Faint stars in the Ursa Minor dwarf spheroidal galaxy: implications for the low-mass stellar initial mass function at high redshift. , 7:395–433, October 2002.

3 Figure 2: Tantalizing 3.6m Canada-France-Hawaii Telescope (CFHT) data indicating a potential IMF deviation in Coma Berenices. However, deeper data are necessary to prove this conclusively. For reference, the Salpeter IMF is at α=-2.3

Description of the Observations

Observations in two filters are required in order to filter out unresolved galaxies and fore- ground stars. F606W and F814W are selected as they bear a strong resemblance to the Johnson V and R bands, which facilitates comparison to other IMF studies. They are also filters broad enough to construct an accurate color-magnitude diagram. The magnitude limit of 27.3 in F814W is chosen as the corresponding detection limit to 28.5 in F606W for an 0.25 M star. ⊙ For a signal-to-noise ratio of 10 and an E(B-V) value of 0.03, the exposure time calcula- tor estimates it will take 12,000 seconds to reach down to a F606W magnitude of 28.5- five magnitude below the main-sequence turnoff- and another 15,000 seconds to reach an F814W magnitude of 27.3. UMi is visible for roughly 60 minutes per orbit: assuming an overhead of 6 minutes for acquisition and 1 minute for readout, this leaves 53 minutes per orbit for science. Thus, I request 5 orbits in F606W and 6 orbits in F814W to reach the desired magnitude limits. Standard dithering techniques will be employed in order to facilitate hot pixel removal and avoid contamination from bad columns.

Image reduction will be performed with the MULTIDRIZZLE package from STSCi. Photom- etry will be performed with DAOphot, and based on the low stellar density of ground-based data, crowding will not be an issue. A luminosity function will be constructed from the photometric data and from it, an IMF will be inferred.

4 Special Requirements

None. Coordinated Observations

None. Justify Duplications

Extant WFPC2 imaging data currently exists in the online archive. These data will be substantially deeper and image a diﬀerent ﬁeld in Ursa Minor, thus preventing direct overlap. Past HST Usage and Current Commitments

Previously performed data reduction and photometry on archival WFPC2 Ursa Minor data.