Star Formation In M31 Research Articles

Measuring Star Formation Histories, Distances, and Metallicities with Pixel Color–Magnitude Diagrams. II. Applications to Nearby Elliptical Galaxies

Abstract We present spatially resolved measurements of star formation histories (SFHs), metallicities, and distances in three nearby elliptical galaxies and the bulge of M31 derived using the pixel color–magnitude diagram (pCMD) technique. We compute pCMDs from archival Hubble Space Telescope photometry of M87, M49, NGC 3377, and M31, and we fit the data using the new code PCMDPy. We measure distances to each system that are accurate to ∼10%. The recovered nonparametric SFHs place reasonable (±1 dex) constraints on the recent (<2 Gyr) star formation in M31 and NGC 3377, both of which show evidence of inside-out growth. The SFHs in M87 and M49 are constrained only at the oldest ages. The pCMD technique is a promising new avenue for studying the evolutionary history of the nearby universe and is highly complementary to existing stellar population modeling techniques.

Elemental Abundances in M31: A Comparative Analysis of Alpha and Iron Element Abundances in the the Outer Disk, Giant Stellar Stream, and Inner Halo of M31

Abstract We measured [Fe/H] and [α/Fe] using spectral synthesis of low-resolution stellar spectroscopy for 70 individual red-giant-branch stars across four fields spanning the outer disk, Giant Stellar Stream (GSS), and inner halo of M31. Fields at M31-centric projected distances of 23 kpc in the halo, 12 kpc in the halo, 22 kpc in the GSS, and 26 kpc in the outer disk are α-enhanced, with [α/Fe] = 0.43, 0.50, 0.41, and 0.58, respectively. The 23 and 12 kpc halo fields are relatively metal-poor, with [Fe/H] = −1.54 and −1.30, whereas the 22 kpc GSS and 26 kpc outer disk fields are relatively metal-rich with [Fe/H] = −0.84 and −0.92, respectively. For fields with substructure, we separated the stellar populations into kinematically hot stellar halo components and kinematically cold components. We did not find any evidence of a radial [α/Fe] gradient along the high surface brightness core of the GSS between ∼17 and 22 kpc. However, we found tentative suggestions of a negative radial [α/Fe] gradient in the stellar halo, which may indicate that different progenitor(s) or formation mechanisms contributed to the build up of the inner versus outer halo. Additionally, the [α/Fe] distribution of the metal-rich ([Fe/H] > −1.5), smooth inner stellar halo (rproj ≲ 26 kpc) is inconsistent with having formed from the disruption of a progenitor(s) similar to present-day M31 satellite galaxies. The 26 kpc outer disk is most likely associated with the extended disk of M31, where its high α-enhancement provides support for an episode of rapid star formation in M31's disk possibly induced by a major merger.

A Dramatic Decrease in Carbon Star Formation in M31

Abstract We analyze resolved stellar near-infrared photometry of 21 Hubble Space Telescope (HST) fields in M31 to constrain the impact of metallicity on the formation of carbon stars. Observations of nearby galaxies show that carbon stars are increasingly rare at higher metallicity. Models indicate that carbon star formation efficiency drops due to the decrease in dredge-up efficiency in metal-rich thermally pulsing Asymptotic Giant Branch (TP-AGB) stars, coupled to a higher initial abundance of oxygen. However, while models predict a metallicity ceiling above which carbon stars cannot form, previous observations have not yet pinpointed this limit. Our new observations reliably separate carbon stars from M-type TP-AGB stars across 2.6–13.7 kpc of M31's metal-rich disk using HST WFC3/IR medium-band filters. We find that the ratio of C to M stars (C/M) decreases more rapidly than extrapolations of observations in more metal-poor galaxies, resulting in a C/M that is too low by more than a factor of 10 in the innermost fields and indicating a dramatic decline in C star formation efficiency at metallicities higher than [M/H] ≈ −0.1 dex. The metallicity ceiling remains undetected, but must occur at metallicities higher than what is measured in M31's inner disk ([M/H] ≳ +0.06 dex).

Discrete clouds of neutral gas between the galaxies M31 and M33

Spiral galaxies must acquire gas to maintain their observed level of star formation beyond the next few billion years. A source of this material may be the gas that resides between galaxies, but our understanding of the state and distribution of this gas is incomplete. Radio observations of the Local Group of galaxies have revealed hydrogen gas extending from the disk of the galaxy M31 at least halfway to M33. This feature has been interpreted to be the neutral component of a condensing intergalactic filament, which would be able to fuel star formation in M31 and M33, but simulations suggest that such a feature could also result from an interaction between both galaxies within the past few billion years (ref. 5). Here we report radio observations showing that about 50 per cent of this gas is composed of clouds, with the rest distributed in an extended, diffuse component. The clouds have velocities comparable to those of M31 and M33, and have properties suggesting that they are unrelated to other Local Group objects. We conclude that the clouds are likely to be transient condensations of gas embedded in an intergalactic filament and are therefore a potential source of fuel for future star formation in M31 and M33.

The cosmic web in focus

Detection of the trace neutral fraction of hydrogen gas that stretches between the nearby Andromeda and Triangulum galaxies has allowed resolved spectral imaging of this elusive intergalactic medium. See Letter p.224 Previous radio observations have identified neutral hydrogen gas lying between the Local Group galaxies Andromeda (M31) and Triangulum (M33). New data at higher resolution provide a more detailed view of this gas. About half is present as clouds, with the remainder distributed in an extended, diffuse component. The authors propose that the clouds form as transient condensations of gas embedded in an intergalactic filament, and are therefore a potential source of fuel for future star formation in M31 and M33.

AN UPDATED CATALOG OF M31 GLOBULAR-LIKE CLUSTERS:UBVRIPHOTOMETRY, AGES, AND MASSES

We present an updated UBVRI photometric catalog containing 970 objects in the field of M31, selected from the Revised Bologna Catalog (RBC v.4.0), including 965, 967, 965, 953, and 827 sources in the individual UBVRI bands, respectively, of which 205, 123, 14, 126, and 109 objects do not have previously published photometry. Photometry is performed using archival images from the Local Group Galaxies Survey, which covers 2.2 deg^2 along the major axis of M31. We focus on 445 confirmed `globular-like' clusters and candidates, comprising typical globular and young massive clusters. The ages and masses of these objects are derived by comparison of their observed spectral-energy distributions with simple stellar population synthesis. Approximately half of the clusters are younger than 2 Gyr, suggesting that there has been significant recent active star formation in M31, which is consistent with previous results. We note that clusters in the halo (r_ projected>30kpc) are composed of two different components, older clusters with ages >10 Gyr and younger clusters with ages around 1 Gyr. The spatial distributions show that the young clusters (<2 Gyr) are spatially coincident with the galaxy's disk, including the `10 kpc ring,' the `outer ring,' and the halo of M31, while the old clusters (> 2 Gyr) are spatially correlated with the bulge and halo. We also estimate the masses of the 445 confirmed clusters and candidates in M31 and find that our estimates agree well with previously published values. We find that none of the young disk clusters can survive the inevitable encounters with giant molecular clouds in the galaxy's disk and that they will eventually disrupt on timescales of a few Gyr. Specifically, young disk clusters with a mass of 10^4 M_\odot are expected to dissolve within 3.0 Gyr and will, thus, not evolve to become globular clusters.

AN ULTRAVIOLET STUDY OF STAR-FORMING REGIONS IN M31

We present a comprehensive study of star-forming (SF) regions in the nearest large spiral galaxy M31. We use Galaxy Evolution Explorer (GALEX) far-UV (1344-1786 A, FUV) and near-UV (1771-2831 A, NUV) imaging to detect young massive stars and trace the recent star formation across the galaxy. The FUV and NUV flux measurements of the SF regions, combined with ground-based data for estimating the reddening by interstellar dust from the massive stars they contain, are used to derive their ages and masses. The GALEX imaging, combining deep sensitivity and coverage of the entire galaxy, provides a complete picture of the recent star formation in M31 and its variation with environment throughout the galaxy. The FUV and NUV measurements are sensitive to detect stellar populations younger than a few hundred Myr. We detected 894 SF regions, with size ≥ 1600 pc2 above an average FUV flux limit of ~26 ABmag arcsecond–2, over the whole 26 kpc (radius) galaxy disk. We derive the star formation history of M31 within this time span. The star formation rate (SFR) from the youngest UV sources (age ≤ 10 Myr) is comparable to that derived from Hα, as expected. We show the dependence of the results on the assumed metallicity. When star formation detected from IR measurements of the heated dust is added to the UV-measured star formation (from the unobscured populations) in the recent few Myr, we find the SFR has slightly decreased in recent epochs, with a possible peak between 10 and 100 Myr, and an average value of SFR ~0.6 or 0.7 M ☉ yr–1 (for metallicity Z = 0.02 or 0.05, respectively) over the last 400 Myr.

The Period‐Luminosity Relation for Long‐Period Variables in M31

A significant fraction of the disk of M31 has been surveyed for long-period variable stars. We report the results, including near-infrared photometry of almost 2000 variables, and light curves in the i band. The period-luminosity relation suggests that most of the variables are asymptotic giant branch stars, and their luminosity function can be understood in terms of mass-loss rates, which increase with increasing luminosity on the giant branch, and star formation rates, which were a few times higher a billion years ago in M31 than they are today. We see some supergiant long-period variables, but somewhat fewer than expected based on their frequency in M33, and confined to the ring of star formation in M31 seen in the IRAS and 2MASS surveys.

IRAS colors within M31: Evidence for deficiency of very small grains?

Significant differences are found in the IRAS color-color diagrams of small regions (2 min x 2 min, or 0.4 x 1.8 kpc) within the disk of M31 compared to Galactic cirrus, most noticeably demonstrated by a trend of low 60 to 100 micrometer surface brightness ratio and high 12 to 25 micrometer ratio. Based on physical arguments, we conclude that these color differences are best explained by assuming that 'very small grains' (VSG; but not polycylic aromatic hydrocarbons) are only half as abundant in M31 as they are in Galactic cirrus. We confirm this conclusion and test its detailed agreement with data by using the phenomenological model of Desert et al. (1990). In particular, we show that the data cannot be explained by postulating weaker UV heating in the disk of M31. We also show that the VSG-deficient model predicts correctly the correspondence between the IRAS colors and the 100 micrometer emissivity per H I atom in the outer disk of M31. 'Very small grains' are a leading candidate for the carrier of the 2175 A bump in the extinction curve. Our suggested VSG deficiency in M31 is thus consistent with recent Hubble Space Telescope (HST) observations which show evidence for a weaker and narrower 2175 A bump on the M31 extinction curve. Some speculation is offered as to possible links between very small grains and the low rate of current star formation in M31.

On the Rate of Star Formation in M31

view Abstract Citations (31) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Rate of Star Formation in M31 Hartwick, F. D. A. Abstract A correlation has been made between the surface distribution of H ii regions and the neutral hydrogen gas in M3 1 It is found that the density of H ii regions is related to the gas density by the relation PH ii 50+0 12 Publication: The Astrophysical Journal Pub Date: January 1971 DOI: 10.1086/150785 Bibcode: 1971ApJ...163..431H full text sources ADS | data products NED (1)