near-IR Color-magnitude Diagrams Research Articles

A New Identity Card for the Bulge Globular Cluster NGC 6440 from Resolved Star Counts*

Abstract We present a new identity card for the cluster NGC 6440 in the Galactic Bulge. We have used a combination of high-resolution Hubble Space Telescope images, wide-field ground-based observations performed with the ESO-FORS2, and the public survey catalog Pan-STARRS to determine the gravitational center, projected density profile, and structural parameters of this globular from resolved star counts. The new determination of the cluster center differs by ∼2″ (corresponding to 0.08 pc) from the previous estimate, which was based on the surface brightness peak. The star density profile, extending out to 700″ from the center and suitably decontaminated from the Galactic field contribution, is best fit by a King model with a significantly higher concentration (c = 1.86 ± 0.06) and smaller core radius (r c = 6.″4 ± 0.″3) with respect to the literature values. By taking advantage of high-quality optical and near-IR color–magnitude diagrams, we also estimated the cluster age, distance, and reddening. The luminosity of the red giant branch bump was also determined. This study indicates that the extinction coefficient in the bulge in the direction of the cluster has a value (R V = 2.7) that is significantly lower than that traditionally used for the Galaxy (R V = 3.1). The corresponding best-fit values of the age, distance, and color excess of NGC 6440 are 13 Gyr, 8.3 kpc, and E(B − V) ∼ 1.27. These new determinations also allowed us to update the values of the central (t rc = 2.5 107 yr) and half-mass (t rh = 109 yr) relaxation times, suggesting that NGC 6440 is in a dynamically evolved stage.

Anomalous Stellar Populations in Low-surface-brightness Galaxies

Abstract We present new Hubble Space Telescope WFC3 near-IR observations of the color–magnitude diagrams (CMDs) in two low-surface-brightness galaxies, F575-3 and F615-1, notable for having no current star formation based on a lack of Hα emission. Key features of the near-IR CMDs are resolved, such as the red giant branch (RGB), the asymptotic giant branch (AGB) region, and the top of the blue main sequence. F575-3 has the bluest RGB of any CMD in the literature, indicating an extremely low mean metallicity. F615-1 has unusually wide RGB and AGB sequences, suggesting multiple episodes of star formation from metal-poor gas, and possibly infalling material. Both galaxies have an unusual population of stars to the red of the RGB and lower in luminosity than typical AGB stars. These stars have normal optical colors but abnormal near-IR colors. We suggest that this population of stars might be analogous to local peculiar stars like Be stars with strong near-IR excesses owing to a surrounding disk of hot gas.

VVV Survey Microlensing: Candidate Events with a Source in the Far Disk

Abstract The VVV microlensing search has recently surveyed the region of the Galactic plane at b = 0 within −10.00 ≤ l ≤ 10.44 deg. in the near-infrared (IR) discovering hundreds of microlensing events. In this paper we explore the microlensing events with background sources that might be located in the far disk of the Galaxy, beyond the bulge. We discuss the possible configurations for the microlensing lenses and sources located at different places within the Galactic plane. Then we search for these events using the local red clump centroids of the VVV near-IR color–magnitude diagrams. According to the estimated distances and proper motions, N = 20 events may have sources located in the far disk. The candidates for far-disk sources show on average longer timescales (t E = 49.3 ± 7.9 days) than the mean of the timescale distribution for the bulge red clump sources (t E = 36.4 ± 1.1 days). We conclude that the population of microlensing events in the region −10.00 ≤ l ≤ 10.44, −0.46 ≤ b ≤ 0.65 deg. contains a non-negligible number of events with candidate far-disk sources (∼11%). Our results are relevant in view of future microlensing plans with the Roman Space Telescope (formerly WFIRST) in the near-IR.

The First Galaxy Cluster Discovered by the VISTA Variables in the Vía Láctea Survey

Abstract We report the first confirmed detection of the galaxy cluster VVV-J144321-611754 at very low latitudes (l = 315.°836, b = −1.°650) located in the tile d015 of the VISTA Variables in the Vía Láctea (VVV) survey. We defined the region of 30 × 30 arcmin2 centered in the brightest galaxy finding 25 galaxies. For these objects, extinction-corrected median colors of (H−K s ) = 0.34 ± 0.05 mag, (J−H) = 0.57 ± 0.08 mag, and (J−K s ) = 0.87 ± 0.06 mag; R 1/2 = 1.59 ± 0.″16; C = 3.01 ± 0.08; and Sérsic index n = 4.63 ± 0.39 were estimated. They were visually confirmed showing characteristics of early-type galaxies in the near-IR images. An automatic clustering analysis performed in the whole tile found that the concentration of galaxies VVV-J144321-611754 is a real, compact concentration of early-type galaxies. Assuming a typical galaxy cluster with low X-ray luminosity, the photometric redshift of the brightest galaxy is z = 0.196 ± 0.025. Follow-up near-IR spectroscopy with FLAMINGOS-2 at the Gemini-South telescope revealed that the two brighter cluster galaxies have typical spectra of early-type galaxies and the estimated redshift for the brightest galaxy VVV-J144321.06-611753.9 is z = 0.234 ± 0.022 and that for VVV-J144319.02-611746.1 is z = 0.232 ± 0.019. Finally, these galaxies clearly follow the cluster red sequence in the rest-frame near-IR color–magnitude diagram with a slope similar to a galaxy cluster at a redshift of 0.2. These results are consistent with the presence of a bona fide galaxy cluster beyond the Milky Way disk.

Confirmation of a New Metal-poor Globular Cluster in the Galactic Bulge∗

Abstract We use deep near-IR photometry of the VISTA Variables in the Vía Láctea (VVV) Survey and deep DECam Plane Survey (DECaPS) optical photometry to confirm the physical reality of the candidate globular cluster (GC) Minni 22, which is located in the Galactic bulge. This object, which was detected as a high density region in our maps of bulge red giants, is now confirmed as a real GC based on the optical and near-IR color–magnitude diagrams. We also recover three known fundamental mode (ab type) RR Lyrae stars within 2 arcmin of the cluster center. The presence of RR Lyrae stars also seems to confirm Minni 22 as a bonafide old and metal-poor GC. We estimate a cluster reddening mag and determine its heliocentric distance D = 7.4 ± 0.3 kpc. The optical and near-IR color–magnitude diagrams reveal well-defined red giant branches in all cases, including a red giant branch bump at Ks = 13.30 ± 0.05 mag. The comparison with theoretical isochrones yields a mean metallicity of [Fe/H] = −1.3 ± 0.3 dex, and age of t ∼ 11.2 Gyr. This is a good example of a new low-luminosity (M V = −6.2 mag) GC found in the central bulge of the Milky Way. After discussing the different ways to confirm the existence of bulge GC candidates, we find that one of the best methods is to use the CMDs from the combination of the DECaPS + VVV photometries.

VVV Survey Microlensing: The Galactic Longitude Dependence

Abstract We completed the search for microlensing events in the zero latitude area of the Galactic bulge using the VVV Survey near-infrared (near-IR) data obtained between 2010 and 2015. We have now a total sample of N = 630 events Using the near-IR color–magnitude diagram we selected the red clump (RC) sources to analyze the longitude dependence of microlensing across the central region of the Galactic plane. The events show a homogeneous distribution, smoothly increasing in numbers toward the Galactic center, as predicted by different models. We find a slight asymmetry, with a larger number of events toward negative longitudes than positive longitudes. This asymmetry is seen both in the complete sample and the subsample of RC giant sources, and it is possibly related with the inclination of the bar along the line of sight. The timescale distribution is fairly symmetric with a peak in 17.4 ± 1.0 days for the complete sample (N = 630 events), and 20.7 ± 1.0 days for the RC stars (N = 291 events), in agreement with previous results.

New VVV Survey Globular Cluster Candidates in the Milky Way Bulge*

Abstract It is likely that a number of Galactic globular clusters remain to be discovered, especially toward the Galactic bulge. High stellar density combined with high and differential interstellar reddening are the two major problems for finding globular clusters located toward the bulge. We use the deep near-IR photometry of the VISTA Variables in the Vía Láctea (VVV) Survey to search for globular clusters projected toward the Galactic bulge, and hereby report the discovery of 22 new candidate globular clusters. These objects, detected as high density regions in our maps of bulge red giants, are confirmed as globular cluster candidates by their color–magnitude diagrams. We provide their coordinates as well as their near-IR color–magnitude diagrams, from which some basic parameters are derived, such as reddenings and heliocentric distances. The color–magnitude diagrams reveal well defined red giant branches in all cases, often including a prominent red clump. The new globular cluster candidates exhibit a variety of extinctions (0.06 < A Ks < 2.77) and distances (5.3 < D < 9.5 kpc). We also classify the globular cluster candidates into 10 metal-poor and 12 metal-rich clusters, based on the comparison of their color–magnitude diagrams with those of known globular clusters also observed by the VVV Survey. Finally, we argue that the census for Galactic globular clusters still remains incomplete, and that many more candidate globular clusters (particularly the low luminosity ones) await to be found and studied in detail in the central regions of the Milky Way.

FSR 1716: A New Milky Way Globular Cluster Confirmed Using VVV RR Lyrae Stars

Abstract We use deep multi-epoch near-IR images of the VISTA Variables in the Vía Láctea (VVV) Survey to search for RR Lyrae stars toward the Southern Galactic plane. Here, we report the discovery of a group of RR Lyrae stars close together in VVV tile d025. Inspection of the VVV images and PSF photometry reveals that most of these stars are likely to belong to a globular cluster that matches the position of the previously known star cluster FSR 1716. The stellar density map of the field yields a >100σ detection for this candidate globular cluster that is centered at equatorial coordinates R.A. J2000 = 16:10:30.0, decl. J2000 = −53:44:56 and galactic coordinates l = 329.77812, b = −1.59227. The color–magnitude diagram of this object reveals a well-populated red giant branch, with a prominent red clump at K s = 13.35 ± 0.05, and J − K s = 1.30 ± 0.05. We present the cluster RR Lyrae positions, magnitudes, colors, periods, and amplitudes. The presence of RR Lyrae indicates an old globular cluster, with an age >10 Gyr. We classify this object as an Oosterhoff type I globular cluster, based on the mean period of its RR Lyrae type ab, days, and argue that this is a relatively metal-poor cluster with [Fe/H] = −1.5 ± 0.4 dex. The mean extinction and reddening for this cluster are and E(J − K s ) = 0.72 ± 0.02 mag, respectively, as measured from the RR Lyrae colors and the near-IR color–magnitude diagram. We also measure the cluster distance using the RR Lyrae type ab stars. The cluster mean distance modulus is (m − M)0 = 14.38 ± 0.03 mag, implying a distance D = 7.5 ± 0.2 kpc and a Galactocentric distance R G = 4.3 kpc.

DEEP NEAR-IR OBSERVATIONS OF THE GLOBULAR CLUSTER M4: HUNTING FOR BROWN DWARFS

ABSTRACT We present an analysis of deep Hubble Space Telescope (HST)/Wide Field Camera 3 near-IR (NIR) imaging data of the globular cluster (GC) M4. The best-photometry NIR color–magnitude diagram (CMD) clearly shows the main sequence extending toward the expected end of the hydrogen-burning limit and going beyond this point toward fainter sources. The white dwarf (WD) sequence can be identified. As such, this is the deepest NIR CMD of a GC to date. Archival HST optical data were used for proper-motion cleaning of the CMD and for distinguishing the WDs from brown dwarf (BD) candidates. Detection limits in the NIR are around F110W ≈ 26.5 mag and F160W ≈ 27 mag, and in the optical around F775W ≈ 28 mag. Comparing our observed CMDs with theoretical models, we conclude that we have reached beyond the H-burning limit in our NIR CMD and are probably just above or around this limit in our optical–NIR CMDs. Thus, any faint NIR sources that have no optical counterpart are potential BD candidates, since the optical data are not deep enough to detect them. We visually inspected the positions of NIR sources that are fainter than the H-burning limit in F110W and for which the optical photometry did not return a counterpart. We found in total five sources for which we did not get an optical measurement. For four of these five sources, a faint optical counterpart could be visually identified, and an upper optical magnitude was estimated. Based on these upper optical magnitude limits, we conclude that one source is likely a WD, one source could be either a WD or BD candidate, and the remaining two sources agree with being BD candidates. No optical counterpart could be detected for just one source, which makes this source a good BD candidate. We conclude that we found in total four good BD candidates.

THE INFRARED EYE OF THE WIDE-FIELD CAMERA 3 ON THE HUBBLE SPACE TELESCOPE REVEALS MULTIPLE MAIN SEQUENCES OF VERY LOW MASS STARS IN NGC 2808

We use images taken with the infrared channel of the Wide Field Camera 3 on the Hubble Space Telescope (HST) to study the multiple main sequences (MSs) of NGC 2808. Below the turn off, the red, the middle, and the blue MS, previously detected from visual-band photometry, are visible over an interval of about 3.5 F160W magnitudes. The three MSs merge together at the level of the MS bend. At fainter magnitudes, the MS again splits into two components containing ~65% and ~35% of stars, with the most-populated MS being the bluest one. Theoretical isochrones suggest that the latter is connected to the red MS discovered in the optical color-magnitude diagram (CMD), and hence corresponds to the first stellar generation, having primordial helium and enhanced carbon and oxygen abundances. The less-populated MS in the faint part of the near-IR CMD is helium-rich and poor in carbon and oxygen, and it can be associated with the middle and the blue MS of the optical CMD. The finding that the photometric signature of abundance anticorrelation are also present in fully convective MS stars reinforces the inference that they have a primordial origin.

THE MASSIVE STAR-FORMING REGION CYGNUS OB2. II. INTEGRATED STELLAR PROPERTIES AND THE STAR FORMATION HISTORY

Cygnus OB2 is the nearest example of a massive star forming region, containing over 50 O-type stars and hundreds of B-type stars. We have analysed the properties of young stars in two fields in Cyg OB2 using the recently published deep catalogue of Chandra X-ray point sources with complementary optical and near-IR photometry. Our sample is complete to 1 Msun (excluding A and B-type stars that do not emit X-rays), making this the deepest study of the stellar properties and star formation history in Cyg OB2 to date. From Siess et al. (2000) isochrone fits to the near-IR color-magnitude diagram, we derive ages of 3.5 (+0.75/-1.0) and 5.25 (+1.5/-1.0) Myrs for sources in the two fields, both with considerable spreads around the pre-MS isochrones. The presence of a stellar population somewhat older than the present-day O-type stars, also fits in with the low fraction of sources with inner circumstellar disks (as traced by the K-band excess) that we find to be very low, but appropriate for a population of age ~5 Myrs. We also find that the region lacks a population of highly embedded sources that is often observed in young star forming regions, suggesting star formation in the vicinity has declined. We measure the stellar mass functions in this limit and find a power-law slope of Gamma = -1.09 +/- 0.13, in good agreement with the global mean value estimated by Kroupa (2002). A steepening of the slope at higher masses is observed and suggested as due to the presence of the previous generation of stars that have lost their most massive members. Finally, combining our mass function and an estimate of the radial density profile of the association suggests a total mass of Cyg OB2 of ~3 x 10^4 Msun, similar to that of many of our Galaxy's most massive star forming regions.

THE ASYMPTOTIC GIANT BRANCH AND THE TIP OF THE RED GIANT BRANCH AS PROBES OF STAR FORMATION HISTORY: THE NEARBY DWARF IRREGULAR GALAXY KKH 98

We investigate the utility of the asymptotic giant branch (AGB) and the red giant branch (RGB) as probes of the star formation history (SFH) of the nearby (D=2.5 Mpc) dwarf irregular galaxy, KKH 98. Near-infrared (IR) Keck Laser Guide Star Adaptive Optics (AO) images resolve 592 IR bright stars reaching over 1 magnitude below the Tip of the Red Giant Branch. Significantly deeper optical (F475W and F814W) Hubble Space Telescope images of the same field contain over 2500 stars, reaching to the Red Clump and the Main Sequence turn-off for 0.5 Gyr old populations. Compared to the optical color magnitude diagram (CMD), the near-IR CMD shows significantly tighter AGB sequences, providing a good probe of the intermediate age (0.5 - 5 Gyr) populations. We match observed CMDs with stellar evolution models to recover the SFH of KKH 98. On average, the galaxy has experienced relatively constant low-level star formation (5 x 10^-4 Mo yr^-1) for much of cosmic time. Except for the youngest main sequence populations (age < 0.1 Gyr), which are typically fainter than the AO data flux limit, the SFH estimated from the the 592 IR bright stars is a reasonable match to that derived from the much larger optical data set. Differences between the optical and IR derived SFHs for 0.1 - 1 Gyr populations suggest that current stellar evolution models may be over-producing the AGB by as much as a factor of three in this galaxy. At the depth of the AO data, the IR luminous stars are not crowded. Therefore these techniques can potentially be used to determine the stellar populations of galaxies at significantly further distances.

The Star Formation and Extinction Coevolution of UV‐Selected Galaxies over 0.05 &lt; z &lt; 1.2

We use a new stacking technique to obtain mean mid-IR and far-IR to far-UV flux ratios over the rest-frame near-UV, near-IR color-magnitude diagram. We employ COMBO-17 redshifts and COMBO-17 optical, GALEX far- and near-UV, and Spitzer IRAC and MIPS mid-IR photometry. This technique permits us to probe the infrared excess (IRX), the ratio of far-IR to far-UV luminosity, and the specific star formation rate (SSFR) and their coevolution over 2 orders of magnitude of stellar mass and over redshift 0.1 < z < 1.2. We find that the SSFR and the characteristic mass (M_0) above which the SSFR drops increase with redshift (downsizing). At any given epoch, the IRX is an increasing function of mass up to M_0. Above this mass the IRX falls, suggesting gas exhaustion. In a given mass bin below M_0, the IRX increases with time in a fashion consistent with enrichment. We interpret these trends using a simple model with a Schmidt-Kennicutt law and extinction that tracks gas density and enrichment. We find that the average IRX and SSFR follow a galaxy age parameter ξ, which is determined mainly by the galaxy mass and time since formation. We conclude that blue-sequence galaxies have properties which show simple, systematic trends with mass and time such as the steady buildup of heavy elements in the interstellar media of evolving galaxies and the exhaustion of gas in galaxies that are evolving off the blue sequence. The IRX represents a tool for selecting galaxies at various stages of evolution.

Red-giant branch morphology of metal-poor globular clusters in the Galactic bulge

AbstractUsing the (J − K, K) color-magnitude diagrams (CMDs) of 16 metal-poor globular clusters in the Galactic bulge, we investigate the morphological properties of their red-giant branch (RGB), comparing with those of metal-rich clusters in the Galactic bulge and metal-poor clusters in the Galactic halo. The RGB morphological parameters, such as colors at fixed magnitudes, magnitudes at a fixed color, the RGB slope, and a difference of color indices at two fixed magnitudes have been derived from the near-IR CMDs for each cluster. Metal-poor Galactic bulge clusters follow the previous empirical relations between colors at fixed magnitudes and magnitudes at a fixed color of the RGB and the cluster's metallicity. However, the RGB slope and the color difference parameters of some bulge clusters deviate slightly from the previous empirical linear relations for the other globular clusters, implying that the metal-poor bulge clusters may have different formation origin from the other globular clusters in the Galaxy.

Near-Infrared Properties of 24 Globular Clusters in the Galactic Bulge

We present near-IR color-magnitude diagrams and physical parameters for a sample of 24 Galactic globular clusters toward the bulge. In this paper we discuss the properties of 12 new clusters (out of the 24) in addition to those previously studied and published by our group. The compilation includes measurements of the cluster reddening, distance, photometric metallicity, horizontal branch red clump, and red giant branch morphological (e.g., mean ridgelines) and evolutionary (e.g., bump and tip) features. The compilation is available in electronic form on the World Wide Web, and it will be updated regularly.

An Empirical Tool to Derive Metallicity, Reddening, and Distance for Old Stellar Populations from Near‐Infrared Color‐Magnitude Diagrams

We present an empirical method to derive photometric metallicity, reddening, and distance to old stellar populations by using a few major features of the red giant branch (RGB) in near-IR color-magnitude diagrams. We combine the observed RGB features with a set of equations linking the global metallicity [M/H] to suitable RGB parameters (colors, magnitudes, and slope), as calibrated from a homogeneous sample of Galactic globular clusters with different metallicities. This technique can be applied to efficiently derive the main population parameters of old stellar systems, with the goal of using ground-based adaptive optics and space facilities to probe the stellar content of remote galaxies.

Evolutionary models for cool brown dwarfs and extrasolar giant planets. The case of HD 209458

We present evolutionary models for cool brown dwarfs and extra-solar giant planets. The models reproduce the main trends of observed methane dwarfs in near-IR color-magnitude diagrams. We also present evolutionary models for irradiated planets, coupling for the first time irradiated atmosphere profiles and inner structures. We focus on HD 209458-like systems and show that irradiation effects can substantially affect the radius of sub-jovian mass giant planets. Irradiation effects, however, cannot alone explain the large observed radius of HD 209458b. Adopting assumptions which optimise irradiation effects and taking into account the extension of the outer atmospheric layers, we still find ~20% discrepancy between observed and theoretical radii. An extra source of energy seems to be required to explain the observed value of the first transit planet.

Magellanic Cloud Structure from Near-Infrared Surveys. I. The Viewing Angles of the Large Magellanic Cloud

We present a detailed study of the viewing angles of the LMC disk plane. We find that our viewing direction differs considerably from the commonly accepted values, which has important implications for the structure of the LMC. The discussion is based on an analysis of spatial variations in the apparent magnitude of features in the near-IR color-magnitude diagrams extracted from the Deep Near-Infrared Southern Sky Survey (DENIS) and Two Micron All-Sky Survey (2MASS). Sinusoidal brightness variations with a peak-to-peak amplitude of ~0.25 mag are detected as a function of position angle. The same variations are detected for asymptotic giant branch stars (using the mode of their luminosity function) and for red giant branch stars (using the tip of their luminosity function), and these variations are seen consistently in all of the near-IR photometric bands in both DENIS and 2MASS data. The observed spatial brightness variations are naturally interpreted as the result of distance variations because of one side of the LMC plane being closer to us than the opposite side. There is no evidence that any complicating effects, such as possible spatial variations in dust absorption or the age/metallicity of the stellar population, cause large-scale brightness variations in the near-IR at a level that exceeds the formal errors (~0.03 mag). The best-fitting geometric model of an inclined plane yields an inclination angle i = 347 ? 62 and line-of-nodes position angle ? = 1225 ? 83. The quoted errors are conservative estimates that take into account the possible influence of systematic errors; the formal errors are much smaller, 07 and 16, respectively. There is tentative evidence for variations of ~10? in the viewing angles with distance from the LMC center, suggesting that the LMC disk plane may be warped. Traditional methods to estimate the position angle of the line of nodes have used either the major-axis position angle ?maj of the spatial distribution of tracers on the sky or the position angle ?max of the line of maximum gradient in the velocity field, given that for a circular disk ?maj = ?max = ?. The present study does not rely on the assumption of circular symmetry and is considerably more accurate than previous studies of its kind. We find that the actual position angle of the line of nodes differs considerably from both ?maj and ?max, for which measurements have fallen in the range 140??190?. This indicates that the intrinsic shape of the LMC disk is not circular but elliptical. Paper II of this series explores the implications of this result through a detailed study of the shape and structure of the LMC. The inclination angle inferred here is consistent with previous estimates, but this is to some extent a coincidence, given that also for the inclination angle most previous estimates were based on the incorrect assumption of circular symmetry.

The Star Formation Histories of Blue Compact Galaxies: Where are the GAPs?

The star formation histories of Blue Compact Dwarf (BCD) galaxies are commonly viewed as episodic: short starbursts separated by long quiescent periods. However, the near-IR color magnitude diagrams of three nearby type `iE' BCDs obtained with the NICMOS camera aboard HST do not exhibit gaps in star formation. Stars are instead found across all mass ranges and in all of the major phases of stellar evolution (which are above our detection limits). To overcome the limitations of our data, i.e., photometric errors and incompleteness, we investigate episodic modes of star formation with the help of simulations. As a case study, we here provide simulated star formation histories of I Zw 36. We show that if we allow short stops (10–100 Myr) in star formation we can still produce acceptable color magnitude diagrams (CMD) which match the data. Once, however, the formation of stars is interruped by several100 Myr, the morphology of the CMD changes so drastically that we conclude long gaps cannot be hidden in the data.

A Near-Infrared Stellar Census of Blue Compact Dwarf Galaxies: NICMOS Detection of Red Giant Stars in the Wolf-Rayet Galaxy Markarian 178

We observed the blue compact dwarf/Wolf-Rayet galaxy Mrk 178 with the NICMOS camera aboard Hubble Space Telescope. The galaxy is well resolved into individual stars in the near-IR; photometry in J and H yields color-magnitude diagrams containing 791 individual point sources. We discuss the stellar content, drawing particular attention to the intermediate-age and/or old stars. Mrk 178 is only the second blue compact dwarf galaxy in which the red giant branch has been resolved, indicating stars with ages of at least 1–2 Gyr. This allows us to derive a distance of ≥4.2(±0.5) Mpc. The near-IR color-magnitude diagram also exhibits an abundance of luminous, asymptotic giant branch stars. We find that this requires vigorous star formation several hundred Myr ago. Some candidate carbon stars are identified via their extreme near-IR color. We argue that Mrk 178 is fundamentally an old galaxy, based on the NICMOS detection of red giants underlying the blue, starburst core, and its extended, faint halo of redder color.